Viral Evolution and Its Relevance for Food-Borne Virus Epidemiology

The adaptation of viruses to changing environments determines the types and numbers of viral particles present in any physical context, be it food or other, and hence the probability of encountering a susceptible host to cause disease. Genes encoding nonstructural proteins of viruses are generally more highly conserved than are genes encoding structural proteins such as capsid proteins of naked viruses and surface glycoproteins of enveloped viruses. Mutation is the most universal form of genetic variation of viruses. The mechanisms of genetic variation of viruses are blind processes that generate repertoires of variants which are then subjected to selection and random (chance) expansions in each particular environment in which virus replication takes place. In general, virus transmission by food or water should be considered a chance event; it should be regarded as a special case of fecal-oral transmission, with the virus being present in food or the environment for short or long periods. Transmission via food is an opportunity for an unlikely transmission event to occur relatively easily. Cross-species transmission is an event with strong selective pressure at the level of virus entry. Since virus entry involves protein rather than nucleic acid, the relevance of genetic variation is limited if it does not result in phenotypic variation. Contamination at the source occurs when irrigation water contains sewage or when filter-feeding bivalve mollusks grow in sewage-contaminated water.