Handbook of Clinical Neurology, Vol. 123 (3rd series) Neurovirology A.C. Tselis and J. Booss, Editors © 2014 Elsevier B.V. All rights reserved
Chapter 11
Herpes simplex virus REBECCA W. WIDENER AND RICHARD J. WHITLEY* Department of Pediatrics, University of Alabama at Birmingham, Birmingham, AL, USA
INTRODUCTION Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) are members of the large family of herpesviruses. They are further categorized in the subfamily of Alphaherpesvirinae, along with varicella-zoster. These viruses are recognized by their short reproductive cycle, prompt destruction of the host cell, and ability to establish latency within sensory ganglia (Whitley, 2004). Though not exclusive of each other, HSV-1 is well known for causing orofacial lesions and encephalitis of children and adults. HSV-2 causes genital herpes, aseptic meningitis, and devastating infections of the neonate. Significant advances have been made in the diagnosis and treatment of these diseases.
VIRAL COMPOSITION AND REPLICATION HSV-1 and HSV-2 are composed of a large, doublestranded DNA core, an icosohedral capsid, an amorphous protein tegument, and an outer lipid bilayer envelope. Through cryoelectron tomography, the virion is seen as a spherical particle with spikes located on the outer envelope, extending its diameter to 225 nm. The capsid is offset, eccentrically located with one pole close to the envelope and the other approximately 30 nm from the envelope (Grunewald et al., 2003). The viral DNA has 152–155 kbp with a molecular weight of approximately 100 106 Da. It is delineated into a unique long segment (UL) and a unique short segment (US) that is flanked by inverted repeats. The L and S components are covalently linked and capable of inverting about each other, producing orientations that lead to four different DNA isomers (Hayward et al., 1975). The DNA encodes at least 84 multifunctional proteins (Ward and Roizman, 1994). HSV-1 and HSV-2 are closely related, sharing 83% identically aligned
nucleotides and approximately 50% sequence homology (Kieff et al., 1972; Dolan et al., 1998). The tegument layer is an amorphous collection of at least 20 proteins critical to ensuring viral fecundity within the cell (Roizman et al., 2007). Notable proteins comprising the tegument include the virion host shutoff (VHS) protein encoded by the UL41 gene, which interacts with cellular proteins to facilitate mRNA degradation after infection, and VP16 virion transactivating protein, responsible for enhancing the viral replication genes of alpha transcripts (Whitley and Roizman, 2001). A more comprehensive catalog of herpes simplex gene products is summarized by Roizman et al. (2007). The lipid envelope that surrounds the virus contains 11 viral encoded glycoproteins (gB–gM) and is responsible for attachment and penetration into the host cell, as well as inciting the host inflammatory response. The lipids within the bilayer are acquired from the host (Roizman et al., 2007). Studies suggest the lipids are incorporated into the envelope during budding with vesicles of the Golgi network (van Genderen et al., 1994). HSV attaches to the host cell via three different receptors. The envelope primarily fuses with the plasma membrane, releasing the capsid into the cytoplasm. Once it reaches the nuclear pore, the capsid releases the viral DNA into the nucleus. Viral genome transcription, viral DNA replication, and assembly of new capsids take place in the nucleus. Transcription of viral proteins follows a temporal course of immediate early (alpha), early (beta), and late (gamma) gene products. To commence transcription, the host RNA polymerase II transcribes the viral DNA into alpha mRNA. The alpha gene products are responsible for regulation of viral replication, as well as ensuring transcription of beta proteins. Beta proteins are involved in DNA synthesis and packaging. Synthesis of viral DNA requires at least seven viral proteins. The viral proteins thymidine kinase, ribonucleotide
*Correspondence to: Richard J. Whitley, M.D., Department of Pediatrics, University of Alabama at Birmingham, CHB 303P, 1600 7th Avenue South, Birmingham, AL 35233-1711, USA. Tel: þ1-205-934-5316, E-mail: [email protected]
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reductase, dUTPase, and uracyl DNA glucosylase also regulate viral DNA synthesis. Following viral DNA synthesis, structural components and assembly of the capsid, tegument, and envelope are encoded by the gamma proteins. Viral proteins aid in packaging the DNA into preformed capsids (Roizman et al., 2007). The newly made virions are released from the nuclei and acquire the surrounding lipid envelope. This may occur in several ways: obtaining an envelope from the inner nuclear membrane, de-enveloped by the outer nuclear membrane, then subsequently re-enveloped by the Golgi membrane; budding from the inner nuclear membrane into the perinuclear space, enabling intraluminal transportation from the rough endoplasmic reticulum to Golgi cisternae; or by escaping out of dilated nuclear pores and obtaining cytoplasmic envelopment (van Genderen et al., 1994; Leuzinger et al., 2005; Wild et al., 2005). This entire process can take 18–20 hours (Roizman et al., 2007).
IMMUNOLOGYAND VIRAL DEFENSE There are several defenses used by HSV to combat the host response. It is believed that a large proportion of the HSV gene product is responsible for this very task. The viral protein ICP47 binds to the cellular transporter protein TAP1 or TAP2, thereby blocking degraded viral and cellular products from transport to the endoplasmic reticulum for major histocompatibility complex class I presentation on the cell surface. Viral proteins Us3, gJ, and gD block programmed cell death, as would normally be initiated by a disturbance in the cellular milieu. Herpes simplex selectively degrades mRNA, seemingly shutting down all cellular protein synthesis. As mentioned above, the VHS protein encoded by the UL41 gene mediates host and viral mRNA degradation (Whitley and Roizman, 2001). The viral protein ICP27 collaborates with VHS to further inhibit host mRNA biogenesis (Hardwicke and Sandri-Goldin, 1994; Song et al., 2001). The cellular protein eIF-2a is capable, once phosphorylated, of blocking all protein synthesis within the cell. However, the protein g134.5 is a viral product that dephosphorylates eIF-2-alpha, thereby ensuring that viral protein synthesis commences (Chou et al., 1995; He et al., 1997). This diploid gene also plays a critical role in the neurovirulence of HSV, as will be noted below.
LATENCY, REACTIVATION, AND NEUROVIRULENCE Once infection is initiated on mucosal surfaces, HSV-1 and HSV-2 fuse with the axon termini and travel in a retrograde fashion along sensory fibers to establish latency in the trigeminal and sacral ganglion nuclei, respectively. During latency, the viral genome assumes a circular conformation, and latency-associated transcripts (LAT) are
the primary viral gene products expressed (Roizman et al., 2007). The role of LATs has been extensively studied and they have been shown to be protective against neuronal cell death by preventing apoptosis, promoting axon regeneration, or reducing viral gene expression (Perng et al., 2000; Bloom, 2004; Hamza et al., 2007; Li et al., 2010). The sensory ganglia serve as a reservoir for the virus until time of reactivation, upon which the virus travels back down the sensory nerves to the cutaneous nerve endings, resulting in a localized vesicular eruption, usually at the site of the initial infection. There are many triggers to reactivation, including physical or emotional stress, local trauma, fever, immunosuppression, exposure to ultraviolet light, menstruation, or hormonal imbalances. The exact mechanism of signal transduction between the inciting trigger and reactivation is unknown (Roizman et al., 2007). Induction of reactivation brings with it decreased levels of LAT while lytic gene transcripts are activated (Spivack and Fraser, 1988). HSV has the distinguishing property of neurovirulence; it has the capacity both to invade and replicate in neural tissue. This ability is achieved by several viral genes, but most importantly g134.5. The mechanism by which this gene functions in neurovirulence is mediated through PKR and eIF-2 alpha. Mutants deficient of this gene are unable to invade and replicate in the central nervous system (CNS) and latency is significantly diminished. For these reasons, g134.5-deficient mutants have become a target of promising vectors for gene therapy and vaccine production (Roizman et al., 2007).
EPIDEMIOLOGY Inasmuch as HSV-1 and HSV-2 share genetic homology, their epidemiology also has similarities. They both occur worldwide in developed and developing countries without seasonal variation. Humans are the only natural host. Viral transmission occurs with close contact, commonly presents asymptomatically, rarely causes a fatal infection, and establishes latency, as described above. For these reasons, the seroprevalence of herpes simplex remains high: over one-third of the world’s population has clinically recurrent HSV infections (Whitley and Roizman, 2009). Recurrent orolabial herpes infection is largely due to HSV-1; one study highlighted cases of orolabial herpes due to HSV-2 at a minimum of 6% (Cowan et al., 1996). Genital herpes infections, however, are caused by either HSV-1 or HSV-2. When a person who is seronegative for both HSV-1 and HSV-2 develops a herpes simplex infection, this is known as a “primary infection.” If the person is seropositive for either and develops a recurrence of this infection, this is called a “recurrent infection.” If a person is seropositive for one type and develops a herpetic infection of the other
HERPES SIMPLEX VIRUS 253 type, it is pathologically distinguished as an “initial Heterosexual men show a slightly less frequent seroconinfection,” or “non-primary first-episode” infection version, of zero, 20%, 35%, and 70% for one lifetime (Whitley et al., 1998). partner, 2–10 partners, 11–50 partners, and Other demographic information, such as geographic >50 partners, respectively (Whitley et al., 1998). The location, socioeconomic status, age, and race also factor seroprevalence in the university setting was most into the prevalence of the virus. In developing countries, recently found to be 3.4%, with a yearly acquisition rate HSV-1 seroconversion occurs at a young age. By age 5, of 2% (Nahmias et al., 1990; Mark et al., 2008). approximately one-third of the population has antibodies Compared to primary genital herpes infections, to HSV-1. This peaks to 70–80% during adolescence. By recurrent genital herpes simplex infections vary slightly comparison, the middle and upper classes of developed less between males and females. Males have 2.7 recurcountries seroconvert later in life – only about one-fifth rent infections per 100 days compared to 1.9 recurrences of the population by age 5 – with no substantial increase for females (Corey et al., 1983). Recurrences occur in frequency until it peaks at age 20–40 years at 40–60% more frequently in the first year following the initial (Whitley and Roizman, 2009). In 1990, Nahmias et al. infection and are increased with HSV-2 viral types comreported a 5–10% incidence of HSV-1 in university stupared to HSV-1 (Lafferty et al., 1987; Whitley and dents. That percentage is likely much higher today. In Roizman, 2009). the United States, seroprevalence is also influenced by Asymptomatic shedding of genital HSV is common. race. African Americans have a higher prevalence of It is also more frequent in the first year following a priHSV-1 by age 5 compared to Caucasians – 35% and mary episode and with HSV-2 compared to HSV-1. Sub18%, respectively. According to the Centers for Disease clinical shedding is detected in 12%, 18%, and 23% of Control and Prevention National Health and Nutrition women with primary HSV-1, primary HSV-2, and recurExamination Surveys (NHANES), this twofold differrent HSV-2 infections, respectively, using viral culture ence is maintained throughout adolescence (Centers (Adam et al., 1979; Koelle et al., 1992). The rate of detecfor Disease Control and Prevention, 2009). By the fourth tion of genital HSV by polymerase chain reaction (PCR) decade of life this disparity narrows to approximately has been shown to be 3.5 times higher than viral isolation 60% of Caucasians having HSV-1 antibodies, compared (Wald et al., 1997). Asymptomatic shedding of recurrent to 80% of African Americans (Xu et al., 2006). HSV-2 can be detected in 1–5% of days when cultured HSV-2 infections are transmitted by sexual contact, (Whitley et al., 1998) and up to 25% of all days by thus seroconversion does not usually occur until the PCR detection of viral DNA. Interestingly, shedding onset of sexual activity. Among 14–19-year-olds, acquican be intermittent on the same day, as demonstrated sition of HSV-2 infection, as demonstrated by the develby PCR. opment of type-specific antibodies, occurs in 1.4% of the Importantly, HSV-2 has a disastrous synergy with population, increasing to 26% by age 50. The most recent human immunodeficiency virus (HIV). The risk of NHANES reports that of those found to be infected with acquiring HIV is increased two- to threefold if a person HSV-2, 80% had not yet been diagnosed (Centers for is infected with HSV-2 (Wald and Link, 2002; Freeman Disease Control and Prevention, 2010). Similar to et al., 2006). Mucosal disruption from herpetic genital HSV-1, race also has an influence on the acquisition of ulcers, even seemingly microscopic ulcerations, faciliinfection. African Americans show a threefold increased tates an adequate portal of entry for HIV. The herpetic prevalence compared to Caucasians (39% versus 12%), a ulcers, in turn, have an influx of CD4 lymphocytes, prodisparity that has remained stable since 1988. Females viding a large reservoir of cells for HIV to target (Wald also have a disproportionately higher seroprevalence and Link, 2002). HSV upregulates HIV replication; HIV (21%) compared to males (11%) (Xu et al., 2006; infection reciprocates, increasing the frequency of Centers for Disease Control and Prevention, 2010). recurrent HSV-2 outbreaks (Mosca et al., 1987; Females are more susceptible to herpes genital infections Margolis et al., 1992; Celum, 2010). Patients who are than men, most likely due to an increased mucosal seropositive for both HSV-2 and HIV have shown benesurface. fit with acyclovir or valacyclovir suppressive therapy by As would be expected, the number of sexual partners decreasing plasma HIV viral load. This was found durhas a direct effect on the seroconversion rate. In the ing times of symptomatic and asymptomatic HSV-2 United States, for heterosexual women who have one shedding (Schacker et al., 2002; Nagot et al., 2007; partner, the probability of acquiring HSV-2 infection Celum et al., 2010). is 80% if she has 11–50 HSV-2 during pregnancy is 2.5% with each gestation. partners and >50 partners, respectively. The rates of Transmission from a pregnant mother to her unborn HSV-2 acquisition for homosexual men are similar. child is most often related to shedding of the virus at
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the time of delivery and the type of maternal infection (primary versus recurrent). Regardless of history, 0.5–1% of women are excreting the virus during that time (Whitley et al., 1998).
oligodendrocytic involvement, gliosis, and astrocytosis are common (Booss and Kim, 1984; Whitley, 2004).
CENTRAL NERVOUS SYSTEM PATHOLOGY
The incidence of neonatal herpes is estimated at 1 in 3200 live births, resulting in approximately 1500 cases annually in the United States. The vast majority is caused by primary HSV genital maternal infections, predominantly HSV-2. The maternal acquisition of genital herpes has been thoroughly evaluated by Brown et al. In a study following 7046 susceptible women from their first prenatal visit, they found 2% acquired HSV infection during pregnancy, as determined by seroconversion and the development of type-specific antibodies. This acquisition rate was equal throughout each trimester. Risk factors associated with converting were a younger age, mothers who were not married, and co-occurrence of other sexually transmitted diseases. Of the pregnant women who seroconverted for either type, two-thirds of them did so without any symptoms. Of those who had symptomatic infection, the majority of infection manifested as genital lesions (Brown et al., 1997). Indeed, in the overwhelming majority of neonatal HSV infections there is no maternal history of a genital infection or symptoms at time of delivery, nor is there a history of a sexual partner with recurrent lesions (Whitley et al., 1988). Although the rate of acquiring a new HSV infection is equal throughout each trimester, the timing of the acquisition is important to the development of neonatal disease. The chance of infecting the newborn is less than 3% if the mother seroconverts prior to labor. This is increased when the maternal infection is encountered near the time of labor, sparing the baby of otherwise protective transplacental antibodies (Brown et al., 1997). During labor, the rate of transmitting the virus from mother to the newborn is also dependent on, among other factors, maternal primary versus recurrent infection or the application of fetal scalp monitors. There is a much greater risk of transmitting the virus to the newborn for mothers with primary infection (57%) during the third trimester of gestation, regardless of virus type, versus non-primary, first-episode infection (25%) or recurrent infection (2%). This highlights the increased viral burden seen during primary infections and the type-specific maternal antibodies, specifically to HSV2, likely conveying some degree of protection to the infant during non-primary infections. Duration of rupture of membranes greater than 6 hours, mode of delivery, and integrity of the newborn’s mucocutaneous barriers (to include placement of fetal scalp electrodes) are also influential (Brown et al., 2003; Kimberlin, 2004).
HSV causes two devastating infections of the CNS that must be distinguished. These are neonatal HSV infection that occurs in children less than 1 month of age, and herpes simplex encephalitis (HSE), which occurs in individuals generally over 3 months of age. Both of these infections are life-threatening and can result in significant neurologic morbidity. Additionally, less severe manifestations of disease occur, such as HSV aseptic meningitis and the radiculopathies. Grossly, herpetic encephalitis produces inflammation, congestion, and/or hemorrhage, leading to liquefactive necrosis of the brain tissue (Fig. 11.1). The pathologic findings with CNS disease are similar from a microscopic perspective. Early in the infection, changes are non-specific. Capillary congestion within the cortex and subcortical white matter are appreciated. Petechiae, hemorrhagic necrosis, and perivascular cuffing can be observed as a result. During the first week of infection, eosinophilic nuclear inclusions (Cowdry type A) may be seen in half of patients, supporting the diagnosis of a viral infection. These have a homogeneous appearance surrounded by a halo secured by a marginated rim of chromatin. After the second week of infection, glial nodules are commonly seen. Extensive hemorrhagic necrosis sets in. Inflammation becomes evident, with meningeal infiltration, perivascular subarachnoid mononuclear infiltration, perivascular infiltration of the gray and white matter, non-specific gliosis, and satellitosis neuronophagia. Later in the disease process,
Fig. 11.1. Hemorrhagic necrosis of the temporal lobe is seen on a gross pathology specimen of an adult with herpes simplex encephalitis (HSE).
NEONATAL HERPES SIMPLEX INFECTIONS
HERPES SIMPLEX VIRUS Indeed, cesarean section can be preventive in transmitting the virus in a mother who is actively shedding the virus if performed prior to or within 4 hours of membrane rupture (Parvy and Ch’ien, 1980; Brown et al., 2003). Neonatal herpes has occurred, however, despite these delivery precautions. Timing of the infection occurs overwhelmingly in the peripartum period, 85%, followed by postnatal acquisition, 10%, and rarely in utero, 5%. Peripartum and postnatal neonatal herpes is subsequently classified based on clinical symptoms and laboratory findings. Lesions localized to the skin, eyes, and/or mouth only is termed SEM disease and accounts for approximately 45% of neonatal HSV. Neonatal encephalitis, or CNS disease, occurs in 30% of neonatal HSV infections. The remaining 25% of cases are disseminated disease, resulting in multiorgan infection, including the CNS, lungs, liver, adrenal gland, skin, eyes, and/or mouth (Whitley et al., 1988).
In utero HSV disease In utero or congenital neonatal herpes infection is a result of either transplacental infection or ascension from a cervical or vulvar infection late in gestation. It is recognized within the first 48 hours of life. If the transmission occurred during the first 20 weeks of gestation, spontaneous abortion, stillbirth, skin lesions or scarring, limb hypoplasia, and other congenital malformations with severe neurologic insults can result. The spectrum of CNS injuries includes microcephaly, hydranencephaly, cystic encephalomalacia, cortical and cerebellar atrophy, intracranial hemorrhage and calcifications, chorioretinitis, and micropthalmia (South et al., 1969; Hutto et al., 1987; Baldwin and Whitley, 1989; Gray et al., 1992; Johansson et al., 2003; Lee et al., 2003).
Disseminated disease Of the peri- and postnatally acquired HSV infections, disseminated disease has the most devastating prognosis, with a 30% mortality rate despite proper treatment (Kimberlin et al., 2001a). Newborns present clinically between 10 and 12 days of life with multiorgan involvement characterized as lethargy, irritability, respiratory distress, disseminated intravascular coagulopathy, seizures, jaundice, and/or vesicular rash. Importantly, the skin findings will be absent in approximately 20% of cases. Encephalitis commonly occurs in 60–75% of infants with disseminated disease (Whitley et al., 1998; Kimberlin et al., 2001b). For this classification of disease, the significant predictors of mortality are prematurity, lethargy, a state of coma, disseminated intravascular coagulopathy, or pneumonitis (Whitley et al., 1991).
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Neonatal CNS disease (encephalitis) Almost half of neonatal HSV infections will involve the CNS, either as its sole manifestation or with disseminated disease. CNS disease in neonates presents to medical attention later than SEM or disseminated disease, at around 15–19 days of life, with lethargy, irritability, poor feeding, temperature instability, seizures, bulging fontanel, and pyramidal tract signs. Notably, newborns can develop diffuse encephalitis as compared to the classic focal disease found in older children and adults. This diffuse process will usually result in encephalomalacia. Characteristic skin findings will be absent in 40% of cases (Whitley et al., 1998). Seizures can be focal or generalized, and if present at the time of starting antivirals, are shown to be a poor prognostic factor on development by 12 months of age (Whitley et al., 1991; Kimberlin et al., 2001b). Mortality for localized CNS disease with treatment is 6%, although morbidity remains high, with 70% having neurologic sequelae (Kimberlin et al., 2001a).
SEM disease Disease that is localized to the skin, eyes, and/or mouth only is found in 40% of neonates with HSV infection. These findings appear near 10–12 days of life. Clinical manifestations include a combination of discrete lesions of the skin, found in approximately 80% of localized disease, mouth, and keratoconjunctivitis (Kimberlin et al., 2001b). With proper treatment, mortality is absent. Recurrences following treatment are common over the first 6 months of life, and may extend longer (Whitley et al., 1998).
Diagnosis Although it is not recommended that every neonate who presents for medical attention for a sepsis rule-out be evaluated and treated for HSV, a high index of suspicion must be maintained if a newborn has clinical findings or maternal history concerning for exposure. This is especially true in infants under 1 month of age and in illappearing infants, even in the absence of skin lesions, as 20–40% of cases never develop lesions. The diagnosis of neonatal HSV should include evaluating for dissemination or CNS disease with a complete blood cell count, liver function tests, chest X-ray if respiratory disease is present, and cerebrospinal fluid (CSF) studies. Surface specimens should be collected and sent for isolation of the virus. This should include viral cultures of the skin or mucosal lesions if present, conjunctiva, oropharynx, and rectum. Urine and CSF should also be sent for viral culture. Herpes simplex generally grows rapidly in culture from skin and mucosal surfaces,
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producing relatively rapid and sensitive results if present. Cytopathic effects can be seen in 80% of cultures within 2 days of incubation and 94% of cultures by the fourth day (Callihan and Menegus, 1984). More than 90% of viral cultures will be positive from the skin or conjunctiva, regardless of neonatal disease classification, about half of oropharyngeal cultures yield a positive result, and 40% of CSF or brain biopsy cultures in neonates with CNS involvement are positive (Kimberlin et al., 2001b). While PCR has largely replaced virus culture as the diagnostic method of choice, a virus isolate is of value for both rapid typing and assessment of resistance, if necessary. The utilization of PCR has been of significant value for diagnostic use as well as for the study of disease pathogenesis. CSF should be sent for HSV PCR, as its increased sensitivity over culture has proven itself consistently as the gold standard for diagnosing neurologic involvement. This is as true for children and adults with HSE as it is for neonates. The overall sensitivity of PCR in neonates has been reported as 80–100% (Kimura et al., 1990; Troendle-Atkins et al., 1993; Kimberlin et al., 1996a). CSF PCR has yielded a positive result in 25% of neonates with localized SEM infection, reflecting asymptomatic infection of the CNS (Kimberlin et al., 1996a). Serum can also be sent for PCR. HSV has been detected by PCR in the serum of neonates infected with localized SEM disease and CNS disease, in contrast with the serum of children and adults with HSE, where it is not seen. The detection of HSV DNA in the CNS and serum of infants with SEM disease is of concern, as it suggests the possibility of dissemination, highlighting neonatal HSV disease as more of a spectrum rather than a discrete categorization. Indeed, PCR is also positive in the serum of neonates with disseminated disease, in keeping with the theory of viremic spread in neonates with multiorgan involvement. Prolonged duration of detection following initiation of therapy has been seen in the serum during disseminated disease and in the CSF of neonates with CNS involvement of up to 3 weeks (Kimura et al., 1990). PCR of the CSF is also pertinent in guiding therapy. All patients with HSV CNS involvement should have a repeat lumbar puncture at the end of the intravenous acyclovir course to document clearance of the virus. Babies with a persistently positive PCR from the CSF upon completion of therapy have a worse outcome. A repeat positive result would warrant a prolonged duration of intravenous treatment, as discussed below (Kimberlin et al., 1996a; Malm and Forsgren, 1999; Mejias et al., 2009). Similar to HSE of children and adults, and herpes simplex meningitis, serologic studies do not have a role in diagnosing neonatal herpes infections. Serologic testing of the mother during pregnancy may help identify
discordant couples, and thus prompt counseling of women who are seronegative on the rates of seroconverting and the higher risk of transmitting a primary herpesvirus during pregnancy (Kimberlin, 2004). Neuroimaging is an integral adjuvant to the diagnosis. As opposed to HSE in older individuals caused by HSV1, neonatal HSV-2 encephalitis usually affects the brain more diffusely, although it can selectively affect the temporal lobe. A computed tomography (CT) scan early in the disease course often may be normal or only have very subtle pathologic findings. It can reveal hypodensities in the periventricular white matter, with relative sparing of the basal ganglia, thalami, and posterior fossa. Later in the disease, focal hemorrhagic necrosis, calcifications, and/or encephalomalacia may be seen (Tien et al., 1993). Early magnetic resonance imaging (MRI) studies show diffuse loss of gray- and whitematter differentiation on the T1- and T2-weighted images. The basal ganglia and deep gray-matter regions may very well be involved. Diffusion-weighted images can delineate more extensive involvement over conventional MRI or CT. This appears to be especially true for neonates, as their brains have increased water content, making it difficult to discern from infection on early conventional MRI images. Images produced by fluid attenuated inversion recovery are poor in a neonate, accounting for a more immature myelination compared to children and adults (Vossough et al., 2008). Figure 11.2 illustrates a rare case of congenital HSV disease, highlighting the severe, diffuse nature of CNS involvement with resulting encephalomalacia.
Treatment Current antiviral therapy has reduced mortality in neonatal disseminated disease from 85% in the pretreatment era to 30% and those with CNS disease from 50% to 4%. Neonatal herpes, regardless of classification, should be treated with intravenous acyclovir for the entire duration of therapy. Acyclovir at 60 mg/kg/day divided in three doses has been shown to reduce mortality drastically compared to standard dosing. The duration for disseminated and CNS disease is 21 days; SEM disease warrants 14 days of therapy. Due to its devastating nature, treatment should not be delayed pending laboratory results. For any process with CNS involvement, a repeat lumbar puncture at the end of therapy has become the standard to document a negative CSF PCR result, thereby allowing cessation of therapy. If the repeat PCR remains positive, then therapy needs to continue until a further repeat lumbar puncture is negative. The most common side-effect of acyclovir is neutropenia, and serial absolute neutrophil counts are followed during therapy (Kimberlin et al., 2001a). Suppressive
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Fig. 11.2. In utero herpes simplex virus type 2 infection in a baby born at 27 weeks. (A) Skin lesions were apparent at delivery. Keratitis and chorioretinitis were also found on ophthalmologic exam. (B) Magnetic resonance imaging obtained at 6 days old shows T1 bilateral periventricular hyperintensities and hydrocephalus. (C) Follow-up computed tomography scan with contrast enhancement at 6 months of age shows significant cerebral atrophy and calcifications within the basal ganglia and periventricular regions.
therapy with oral acyclovir at a dose of 300 mg/m2/dose given two to three times a day for at least 6 months has been shown to reduce HSV recurrences in neonates with SEM disease and has benefit in those with CNS disease on neurologic outcome (Kimberlin et al., 1996b, 2011).
HSV ENCEPHALITIS HSE is the most common cause of sporadic, fatal encephalitis globally (Whitley et al., 1998). Outside of the neonatal period, essentially all cases of HSE in otherwise healthy children and adults are caused by HSV-1; a small portion (10%) are due to HSV-2 and are more commonly described in immunocompromised patients (Aurelius et al., 1993; Schiff and Rosenblum, 1998; Mommeja-Marin et al., 2003; Omland et al., 2008; Bradford et al., 2009). In the setting of HSV-1, HSE results from primary infections in one-third of cases and reactivation in two-thirds of cases (Nahmias et al., 1982). The small percentage of HSE due to type 2 is attributed mainly to primary disease (Aurelius et al., 1993). The US incidence is approximately 1250 cases per year. It has a bimodal distribution, occurring in one-third of cases in people under 20 years old, and one-half of cases over 50 years (Tyler, 2004). HSE causes localized pathology, resulting in inflammation and necrosis most commonly in the medialtemporal lobes and orbital-frontal regions of the brain. Herpetic brainstem encephalitis has also been described sparsely in the literature, where the extent of injury is concentrated within the brainstem rather than the cerebrum (Tyler et al., 1995). The initiating event and subsequent pathogenesis of HSE remain unclear. It has been speculated in the course of primary HSV-1 infections that the virus invades the olfactory bulbs by way of the nares, thereby gaining access to the orbitofrontal and medial temporal lobe. Others have postulated infection by route of the trigeminal nerve following a primary infection of
the oral cavity (Barnett et al., 1994). HSE caused by reactivation has been proposed as the latent virus in the trigeminal ganglion gaining transneuronal spread to the known locales in the brain (Davis and Johnson, 1979). This is also the proposed mechanism of Tyler et al. (1995), in a case of confirmed HSV recurrent brainstem encephalitis in a patient with recurrent herpes labialis. Clinically, HSE can present with headache, fever, speech disturbances, confusion, personality changes, and localized neurologic signs and seizures. The signs are indicative of the area of brain it is affecting. Symptoms develop acutely over a few days (Whitley et al., 1982). Decreased level of consciousness progresses and without treatment mortality exceeds 70%; of those who survive, only 2.5% recover normal neurologic status (Ward and Roizman, 1994).
Diagnosis To aid in diagnosing HSE, routine procedures are employed using CSF evaluation, electroencephalography (EEG), and neuroimaging. CSF findings characteristically reveal elevated white blood cell counts (WBCs). In the early phase of infection, the total number of WBC may be normal or have a neutrophil predominance. Nevertheless, the CSF findings change with WBCs, rapidly increasing to a few hundred million cells per liter with a mononuclear predominance. The amount of red blood cells in half of cases is 10 106 cells per liter. Many are even reported at 100–1000 106 cells per liter. These elevations may be indicative of intracerebral hemorrhage, damaged blood–brain barrier, or traumatic procedure. In the first week of infection, half of patients have a normal CSF protein level. The protein then increases well above the upper level of normal (Whitley et al., 1982; Koskiniemi et al., 1984). Glucose traditionally is normal; however
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hypoglycorrhachia may also be seen. One series reported 50% of 34 encephalitis and meningitis HSV-PCRpositive CSF samples to have hypoglycorrhachia (Davis et al., 2004). Importantly, 3% of cases will have totally normal CSF findings and about 20% of CSF studies never have evidence of red blood cells (Whitley et al., 1982). While isolation of the virus is, of course, integral to confirming the diagnosis, historically only brain biopsy provided tissue for virus isolation. PCR has now become the gold standard for the assessment of HSV involvement of the CNS, not only for initial diagnostics, but for evaluating response to treatment as well. It has repeatedly proven itself as highly specific, sensitive, and rapid. The specificity and sensitivity approach nearly 100% for both children and adults. HSV PCR will continue to detect viral DNA in the spinal fluid within the first week following therapy (Aurelius et al., 1991; Lakeman and Whitley, 1995). In the second week of therapy, the PCR will remain positive in 50% of patients. Beyond day 15 of therapy, 20% or less will have a positive result (Lakeman and Whitley, 1995). CSF cultures of HSV rarely are positive in children >6 months and adults with HSE, and thus are not routinely recommended (Nahmias et al., 1982; Boivin, 2004; Tyler, 2004). CSF serology for HSV is also not of benefit for acutely diagnosing encephalitis. Whereas intrathecal HSV antibodies do increase significantly, it can take weeks, thereby delaying the diagnosis. It is impossible to discern if the presence of increased intrathecal antibodies is due to local production, or leakage from the disrupted blood–brain barrier. CSF serology may, however, aid in retrospectively diagnosing the patient by seeing increasing titers in the CSF (with a sensitivity of 70–90% and specificity of 81–88%) or decreasing serum-to-CSF antibody ratios of 20 (a sensitivity of 50% and specificity of 81% in the first 10 days of infection) (Nahmias et al., 1982). Serum serology will provide information on whether the infection is of primary or recurrent nature only. There is currently no value in knowing the serum antibody status in regard to clinical features, treatment, or prognosis (Tyler, 2004). The EEG during the first week of infection shows non-specific focal slowing, followed by spike and wave patterning of the temporal region. Periodic lateralizing epileptiform discharges may develop and help further suggest a diagnosis of HSE, though are not pathognomonic. Neuroimaging may show edema early in the course, followed by hemorrhage and midline shift (Whitley et al., 1998). MRI is the modality of choice over CT, given its higher sensitivity and enhanced resolution. CT scans may often be normal or show non-specific hypodensities of the temporal lobes (Tien et al., 1993; Raschilas et al., 2002). Early MRI highlights the edema as high signal intensities on T2-weighted images, usually
Fig. 11.3. Herpes simplex encephalitis infection in an adult showing localized temporal lobe involvement on magnetic resonance imaging T2-weighted image.
involving the orbital surfaces of the frontal lobes, inferomedial temporal lobes, insular cortex, and external capsula (Fig 11.3). The cingulate gyrus has also been known to be involved later in the infection (Tien et al., 1993). There is a significant correlation between CSF PCRpositive patients and temporal lobe lesions seen on MRI. Brainstem, temporal superficial cortex, and basal ganglia are less commonly affected sites (Domingues et al., 1998). When findings on conventional MRI images are subtle or normal, diffusion-weighted imaging may aid in increasing the sensitivity (McCabe et al., 2003).
Treatment Due to its fatal nature if left untreated and a high propensity to cause permanent neurologic damage in those who do survive, a high index of suspicion should be maintained in patients who present with a decreasing level of consciousness, abnormal CSF indices, and focal neurologic exam. Treatment for suspected or confirmed HSE is with intravenous acyclovir, 30 mg/kg/day for 14–21 days (Skoldenberg et al., 1984; Whitley et al., 1986). Higher doses have been associated with neurotoxicity and impaired renal function. Treatment should not be withheld pending results of confirmatory lab data. A lumbar puncture for HSV should be performed at the end of therapy to document clearance of the virus as detected by PCR. Should this persist, an extended acyclovir course is warranted (Kimberlin, 2007). Resistance to acyclovir is possible with HSV infections, especially in the immunocompromised patient. This occurs through mutations in the viral gene for thymidine kinase (TK). Either TK-deficient viral progeny are made, or TK that is unable to phosphorylate the acyclovir is the mutation. In vitro, these resistant mutants are susceptible to foscarnet and vidarabine; however only foscarnet has been shown to be clinically effective (Whitley et al., 1998).
HERPES SIMPLEX VIRUS
Prognosis Even in the face of adequate treatment, the mortality and morbidity for HSE remain high. Although acyclovir has reduced mortality from 70% in those untreated to 20%, approximately 60% of survivors continue to have impaired neurologic function. Factors shown to carry a poor prognosis include patients with a Glasgow coma score
Chapter 11
Herpes simplex virus REBECCA W. WIDENER AND RICHARD J. WHITLEY* Department of Pediatrics, University of Alabama at Birmingham, Birmingham, AL, USA
INTRODUCTION Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) are members of the large family of herpesviruses. They are further categorized in the subfamily of Alphaherpesvirinae, along with varicella-zoster. These viruses are recognized by their short reproductive cycle, prompt destruction of the host cell, and ability to establish latency within sensory ganglia (Whitley, 2004). Though not exclusive of each other, HSV-1 is well known for causing orofacial lesions and encephalitis of children and adults. HSV-2 causes genital herpes, aseptic meningitis, and devastating infections of the neonate. Significant advances have been made in the diagnosis and treatment of these diseases.
VIRAL COMPOSITION AND REPLICATION HSV-1 and HSV-2 are composed of a large, doublestranded DNA core, an icosohedral capsid, an amorphous protein tegument, and an outer lipid bilayer envelope. Through cryoelectron tomography, the virion is seen as a spherical particle with spikes located on the outer envelope, extending its diameter to 225 nm. The capsid is offset, eccentrically located with one pole close to the envelope and the other approximately 30 nm from the envelope (Grunewald et al., 2003). The viral DNA has 152–155 kbp with a molecular weight of approximately 100 106 Da. It is delineated into a unique long segment (UL) and a unique short segment (US) that is flanked by inverted repeats. The L and S components are covalently linked and capable of inverting about each other, producing orientations that lead to four different DNA isomers (Hayward et al., 1975). The DNA encodes at least 84 multifunctional proteins (Ward and Roizman, 1994). HSV-1 and HSV-2 are closely related, sharing 83% identically aligned
nucleotides and approximately 50% sequence homology (Kieff et al., 1972; Dolan et al., 1998). The tegument layer is an amorphous collection of at least 20 proteins critical to ensuring viral fecundity within the cell (Roizman et al., 2007). Notable proteins comprising the tegument include the virion host shutoff (VHS) protein encoded by the UL41 gene, which interacts with cellular proteins to facilitate mRNA degradation after infection, and VP16 virion transactivating protein, responsible for enhancing the viral replication genes of alpha transcripts (Whitley and Roizman, 2001). A more comprehensive catalog of herpes simplex gene products is summarized by Roizman et al. (2007). The lipid envelope that surrounds the virus contains 11 viral encoded glycoproteins (gB–gM) and is responsible for attachment and penetration into the host cell, as well as inciting the host inflammatory response. The lipids within the bilayer are acquired from the host (Roizman et al., 2007). Studies suggest the lipids are incorporated into the envelope during budding with vesicles of the Golgi network (van Genderen et al., 1994). HSV attaches to the host cell via three different receptors. The envelope primarily fuses with the plasma membrane, releasing the capsid into the cytoplasm. Once it reaches the nuclear pore, the capsid releases the viral DNA into the nucleus. Viral genome transcription, viral DNA replication, and assembly of new capsids take place in the nucleus. Transcription of viral proteins follows a temporal course of immediate early (alpha), early (beta), and late (gamma) gene products. To commence transcription, the host RNA polymerase II transcribes the viral DNA into alpha mRNA. The alpha gene products are responsible for regulation of viral replication, as well as ensuring transcription of beta proteins. Beta proteins are involved in DNA synthesis and packaging. Synthesis of viral DNA requires at least seven viral proteins. The viral proteins thymidine kinase, ribonucleotide
*Correspondence to: Richard J. Whitley, M.D., Department of Pediatrics, University of Alabama at Birmingham, CHB 303P, 1600 7th Avenue South, Birmingham, AL 35233-1711, USA. Tel: þ1-205-934-5316, E-mail: [email protected]
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reductase, dUTPase, and uracyl DNA glucosylase also regulate viral DNA synthesis. Following viral DNA synthesis, structural components and assembly of the capsid, tegument, and envelope are encoded by the gamma proteins. Viral proteins aid in packaging the DNA into preformed capsids (Roizman et al., 2007). The newly made virions are released from the nuclei and acquire the surrounding lipid envelope. This may occur in several ways: obtaining an envelope from the inner nuclear membrane, de-enveloped by the outer nuclear membrane, then subsequently re-enveloped by the Golgi membrane; budding from the inner nuclear membrane into the perinuclear space, enabling intraluminal transportation from the rough endoplasmic reticulum to Golgi cisternae; or by escaping out of dilated nuclear pores and obtaining cytoplasmic envelopment (van Genderen et al., 1994; Leuzinger et al., 2005; Wild et al., 2005). This entire process can take 18–20 hours (Roizman et al., 2007).
IMMUNOLOGYAND VIRAL DEFENSE There are several defenses used by HSV to combat the host response. It is believed that a large proportion of the HSV gene product is responsible for this very task. The viral protein ICP47 binds to the cellular transporter protein TAP1 or TAP2, thereby blocking degraded viral and cellular products from transport to the endoplasmic reticulum for major histocompatibility complex class I presentation on the cell surface. Viral proteins Us3, gJ, and gD block programmed cell death, as would normally be initiated by a disturbance in the cellular milieu. Herpes simplex selectively degrades mRNA, seemingly shutting down all cellular protein synthesis. As mentioned above, the VHS protein encoded by the UL41 gene mediates host and viral mRNA degradation (Whitley and Roizman, 2001). The viral protein ICP27 collaborates with VHS to further inhibit host mRNA biogenesis (Hardwicke and Sandri-Goldin, 1994; Song et al., 2001). The cellular protein eIF-2a is capable, once phosphorylated, of blocking all protein synthesis within the cell. However, the protein g134.5 is a viral product that dephosphorylates eIF-2-alpha, thereby ensuring that viral protein synthesis commences (Chou et al., 1995; He et al., 1997). This diploid gene also plays a critical role in the neurovirulence of HSV, as will be noted below.
LATENCY, REACTIVATION, AND NEUROVIRULENCE Once infection is initiated on mucosal surfaces, HSV-1 and HSV-2 fuse with the axon termini and travel in a retrograde fashion along sensory fibers to establish latency in the trigeminal and sacral ganglion nuclei, respectively. During latency, the viral genome assumes a circular conformation, and latency-associated transcripts (LAT) are
the primary viral gene products expressed (Roizman et al., 2007). The role of LATs has been extensively studied and they have been shown to be protective against neuronal cell death by preventing apoptosis, promoting axon regeneration, or reducing viral gene expression (Perng et al., 2000; Bloom, 2004; Hamza et al., 2007; Li et al., 2010). The sensory ganglia serve as a reservoir for the virus until time of reactivation, upon which the virus travels back down the sensory nerves to the cutaneous nerve endings, resulting in a localized vesicular eruption, usually at the site of the initial infection. There are many triggers to reactivation, including physical or emotional stress, local trauma, fever, immunosuppression, exposure to ultraviolet light, menstruation, or hormonal imbalances. The exact mechanism of signal transduction between the inciting trigger and reactivation is unknown (Roizman et al., 2007). Induction of reactivation brings with it decreased levels of LAT while lytic gene transcripts are activated (Spivack and Fraser, 1988). HSV has the distinguishing property of neurovirulence; it has the capacity both to invade and replicate in neural tissue. This ability is achieved by several viral genes, but most importantly g134.5. The mechanism by which this gene functions in neurovirulence is mediated through PKR and eIF-2 alpha. Mutants deficient of this gene are unable to invade and replicate in the central nervous system (CNS) and latency is significantly diminished. For these reasons, g134.5-deficient mutants have become a target of promising vectors for gene therapy and vaccine production (Roizman et al., 2007).
EPIDEMIOLOGY Inasmuch as HSV-1 and HSV-2 share genetic homology, their epidemiology also has similarities. They both occur worldwide in developed and developing countries without seasonal variation. Humans are the only natural host. Viral transmission occurs with close contact, commonly presents asymptomatically, rarely causes a fatal infection, and establishes latency, as described above. For these reasons, the seroprevalence of herpes simplex remains high: over one-third of the world’s population has clinically recurrent HSV infections (Whitley and Roizman, 2009). Recurrent orolabial herpes infection is largely due to HSV-1; one study highlighted cases of orolabial herpes due to HSV-2 at a minimum of 6% (Cowan et al., 1996). Genital herpes infections, however, are caused by either HSV-1 or HSV-2. When a person who is seronegative for both HSV-1 and HSV-2 develops a herpes simplex infection, this is known as a “primary infection.” If the person is seropositive for either and develops a recurrence of this infection, this is called a “recurrent infection.” If a person is seropositive for one type and develops a herpetic infection of the other
HERPES SIMPLEX VIRUS 253 type, it is pathologically distinguished as an “initial Heterosexual men show a slightly less frequent seroconinfection,” or “non-primary first-episode” infection version, of zero, 20%, 35%, and 70% for one lifetime (Whitley et al., 1998). partner, 2–10 partners, 11–50 partners, and Other demographic information, such as geographic >50 partners, respectively (Whitley et al., 1998). The location, socioeconomic status, age, and race also factor seroprevalence in the university setting was most into the prevalence of the virus. In developing countries, recently found to be 3.4%, with a yearly acquisition rate HSV-1 seroconversion occurs at a young age. By age 5, of 2% (Nahmias et al., 1990; Mark et al., 2008). approximately one-third of the population has antibodies Compared to primary genital herpes infections, to HSV-1. This peaks to 70–80% during adolescence. By recurrent genital herpes simplex infections vary slightly comparison, the middle and upper classes of developed less between males and females. Males have 2.7 recurcountries seroconvert later in life – only about one-fifth rent infections per 100 days compared to 1.9 recurrences of the population by age 5 – with no substantial increase for females (Corey et al., 1983). Recurrences occur in frequency until it peaks at age 20–40 years at 40–60% more frequently in the first year following the initial (Whitley and Roizman, 2009). In 1990, Nahmias et al. infection and are increased with HSV-2 viral types comreported a 5–10% incidence of HSV-1 in university stupared to HSV-1 (Lafferty et al., 1987; Whitley and dents. That percentage is likely much higher today. In Roizman, 2009). the United States, seroprevalence is also influenced by Asymptomatic shedding of genital HSV is common. race. African Americans have a higher prevalence of It is also more frequent in the first year following a priHSV-1 by age 5 compared to Caucasians – 35% and mary episode and with HSV-2 compared to HSV-1. Sub18%, respectively. According to the Centers for Disease clinical shedding is detected in 12%, 18%, and 23% of Control and Prevention National Health and Nutrition women with primary HSV-1, primary HSV-2, and recurExamination Surveys (NHANES), this twofold differrent HSV-2 infections, respectively, using viral culture ence is maintained throughout adolescence (Centers (Adam et al., 1979; Koelle et al., 1992). The rate of detecfor Disease Control and Prevention, 2009). By the fourth tion of genital HSV by polymerase chain reaction (PCR) decade of life this disparity narrows to approximately has been shown to be 3.5 times higher than viral isolation 60% of Caucasians having HSV-1 antibodies, compared (Wald et al., 1997). Asymptomatic shedding of recurrent to 80% of African Americans (Xu et al., 2006). HSV-2 can be detected in 1–5% of days when cultured HSV-2 infections are transmitted by sexual contact, (Whitley et al., 1998) and up to 25% of all days by thus seroconversion does not usually occur until the PCR detection of viral DNA. Interestingly, shedding onset of sexual activity. Among 14–19-year-olds, acquican be intermittent on the same day, as demonstrated sition of HSV-2 infection, as demonstrated by the develby PCR. opment of type-specific antibodies, occurs in 1.4% of the Importantly, HSV-2 has a disastrous synergy with population, increasing to 26% by age 50. The most recent human immunodeficiency virus (HIV). The risk of NHANES reports that of those found to be infected with acquiring HIV is increased two- to threefold if a person HSV-2, 80% had not yet been diagnosed (Centers for is infected with HSV-2 (Wald and Link, 2002; Freeman Disease Control and Prevention, 2010). Similar to et al., 2006). Mucosal disruption from herpetic genital HSV-1, race also has an influence on the acquisition of ulcers, even seemingly microscopic ulcerations, faciliinfection. African Americans show a threefold increased tates an adequate portal of entry for HIV. The herpetic prevalence compared to Caucasians (39% versus 12%), a ulcers, in turn, have an influx of CD4 lymphocytes, prodisparity that has remained stable since 1988. Females viding a large reservoir of cells for HIV to target (Wald also have a disproportionately higher seroprevalence and Link, 2002). HSV upregulates HIV replication; HIV (21%) compared to males (11%) (Xu et al., 2006; infection reciprocates, increasing the frequency of Centers for Disease Control and Prevention, 2010). recurrent HSV-2 outbreaks (Mosca et al., 1987; Females are more susceptible to herpes genital infections Margolis et al., 1992; Celum, 2010). Patients who are than men, most likely due to an increased mucosal seropositive for both HSV-2 and HIV have shown benesurface. fit with acyclovir or valacyclovir suppressive therapy by As would be expected, the number of sexual partners decreasing plasma HIV viral load. This was found durhas a direct effect on the seroconversion rate. In the ing times of symptomatic and asymptomatic HSV-2 United States, for heterosexual women who have one shedding (Schacker et al., 2002; Nagot et al., 2007; partner, the probability of acquiring HSV-2 infection Celum et al., 2010). is 80% if she has 11–50 HSV-2 during pregnancy is 2.5% with each gestation. partners and >50 partners, respectively. The rates of Transmission from a pregnant mother to her unborn HSV-2 acquisition for homosexual men are similar. child is most often related to shedding of the virus at
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the time of delivery and the type of maternal infection (primary versus recurrent). Regardless of history, 0.5–1% of women are excreting the virus during that time (Whitley et al., 1998).
oligodendrocytic involvement, gliosis, and astrocytosis are common (Booss and Kim, 1984; Whitley, 2004).
CENTRAL NERVOUS SYSTEM PATHOLOGY
The incidence of neonatal herpes is estimated at 1 in 3200 live births, resulting in approximately 1500 cases annually in the United States. The vast majority is caused by primary HSV genital maternal infections, predominantly HSV-2. The maternal acquisition of genital herpes has been thoroughly evaluated by Brown et al. In a study following 7046 susceptible women from their first prenatal visit, they found 2% acquired HSV infection during pregnancy, as determined by seroconversion and the development of type-specific antibodies. This acquisition rate was equal throughout each trimester. Risk factors associated with converting were a younger age, mothers who were not married, and co-occurrence of other sexually transmitted diseases. Of the pregnant women who seroconverted for either type, two-thirds of them did so without any symptoms. Of those who had symptomatic infection, the majority of infection manifested as genital lesions (Brown et al., 1997). Indeed, in the overwhelming majority of neonatal HSV infections there is no maternal history of a genital infection or symptoms at time of delivery, nor is there a history of a sexual partner with recurrent lesions (Whitley et al., 1988). Although the rate of acquiring a new HSV infection is equal throughout each trimester, the timing of the acquisition is important to the development of neonatal disease. The chance of infecting the newborn is less than 3% if the mother seroconverts prior to labor. This is increased when the maternal infection is encountered near the time of labor, sparing the baby of otherwise protective transplacental antibodies (Brown et al., 1997). During labor, the rate of transmitting the virus from mother to the newborn is also dependent on, among other factors, maternal primary versus recurrent infection or the application of fetal scalp monitors. There is a much greater risk of transmitting the virus to the newborn for mothers with primary infection (57%) during the third trimester of gestation, regardless of virus type, versus non-primary, first-episode infection (25%) or recurrent infection (2%). This highlights the increased viral burden seen during primary infections and the type-specific maternal antibodies, specifically to HSV2, likely conveying some degree of protection to the infant during non-primary infections. Duration of rupture of membranes greater than 6 hours, mode of delivery, and integrity of the newborn’s mucocutaneous barriers (to include placement of fetal scalp electrodes) are also influential (Brown et al., 2003; Kimberlin, 2004).
HSV causes two devastating infections of the CNS that must be distinguished. These are neonatal HSV infection that occurs in children less than 1 month of age, and herpes simplex encephalitis (HSE), which occurs in individuals generally over 3 months of age. Both of these infections are life-threatening and can result in significant neurologic morbidity. Additionally, less severe manifestations of disease occur, such as HSV aseptic meningitis and the radiculopathies. Grossly, herpetic encephalitis produces inflammation, congestion, and/or hemorrhage, leading to liquefactive necrosis of the brain tissue (Fig. 11.1). The pathologic findings with CNS disease are similar from a microscopic perspective. Early in the infection, changes are non-specific. Capillary congestion within the cortex and subcortical white matter are appreciated. Petechiae, hemorrhagic necrosis, and perivascular cuffing can be observed as a result. During the first week of infection, eosinophilic nuclear inclusions (Cowdry type A) may be seen in half of patients, supporting the diagnosis of a viral infection. These have a homogeneous appearance surrounded by a halo secured by a marginated rim of chromatin. After the second week of infection, glial nodules are commonly seen. Extensive hemorrhagic necrosis sets in. Inflammation becomes evident, with meningeal infiltration, perivascular subarachnoid mononuclear infiltration, perivascular infiltration of the gray and white matter, non-specific gliosis, and satellitosis neuronophagia. Later in the disease process,
Fig. 11.1. Hemorrhagic necrosis of the temporal lobe is seen on a gross pathology specimen of an adult with herpes simplex encephalitis (HSE).
NEONATAL HERPES SIMPLEX INFECTIONS
HERPES SIMPLEX VIRUS Indeed, cesarean section can be preventive in transmitting the virus in a mother who is actively shedding the virus if performed prior to or within 4 hours of membrane rupture (Parvy and Ch’ien, 1980; Brown et al., 2003). Neonatal herpes has occurred, however, despite these delivery precautions. Timing of the infection occurs overwhelmingly in the peripartum period, 85%, followed by postnatal acquisition, 10%, and rarely in utero, 5%. Peripartum and postnatal neonatal herpes is subsequently classified based on clinical symptoms and laboratory findings. Lesions localized to the skin, eyes, and/or mouth only is termed SEM disease and accounts for approximately 45% of neonatal HSV. Neonatal encephalitis, or CNS disease, occurs in 30% of neonatal HSV infections. The remaining 25% of cases are disseminated disease, resulting in multiorgan infection, including the CNS, lungs, liver, adrenal gland, skin, eyes, and/or mouth (Whitley et al., 1988).
In utero HSV disease In utero or congenital neonatal herpes infection is a result of either transplacental infection or ascension from a cervical or vulvar infection late in gestation. It is recognized within the first 48 hours of life. If the transmission occurred during the first 20 weeks of gestation, spontaneous abortion, stillbirth, skin lesions or scarring, limb hypoplasia, and other congenital malformations with severe neurologic insults can result. The spectrum of CNS injuries includes microcephaly, hydranencephaly, cystic encephalomalacia, cortical and cerebellar atrophy, intracranial hemorrhage and calcifications, chorioretinitis, and micropthalmia (South et al., 1969; Hutto et al., 1987; Baldwin and Whitley, 1989; Gray et al., 1992; Johansson et al., 2003; Lee et al., 2003).
Disseminated disease Of the peri- and postnatally acquired HSV infections, disseminated disease has the most devastating prognosis, with a 30% mortality rate despite proper treatment (Kimberlin et al., 2001a). Newborns present clinically between 10 and 12 days of life with multiorgan involvement characterized as lethargy, irritability, respiratory distress, disseminated intravascular coagulopathy, seizures, jaundice, and/or vesicular rash. Importantly, the skin findings will be absent in approximately 20% of cases. Encephalitis commonly occurs in 60–75% of infants with disseminated disease (Whitley et al., 1998; Kimberlin et al., 2001b). For this classification of disease, the significant predictors of mortality are prematurity, lethargy, a state of coma, disseminated intravascular coagulopathy, or pneumonitis (Whitley et al., 1991).
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Neonatal CNS disease (encephalitis) Almost half of neonatal HSV infections will involve the CNS, either as its sole manifestation or with disseminated disease. CNS disease in neonates presents to medical attention later than SEM or disseminated disease, at around 15–19 days of life, with lethargy, irritability, poor feeding, temperature instability, seizures, bulging fontanel, and pyramidal tract signs. Notably, newborns can develop diffuse encephalitis as compared to the classic focal disease found in older children and adults. This diffuse process will usually result in encephalomalacia. Characteristic skin findings will be absent in 40% of cases (Whitley et al., 1998). Seizures can be focal or generalized, and if present at the time of starting antivirals, are shown to be a poor prognostic factor on development by 12 months of age (Whitley et al., 1991; Kimberlin et al., 2001b). Mortality for localized CNS disease with treatment is 6%, although morbidity remains high, with 70% having neurologic sequelae (Kimberlin et al., 2001a).
SEM disease Disease that is localized to the skin, eyes, and/or mouth only is found in 40% of neonates with HSV infection. These findings appear near 10–12 days of life. Clinical manifestations include a combination of discrete lesions of the skin, found in approximately 80% of localized disease, mouth, and keratoconjunctivitis (Kimberlin et al., 2001b). With proper treatment, mortality is absent. Recurrences following treatment are common over the first 6 months of life, and may extend longer (Whitley et al., 1998).
Diagnosis Although it is not recommended that every neonate who presents for medical attention for a sepsis rule-out be evaluated and treated for HSV, a high index of suspicion must be maintained if a newborn has clinical findings or maternal history concerning for exposure. This is especially true in infants under 1 month of age and in illappearing infants, even in the absence of skin lesions, as 20–40% of cases never develop lesions. The diagnosis of neonatal HSV should include evaluating for dissemination or CNS disease with a complete blood cell count, liver function tests, chest X-ray if respiratory disease is present, and cerebrospinal fluid (CSF) studies. Surface specimens should be collected and sent for isolation of the virus. This should include viral cultures of the skin or mucosal lesions if present, conjunctiva, oropharynx, and rectum. Urine and CSF should also be sent for viral culture. Herpes simplex generally grows rapidly in culture from skin and mucosal surfaces,
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producing relatively rapid and sensitive results if present. Cytopathic effects can be seen in 80% of cultures within 2 days of incubation and 94% of cultures by the fourth day (Callihan and Menegus, 1984). More than 90% of viral cultures will be positive from the skin or conjunctiva, regardless of neonatal disease classification, about half of oropharyngeal cultures yield a positive result, and 40% of CSF or brain biopsy cultures in neonates with CNS involvement are positive (Kimberlin et al., 2001b). While PCR has largely replaced virus culture as the diagnostic method of choice, a virus isolate is of value for both rapid typing and assessment of resistance, if necessary. The utilization of PCR has been of significant value for diagnostic use as well as for the study of disease pathogenesis. CSF should be sent for HSV PCR, as its increased sensitivity over culture has proven itself consistently as the gold standard for diagnosing neurologic involvement. This is as true for children and adults with HSE as it is for neonates. The overall sensitivity of PCR in neonates has been reported as 80–100% (Kimura et al., 1990; Troendle-Atkins et al., 1993; Kimberlin et al., 1996a). CSF PCR has yielded a positive result in 25% of neonates with localized SEM infection, reflecting asymptomatic infection of the CNS (Kimberlin et al., 1996a). Serum can also be sent for PCR. HSV has been detected by PCR in the serum of neonates infected with localized SEM disease and CNS disease, in contrast with the serum of children and adults with HSE, where it is not seen. The detection of HSV DNA in the CNS and serum of infants with SEM disease is of concern, as it suggests the possibility of dissemination, highlighting neonatal HSV disease as more of a spectrum rather than a discrete categorization. Indeed, PCR is also positive in the serum of neonates with disseminated disease, in keeping with the theory of viremic spread in neonates with multiorgan involvement. Prolonged duration of detection following initiation of therapy has been seen in the serum during disseminated disease and in the CSF of neonates with CNS involvement of up to 3 weeks (Kimura et al., 1990). PCR of the CSF is also pertinent in guiding therapy. All patients with HSV CNS involvement should have a repeat lumbar puncture at the end of the intravenous acyclovir course to document clearance of the virus. Babies with a persistently positive PCR from the CSF upon completion of therapy have a worse outcome. A repeat positive result would warrant a prolonged duration of intravenous treatment, as discussed below (Kimberlin et al., 1996a; Malm and Forsgren, 1999; Mejias et al., 2009). Similar to HSE of children and adults, and herpes simplex meningitis, serologic studies do not have a role in diagnosing neonatal herpes infections. Serologic testing of the mother during pregnancy may help identify
discordant couples, and thus prompt counseling of women who are seronegative on the rates of seroconverting and the higher risk of transmitting a primary herpesvirus during pregnancy (Kimberlin, 2004). Neuroimaging is an integral adjuvant to the diagnosis. As opposed to HSE in older individuals caused by HSV1, neonatal HSV-2 encephalitis usually affects the brain more diffusely, although it can selectively affect the temporal lobe. A computed tomography (CT) scan early in the disease course often may be normal or only have very subtle pathologic findings. It can reveal hypodensities in the periventricular white matter, with relative sparing of the basal ganglia, thalami, and posterior fossa. Later in the disease, focal hemorrhagic necrosis, calcifications, and/or encephalomalacia may be seen (Tien et al., 1993). Early magnetic resonance imaging (MRI) studies show diffuse loss of gray- and whitematter differentiation on the T1- and T2-weighted images. The basal ganglia and deep gray-matter regions may very well be involved. Diffusion-weighted images can delineate more extensive involvement over conventional MRI or CT. This appears to be especially true for neonates, as their brains have increased water content, making it difficult to discern from infection on early conventional MRI images. Images produced by fluid attenuated inversion recovery are poor in a neonate, accounting for a more immature myelination compared to children and adults (Vossough et al., 2008). Figure 11.2 illustrates a rare case of congenital HSV disease, highlighting the severe, diffuse nature of CNS involvement with resulting encephalomalacia.
Treatment Current antiviral therapy has reduced mortality in neonatal disseminated disease from 85% in the pretreatment era to 30% and those with CNS disease from 50% to 4%. Neonatal herpes, regardless of classification, should be treated with intravenous acyclovir for the entire duration of therapy. Acyclovir at 60 mg/kg/day divided in three doses has been shown to reduce mortality drastically compared to standard dosing. The duration for disseminated and CNS disease is 21 days; SEM disease warrants 14 days of therapy. Due to its devastating nature, treatment should not be delayed pending laboratory results. For any process with CNS involvement, a repeat lumbar puncture at the end of therapy has become the standard to document a negative CSF PCR result, thereby allowing cessation of therapy. If the repeat PCR remains positive, then therapy needs to continue until a further repeat lumbar puncture is negative. The most common side-effect of acyclovir is neutropenia, and serial absolute neutrophil counts are followed during therapy (Kimberlin et al., 2001a). Suppressive
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Fig. 11.2. In utero herpes simplex virus type 2 infection in a baby born at 27 weeks. (A) Skin lesions were apparent at delivery. Keratitis and chorioretinitis were also found on ophthalmologic exam. (B) Magnetic resonance imaging obtained at 6 days old shows T1 bilateral periventricular hyperintensities and hydrocephalus. (C) Follow-up computed tomography scan with contrast enhancement at 6 months of age shows significant cerebral atrophy and calcifications within the basal ganglia and periventricular regions.
therapy with oral acyclovir at a dose of 300 mg/m2/dose given two to three times a day for at least 6 months has been shown to reduce HSV recurrences in neonates with SEM disease and has benefit in those with CNS disease on neurologic outcome (Kimberlin et al., 1996b, 2011).
HSV ENCEPHALITIS HSE is the most common cause of sporadic, fatal encephalitis globally (Whitley et al., 1998). Outside of the neonatal period, essentially all cases of HSE in otherwise healthy children and adults are caused by HSV-1; a small portion (10%) are due to HSV-2 and are more commonly described in immunocompromised patients (Aurelius et al., 1993; Schiff and Rosenblum, 1998; Mommeja-Marin et al., 2003; Omland et al., 2008; Bradford et al., 2009). In the setting of HSV-1, HSE results from primary infections in one-third of cases and reactivation in two-thirds of cases (Nahmias et al., 1982). The small percentage of HSE due to type 2 is attributed mainly to primary disease (Aurelius et al., 1993). The US incidence is approximately 1250 cases per year. It has a bimodal distribution, occurring in one-third of cases in people under 20 years old, and one-half of cases over 50 years (Tyler, 2004). HSE causes localized pathology, resulting in inflammation and necrosis most commonly in the medialtemporal lobes and orbital-frontal regions of the brain. Herpetic brainstem encephalitis has also been described sparsely in the literature, where the extent of injury is concentrated within the brainstem rather than the cerebrum (Tyler et al., 1995). The initiating event and subsequent pathogenesis of HSE remain unclear. It has been speculated in the course of primary HSV-1 infections that the virus invades the olfactory bulbs by way of the nares, thereby gaining access to the orbitofrontal and medial temporal lobe. Others have postulated infection by route of the trigeminal nerve following a primary infection of
the oral cavity (Barnett et al., 1994). HSE caused by reactivation has been proposed as the latent virus in the trigeminal ganglion gaining transneuronal spread to the known locales in the brain (Davis and Johnson, 1979). This is also the proposed mechanism of Tyler et al. (1995), in a case of confirmed HSV recurrent brainstem encephalitis in a patient with recurrent herpes labialis. Clinically, HSE can present with headache, fever, speech disturbances, confusion, personality changes, and localized neurologic signs and seizures. The signs are indicative of the area of brain it is affecting. Symptoms develop acutely over a few days (Whitley et al., 1982). Decreased level of consciousness progresses and without treatment mortality exceeds 70%; of those who survive, only 2.5% recover normal neurologic status (Ward and Roizman, 1994).
Diagnosis To aid in diagnosing HSE, routine procedures are employed using CSF evaluation, electroencephalography (EEG), and neuroimaging. CSF findings characteristically reveal elevated white blood cell counts (WBCs). In the early phase of infection, the total number of WBC may be normal or have a neutrophil predominance. Nevertheless, the CSF findings change with WBCs, rapidly increasing to a few hundred million cells per liter with a mononuclear predominance. The amount of red blood cells in half of cases is 10 106 cells per liter. Many are even reported at 100–1000 106 cells per liter. These elevations may be indicative of intracerebral hemorrhage, damaged blood–brain barrier, or traumatic procedure. In the first week of infection, half of patients have a normal CSF protein level. The protein then increases well above the upper level of normal (Whitley et al., 1982; Koskiniemi et al., 1984). Glucose traditionally is normal; however
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hypoglycorrhachia may also be seen. One series reported 50% of 34 encephalitis and meningitis HSV-PCRpositive CSF samples to have hypoglycorrhachia (Davis et al., 2004). Importantly, 3% of cases will have totally normal CSF findings and about 20% of CSF studies never have evidence of red blood cells (Whitley et al., 1982). While isolation of the virus is, of course, integral to confirming the diagnosis, historically only brain biopsy provided tissue for virus isolation. PCR has now become the gold standard for the assessment of HSV involvement of the CNS, not only for initial diagnostics, but for evaluating response to treatment as well. It has repeatedly proven itself as highly specific, sensitive, and rapid. The specificity and sensitivity approach nearly 100% for both children and adults. HSV PCR will continue to detect viral DNA in the spinal fluid within the first week following therapy (Aurelius et al., 1991; Lakeman and Whitley, 1995). In the second week of therapy, the PCR will remain positive in 50% of patients. Beyond day 15 of therapy, 20% or less will have a positive result (Lakeman and Whitley, 1995). CSF cultures of HSV rarely are positive in children >6 months and adults with HSE, and thus are not routinely recommended (Nahmias et al., 1982; Boivin, 2004; Tyler, 2004). CSF serology for HSV is also not of benefit for acutely diagnosing encephalitis. Whereas intrathecal HSV antibodies do increase significantly, it can take weeks, thereby delaying the diagnosis. It is impossible to discern if the presence of increased intrathecal antibodies is due to local production, or leakage from the disrupted blood–brain barrier. CSF serology may, however, aid in retrospectively diagnosing the patient by seeing increasing titers in the CSF (with a sensitivity of 70–90% and specificity of 81–88%) or decreasing serum-to-CSF antibody ratios of 20 (a sensitivity of 50% and specificity of 81% in the first 10 days of infection) (Nahmias et al., 1982). Serum serology will provide information on whether the infection is of primary or recurrent nature only. There is currently no value in knowing the serum antibody status in regard to clinical features, treatment, or prognosis (Tyler, 2004). The EEG during the first week of infection shows non-specific focal slowing, followed by spike and wave patterning of the temporal region. Periodic lateralizing epileptiform discharges may develop and help further suggest a diagnosis of HSE, though are not pathognomonic. Neuroimaging may show edema early in the course, followed by hemorrhage and midline shift (Whitley et al., 1998). MRI is the modality of choice over CT, given its higher sensitivity and enhanced resolution. CT scans may often be normal or show non-specific hypodensities of the temporal lobes (Tien et al., 1993; Raschilas et al., 2002). Early MRI highlights the edema as high signal intensities on T2-weighted images, usually
Fig. 11.3. Herpes simplex encephalitis infection in an adult showing localized temporal lobe involvement on magnetic resonance imaging T2-weighted image.
involving the orbital surfaces of the frontal lobes, inferomedial temporal lobes, insular cortex, and external capsula (Fig 11.3). The cingulate gyrus has also been known to be involved later in the infection (Tien et al., 1993). There is a significant correlation between CSF PCRpositive patients and temporal lobe lesions seen on MRI. Brainstem, temporal superficial cortex, and basal ganglia are less commonly affected sites (Domingues et al., 1998). When findings on conventional MRI images are subtle or normal, diffusion-weighted imaging may aid in increasing the sensitivity (McCabe et al., 2003).
Treatment Due to its fatal nature if left untreated and a high propensity to cause permanent neurologic damage in those who do survive, a high index of suspicion should be maintained in patients who present with a decreasing level of consciousness, abnormal CSF indices, and focal neurologic exam. Treatment for suspected or confirmed HSE is with intravenous acyclovir, 30 mg/kg/day for 14–21 days (Skoldenberg et al., 1984; Whitley et al., 1986). Higher doses have been associated with neurotoxicity and impaired renal function. Treatment should not be withheld pending results of confirmatory lab data. A lumbar puncture for HSV should be performed at the end of therapy to document clearance of the virus as detected by PCR. Should this persist, an extended acyclovir course is warranted (Kimberlin, 2007). Resistance to acyclovir is possible with HSV infections, especially in the immunocompromised patient. This occurs through mutations in the viral gene for thymidine kinase (TK). Either TK-deficient viral progeny are made, or TK that is unable to phosphorylate the acyclovir is the mutation. In vitro, these resistant mutants are susceptible to foscarnet and vidarabine; however only foscarnet has been shown to be clinically effective (Whitley et al., 1998).
HERPES SIMPLEX VIRUS
Prognosis Even in the face of adequate treatment, the mortality and morbidity for HSE remain high. Although acyclovir has reduced mortality from 70% in those untreated to 20%, approximately 60% of survivors continue to have impaired neurologic function. Factors shown to carry a poor prognosis include patients with a Glasgow coma score
