REVIEW For reprint orders, please contact: [email protected]
Noroviruses: epidemiology, immunity and prospects for prevention Kimberly Pringle*,1,2, Benjamin Lopman1, Everardo Vega1, Jan Vinje1, Umesh D Parashar1 & Aron J Hall1
ABSTRACT In recent years, noroviruses have become recognized as an important cause of both sporadic and epidemic acute gastroenteritis (AGE), largely due to the improved availability of broadly reactive real-time RT-PCR (TaqMan-based RT-PCR) assays. While there is substantial diversity among noroviruses, one specific genotype, GII.4, is the most common etiology in sporadic and epidemic AGE. Outbreaks of norovirus AGE most commonly occur in healthcare facilities and restaurants and result in significant morbidity and mortality and substantial healthcare costs. Norovirus vaccine development is progressing, and Phase I and II human trials have shown proof-of-principle that norovirus vaccines can reduce illness and infection.
Norwalk virus, the prototype norovirus strain, was first visualized in 1972 by Kapikian et al. using immune electron microscopy on specimens from an outbreak of acute gastroenteritis that affected students at an elementary school in Norwalk, (OH, USA) 4 years earlier [1] . Attempts to grow norovirus in cell culture and organ culture have largely been unsuccessful [2] . In the 1990s, the cloning and sequencing of the norovirus genome ushered in a new diagnostic era with the development of sensitive and specific molecular RT-PCR assays for detection of viral norovirus RNA. These first generation conventional RT-PCR assays have been succeeded with real-time TaqMan-based RT-PCR (RT-qPCR) assays which are now considered the gold standard and are widely used in many public health laboratories globally. The use of these assays has dramatically improved our understanding of the epidemiology and burden of norovirus-associated acute gastroenteritis (AGE). This review summarizes the current knowledge of the epidemiology, immunity, and prospects for prevention of noroviruses.
KEYWORDS
• Caliciviridae • GII.4 • herd immunity • norovirus • vaccine
Virology & diagnostics Noroviruses are positive sense single-stranded RNA viruses belonging to the genus Norovirus within the family Caliciviridae, which also include at least four other genera Sapovirus, Vesivirus, Lagovirus and Nebovirus [3] . Because noroviruses cannot be grown in cell culture, they are classified in genetic groups or genogroups (G) of which viruses belonging to GI, GII, GIV infect humans. These genogroups are further divided into at least 32 genotypes [4] . The RNA genome of noroviruses consists of three open reading frames (ORF): ORF1 encodes six nonstructural proteins including Division of Viral Diseases, National Center for Immunization & Respiratory Diseases, Centers for Disease Control & Prevention, 1600 Clifton Road, Mailstop A-34, Atlanta, GA 30333, USA 2 Epidemic Intelligence Service, Centers for Disease Control & Prevention, 1600 Clifton Road, Mailstop E-92, Atlanta, GA 30333, USA *Author for correspondence: [email protected] 1
10.2217/FMB.14.102 10.2217/FMB.14.102
Future Microbiol. (2015) 10(1), 53–67
part of
ISSN 1746-0913
53
Review Pringle, Lopman, Vega, Vinje, Parashar & Hall
A
B
Figure 1. A 3D structure of Norwalk virus was created using the Norwalk crystal structure (PDB number 1IHM). The full capsid was edited in Chimera [5] to show the VP1 capsid monomer (left) and the dimer (right). The S domain is colored blue and the P domain is colored yellow and red. The P1 and P2 subdomain are colored yellow and red, respectively.
the RNA dependent RNA polymerase (RdRp); ORF2 encodes the major capsid (VP1), which consists of two protruding domains (P1 and P2) and a shell domain (S) that determine the antigenicity of the virus (Figure 1) [4] ; and ORF3 which encodes the minor capsid (VP2). In the 1970s and 1980s, electron microscopy and radioimmunoassays were used to detect norovirus but these methods were cumbersome and had limited sensitivity. With the advent of PCR diagnostics in the 1990s, these molecular assays have now become the gold standard for diagnosis. The ORF1-ORF2 junction is the most conserved region of the norovirus genome, making this region ideal for designing broadly reactive primers and probes for RT-qPCR. Because of their high analytic sensitivity, RT-qPCR assays can detect very low amounts of virus that might be present in samples from persons who are asymptomatically infected or have recovered within the past few weeks from norovirus AGE. Thus, when determining the etiology of AGE in samples collected more than 3–5 days after onset of symptoms, RT-qPCR results with high cycle threshold (CT) values (i.e., low viral load) should be interpreted with caution. Recently, commercial enzyme immunoassays (EIA) have become available. However, given the large variation among noroviruses and variable viral loads found in stool samples, these antigen-capture
54
Future Microbiol. (2015) 10(1)
EIAs are less sensitive than RT-qPCR assays. Consequently, EIAs have limitations when used to diagnosis norovirus in individual patients but these assays may be useful in outbreak settings where a large number of stool samples are available [6–10] . Clinical presentation & epidemiology Norovirus disease is characterized by sudden onset of vomiting, diarrhea and abdominal cramps lasting 2–3 days, with subsequent resolution to baseline health [11] . The host’s age affects the symptom profile with children sometimes having a longer duration of diarrhea [12] . The elderly generally have less vomiting [13] and commonly report anorexia, headache and abdominal pain [14] . Noroviruses are also more likely to cause severe disease, and sometimes even death, in the elderly [15,16] . A recent study in the USA found substantially elevated hospitalization and mortality rates in nursing homes during norovirus AGE outbreak periods, defined as beginning 2 days prior to symptom onset of the first recognized case and ending the day of symptom onset of the last case, when compared with nonoutbreak periods [17] . The findings remained even after controlling for other wintertime illnesses, including influenza. These data challenge the dogma that norovirus is a mild, self-limiting illness, finding a 20–30%
future science group
Noroviruses: epidemiology, immunity & prospects for prevention increase risk of death and hospitalization during outbreaks among individuals aged more than 90 years [17,18] Of the estimated 800 annual norovirus deaths each year in the USA, 90% are persons aged 65 years or greater [19] . Viral shedding & transmission In the absence of an ability to cultivate norovirus in vitro, data on shedding are based on detection of viral RNA. After challenging volunteers with Norwalk virus (GI.1), Atmar et al. found that viral shedding peaks a median of 4 days after inoculation independent of symptom manifestation. Infected participants began shedding a mean of 8 h (median = 36) after inoculation; the highest concentration of shedding occurred after resolution of symptoms in 69% of the volunteers [20] . Improved understanding of the factors associated with duration and quantity of viral shedding may have important implications for transmission prevention. Both experimental and observational studies have found that duration of viral shedding varies by age and typically lasts 20–30 days for adults [20,21] . Observational studies have shown that viral shedding may be of longer duration in infants, the elderly and the immunocompromised [22–24] . Studies testing the hypotheses that age, clinical severity and duration of illness are correlated with level and duration of viral shedding have yielded mixed evidence [12,21,24–28] . Noroviruses are transmitted either directly from person-to-person, via the fecal-oral route or aerosolized vomitus, or indirectly through contaminated food, water and environmental surfaces. Noroviruses can spread easily as studies estimate as few as 18–2800 viral particles [29,30] can cause infection. With each gram of stool from an infected individual containing greater than 1 × 1010 RNA copies at peak [31] , a single infected individual has the potential to infect several hundred or even thousands of others. Compounding this copious viral shedding is the potential for pre-, post-, and asymptomatic shedding as well as the environmental stability, resistance to disinfectants and thermal stability of the virus [32] . These characteristics further underscore the public health challenges posed by noroviruses. Epidemic norovirus gastroenteritis Early norovirus studies focused on outbreaks from developed countries, where noroviruses are
future science group
Review
estimated to cause about 50% of all outbreaks of AGE [33,34] . In the USA, long-term care facilities (LTCFs) and other healthcare facilities are the most commonly reported setting of norovirus outbreaks (62–64%), followed by restaurants and catered events (15–20%) and schools and day-care facilities (5–6%) [35,36] . Studies from Finland, Germany, England and Wales and Australia demonstrated similar patterns in terms of settings with hospitals, nursing homes and other inpatient care facilities being the site for the most norovirus outbreaks (Table 1) [37–41] . However, a systematic review published by Matthews found that 35% of outbreaks reported in the literature occurred in food service and 27% occurred in healthcare settings [42] . These conflicting results likely reflect differences in sensitivity and completeness of surveillance systems across different countries and publication bias of foodborne outbreaks. For example, there is long-standing national foodborne disease surveillance existent in the USA where as healthcare surveillance for norovirus has been more recently incorporated into these systems. However, it is important to note that outside of high-income countries, there are few published articles examining incidence of norovirus outbreaks over multiple seasons in different settings. In low- and middle-income countries, the few long-term hospital and community based studies examined sporadic AGE in children [43–50] . GII noroviruses, and specifically GII.4, has been the predominant strain detected in norovirus outbreaks globally, at least for the last decade [38,42,51–52] . New variant GII.4 strains displacing previous dominant strains have emerged every 2–3 years, and have been associated with increased norovirus activity for most, but not all, GII.4 variants (Table 2) [14,53–54] . For example, 95/96_US viruses were associated with increased norovirus activity on five continents [55] , and Farmington_Hills_2002 viruses were associated with an increased number of outbreaks throughout Europe and the USA [56,57] . However, not all new GII.4 variants that replace previously dominant variants have been associated with increased disease activity. For example, the emergence of New Orleans_2009 and Sydney_2012 viruses was not associated with an increased number of outbreaks in the USA [58,59] . In addition, there are strains that gain regional dominance, but never reach a global pandemic level like Asia_2003, Bristol_1987, Yerseke_2006 and Osaka_2007. It is hypothesized that some strains are restricted
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55
Review Pringle, Lopman, Vega, Vinje, Parashar & Hall Table 1. Number of norovirus outbreaks by country, author year, setting, genotype and mode of transmission. Country
Study (year)
Time period
Norovirus Setting outbreaks (n)
Finland USA
Maunula et al. (2005) Vega et al. (2014)
1998 –2002 2009–2013
416 3960
Germany
Bernard et al. (2014)
2001–2009
31,644
England and Wales Australia
Lopman et al. (2003)
1992–2000
1877
Marshall et al. (2005)
2001
30
Spain
Buesa et al. (2002)
2001
30
Norovirus outbreaks (%) by setting
GII.4 (%) Mode of transmission
Norovirus outbreaks (%) by mode of transmission
Hospitals 31 Food service 14 LTCF 63
86.9† 84.1
N/A 83.7
Restaurants Schools Hospitals Nursing homes Households Hospitals
10 6 75 28
49.6 17.2 85 82
N/A Person-toperson Foodborne N/A
24 40
50 N/A
85.2
LTCF Hotels Schools Hostels/LTCF Foodservice School/ daycare Schools Nursing homes
39 8 4 27 27 17
63
Person-toperson Foodborne N/A
43 21
79†
N/A
N/A
Ref.
16.1 N/A
5.0 N/A
[37] [35]
[38]
[39]
[40]
[41]
Percentage of GII outbreaks. LTCF: Long-term care facility; N/A: Not available. †
regionally because they share epitopes with the pandemic strain, resulting in a reduced susceptible population [60] . Finally, other strains circulate widely but in low frequencies, like Japan_2001 and Henry_2001 [61] . GII.4 strains are a common cause of AGE outbreaks in healthcare settings, including hospitals and long-term care facilities [35,37,51,53,62–64] ; a study analyzing 5 years of surveillance data found a significant association between the GII.4 strain and person-to-person transmission (OR = 3.0; p < 0.05) [35] . In another study, norovirus were found to be the causative etiology in 86% of AGE outbreaks in LTCFs suspected to be of viral origin [25] . Although GII.4 viruses are the most frequently identified genotype in foodborne norovirus outbreaks, non-GII.4 genotypes are more likely to cause foodborne outbreaks than person-to-person outbreaks [35,53,65,66] . GII.4 appears to cause more severe clinical disease, including increased nausea, vomiting, abdominal cramps, fever and mucous when compared with non-GII.4 outbreaks [24–25,67,68] .
56
Future Microbiol. (2015) 10(1)
However, these differences appear to be in part age-dependent, with younger healthcare staff reporting milder symptoms when compared with elderly residents in a LTCF within a given outbreak [24,68] . It also appears that GII.4 outbreaks have higher attack rates in the elderly (32 vs 24%) when compared with non-GII.4 outbreaks [68] . A systematic review by Desai et al. found in a multivariate analysis that GII.4 strains caused higher hospitalization and mortality rates, with deaths more commonly occurring in hospital settings when compared with community settings after controlling for age and mode of transmission [18] . Sporadic norovirus gastroenteritis A recent systematic review examining the global prevalence of norovirus among cases of acute gastroenteritis found a pooled prevalence of 17% in hospitalized and 24% in community samples [50] . Although affecting the entire age range, studies conducted on multiple continents consistently found that norovirus gastroenteritis
future science group
Noroviruses: epidemiology, immunity & prospects for prevention frequently occurs among children, with many episodes occurring in children younger than 2 years old and high antibody prevalence by 5 years of age [26,43,69–77] . Globally, norovirus activity peaks in the colder winter months [78] . Historically, most studies found rotavirus to be the most common cause of AGE in hospitalized children [46–47,72,74–75,79–80] . However, none of these countries had introduced rotavirus vaccine into their national immunization program during stool sample collection (Table 3) . A study conducted in the USA 3 years after the introduction of the rotavirus vaccine demonstrated that norovirus had become the most common etiology of AGE in hospitalized children [71] . This study suggests that, as countries introduce the rotavirus vaccine to their national immunization, and rates of rotavirus disease decline, norovirus may become the leading cause of severe AGE in children. Norovirus can cause severe AGE in children though it appears to be less severe than rotavirus AGE [70,82,84] with children having fewer hospitalizations and episodes of vomiting or diarrhea [84,85] . Norovirus severity is measured using various measurements including the World Health Organization scale [86] , modified-Vesikari and Vesikari scales [49,87] , which estimate gastroenteritis severity by calculating a score based on signs and symptoms of disease [45,49,70,76,88–89] , by reported symptoms [79,80] , or by comparing hospitalization rates and length of stay with rotavirus [75,82] . A recent systematic review found
Review
GII.4 (67.2%) to be the most common genotype causing sporadic norovirus AGE in children followed by GII.3 (16.3%) [90] . Whether different norovirus strains cause different disease severity in children remains unclear [84,91] . Norovirus is frequently detected in stool samples from hospitalized patients without AGE [43,72] as well as in asymptomatic persons in the community [45,48,68,92–93] . The significance of these asymptomatic infections is still unclear. Some have hypothesized asymptomatic people may serve as a reservoir of newly emerging epidemic and endemic strains. For example, norovirus strains in asymptomatic children in Malawi and Sweden later became dominant strains in adults [43,94] . These asymptomatic infections may also represent prolonged viral shedding following a symptomatic infection or indicate a greater overall frequency of exposure and force of infection [95] , as would be the case in densely populated, poor hygiene settings of developing countries. Molecular epidemiology & evolution of noroviruses Norovirus molecular evolution provides important clues regarding immunogenicity and virushost interactions. Because noroviruses replicate with error prone RdRp, a certain level of genetic diversity is expected. Norovirus strains, including GII.4 strains, have similar error rates compared with other RNA viruses, like poliovirus or hepatitis C virus [51] ranging from 1.37 to
Table 2. GII.4 variant strains, years of circulation, epidemic season and other strain names used in the literature. Norovirus strain Gen Bank Years of accession number† circulation
Increase in number Other names used in of outbreaks in the literature USA
Bristol
X75716
1987–1994
None
95/96-US Henry Farmington Hills Hunter Asia Yerseke Osaka Den Haag New Orleans Sydney
AJ004864‡ EU310927 AY485642‡ AY883096‡ AB220921 EF126963 AB279553 EF126965‡ GU445325 JX459908
1995–2002 2000–2002 2002–2004 2003–2006 2004–2006 2006–2008 2005–2007 2006–2009 2009–present 2012–present
1995–1996 None 2002–2003 None None 2006–2007 None 2006–2007 None None
† ‡
Lordsdale, Camberwell, and MD145–12 Grimsby 2002 Variant 2004 variant Chiba, Sakai, Asia_2003 Laurens, 2006a variant Minerva, 2006b variant
Gen Bank accession number of the first submitted capsid sequence of this variant. Pandemic strains.
future science group
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57
Review Pringle, Lopman, Vega, Vinje, Parashar & Hall Table 3. Prevalence of rotavirus norovirus and coinfection by country, reference, patient setting, age range, year of rotavirus vaccine introduction to National Immunization Program and most common genogroup and genotype. Country
Study (year)
NoV (%)
RV and Sample NoV number (%)
Clinical setting and symptoms
Age Year RV (years) vaccine in NIP†
Most common NoV genogroup/ genotype (%)
2009–2012 36.9
11.1
N/A
878
H & O + AGE
≤5
2012
GII (8.6)
[81]
1997–2007 32.1
11.3
1.5
2,458
H, + AGE
Noroviruses: epidemiology, immunity and prospects for prevention Kimberly Pringle*,1,2, Benjamin Lopman1, Everardo Vega1, Jan Vinje1, Umesh D Parashar1 & Aron J Hall1
ABSTRACT In recent years, noroviruses have become recognized as an important cause of both sporadic and epidemic acute gastroenteritis (AGE), largely due to the improved availability of broadly reactive real-time RT-PCR (TaqMan-based RT-PCR) assays. While there is substantial diversity among noroviruses, one specific genotype, GII.4, is the most common etiology in sporadic and epidemic AGE. Outbreaks of norovirus AGE most commonly occur in healthcare facilities and restaurants and result in significant morbidity and mortality and substantial healthcare costs. Norovirus vaccine development is progressing, and Phase I and II human trials have shown proof-of-principle that norovirus vaccines can reduce illness and infection.
Norwalk virus, the prototype norovirus strain, was first visualized in 1972 by Kapikian et al. using immune electron microscopy on specimens from an outbreak of acute gastroenteritis that affected students at an elementary school in Norwalk, (OH, USA) 4 years earlier [1] . Attempts to grow norovirus in cell culture and organ culture have largely been unsuccessful [2] . In the 1990s, the cloning and sequencing of the norovirus genome ushered in a new diagnostic era with the development of sensitive and specific molecular RT-PCR assays for detection of viral norovirus RNA. These first generation conventional RT-PCR assays have been succeeded with real-time TaqMan-based RT-PCR (RT-qPCR) assays which are now considered the gold standard and are widely used in many public health laboratories globally. The use of these assays has dramatically improved our understanding of the epidemiology and burden of norovirus-associated acute gastroenteritis (AGE). This review summarizes the current knowledge of the epidemiology, immunity, and prospects for prevention of noroviruses.
KEYWORDS
• Caliciviridae • GII.4 • herd immunity • norovirus • vaccine
Virology & diagnostics Noroviruses are positive sense single-stranded RNA viruses belonging to the genus Norovirus within the family Caliciviridae, which also include at least four other genera Sapovirus, Vesivirus, Lagovirus and Nebovirus [3] . Because noroviruses cannot be grown in cell culture, they are classified in genetic groups or genogroups (G) of which viruses belonging to GI, GII, GIV infect humans. These genogroups are further divided into at least 32 genotypes [4] . The RNA genome of noroviruses consists of three open reading frames (ORF): ORF1 encodes six nonstructural proteins including Division of Viral Diseases, National Center for Immunization & Respiratory Diseases, Centers for Disease Control & Prevention, 1600 Clifton Road, Mailstop A-34, Atlanta, GA 30333, USA 2 Epidemic Intelligence Service, Centers for Disease Control & Prevention, 1600 Clifton Road, Mailstop E-92, Atlanta, GA 30333, USA *Author for correspondence: [email protected] 1
10.2217/FMB.14.102 10.2217/FMB.14.102
Future Microbiol. (2015) 10(1), 53–67
part of
ISSN 1746-0913
53
Review Pringle, Lopman, Vega, Vinje, Parashar & Hall
A
B
Figure 1. A 3D structure of Norwalk virus was created using the Norwalk crystal structure (PDB number 1IHM). The full capsid was edited in Chimera [5] to show the VP1 capsid monomer (left) and the dimer (right). The S domain is colored blue and the P domain is colored yellow and red. The P1 and P2 subdomain are colored yellow and red, respectively.
the RNA dependent RNA polymerase (RdRp); ORF2 encodes the major capsid (VP1), which consists of two protruding domains (P1 and P2) and a shell domain (S) that determine the antigenicity of the virus (Figure 1) [4] ; and ORF3 which encodes the minor capsid (VP2). In the 1970s and 1980s, electron microscopy and radioimmunoassays were used to detect norovirus but these methods were cumbersome and had limited sensitivity. With the advent of PCR diagnostics in the 1990s, these molecular assays have now become the gold standard for diagnosis. The ORF1-ORF2 junction is the most conserved region of the norovirus genome, making this region ideal for designing broadly reactive primers and probes for RT-qPCR. Because of their high analytic sensitivity, RT-qPCR assays can detect very low amounts of virus that might be present in samples from persons who are asymptomatically infected or have recovered within the past few weeks from norovirus AGE. Thus, when determining the etiology of AGE in samples collected more than 3–5 days after onset of symptoms, RT-qPCR results with high cycle threshold (CT) values (i.e., low viral load) should be interpreted with caution. Recently, commercial enzyme immunoassays (EIA) have become available. However, given the large variation among noroviruses and variable viral loads found in stool samples, these antigen-capture
54
Future Microbiol. (2015) 10(1)
EIAs are less sensitive than RT-qPCR assays. Consequently, EIAs have limitations when used to diagnosis norovirus in individual patients but these assays may be useful in outbreak settings where a large number of stool samples are available [6–10] . Clinical presentation & epidemiology Norovirus disease is characterized by sudden onset of vomiting, diarrhea and abdominal cramps lasting 2–3 days, with subsequent resolution to baseline health [11] . The host’s age affects the symptom profile with children sometimes having a longer duration of diarrhea [12] . The elderly generally have less vomiting [13] and commonly report anorexia, headache and abdominal pain [14] . Noroviruses are also more likely to cause severe disease, and sometimes even death, in the elderly [15,16] . A recent study in the USA found substantially elevated hospitalization and mortality rates in nursing homes during norovirus AGE outbreak periods, defined as beginning 2 days prior to symptom onset of the first recognized case and ending the day of symptom onset of the last case, when compared with nonoutbreak periods [17] . The findings remained even after controlling for other wintertime illnesses, including influenza. These data challenge the dogma that norovirus is a mild, self-limiting illness, finding a 20–30%
future science group
Noroviruses: epidemiology, immunity & prospects for prevention increase risk of death and hospitalization during outbreaks among individuals aged more than 90 years [17,18] Of the estimated 800 annual norovirus deaths each year in the USA, 90% are persons aged 65 years or greater [19] . Viral shedding & transmission In the absence of an ability to cultivate norovirus in vitro, data on shedding are based on detection of viral RNA. After challenging volunteers with Norwalk virus (GI.1), Atmar et al. found that viral shedding peaks a median of 4 days after inoculation independent of symptom manifestation. Infected participants began shedding a mean of 8 h (median = 36) after inoculation; the highest concentration of shedding occurred after resolution of symptoms in 69% of the volunteers [20] . Improved understanding of the factors associated with duration and quantity of viral shedding may have important implications for transmission prevention. Both experimental and observational studies have found that duration of viral shedding varies by age and typically lasts 20–30 days for adults [20,21] . Observational studies have shown that viral shedding may be of longer duration in infants, the elderly and the immunocompromised [22–24] . Studies testing the hypotheses that age, clinical severity and duration of illness are correlated with level and duration of viral shedding have yielded mixed evidence [12,21,24–28] . Noroviruses are transmitted either directly from person-to-person, via the fecal-oral route or aerosolized vomitus, or indirectly through contaminated food, water and environmental surfaces. Noroviruses can spread easily as studies estimate as few as 18–2800 viral particles [29,30] can cause infection. With each gram of stool from an infected individual containing greater than 1 × 1010 RNA copies at peak [31] , a single infected individual has the potential to infect several hundred or even thousands of others. Compounding this copious viral shedding is the potential for pre-, post-, and asymptomatic shedding as well as the environmental stability, resistance to disinfectants and thermal stability of the virus [32] . These characteristics further underscore the public health challenges posed by noroviruses. Epidemic norovirus gastroenteritis Early norovirus studies focused on outbreaks from developed countries, where noroviruses are
future science group
Review
estimated to cause about 50% of all outbreaks of AGE [33,34] . In the USA, long-term care facilities (LTCFs) and other healthcare facilities are the most commonly reported setting of norovirus outbreaks (62–64%), followed by restaurants and catered events (15–20%) and schools and day-care facilities (5–6%) [35,36] . Studies from Finland, Germany, England and Wales and Australia demonstrated similar patterns in terms of settings with hospitals, nursing homes and other inpatient care facilities being the site for the most norovirus outbreaks (Table 1) [37–41] . However, a systematic review published by Matthews found that 35% of outbreaks reported in the literature occurred in food service and 27% occurred in healthcare settings [42] . These conflicting results likely reflect differences in sensitivity and completeness of surveillance systems across different countries and publication bias of foodborne outbreaks. For example, there is long-standing national foodborne disease surveillance existent in the USA where as healthcare surveillance for norovirus has been more recently incorporated into these systems. However, it is important to note that outside of high-income countries, there are few published articles examining incidence of norovirus outbreaks over multiple seasons in different settings. In low- and middle-income countries, the few long-term hospital and community based studies examined sporadic AGE in children [43–50] . GII noroviruses, and specifically GII.4, has been the predominant strain detected in norovirus outbreaks globally, at least for the last decade [38,42,51–52] . New variant GII.4 strains displacing previous dominant strains have emerged every 2–3 years, and have been associated with increased norovirus activity for most, but not all, GII.4 variants (Table 2) [14,53–54] . For example, 95/96_US viruses were associated with increased norovirus activity on five continents [55] , and Farmington_Hills_2002 viruses were associated with an increased number of outbreaks throughout Europe and the USA [56,57] . However, not all new GII.4 variants that replace previously dominant variants have been associated with increased disease activity. For example, the emergence of New Orleans_2009 and Sydney_2012 viruses was not associated with an increased number of outbreaks in the USA [58,59] . In addition, there are strains that gain regional dominance, but never reach a global pandemic level like Asia_2003, Bristol_1987, Yerseke_2006 and Osaka_2007. It is hypothesized that some strains are restricted
www.futuremedicine.com
55
Review Pringle, Lopman, Vega, Vinje, Parashar & Hall Table 1. Number of norovirus outbreaks by country, author year, setting, genotype and mode of transmission. Country
Study (year)
Time period
Norovirus Setting outbreaks (n)
Finland USA
Maunula et al. (2005) Vega et al. (2014)
1998 –2002 2009–2013
416 3960
Germany
Bernard et al. (2014)
2001–2009
31,644
England and Wales Australia
Lopman et al. (2003)
1992–2000
1877
Marshall et al. (2005)
2001
30
Spain
Buesa et al. (2002)
2001
30
Norovirus outbreaks (%) by setting
GII.4 (%) Mode of transmission
Norovirus outbreaks (%) by mode of transmission
Hospitals 31 Food service 14 LTCF 63
86.9† 84.1
N/A 83.7
Restaurants Schools Hospitals Nursing homes Households Hospitals
10 6 75 28
49.6 17.2 85 82
N/A Person-toperson Foodborne N/A
24 40
50 N/A
85.2
LTCF Hotels Schools Hostels/LTCF Foodservice School/ daycare Schools Nursing homes
39 8 4 27 27 17
63
Person-toperson Foodborne N/A
43 21
79†
N/A
N/A
Ref.
16.1 N/A
5.0 N/A
[37] [35]
[38]
[39]
[40]
[41]
Percentage of GII outbreaks. LTCF: Long-term care facility; N/A: Not available. †
regionally because they share epitopes with the pandemic strain, resulting in a reduced susceptible population [60] . Finally, other strains circulate widely but in low frequencies, like Japan_2001 and Henry_2001 [61] . GII.4 strains are a common cause of AGE outbreaks in healthcare settings, including hospitals and long-term care facilities [35,37,51,53,62–64] ; a study analyzing 5 years of surveillance data found a significant association between the GII.4 strain and person-to-person transmission (OR = 3.0; p < 0.05) [35] . In another study, norovirus were found to be the causative etiology in 86% of AGE outbreaks in LTCFs suspected to be of viral origin [25] . Although GII.4 viruses are the most frequently identified genotype in foodborne norovirus outbreaks, non-GII.4 genotypes are more likely to cause foodborne outbreaks than person-to-person outbreaks [35,53,65,66] . GII.4 appears to cause more severe clinical disease, including increased nausea, vomiting, abdominal cramps, fever and mucous when compared with non-GII.4 outbreaks [24–25,67,68] .
56
Future Microbiol. (2015) 10(1)
However, these differences appear to be in part age-dependent, with younger healthcare staff reporting milder symptoms when compared with elderly residents in a LTCF within a given outbreak [24,68] . It also appears that GII.4 outbreaks have higher attack rates in the elderly (32 vs 24%) when compared with non-GII.4 outbreaks [68] . A systematic review by Desai et al. found in a multivariate analysis that GII.4 strains caused higher hospitalization and mortality rates, with deaths more commonly occurring in hospital settings when compared with community settings after controlling for age and mode of transmission [18] . Sporadic norovirus gastroenteritis A recent systematic review examining the global prevalence of norovirus among cases of acute gastroenteritis found a pooled prevalence of 17% in hospitalized and 24% in community samples [50] . Although affecting the entire age range, studies conducted on multiple continents consistently found that norovirus gastroenteritis
future science group
Noroviruses: epidemiology, immunity & prospects for prevention frequently occurs among children, with many episodes occurring in children younger than 2 years old and high antibody prevalence by 5 years of age [26,43,69–77] . Globally, norovirus activity peaks in the colder winter months [78] . Historically, most studies found rotavirus to be the most common cause of AGE in hospitalized children [46–47,72,74–75,79–80] . However, none of these countries had introduced rotavirus vaccine into their national immunization program during stool sample collection (Table 3) . A study conducted in the USA 3 years after the introduction of the rotavirus vaccine demonstrated that norovirus had become the most common etiology of AGE in hospitalized children [71] . This study suggests that, as countries introduce the rotavirus vaccine to their national immunization, and rates of rotavirus disease decline, norovirus may become the leading cause of severe AGE in children. Norovirus can cause severe AGE in children though it appears to be less severe than rotavirus AGE [70,82,84] with children having fewer hospitalizations and episodes of vomiting or diarrhea [84,85] . Norovirus severity is measured using various measurements including the World Health Organization scale [86] , modified-Vesikari and Vesikari scales [49,87] , which estimate gastroenteritis severity by calculating a score based on signs and symptoms of disease [45,49,70,76,88–89] , by reported symptoms [79,80] , or by comparing hospitalization rates and length of stay with rotavirus [75,82] . A recent systematic review found
Review
GII.4 (67.2%) to be the most common genotype causing sporadic norovirus AGE in children followed by GII.3 (16.3%) [90] . Whether different norovirus strains cause different disease severity in children remains unclear [84,91] . Norovirus is frequently detected in stool samples from hospitalized patients without AGE [43,72] as well as in asymptomatic persons in the community [45,48,68,92–93] . The significance of these asymptomatic infections is still unclear. Some have hypothesized asymptomatic people may serve as a reservoir of newly emerging epidemic and endemic strains. For example, norovirus strains in asymptomatic children in Malawi and Sweden later became dominant strains in adults [43,94] . These asymptomatic infections may also represent prolonged viral shedding following a symptomatic infection or indicate a greater overall frequency of exposure and force of infection [95] , as would be the case in densely populated, poor hygiene settings of developing countries. Molecular epidemiology & evolution of noroviruses Norovirus molecular evolution provides important clues regarding immunogenicity and virushost interactions. Because noroviruses replicate with error prone RdRp, a certain level of genetic diversity is expected. Norovirus strains, including GII.4 strains, have similar error rates compared with other RNA viruses, like poliovirus or hepatitis C virus [51] ranging from 1.37 to
Table 2. GII.4 variant strains, years of circulation, epidemic season and other strain names used in the literature. Norovirus strain Gen Bank Years of accession number† circulation
Increase in number Other names used in of outbreaks in the literature USA
Bristol
X75716
1987–1994
None
95/96-US Henry Farmington Hills Hunter Asia Yerseke Osaka Den Haag New Orleans Sydney
AJ004864‡ EU310927 AY485642‡ AY883096‡ AB220921 EF126963 AB279553 EF126965‡ GU445325 JX459908
1995–2002 2000–2002 2002–2004 2003–2006 2004–2006 2006–2008 2005–2007 2006–2009 2009–present 2012–present
1995–1996 None 2002–2003 None None 2006–2007 None 2006–2007 None None
† ‡
Lordsdale, Camberwell, and MD145–12 Grimsby 2002 Variant 2004 variant Chiba, Sakai, Asia_2003 Laurens, 2006a variant Minerva, 2006b variant
Gen Bank accession number of the first submitted capsid sequence of this variant. Pandemic strains.
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Review Pringle, Lopman, Vega, Vinje, Parashar & Hall Table 3. Prevalence of rotavirus norovirus and coinfection by country, reference, patient setting, age range, year of rotavirus vaccine introduction to National Immunization Program and most common genogroup and genotype. Country
Study (year)
NoV (%)
RV and Sample NoV number (%)
Clinical setting and symptoms
Age Year RV (years) vaccine in NIP†
Most common NoV genogroup/ genotype (%)
2009–2012 36.9
11.1
N/A
878
H & O + AGE
≤5
2012
GII (8.6)
[81]
1997–2007 32.1
11.3
1.5
2,458
H, + AGE
