Epidemiology of enteric diseases

In this step we will see that enteric viruses

may infect animals in one of two ways: direct contamination and indirect contamination. Animals may eliminate viruses and contaminate the environment with their faeces. Most enteric viruses are hardy viruses that may withstand the harsh conditions of the gastrointestinal tract, with its low pH and many enzymes. Thus, they progress virtually unaltered along the intestine and are eliminated fully infective. A naïve individual may become infected by consuming contaminated food or water. In humans, this would be the case of hepatitis A, and it would be an example of indirect transmission. In the case of animals, they may become infected by sniffing the perianal area, faecal remains or vomit of another infected animal, a common habit in several species.

These would all be examples of direct transmission. A good example of both transmission routes is hepatitis E, which is a zoonosis in which the reservoir species is mainly the pig. Genotypes 1 and 2 of the virus are transmitted by an indirect route, through the ingestion of contaminated water essentially. But with genotype 3, which is predominant in developed countries, humans become infected by consuming uncooked or undercook pork sausage, or by contact with pet pigs. A human disease which has been very crippling for centuries is poliomyelitis, which even though it produces a neurologic syndrome, is also produced by an enterovirus. Poliomyelitis is produced by a member of the Picornaviridae family. The virus is spread through person-to-person contact.

When a child is infected with wild poliovirus, the virus enters the body through the mouth and multiplies in the intestine. It is then shed into the environment through the faeces, where it can spread rapidly through a community, especially in situations of poor hygiene and sanitation. Polio has been eradicated from most countries, but it is still endemic in Pakistan and Afghanistan. In addition, in Ukraine, Guinee, Nigeria, Madagascar, Myanmar and Laos during 2015 the circulation of wild virus, derived from the poliovirus vaccine, was also recorded. The take away message of this step is that enteric viruses are very resistant and that, when excreted with faeces or with vomit, they may contaminate food, water or inanimate objects and be easily transmitted.