Journal of Infection (199 ° )
20, 189-192
Editorial Norwalk
agent comes
of age
It is now 21 years since an outbreak of non-bacterial gastro-enteritis was reported in an elementary school in Norwalk, Ohio I and i8 years since the causative agent was revealed by electron microscopy (EM) in stool samples related to the outbreak. 2 Although all readers will be familiar with this agent and other morphologically indistinguishable particles, advances in our knowledge have been painfully slow over the two decades. This has been mainly due to the difficulty in purifying the viruses from clinical samples and the failure to culture them in vitro. As yet, fundamental information about their taxonomic status is lacking. T h e presumption, based on parameters such as size, buoyant density, major polypeptide and limited serological data is that they are members of the caliciviridae. Although the name is not internationally accepted, these viruses are now generally referred to as small round structured viruses (SRSVs) in the U.K. 3 T h e most outstanding practical problem is the difficulty in confirming the diagnosis. T h e period of virus excretion is usually only 2-3 days, relatively small numbers are excreted and the viruses are technically difficult to visualise so that the standard method of E M has a sensitivity of only 20--30 % even in the best hands. Recently a modification to E M called Solid Phase I m m u n e E M (SPIEM), 4 in which virus particles are trapped on the grid using a human convalescent serum, has been applied and has proved to be of significant benefit both in increasing sensitivity and the ease of detection of SRSVs. Although in Britain E M remains the only routine method for laboratory diagnosis, in the U.S.A. radioimmunoassay5 and subsequently E L I SA tests 6 for antigen detection in faeces have been available in some centres for several years. Production of sensitive specific assays is dogged by difficulties in preparing high quality reagents. Thus, h u m a n convalescent sera have to be used for trapping antigen and inevitably these will bind many antigens present in normal faeces severely impairing the performance of assays. T h e other approach to laboratory diagnosis, particularly in outbreaks, is through the use of serology. A range of tests for antibody detection have been described including immune EM, blocking radioimmunoassay,5 E L I S A 7 and most recently an IgM capture ELISA. 8 As with antigen detection, these methods have not been routinely available in the U.K. and, apart from the IgM capture assay, have required acute and convalescent sera limiting their usefulness. It became clear early in the history of SRSVs that some were immunologically distinct and others clearly related. Initial serotyping work was done purely by the technique of immune E M 9 subsequently other workers have applied E L I S A , 1° S P I E M 1~ and Western blotting. ~2 When better diagnostic assays become available and the epidemiology of these viruses can be studied more precisely, a reliable typing scheme will become invaluable. It, therefore, behoves those working in this area to try to collate the existing fragmented data into a unified scheme. T h e clinical illness associated with SRSVs is Winter vomiting disease, o163-4453/9o/o3o189 + 04 $02.00/0 8
© 199o T h e British Society for the Study of Infection JIN 20
190
Editorial
which although unpleasant, is rarely severe enough to lead to dehydration. T h e age distribution of illness is quite different to that associated with rotavirus since SRSVs can regularly cause illness in all age groups but do not feature as a significant cause of infantile gastro-enteritis. 13 Seroepidemiological studies have indicated that most adults have evidence of past exposure. 14 Interestingly, limited prospective cohort studies of pre-school children have indicated a significant prevalence in this age group but infection is often symptomless. 1~ T h e forte of SRSVs is in causing epidemics of gastro-enteritis. Diagnostic difficulties have limited study and the number of published outbreaks is small, but there is strong suggestive evidence that they are the commonest causes both of food-borne epidemics and outbreaks of non-bacterial gastro-enteritis involving person to person spread. ~ Thus, despite the mild nature of the illness these viruses pose a significant public health problem. A wide range of foods have been implicated as vehicles, but these fall into two groups. Firstly, outbreaks involving she!!fish, particularly oysters, that have been contaminated on the seabed by filtering virus from sewage; and secondly those in which contamination arises from a food handler who is infected and excreting virus. T h e hi~h-risk foods in this type of outbreak are quite different from those involved in bacterial food poisoning. Typically, they are foods that are handled in the kitchen and then served cold such as salads, fruit, cake icing, sandwiches and cold meats. Hot food has been implicated in only one outbreak suggesting that the virus is readily inactivated by heat. It has been the general assumption that person to person infection was by the classic faecal oral route with food being contaminated via soiled hands. T h e r e is now a growing body of evidence to suggest that this is not the only method of transmission. Firstly, virus has been detected in vgrqit and there is no doubt that the projectile vomitl~'ng which is such a feature of the clinical illness could readily contaminate the environment, whether it be in a toilet, public room, or most importantly, in the kitchen as indeed has been reported in one outbreak. 16 Also, evidence now suggests that air-borne transmission may occur. In a recently reported hospital outbreak, some victims had merely walked through the casualty department without patient or staff contact, x7 This clearly has major implications for attempts at controlling infection. In institutional outbreaks involving person to person spread, transmission usually continues over a period of x-2 weeks, often sustaining an attack rate of 50 % or higher. Interruption of such outbreaks poses a considerable challenge. Infection is most likely to be transmitted to those in close proximity TM and although isolation of infected individuals is often impractical, it should be possible to restrict movement. Emphasis must be placed on basic measures such as hand washing. Experience of outbreaks abroad cruise ships which provide an excellent milieu for person to person virus transmission has shown the importance of environmental decontamination both in toilet areas and in other areas where a victim has vomited. Virus is easily inactivated on hard surfaces by simple disinfection with z % hypochlorite. However, some interesting experiments on carpet artificially contaminated with the related feline calicivirus, have shown that virus inactivation requires prolonged contact with high doses of disinfectants in this setting in which hypochlorite
Editorial
~9
obviously cannot be used (C. Ashley and E. O. Caul, personal communication). Preventive measures in the kitchen are vital. Care must be taken to avoid cross-contamination from shellfish to other foods, e.g. in a refrigerator, by strict segregation. Techniques should be adopted which either avoid the need to handle high-risk foods or keep it to the absolute minimum. Adequate handwashing facilities must be available with a reliable supply of soap and paper towels. T h e most important preventive measure is for symptomatically affected personnel to be excluded from handling food. One of the difficulties about excluding staff whether in the kitchen or in an institution is that many can be so mildly affected that they can continue working throughout the illness. Controversy exists over how long affected individuals should remain off work. 19'2o. Virus excretion is usually undetectable about 2-3 days after the onset of illness but in one outbreak there was some evidence that a food handler may have contaminated food 48 h after recoveryflI Against this, transmission must become less likely when an individual has a formed stool and is not vomiting. Further, in the world of catering, even reliable exclusion of all symptomatic excreters would be remarkable and to expect this to be prolonged for 48 h after recovery is probably unrealistic even if desirable. However, when an outbreak occurs and there is an urgent need to break the chain of transmission, it would seem wise to adopt this period of exclusion. Because ready prepared high-risk foods may already have been contaminated these will need to be discarded. T h e r e are still few recorded outbreaks involving shellfish but the true scale of the problem is indicated by some publications in which series of large outbreaks were reportedfl 2 T h e problem is confined to molluscs rather than crustaceans, and oysters are much the commonest species to be involved. T h r e e factors combine to create this situation. Firstly, oysters are raised in estuarine waters which are often heavily polluted with sewage effluent. Secondly, there is evidence that viruses can be trapped in the flesh of the oyster rendering them immune to removal by the depuration processes so successful with bacterial pathogens. Finally, no gourmet would consider cooking an oyster so that the viruses are protected against this potential safety net. Viral contamination of molluscs has created considerable economic as well as public health problems. T h e U.K. oyster industry is a multi-million-pound enterprise with a considerable export component. Research into providing solutions is urgently required. T h e areas that require consideration on the production side include reducing sewage discharge into estuar!es, treatment of sewage, confining oyster production to cl..ean areas, cleansing of" oysters t~6Wards the end of production by removal to clean water. T h e other aspect is that of possible decontamination of oysters post production by processes such as chlorination, irradiation which do not destroy their gastronomic qualities. Clearly some means of detecting and monitoring contamination by SRSVs is paramount, emphasising yet again the pressing need for technical advances. T h e r e are many questions still to be answered and problems to be solved with this group of viruses. It is to be hoped that these are not completely forgotten in favour of higher profile or more commercially attractive areas of virology. 8-2
I92
Editorial
References
I. Adler JL, Zickl R. Winter vomiting disease. J Infect Dis I969; 119: 668-673. 2. Kapikian AZ, Wyatt RG, Dolin R et al. Visualisation by immune electron microscopy of a 27 nm particle associated with acute infectious non bacterial gastroenteritis. J Virol I972; I0:
IO75--IO8I.
3. Caul EO, Appleton H. T h e electron microscopical and physical characteristics of small round human faecal viruses: an interim scheme for classification, ff Med Virol t982; 9: 257-265 . 4. Rubenstein AS, Miller M F . Comparison of an enzyme immunoassay with electron microscopic procedures for detecting rotavirus. ,7 Clin Microbiol I982; 15: 938-944. 5. Greenberg HB, Wyatt RG, Valdesuso J e t al. Solid-phase microtitre radioimmunoassay for detection of the Norwalk strain of acute nonbacterial epidemic gastroenteritis virus and its antibodies, ff A/led Virol I978; 2: 97-Io8. 6. Hermann JE, Nowak NA, Blacklow NR. Detection of Norwalk virus in stools by enzyme immunoassay. ,7 A/led Virol I985; I7: I27-I33. 7- Gary G W , Kaplan JE, Stone S E e t al. Detection of Norwalk virus antibodies and antigen with a biotin-avidin immunoassay, ff Clin A/licrobiol I985; 22: 274-278. 8. Erdman D D , Gary GW, Anderson LJ. Development and evaluation of an I g M capture enzyme immunoassay for diagnosis of recent Norwalk virus infection. J Virol Methods I989; 24: 57-66. 9. Thomhill T S , Wyatt RG, Kalica A R et al. Detection by immune electron microscopy of 26-27 nm virus-like particles associated with two family outbreaks of gastroenteritis, ff Infect Dis I977; I35: 20-27. IO. Cubitt WD, Blacklow NR, Herrmann J E e t al. Antigenic relationships between human caliciviruses and Norwalk virus. :7 Infect Dis I987; 156: 8o8-8 I4. I I. Lewis DC, Lightfood NR, Pether JVS. Solid phase immune electron microscopy with human Immunoglobulin M for serotyping of Norwalk-like v!ruses, ff Clin Microbiol I988; 26 : 938-942. I2. Hayashi Y, Ando T, Utagawa E et al. Western blot assay of small round structured virus associated with an acute gastroenteritis outbreak in Tokyo. ff Clin Microbiol I989; 27: I728-I733. 13. Ellis ME, Watson B, Mandal BK et al. Micro-organisms in gastroenteritis. Arch Dis Child I984; 59: 848-855. I4. Picketing L K , DuPont H L , Blacklow NR, Cakar G. Diarrhoea due to Norwalk virus in families. ,7 Infect Dis I982; 146: 116-II7. I5. Riordan T. Norwalk virus disease. In: Morgan-Capner P, Ed. Current topics in virology. London: PHLS. In press. I6. Reid JA, Csul EO, White D G , Palmer SR. Role of infected food handler in hotel outbreak of Norwalk-like viral gastroenteritis: implications for control. Lancet I988; ii: 321-323. I7. Kaplan JE, Schonberger LB, Vasaro G et al. An outbreak of acute nonbacterial gastroenteritis in a nursing home. Demonstration of person to person transmission by temporal clustering of cases. A m ,7 Epidemiol I982; 116: 940-948. I8. Sawyer LA, Murphy JJ, Kaplan JE et al. 25-3 ° n m Virus particle associated with a hospital outbreak of acute gastroenteritis with evidence for airborne transmission. A m ff Epidemiol 1988; I27: 1261-1271. 19. Harries PG, Lumley KPS, Baxendine D M et al. Gastroenteritis in food handlers. Lancet 1988 ; ii: 9o 9. 2o. Palmer SR. Viral gastroenteritis in food handlers. Lancet 1988; ii: 1247. 21. White KE, Osterholm M T , Mariotti JA et al. A foodbome outbreak of Norwalk virus gastroenteritis. Evidence for post recovery transmission. A m ff Epidemiol 1986; 124: I20-126. 22. Sekine S, Okada S, Hayashi Y et al. Prevalence of small round structured virus infections in acute gastroenteritis outbreaks in Tokyo. A/licrobiol Immunol 1989; 33: 2o7-217.
20, 189-192
Editorial Norwalk
agent comes
of age
It is now 21 years since an outbreak of non-bacterial gastro-enteritis was reported in an elementary school in Norwalk, Ohio I and i8 years since the causative agent was revealed by electron microscopy (EM) in stool samples related to the outbreak. 2 Although all readers will be familiar with this agent and other morphologically indistinguishable particles, advances in our knowledge have been painfully slow over the two decades. This has been mainly due to the difficulty in purifying the viruses from clinical samples and the failure to culture them in vitro. As yet, fundamental information about their taxonomic status is lacking. T h e presumption, based on parameters such as size, buoyant density, major polypeptide and limited serological data is that they are members of the caliciviridae. Although the name is not internationally accepted, these viruses are now generally referred to as small round structured viruses (SRSVs) in the U.K. 3 T h e most outstanding practical problem is the difficulty in confirming the diagnosis. T h e period of virus excretion is usually only 2-3 days, relatively small numbers are excreted and the viruses are technically difficult to visualise so that the standard method of E M has a sensitivity of only 20--30 % even in the best hands. Recently a modification to E M called Solid Phase I m m u n e E M (SPIEM), 4 in which virus particles are trapped on the grid using a human convalescent serum, has been applied and has proved to be of significant benefit both in increasing sensitivity and the ease of detection of SRSVs. Although in Britain E M remains the only routine method for laboratory diagnosis, in the U.S.A. radioimmunoassay5 and subsequently E L I SA tests 6 for antigen detection in faeces have been available in some centres for several years. Production of sensitive specific assays is dogged by difficulties in preparing high quality reagents. Thus, h u m a n convalescent sera have to be used for trapping antigen and inevitably these will bind many antigens present in normal faeces severely impairing the performance of assays. T h e other approach to laboratory diagnosis, particularly in outbreaks, is through the use of serology. A range of tests for antibody detection have been described including immune EM, blocking radioimmunoassay,5 E L I S A 7 and most recently an IgM capture ELISA. 8 As with antigen detection, these methods have not been routinely available in the U.K. and, apart from the IgM capture assay, have required acute and convalescent sera limiting their usefulness. It became clear early in the history of SRSVs that some were immunologically distinct and others clearly related. Initial serotyping work was done purely by the technique of immune E M 9 subsequently other workers have applied E L I S A , 1° S P I E M 1~ and Western blotting. ~2 When better diagnostic assays become available and the epidemiology of these viruses can be studied more precisely, a reliable typing scheme will become invaluable. It, therefore, behoves those working in this area to try to collate the existing fragmented data into a unified scheme. T h e clinical illness associated with SRSVs is Winter vomiting disease, o163-4453/9o/o3o189 + 04 $02.00/0 8
© 199o T h e British Society for the Study of Infection JIN 20
190
Editorial
which although unpleasant, is rarely severe enough to lead to dehydration. T h e age distribution of illness is quite different to that associated with rotavirus since SRSVs can regularly cause illness in all age groups but do not feature as a significant cause of infantile gastro-enteritis. 13 Seroepidemiological studies have indicated that most adults have evidence of past exposure. 14 Interestingly, limited prospective cohort studies of pre-school children have indicated a significant prevalence in this age group but infection is often symptomless. 1~ T h e forte of SRSVs is in causing epidemics of gastro-enteritis. Diagnostic difficulties have limited study and the number of published outbreaks is small, but there is strong suggestive evidence that they are the commonest causes both of food-borne epidemics and outbreaks of non-bacterial gastro-enteritis involving person to person spread. ~ Thus, despite the mild nature of the illness these viruses pose a significant public health problem. A wide range of foods have been implicated as vehicles, but these fall into two groups. Firstly, outbreaks involving she!!fish, particularly oysters, that have been contaminated on the seabed by filtering virus from sewage; and secondly those in which contamination arises from a food handler who is infected and excreting virus. T h e hi~h-risk foods in this type of outbreak are quite different from those involved in bacterial food poisoning. Typically, they are foods that are handled in the kitchen and then served cold such as salads, fruit, cake icing, sandwiches and cold meats. Hot food has been implicated in only one outbreak suggesting that the virus is readily inactivated by heat. It has been the general assumption that person to person infection was by the classic faecal oral route with food being contaminated via soiled hands. T h e r e is now a growing body of evidence to suggest that this is not the only method of transmission. Firstly, virus has been detected in vgrqit and there is no doubt that the projectile vomitl~'ng which is such a feature of the clinical illness could readily contaminate the environment, whether it be in a toilet, public room, or most importantly, in the kitchen as indeed has been reported in one outbreak. 16 Also, evidence now suggests that air-borne transmission may occur. In a recently reported hospital outbreak, some victims had merely walked through the casualty department without patient or staff contact, x7 This clearly has major implications for attempts at controlling infection. In institutional outbreaks involving person to person spread, transmission usually continues over a period of x-2 weeks, often sustaining an attack rate of 50 % or higher. Interruption of such outbreaks poses a considerable challenge. Infection is most likely to be transmitted to those in close proximity TM and although isolation of infected individuals is often impractical, it should be possible to restrict movement. Emphasis must be placed on basic measures such as hand washing. Experience of outbreaks abroad cruise ships which provide an excellent milieu for person to person virus transmission has shown the importance of environmental decontamination both in toilet areas and in other areas where a victim has vomited. Virus is easily inactivated on hard surfaces by simple disinfection with z % hypochlorite. However, some interesting experiments on carpet artificially contaminated with the related feline calicivirus, have shown that virus inactivation requires prolonged contact with high doses of disinfectants in this setting in which hypochlorite
Editorial
~9
obviously cannot be used (C. Ashley and E. O. Caul, personal communication). Preventive measures in the kitchen are vital. Care must be taken to avoid cross-contamination from shellfish to other foods, e.g. in a refrigerator, by strict segregation. Techniques should be adopted which either avoid the need to handle high-risk foods or keep it to the absolute minimum. Adequate handwashing facilities must be available with a reliable supply of soap and paper towels. T h e most important preventive measure is for symptomatically affected personnel to be excluded from handling food. One of the difficulties about excluding staff whether in the kitchen or in an institution is that many can be so mildly affected that they can continue working throughout the illness. Controversy exists over how long affected individuals should remain off work. 19'2o. Virus excretion is usually undetectable about 2-3 days after the onset of illness but in one outbreak there was some evidence that a food handler may have contaminated food 48 h after recoveryflI Against this, transmission must become less likely when an individual has a formed stool and is not vomiting. Further, in the world of catering, even reliable exclusion of all symptomatic excreters would be remarkable and to expect this to be prolonged for 48 h after recovery is probably unrealistic even if desirable. However, when an outbreak occurs and there is an urgent need to break the chain of transmission, it would seem wise to adopt this period of exclusion. Because ready prepared high-risk foods may already have been contaminated these will need to be discarded. T h e r e are still few recorded outbreaks involving shellfish but the true scale of the problem is indicated by some publications in which series of large outbreaks were reportedfl 2 T h e problem is confined to molluscs rather than crustaceans, and oysters are much the commonest species to be involved. T h r e e factors combine to create this situation. Firstly, oysters are raised in estuarine waters which are often heavily polluted with sewage effluent. Secondly, there is evidence that viruses can be trapped in the flesh of the oyster rendering them immune to removal by the depuration processes so successful with bacterial pathogens. Finally, no gourmet would consider cooking an oyster so that the viruses are protected against this potential safety net. Viral contamination of molluscs has created considerable economic as well as public health problems. T h e U.K. oyster industry is a multi-million-pound enterprise with a considerable export component. Research into providing solutions is urgently required. T h e areas that require consideration on the production side include reducing sewage discharge into estuar!es, treatment of sewage, confining oyster production to cl..ean areas, cleansing of" oysters t~6Wards the end of production by removal to clean water. T h e other aspect is that of possible decontamination of oysters post production by processes such as chlorination, irradiation which do not destroy their gastronomic qualities. Clearly some means of detecting and monitoring contamination by SRSVs is paramount, emphasising yet again the pressing need for technical advances. T h e r e are many questions still to be answered and problems to be solved with this group of viruses. It is to be hoped that these are not completely forgotten in favour of higher profile or more commercially attractive areas of virology. 8-2
I92
Editorial
References
I. Adler JL, Zickl R. Winter vomiting disease. J Infect Dis I969; 119: 668-673. 2. Kapikian AZ, Wyatt RG, Dolin R et al. Visualisation by immune electron microscopy of a 27 nm particle associated with acute infectious non bacterial gastroenteritis. J Virol I972; I0:
IO75--IO8I.
3. Caul EO, Appleton H. T h e electron microscopical and physical characteristics of small round human faecal viruses: an interim scheme for classification, ff Med Virol t982; 9: 257-265 . 4. Rubenstein AS, Miller M F . Comparison of an enzyme immunoassay with electron microscopic procedures for detecting rotavirus. ,7 Clin Microbiol I982; 15: 938-944. 5. Greenberg HB, Wyatt RG, Valdesuso J e t al. Solid-phase microtitre radioimmunoassay for detection of the Norwalk strain of acute nonbacterial epidemic gastroenteritis virus and its antibodies, ff A/led Virol I978; 2: 97-Io8. 6. Hermann JE, Nowak NA, Blacklow NR. Detection of Norwalk virus in stools by enzyme immunoassay. ,7 A/led Virol I985; I7: I27-I33. 7- Gary G W , Kaplan JE, Stone S E e t al. Detection of Norwalk virus antibodies and antigen with a biotin-avidin immunoassay, ff Clin A/licrobiol I985; 22: 274-278. 8. Erdman D D , Gary GW, Anderson LJ. Development and evaluation of an I g M capture enzyme immunoassay for diagnosis of recent Norwalk virus infection. J Virol Methods I989; 24: 57-66. 9. Thomhill T S , Wyatt RG, Kalica A R et al. Detection by immune electron microscopy of 26-27 nm virus-like particles associated with two family outbreaks of gastroenteritis, ff Infect Dis I977; I35: 20-27. IO. Cubitt WD, Blacklow NR, Herrmann J E e t al. Antigenic relationships between human caliciviruses and Norwalk virus. :7 Infect Dis I987; 156: 8o8-8 I4. I I. Lewis DC, Lightfood NR, Pether JVS. Solid phase immune electron microscopy with human Immunoglobulin M for serotyping of Norwalk-like v!ruses, ff Clin Microbiol I988; 26 : 938-942. I2. Hayashi Y, Ando T, Utagawa E et al. Western blot assay of small round structured virus associated with an acute gastroenteritis outbreak in Tokyo. ff Clin Microbiol I989; 27: I728-I733. 13. Ellis ME, Watson B, Mandal BK et al. Micro-organisms in gastroenteritis. Arch Dis Child I984; 59: 848-855. I4. Picketing L K , DuPont H L , Blacklow NR, Cakar G. Diarrhoea due to Norwalk virus in families. ,7 Infect Dis I982; 146: 116-II7. I5. Riordan T. Norwalk virus disease. In: Morgan-Capner P, Ed. Current topics in virology. London: PHLS. In press. I6. Reid JA, Csul EO, White D G , Palmer SR. Role of infected food handler in hotel outbreak of Norwalk-like viral gastroenteritis: implications for control. Lancet I988; ii: 321-323. I7. Kaplan JE, Schonberger LB, Vasaro G et al. An outbreak of acute nonbacterial gastroenteritis in a nursing home. Demonstration of person to person transmission by temporal clustering of cases. A m ,7 Epidemiol I982; 116: 940-948. I8. Sawyer LA, Murphy JJ, Kaplan JE et al. 25-3 ° n m Virus particle associated with a hospital outbreak of acute gastroenteritis with evidence for airborne transmission. A m ff Epidemiol 1988; I27: 1261-1271. 19. Harries PG, Lumley KPS, Baxendine D M et al. Gastroenteritis in food handlers. Lancet 1988 ; ii: 9o 9. 2o. Palmer SR. Viral gastroenteritis in food handlers. Lancet 1988; ii: 1247. 21. White KE, Osterholm M T , Mariotti JA et al. A foodbome outbreak of Norwalk virus gastroenteritis. Evidence for post recovery transmission. A m ff Epidemiol 1986; 124: I20-126. 22. Sekine S, Okada S, Hayashi Y et al. Prevalence of small round structured virus infections in acute gastroenteritis outbreaks in Tokyo. A/licrobiol Immunol 1989; 33: 2o7-217.
