Enterically infecting viruses: pathogenicity, transmission and significance for food and waterborne infection

Abstract

Enterically infecting viruses are ubiquitous agents, mostly inducing silent infections. Several are however associated with significant diseases in man from diarrhoea and vomiting to hepatitis and meningitis. These viruses are drawn from a variety of virus families and have different structures and genetic material, yet all are suited to this means of transmission: Normally they are shed in high numbers (assisting environmental transit) and exhibit great particle stability (permitting survival both outside the body and on passage through the stomach). Human activities particularly associated with food and water processing and distribution have the capacity to influence the epidemiology of these viruses. This review provides a description of viruses spreading by these means, their significance as pathogens and considers their behavior in these human-assisted processes. The term virus stems from the Latin virus meaning ‘poison’, and in some ways virus contamination of food resembles toxic contamination more than contamination with other micro-organisms. Viruses are not free living; they are dormant between hosts and have an absolute requirement for living cells in which to replicate. Human viruses require human cells in which to replicate, these are not present in our food and thus such viruses cannot increase in number during storage. The amount of any contaminating viruses should actually decline during storage, and this can be assisted by treatment with chemicals, heat or irradiation. Human viruses do not cause food spoilage and contamination may provide no visible clues to its presence. These features mean that measures to control bacteria will not necessarily control viruses and could actually preserve them. Food-borne viruses are infectious at very low doses and could be introduced at any point in the food chain. Many are difficult (or currently impossible) to culture and detection is no simple task. Outbreaks and sporadic occurrences of food-borne virus infection continue throughout the world. There are simply too many to mention and there is no complete data set. It seems likely that the documented outbreaks are limited only by our ability to document them. The cost of these events is likely to be phenomenal to the community; a single food-borne outbreak of hepatitis A virus (HAV) exposed up to 5000 persons in Colorado. In this case the costs for medical treatment of those infected amounted to approx. $50 000 whilst the cost of tracing and controlling this single outbreak cost over half a million US$ (Dalton et al. 1996). The burden of infectious intestinal disease (IID) in its broader sense is likewise huge, in the UK the cost per case of norovirus (NoV)-induced gastroenteritis involving a GP visit is estimated at £176 and two persons in every 1000 will make such a visit each year (FSA 2000). The interested reader is referred to several recent reviews (Lees 2000; Seymour and Appleton 2001; Sair et al. 2002; Koopmans and Duizer 2004). As enteric viruses cannot replicate outside their hosts, all such virus transmission is in effect person-to-person. Environmental transit time between hosts may be brief or prolonged. Long-distance travel may take place, e.g. through water systems or even the air. Long-distance travel is accompanied by exposure to the environment and dilution; thus viruses having prolonged environmental transit times must be very stable to survive and are (usually) shed in very large numbers. Enteric viruses meet both requirements; they are acid stable and replicate to prodigious titres in the gut before being shed in concentrated doses directly into the sewage system. All potentially food-borne viruses can also be transmitted directly from person to person via faecal contamination of the environment and viewed in this way food is simply another kind of fomite in environmental transmission, it occupies a special niche simply because of its privileged position in terms of its introduction to the body and the potential it may offer for widespread distribution through trade and commerce. The relative importance of food-borne vs more direct person-to-person transmission is unclear; enteric infections are ubiquitous, single occurrences are far too numerous to mention and statistics usually record only outbreaks (when several people are infected in one location or through one common vehicle). However any one outbreak may involve different types of spread; these viruses have a high secondary attack rate and person-to-person transmission will probably follow even if the virus was actually introduced to that setting by food. This can potentially mask food-borne introductions and it is likely therefore that food-borne transmission is underestimated. There are two types of enterically infecting virus – the first are capable of spreading elsewhere in the body. Infection by these viruses is often subclinical but they may induce signs and symptoms of disease in nonintestinal tissues. These viruses inc