key: cord-0008082-kvuy06ln
authors: Koopmans, Marion; Duizer, Erwin
title: Foodborne viruses: an emerging problem
date: 2004-01-01
journal: Int J Food Microbiol
DOI: 10.1016/s0168-1605(03)00169-7
sha: 12c1768d6177262b7414e97bd4ba4c42d74fd304
doc_id: 8082
cord_uid: kvuy06ln

Several groups of viruses may infect persons after ingestion and then are shed via stool. Of these, the norovirus (NoV) and hepatitis A virus (HAV) are currently recognised as the most important human foodborne pathogens with regard to the number of outbreaks and people affected in the Western world. NoV and HAV are highly infectious and may lead to widespread outbreaks. The clinical manifestation of NoV infection, however, is relatively mild. Asymptomatic infections are common and may contribute to the spread of the infection. Introduction of NoV in a community or population (a seeding event) may be followed by additional spread because of the highly infectious nature of NoV, resulting in a great number of secondary infections (50% of contacts). Hepatitis A is an increasing problem because of the decrease in immunity of populations in countries with high standards of hygiene. Molecular-based methods can detect viruses in shellfish but are not yet available for other foods. The applicability of the methods currently available for monitoring foods for viral contamination is unknown. No consistent correlation has been found between the presence of indicator microorganisms (i.e. bacteriophages, E. coli) and viruses. NoV and HAV are highly infectious and exhibit variable levels of resistance to heat and disinfection agents. However, they are both inactivated at 100 °C. No validated model virus or model system is available for studies of inactivation of NoV, although investigations could make use of structurally similar viruses (i.e. canine and feline caliciviruses). In the absence of a model virus or model system, food safety guidelines need to be based on studies that have been performed with the most resistant enteric RNA viruses (i.e. HAV, for which a model system does exist) and also with bacteriophages (for water). Most documented foodborne viral outbreaks can be traced to food that has been manually handled by an infected foodhandler, rather than to industrially processed foods. The viral contamination of food can occur anywhere in the process from farm to fork, but most foodborne viral infections can be traced back to infected persons who handle food that is not heated or otherwise treated afterwards. Therefore, emphasis should be on stringent personal hygiene during preparation. If viruses are present in food preprocessing, residual viral infectivity may be present after some industrial processes. Therefore, it is key that sufficient attention be given to good agriculture practice (GAP) and good manufacturing practice (GMP) to avoid introduction of viruses onto the raw material and into the food-manufacturing environment, and to HACCP to assure adequate management of (control over) viruses present during the manufacturing process. If viruses are present in foods after processing, they remain infectious in most circumstances and in most foods for several days or weeks, especially if kept cooled (at 4 °C). Therefore, emphasis should be on stringent personal hygiene during preparation. For the control of foodborne viral infections, it is necessary to: • Heighten awareness about the presence and spread of these viruses by foodhandlers; • Optimise and standardise methods for the detection of foodborne viruses; • Develop laboratory-based surveillance to detect large, common-source outbreaks at an early stage; and • Emphasise consideration of viruses in setting up food safety quality control and management systems (GHP, GMP, HACCP).

Viruses are very small microorganisms, ranging in size from 15 to 400 nm (examples, see Fig. 1 ). Viruses cause a wide range of diseases in plants, animals and humans. These infections do not occur at random: each group of viruses has its own typical host range and cell preference (called tropism). Viruses can be transmitted in different ways, for example by droplets generated when an infected person coughs, by contamination with stool samples from a person infected with an intestinal virus, by sexual intercourse, by contact with blood from infected persons with bloodborne viruses, by contact with infected animals with zoonotic viruses, or by vectors, such as mosquitoes or ticks for arthropod-borne (arbo-) viruses. Clearly the most relevant in foodborne infections are those viruses that infect the cells lining the intestinal tract and are dispersed by shedding into the stool or through emesis (Table 1) . Some general features of foodborne viral infections and important differences from foodborne bacterial infections are:

Only a few particles are needed to produce illness; High numbers of viral particles are shed in the stools from infected persons (up to 10 11 particles per gram stool reported for rotavirus); Fig. 1 . Electron micrograph and some structural properties of enteric viruses that are commonly (NoV, HAV) or occasionally (other viruses) associated with foodborne or waterborne transmission (Locarnini et al., 1974) . (NoV-Noroviruses, HAV = hepatitis A viruses, PV = poliovirus, EV = enterovirus, HRV = human rotavirus; ss = single-stranded, ds = double-stranded).

Viruses need specific living cells in order to replicate and therefore cannot do so in food or water; and Foodborne viruses typically are quite stable outside the host and are acid-resistant.

Conceivably, current food hygiene guidelines, most of which have been optimised for the prevention of bacterial infections, may be only partially (if at all) effective against viruses. A complicating factor is that most common foodborne viruses grow poorly or not at all in cell culture, so that studies of inactivation of these pathogens are not possible. For this overview, we have reviewed currently available information on foodborne viruses and tried to give an estimate of viral inactivation by looking for parallels in structurally similar viruses that can be grown in cell culture systems in the laboratory.

Numerous viruses can be found in the human gut, but only a few are commonly recognised as important foodborne pathogens. These can be classified into three main groups, according to the type of illness they produce (Table 1) :

Viruses that cause gastroenteritis; Enterically transmitted hepatitis viruses; and A third group of viruses that replicate in the human intestine but cause illness after they migrate to other organs, such as the central nervous system or the liver.

Foodborne illness has been documented for most of these viruses, but recent studies show that the Noroviruses (NoV) and hepatitis A virus (HAV) are by far the most common cause of illness by this mode of transmission (Cliver, 1997) . Some large foodborne outbreaks have occurred with group B and C rotaviruses, and waterborne outbreaks have occurred with hepatitis E virus.

Recent studies have shown that NoV is the single most common cause of gastroenteritis in people of all age groups and is as common as rotavirus in patients who consult their general practitioners for gastroenteritis (Wheeler et al., 1999; Koopmans et al., 2000) . The incidence is highest in young children, but illness also occurs regularly in adults. Asymptomatic infections are common. In addition, the majority of outbreaks of gastroenteritis in institutions such as nursing homes and hospitals is caused by NoV (Codex Alimentarius, 1999) . Although it is not known what proportion of infections can be attributed to the consumption of contaminated food, several reports have shown that foodborne NoV infections are common. Large, even international foodborne outbreaks of NoV have been described (Berg et al., 2000) . Data from seroprevalence studies suggest that NoV infections are found worldwide.

For HAV the picture is different. The incidence of HAV infection varies considerably among and within countries (Mast and Alter, 1993) . In much of the developing world, where HAV infection is endemic, the majority of persons are infected in early childhood and virtually all adults are immune. In these areas, HAV transmission is primarily from person to person. Outbreaks are rare because most infections occur among young children who generally remain asymptomatic. In the developed countries, however, HAV infections become less common as a result of increased standards of living. Very few persons are infected in early childhood, and the majority of adults remain susceptible to infection by HAV. Because virus shedding starts 10 -14 days before the onset of symptoms, there is a clear window for spreading the virus. As a result, the risk of (large) outbreaks of HAV increases in these regions.

In addition, adults are more likely to develop symptoms upon infection, causing enhanced recognition of outbreaks. Indeed, foodborne outbreaks have been reported in most parts of the world and can be large. For example in Shanghai, China, in 1988, 250 ,000 people had HAV after consumption of contaminated clams (Halliday et al., 1991) . Detection of sporadic cases or small clusters of foodborne hepatitis A is problematic because the incubation period can be long. As a result, a possible association with food consumed weeks ago can rarely be investigated at the time of onset of illness. For NoV and HAV, waterborne outbreaks are unusual but have been reported.

For most enteric viruses, host range variants have been found in different animal species. So far, however, the majority appear to be quite host-specific. Recently, NoV was found in a large proportion of calf herds and in some pigs . The strains in animals were genetically distinct from any of the viruses found in people (Sugieda et al., 1998) . No calf-to-human or pig-to-human transmission has been documented so far. The animal viruses, however, are quite similar to the human NoV and continue to change as all RNA viruses do. This implies that zoonotic transmission might occur if the right circumstances arise.

Similarly, hepatitis E virus (HEV) variants were found in pigs and, in this case, almost identical viruses were found in some humans (Meng et al., 1997) . This was taken as the first evidence of zoonotic transmission of HEV. The pig viruses appear to be quite common, even in countries where HEV is rarely diagnosed in humans. This suggests that the risk of zoonotic transmission is rather low and currently of no practical consequence for food handling procedures. Again, however, given the genetic flexibility of RNA viruses, these viruses should be monitored closely for changes in behaviour.

Infection with gastroenteritis viruses is usually diagnosed by the detection of the pathogen in stool samples from sick people, rather than by measuring the antibody response in serum (Tables 2 and 3) . Historically, viruses were diagnosed by scanning a stool suspension under an electron microscope (EM) . This assay still remains the gold standard for virus diagnosis but is rather insensitive and labour-intensive (Table 2) . Routine ELISA assays are available for detection of group A rotaviruses, adenoviruses, and astroviruses, as well as for some of the NoV (Tables 2 and 3 ). For non-group A rotaviruses, Sapoviruses (SaV), and the remaining NoVs, the diagnosis can be made by detection of viral nucleic acid using reverse transcriptase-polymerase chain reaction (RT-PCR) assays. A problem with NoV is the variability of the viral genome, making it difficult to develop a single generic detection test. For the hepatitis viruses, detection of specific IgM antibodies is diagnostic of recent infection. In addition, viruses can be detected in stool and in serum by RT-PCR, but this is not done routinely. There are great Virus detection in food or water has been problematic, even after the introduction of RT-PCR. Because the most important foodborne viruses do not grow (readily) in cell culture, they must be detected directly in food extracts, with all the problems of standardisation, inhibition of enzymes used in the RT-PCR, falsepositive tests, etc. (Lees, 2000; . Because contamination is often caused by foodhandlers, the level of contamination with virus may vary greatly within a product. The combination of variable virus counts and the lack of a culture system is the main reason why virtually no information is available on the variability of test results from sampling or what would be considered representative samples for monitoring purposes. Furthermore, (molecular) diagnostic methods for food or water are not routinely available in food microbiology laboratories (Table 3) . Most successful research has focussed on shellfish, but even with published standard protocols, little is known about the performance of such standards in ''the field'' (e.g. if a batch of oysters does contain some contaminated a Category of laboratory: R = routine; S-R is routinely available in specialised laboratories; S-E = experimentally available in specialised laboratories; S-E/R = routinely available in some of the specialised laboratories, experimentally available in more. b EM = particle detection by electron microscope. HAV < 2 (Hollinger and Ticehurst, 1996) or HAV>4 (Croci et al., 1999; Millard et al., 1987) PV < 2 (Nissen et al., 1996) NoV: incomplete inactivation (Dolin et al., 1972) Medium Inactivation in solid foods lower than in liquids; dependent on fat and protein content HAV>3, HAV < 2, HRV < 2, PV>3 (Abad et al., 1994; Sobsey, 1989) Variable Risk is low for PV but medium for HRV and HAV UV radiation (20 mJ/cm 2 ) PV 3 or less (Sommer et al., 1989) HRV < 3 (Sobsey, 1989) Low Ozone treatment (0.2 mg/l, 10 min) HAV>3, PV 2 or less, HRV < 1 (Kim et al., 1999; Sobsey, 1989) Variable Risk is low for HAV but medium/high for PV and HRV b Unlikely = no reports are known in which NoV, HAV, RV, or PV were found on these food items. Possible = sporadic contamination with NoV, HAV, RV, or PV has been reported on these food items. Likely = contamination with NoV, HAV, RV, or PV is reported frequently on these food items. Negligible risk = product highly unlikely to contain infectious viruses; treatment results in at least 4 log 10 inactivation of common foodborne viruses. Low risk = product unlikely to contain infectious viruses in numbers likely to cause disease in healthy individuals; treatment results in approximately 3 log 10 inactivation of common foodborne viruses. Medium risk = product may contain infectious viruses in numbers that may cause disease; treatment results in approximately 2 log 10 inactivation of common foodborne viruses. High risk = products in which the level of viruses is likely to be high enough to cause disease in healthy individuals; treatment results in less than 1 log 10 inactivation of common foodborne viruses. Variable risk = treatment results in significant differences in inactivation of several common foodborne viruses. c Before spray drying in dried milk processes, a substantial heat step destroys viruses. ones, how likely is it that a virus test will give the right answer?). Therefore, these methods currently cannot be used reliably for quality control and assurance.

For all enteric viruses, strains can be divided into subtypes by analysis of the genome. By doing so, common source outbreaks have been diagnosed, even in cases in which links between different outbreaks had not been suspected on the basis of epidemiological investigation (Berg et al., 2000) . Conversely, molecular strain typing has also been used to disprove links between cases and a suspected source (Marshall et al., 2001) . At present, a European foodborne virus network, including various public health institutes, uses information on strain typing to trace NoV and HAV outbreaks. The participating groups have agreed to exchange epidemiological and virological information through a central database to identify international common source outbreaks as early as possible (QLK1-1999-00594; for information: marion.koopmans@rivm.nl).

A problem in drafting recommendations for virus control and prevention is that some enteric viruses grow poorly (HAV) or not at all (NoV) in cultured cells . In addition, no simple animal models are available for experimental studies of virus inactivation. Thus, detection methods currently rely on genome detection by molecular detection techniques such as RT-PCR. A positive signal indicates an intact segment of viral genomic RNA. This does not provide information on virus infectivity. Completely inactivated particles that pose no threat to public health may still contain intact RNA, thus resulting in a positive virus assay. The RNA will eventually be degraded, but it is unknown how long this will take in different environments. In shellfish, inactivation of the virus was followed by rapid degradation ( < 1 min) of viral RNA (Slomka and Appleton, 1998) .

In seawater, however, RNA persisted for days after inactivation of the virus.

In the absence of a culture system for NoV, a common sense approach is to review information on structurally similar viruses and use these as models for the noncultivatable pathogens. For HAV, a cell-cultureadapted variant has been used, for example in studies addressing heat resistance in different food items (Bidawid et al., 2000a) . For NoV, structurally similar viruses are the enteroviruses, HAV, and astroviruses. These are all viruses with a single-stranded RNA genome and are approximately 7-kb long, approximately 30 nm in size, and similar in capsid structure (no envelope) (Fig. 1 ). Slomka and Appleton (1998) recommended the use of an animal calicivirus (FeCV) for inactivation studies and found that FeCV was clearly less stable than HAV. Because most infectivity and inactivation data are available for the enteroviruses and HAV, we have used these data for our risk estimates (Table 4 ). It should be clear, however, that these remain estimates and will have to be evaluated carefully.

In water quality research, the use of indicators for the presence of human pathogenic viruses has been an area of considerable debate (Lees, 2000) . It is clear from numerous outbreaks that the presence of ''traditional'' bacterial indicators of faecal contamination does not consistently correlate with the presence of pathogenic viruses. Many groups have proposed the use of bacteriophages as indicators. Bacteriophages are viruses that infect and replicate in bacteria. They are present in substantial numbers in human stool samples and, in some respects, are similar to viruses pathogenic to humans. Because of similarities in structure, behaviour, and stability, bacteriophages may be of use in assessing cumulative exposure to human faecal waste. However, care must be taken not to provide a false sense of safety by measuring the presence of bacteriophages only. The observed clear differences in stability of different human pathogenic viruses (described later in this document) that reside in the intestine illustrate that extrapolation of data from one virus to another cannot be relied upon (Slomka and Appleton, 1998) . Similarly, the possible use of phages as surrogates in evaluating the antiviral effectiveness of processes needs to be carefully validated (Mariam and Cliver, 2000a) .

Foods can be contaminated by (Fig. 2) :

Contact with (human) faeces or faecally contaminated water; Contact with faecally soiled materials (including hands); Contact with vomit or water contaminated with vomit; Contact with environments in which infected people were present, even if the surface was not directly contaminated with stool or vomit; and Aerosols generated by infected people.

There is no proof that animal contact, directly or indirectly (pigs, calves, surface-contaminated meat, meat products, or other products derived from those animals), can be a source of foodborne infection.

Central to the issue are infected foodhandlers. These may be:

Infected foodhandlers with symptoms. Shedding of virus occurs during the period of illness;

Infected foodhandlers who have recovered from illness. Shedding of NoV may persist for at least 3 weeks after recovery; Infected foodhandlers without symptoms. Asymptomatic infections are common for all foodborne viruses. For example, carriers of hepatitis A typically shed high quantities of the virus 10-14 days after infection; in the weeks following this period carriers may or may not develop symptoms; and Foodhandlers with contacts with sick people (e.g. people with sick children or relatives).

Note that although most outbreaks can be traced to infected foodhandlers at the end of the food chain, they may be anywhere (e.g. seasonal workers picking berries for use in composite foods, people on recreational boats near shellfish harvesting areas, etc.). A large, multistate outbreak of illness associated with oysters was finally traced back to a sick oyster harvester who had vomited and disposed of the waste overboard (Berg et al., 2000) .

Outbreaks have been documented in association with a long list of food items (e.g. deli meat, sandwiches, bread rolls, bakery products, berries, ice cubes). Dishes containing fresh (or fresh frozen) fruits and vegetables have been the source of numerous outbreaks of foodborne illness. Filter-feeding shellfish are a particular risk, as they concentrate viruses present in their growing water, and numerous outbreaks linked to the consumption of shellfish have been reported.

It is clear that foods requiring either intensive manual handling, manual handling under poor hygienic conditions (e.g. in orchards), or close-to-fork and endproduct manual handling are the products at highest risk (see http://www.who.int/fsf/). Information obtained with HAV suggests that approximately 10% of the virus particles can easily be transferred from faecally contaminated fingers to foods and surfaces (Bidawid et al., 2000b) .

Another factor determining risk for contamination of foods is the stability of some of the foodborne viruses in the environment. For example, rotaviruses in aerosols (generated while vomiting and thought to play a role in the transmission of those viruses) were found to survive in the air up to 9 days at 20 jC (Sattar et al., 1984) . Viruses also may persist for extended periods (1 -60 days for 100-fold reduction in infectivity) on several types of materials commonly found in institutions and domestic environments (e.g. paper, cotton cloth, aluminium, china, glazed tile, latex, and polystyrene; Abad et al., 1997) (Fig. 3 ). Adenoviruses were found to survive for up to 35 days Represented are the number of days after which the virus recovery will be less than 1% (A and B) or 10% (C) of the original contamination. (Data from: Enriquez et al., 1995; Kurdziel et al., 2001; Mbithi et al., 1991; Ward and Irving, 1987) . on a plastic surface in an environment with low relative humidity (Nauheim et al., 1990) . This relation with humidity varies among viruses. A high relative humidity favours the survival of enteroviruses, whereas a low relative humidity favours survival of HAV and human rotavirus (HRV) (Mbithi et al., 1991; Sattar et al., 1986 Sattar et al., , 1988 . Furthermore, HAV remained infectious in dried faeces for 30 days when stored at 25 jC and 42% relative humidity (Hollinger and Ticehurst, 1996) . This stresses the need for virusspecific studies to address virus inactivation. Finally, in artificially contaminated water, viruses may survive for prolonged periods of time, with over 1-year survival of poliovirus and rotavirus in mineral water at 4 jC (Biziagos et al., 1988) . Recent data published by Beuret et al. (2000) on traces of NoV RNA found in bottled water may tend to support this statement. However, as yet, no one has been able to confirm these data.

Viruses, unlike bacteria, are strict intracellular parasites and cannot replicate in food or water. Therefore, viral contamination of food will never increase during processing, transport, or storage, and the contaminated products will look, smell, and taste normal. Moreover, because contamination is often caused by foodhandlers, the level of contamination with virus may vary greatly within a product.

Nonetheless, several recent studies were performed to determine the modes of transfer and inactivation profiles of foodborne viruses. Most food-or waterborne viruses are more resistant to heat, disinfection, and pH changes than are most vegetative bacteria. It is no coincidence that most virus groups implicated in outbreaks are small, nonenveloped particles, rather than large, fragile, enveloped viruses (Fig. 1) . Numerous studies have addressed the stability of viruses under different circumstances (see Table 4 ), but little was done to standardise these studies. An overall conclusion is that HAV and HRV are more resistant to inactivation than enteric adenovirus and poliovirus, but it must be noted that significant differences in survival rates were found for different environmental and substrate conditions. Again, these findings stress the need for independent assessment of behaviour for different viruses.

This poses a problem for NoV, which cannot be grown in cell culture and,, therefore, cannot readily be tested under the experimental conditions described previously. It remains to be seen whether other viruses that can be grown in tissue culture may serve as models for the NoV, as has been suggested for FeCV. In the interim, we recommend using the inactivation profiles of the most stable enteric RNA virus to assess the safety of a process. Thus, for most processes relevant in the food industry, HAV may be considered a good indicator virus.

Information obtained with HAV shows that more than 1000 virus particles can easily be transferred from faecally contaminated fingers to foods and surfaces (Bidawid et al., 2000b) . Based on this information, an inactivation factor of at least 3 log 10 during post-manual-treatment processes would be required. Based on these assumptions, we have tried to estimate the likelihood of survival of the most important foodborne viruses for commonly used food processing methods if foods are contaminated before processing. With the exception of ultrahigh temperature treatment, no methods would completely inactivate more than 3 log 10 of virus, and we estimate that with foods contaminated after processing, viruses will remain active to a significant extent and thus pose a possible risk factor (Fig. 3, Table 4) . Therefore, the emphasis should be strongly on prevention of contamination before or during processing by proper deployment of GHP, GMP, and HACCP. Clearly, the likelihood of virus contamination in primary products will differ for different commodities and is the highest for shellfish and manually handled fruits. For foods contaminated after processing, our estimate is that viruses will remain active in most foods (Fig. 3, Table 4 ).

It is clear that most problems with foodborne viruses occur from contamination of food products during manual handling in combination with minimal processing of foods afterwards. With viral infections (e.g. NoV) being very common, it is wise to assume that the introduction of viruses into the food chain is a likely event that needs to be prevented by stringent hygienic control. Foodhandlers in contact with people with gastroenteritis (e.g. young children) are at special risk of being contaminated and becoming a source of viruses during food manufacture operations. They must be made aware that specific personal hygiene must be ensured. Increasing the awareness of all foodhandlers about transmission of enteric viruses (including the spread of viruses by vomiting) is needed, with special emphasis on the risk of ''silent'' transmission by asymptomatically infected persons and via those who continue to shed virus after recovery from illness. At present, insufficient data are available to determine which steps will be critical for all foods in an HACCP system, but it is clear that at least the following points should be addressed:

Water used in combination with the culturing or preparation of food should be of drinking water quality; and Guidelines specifically aimed at the reduction of viral contamination are needed, as it has become clear that current indicators for water and shellfish quality are insufficient as predictors of viral contamination.

Documented outbreaks of foodborne infections could be reported faster using, for example, the European Foodborne Virus Network, the ''rapid alert system for food'' of the European Union. These networks could operate more effectively if typing information for virus strains were included.

A vaccine is available for hepatitis A, and contacts can be treated with the administration of immunoglobulin within 2 weeks after exposure. The Advisory Committee on Immunization Practices (ACIP, 1996) in the United States has suggested that HAV vaccination should be considered for foodhandlers, although risk assessment will be different for each country given the great differences in seroprevalence of HAV. At present, most countries prefer to stress the use of stringent personal hygiene to prevent infections.

More developmental work is required on methods to detect viruses in food. Such methods should be reasonably simple (as few steps as possible), efficient (in terms of recovery of viruses), and reproducible. The crucial (and most difficult to achieve) step is extraction of virus particles from the food matrix. Research should focus on this process. A standard method to assess virus survival would allow acquisition of comparative data (e.g. responses of different virus types to the same set of environmental conditions). The features of this method would include similar inoculum size, sample size, sampling time, and statistical analysis. A project to develop such a method and apply it in various environments would provide useful data. Efforts to find a cell culture system that will allow propagation of NoV are vital. The above two recommended areas of endeavour will depend upon this to be applicable to NoV. Information is needed on virus survival on different food commodities including thermal resistance. Information is needed on duration of shedding and levels of virus shedding in persons with and without symptoms.

The existing surveillance systems for foodborne viruses are incomplete. Basic virus detection and typing methods are not routinely available in many countries. Rapid detection and reporting networks for foodborne viruses need to be implemented in standard surveillance systems. These networks should combine laboratory and epidemiological information. A reporting strategy for international outbreaks should be established. The detection and prevention of foodborne viral infections should be organised. Foodborne viral infections are diagnosed with increasing frequency. This illustrates the existence of regular breaks in the microbial safety of food. Although the most common pathogens cause relatively mild, selflimiting illness, their high incidence illustrates the potential for large, international foodborne viral epidemics. This includes the risk of foodborne spread of more dangerous pathogens, such as HEV or enteroviruses that may cause paralytic illness. Person-to-person spread is very high. As a result, an initial point-source outbreak may be amplified significantly. Incidents (foodborne outbreaks) should be evaluated carefully by governments, WHO, and NGOs to identify whether changes in the guidelines are needed.

The emphasis should be on GAP. Primary products and raw materials, especially those of agricultural origin, must be protected from contamination by human, animal, domestic, or agricultural wastes that are known sources of viruses/microorganisms. Examples of such products are fresh berries and salad. Foodhandlers, including seasonal workers, need to be educated specifically about microbial safety guidelines and hygiene rules. This includes education about the risk of exposure to viruses through sick children in the household. Managers of agricultural businesses involving produce to be eaten raw need to exclude foodhandlers with symptoms consistent with exposure to infectious foodborne diseases until 48 h after recovery (Cowden et al., 1995) . Foodhandlers returning to work need to be instructed that substantial numbers of NoV may be shed for weeks after recovery from illness and that they need to follow hygiene rules strictly. The microbial safety guidelines for shellfish need to be revised to include viral food safety. Primary products must not be produced in areas where water used for irrigation might constitute a health hazard to the consumer through the food.

Food safety management systems (HACCP, GHP and GMP), safety guidelines, and best-practice documents need to include considerations on the possible risks that infectious foodborne viruses pose during and after processing. This underlines the importance of adherence to good personal hygiene.

Primary products must not be produced in areas where water used for irrigation might constitute a health hazard to the consumer through the food. Foodhandlers, including seasonal workers, need to be educated specifically about the microbial safety guidelines and hygiene rules. This includes education about the risk of exposure to viruses through sick children in the household.

Managers of food manufacturing industries should consider excluding foodhandlers with symptoms consistent with exposure to infectious foodborne diseases until 48 h after recovery (Cowden et al., 1995) . Foodhandlers returning to work need to be instructed that NoV can be shed for weeks following recovery from illness and should be made aware that stringent personal hygiene must be ensured. Microbial food safety guidelines should be revised to include viral food safety (e.g. for codes of practice). Incidents should be reported to public health authorities through existing networks.

Foodhandlers, including seasonal workers, need to be educated specifically about the microbial safety guidelines and hygiene rules. This includes education about the risk of exposure to viruses through sick children in the household. Managers of catering and food service industries need to exclude foodhandlers with symptoms consistent with exposure to infectious foodborne diseases until 48 h after recovery (Cowden et al., 1995) . Foodhandlers returning to work need to be instructed that substantial numbers of NoV may be shed for weeks after recovery from illness. Incidents should be reported to public health authorities through existing networks.

Consumers and physicians need to be specifically educated about microbial safety guidelines and hygiene rules, including those for viruses. 

Human enteric caliciviruses cause gastroenteritis in humans. The human caliciviruses are assigned to two groups, the genera Norovirus (NoV) and Sapovirus (SaV). The NoVs are also known as ''small-roundstructured-viruses'' (SRSV) and the SaVs as ''typical caliciviruses''. The two virus groups differ epidemiologically. The NoVs cause illness in people of all age groups, whereas the SaVs predominantly cause illness in children.

Foodborne transmission of caliciviruses is well known for viruses in the NoV genus. Within this genus is a great diversity of virus types, with genetic differences and differences in the protein composition of the virus particles. To date, 15 distinct genotypes have been recognised, but their number is likely to increase. Infected persons develop immunity, which is shortlived and predominantly type-specific. As a result, one person can have multiple NoV infections, which in part explains the high incidence of NoV infection.

After a 1 -3-day incubation period, infected persons may develop low-grade fever, vomiting, diarrhoea, and headache as prominent symptoms. Symptoms usually subside within 2-3 days, although the course of illness may be protracted in the elderly. Deaths associated with NoV outbreaks have been reported, but a causative relationship remains to be proven. The average attack rate is high (typically 45% or more). The virus is shed via stools and vomit, starting during the incubation period and lasting up to 10 days and possibly longer. NoV infections are highly contagious, resulting in a high rate of transmission to contacts.

NoV infections are among the most important causes of gastroenteritis in adults, and often occur as outbreaks that may be foodborne. The spread can be epidemic. In The Netherlands, approximately 80% of outbreaks of gastroenteritis reported to municipal health services are caused by NoVs. More than half of these outbreaks occur in nursing homes, but this may be an overrepresentation resulting from selection bias. In The Netherlands, foodborne outbreaks are also reported through a network of food inspection services. Preliminary results from studies there suggest that NoVs may also cause a significant number of these outbreaks. Based on studies from the UK and the US, it has been estimated that a substantial proportion of foodborne infections may be caused by NoVs (67% estimated for the US by Mead et al., 1999) . In addition to outbreaks, NoVs also cause numerous sporadic cases of gastroenteritis. Five percent of patients with gastroenteritis who consult a physician have NoV infection, compared with 4% for Salmonella. In addition, caliciviruses are by far the most common cause of sporadic gastroenteritis (NoV accounts for 11% of all cases). Monitoring of sewage samples confirmed that high levels of NoVs circulate in the general population.

It has been established that many different types of NoV cocirculate in the general population, causing sporadic cases and outbreaks. However, occasionally, epidemics occur in which the majority of outbreaks are caused by a single genetic type (e.g. in the Netherlands in 1996).

These epidemics may be widespread and even global. The mechanisms behind the emergence of epidemic types are unknown. Hypotheses range from large-scale foodborne transmission of a single strain to spillover from a reservoir, possibly nonhuman. An indication for the latter was a recent study from Japan in which NoVs were found in stool specimens from pigs, using RT-PCR assays based on caliciviruses of humans (Sugieda et al., 1998) . In the 1990s, the reported incidence of NoV increased, probably as a result of improved diagnostic methods and increased awareness.

Outbreaks of NoV gastroenteritis (not only foodborne) are common in institutions such as nursing homes and hospitals. The high attack rate in both residents and personnel at such institutions leads to major logistic problems (understaffing) during outbreaks. In addition, an unknown but probably large number of sporadic cases occur. The risk factors for these infections are currently under investigation in the United Kingdom and the Netherlands.

NoVs are transmitted by direct contact or indirectly via contaminated water, food, or from the environment. Many foodborne NoV outbreaks have been described, often caused by infected foodhandlers. The NoVs usually are shed in large quantities during the initial stages of the illness, with maximal titres as high as 10 8 virus particles per gram of stool. Although there are some indications for aerogenic transmission of NoV, the importance of this route is still unclear. Infectious viruses can be transmitted not only at the time of illness but also during the incubation period and after recovery, with 30% of cases shedding virus for up to 3 weeks after infection.

In addition to foodborne transmission, waterborne transmission of NoV is common, both directly (e.g. during recreation) or indirectly. NoVs can survive outside the host, are resistant to common disinfectants and extreme pH fluctuations, and are highly infectious. As a result, transmission of virus via fomites is likely.

Filter-feeding shellfish are notorious as a source of foodborne viral infections because they actively concentrate viruses from contaminated water. Infectious viruses can be detected for up to 6 weeks without any loss in quality of the shellfish. Depuration, a practise that may reduce bacterial contamination, is not as effective in reducing the viral load of shellfish.

In addition to shellfish, many food items have been associated with NoV outbreaks. In the literature, several other manually handled foods have been implicated (desserts, fruits, vegetables, salads), but the message is that any food that has been handled manually and not heated (sufficiently) afterwards may be a source of infection.

NoV or SaV infections can be diagnosed by visualisation of virus particles by electron microscopy and with molecular methods (RT-PCR). However, in most countries these methods are not available for routine diagnostics. Stool viruses can be typed by sequence analysis or by reverse-line blotting, and genetic typing may be used to trace common source outbreaks. Using these techniques, outbreaks from geographically distinct regions have been linked.

A.9 . Virus detection in food and water Molecular methods have been adapted for the detection of NoVs in food and water. However, because little is known about their sensitivity under field conditions, these techniques are not yet routinely available. Quality control of food and water on the basis of the detection of indicator organisms for faecal contamination has proven to be an unreliable predictor for viral contamination. When NoVs are detected in food, typing assays can be used to establish transmission routes. However, these techniques are not routinely available in most laboratories.

Some groups of animal caliciviruses have a broad host range, and it is currently a matter of debate whether NoVs can be transmitted between humans and animals. Recently, caliciviruses indistinguishable from NoVs have been found in pigs in Japan and cattle in the United Kingdom, Germany, and the Netherlands. Data from the Netherlands suggest a very high prevalence of NoV in calf herds.

Strict implementation of hygienic rules is currently considered the most important preventive measure. Foodhandlers with gastroenteritis should immediately be removed from the food chain. More problematic are outbreaks linked to asymptomatic, presymptomatic, and postsymptomatic shedders. The kinetics of viral shedding has been studied in only a few infected volunteers and may not reflect real-life situations in which people may have been infected with a low dose of infectious virus. Given the highly infectious nature of NoV and the documented risk of virus transmission to food during the incubation period, it is suggested that guidelines be developed that include the occurrence of gastroenteritis in contacts (e.g. children) of people working at critical points in the food chain. This should be based on data on the kinetics of viral shedding after natural infection.

For prevention of foodborne transmission, it obviously is also essential that food items be not grown or washed in faecally contaminated water. However, the globalisation of the food market has hampered the implementation of control measures to assure safe food, as it is often difficult to exactly trace the food.

Routine monitoring is not yet feasible: first, because there are no good methods, and, second, because end-product testing is not reliable to assure food safety on statistical grounds. Documented outbreaks of foodborne infections could be reported faster using a system such as the ''rapid alert system for food'' of the European Union. However, this would be much more informative if typing information of virus strains were included.

Norwalk virus (one of the prototypes NoV) is resistant to low pH and heat treatment (30 min at 60 jC). The virus reportedly is quite resistant to chlorine; the virus remains infectious after 30 min in the presence of 0.5 -1 mg free chlorine/l. At higher concentrations (>2 mg free chlorine/l), the virus is inactivated. The effect of other disinfectants on NoV infectivity has hardly been studied because of the lack of a tissue culture system or animal model.

Disinfection of human enteric viruses in water by copper and silver in combination with low levels of chlorine

Disinfection of human enteric viruses on fomites

Prevention of hepatitis A through active or passive immunization. Recommendations of the Advisory Committee on Immunization Practices

Diagnosis of noncultivatable gastroenteritis viruses, the human caliciviruses

Detection of enteric viruses in oysters by using PCR

Multistate outbreaks of acute gastroenteritis traced to fecal-contaminated oysters harvested in Louisiana

Norwalk-like virus sequences detected by reverse transcription polymerase chain reaction in mineral water imported into or bottled in Switzerland

Heat inactivation of hepatitis A virus in dairy foods

Contamination of foods by foodhandlers: experiments on hepatitis A virus transfer to food and its interruption

Longterm survival of hepatitis A virus and poliovirus type 1 in mineral water

Virus transmission via food

Discussion Paper on Viruses in Food. FAO/WHO document CX/FH 99/11

Outbreaks of foodborne infectious intestinal disease in England and Wales 1992 -1993

Inactivation of hepatitis A virus in heat-treated mussels

Biological properties of Norwalk agent of acute infectious nonbacterial gastroenteritis

Inactivation of feline calicivirus, a Norwalk virus surrogate

Survival of the enteric adenovirus 40 and 41 in tap, sea and waste water

Norwalk virus gastroenteritis in volunteers consuming depurated oysters

An epidemic of hepatitis A attributable to the ingestion of raw clams in Shanghai

Hepatitis A virus

Inactivation of human viruses by povidone-iodine in comparison with other antiseptics

Application of ozone for enhancing the microbiological safety and quality of foods: a review

Molecular epidemiology of human enteric caliciviruses in the Netherlands

Survival of poliovirus on soft fruit and salad vegetables

Viruses and bivalve shellfish

The relationship between a 27-n virus-like particle and hepatitis A as demonstrated by immune electron microscopy

Small round coliphages as surrogates for human viruses in process assessment

Hepatitis A virus control in strawberry products

Multiple outbreaks of NoV gastroenteritis associated with a Mediterranean-style restaurant

Epidemiology of viral hepatitis

Effect of relative humidity and air temperature on survival of hepatitis A virus on environmental surfaces

Food-related illness and death in the United States

A novel virus in swine is closely related to the human hepatitis E virus

Studies on heat inactivation of hepatitis A virus with special reference to shellfish

Prolonged recoverability of desiccated adenovirus type 19 from various surfaces

Inactivation of hepatitis A and other enteroviruses during heat treatment (pasteurization)

Effect of relative humidity on the airborne survival of rotavirus SA11

Institutional outbreaks of rotavirus diarrhoea: potential role of fomites and environmental surfaces as vehicles for virus transmission

Spread of acute hemorrhagic conjunctivitis due to enterovirus-70: effect of air temperature and relative humidity on virus survival on fomites

Feline calicivirus as a model system for heat inactivation studies of small round structured viruses in shellfish

Inactivation of health-related microorganisms in water by disinfection processes

UV-inaktivierung von Mikroorganismen in Wasser

Detection of Norwalk-like virus genes in the caecum contents of pigs

Evidence that microorganisms cause inactivation of viruses in activated sludge

Virus survival on vegetables sprayirrigated with wastewater

Study of infectious intestinal disease in England: rates in the community, presenting to the general practitioner, and reported to national surveillance

Resistance of poliovirus to inactivation by high hydrostatic pressures