The most important enteric viruses

Enteric viruses are a concern to animal producers, pet owners, veterinarians, public health officials and anyone who cares for animals. Many of these enteropathogens cause severe intestinal lesions, or modify digestion. The result usually is diarrhoea, but the clinical presentation varies. While some of the diarrhoeas are self-limiting, and resolve by themselves, some others may produce the death of the infected animals. In general, they are highly transmissible. In this step we are going to learn which viruses induce gastrointestinal clinical signs and how they produce this effect. Infectious diarrhoea of neonatal animals is one of the most common and economically devastating conditions encountered in husbandry.

By far, most of these diarrhoeas are produced by rotaviruses and coronaviruses, which have also been associated with diarrhoea in adult food animals, but with a lower incidence than in neonates. Other viruses that have been involved in diarrhoea of young farm animals include flavivirus (such as bovine viral diarrhoea virus), parvovirus, enterovirus and paramyxovirus. Other viruses have tropism for the liver. Hepatitis E, an example of these viruses, is a zoonosis which is becoming increasingly important. Every year there are an estimated 3 million symptomatic cases of hepatitis E infection in humans, even though it causes only subclinical infections in pigs. Cattle and sheep may suffer from Rift Valley Fever, a virus that may affect the liver.

This is a very severe zoonosis transmitted through mosquito vectors. The economic impact of enteric virus infections on the poultry industry ranges from insignificant economic effects to those that are severe and cause devastating losses. Enteric diseases tend to predominantly affect young birds, in their first three weeks of life; however, the disease may occur in all age groups. Enteric viruses increase susceptibility to other diseases, decrease feed conversion efficiency, and prolong the time to market. Diarrhoea is a common manifestation of the disease. Parvovirus are very small viruses with correspondingly small genomes. They absolutely require that the cell is dividing for their replication.

As the cells in the intestine are being constantly renewed, they represent an ideal target for these viruses, as we will see later. Parvoviruses produce canine parvovirosis and feline panleukopenia. Coronavirus also reside in the gut of most pets, and in cats, they may produce a fatal disease called feline infectious peritonitis. As for other farm animals, rotavirus are a common cause of diarrhoeas in foals, though they seldom produce disease in adult horses. Diarrhoea is usually seen in foals less than 2 months old, and the younger the foal the more severe the clinical signs. As you can see, there is a wide array of viruses which may reside in the gut and may induce enteric disease in animals.

There are many more RNA viruses involved in diarrhoea outbreaks than DNA viruses. The genome of the RNA viruses is more variable and, thus, they are more difficult to fight against. Even though the number of families of enveloped viruses is roughly similar to the number of families of naked viruses, the severity of the diseases produced by the naked viruses is much greater.

In addition, as they are naked, they are highly resistant 00:04:04.439 –> 00:04:07.439 and permit indirect transmission.

This high resistance also allows naked viruses to enter the body through the mouth. Once in the gut, they infect the enterocytes and may cross the intestinal wall, spreading to other body sites where they cause disease. Contrarily, fragile enveloped viruses bypass the stomach. They enter through other routes and reach the intestine transported by the blood. In this case, they infect the enterocytes from the baso-lateral pole. Regardless of the entry route, they are all shed with the faeces. Intestinal lesions are going to be found in different places depending on the type of viral genome. The mucosa of the intestine is covered with finger-like projections called villi.

At the tip of these villi, enterocytes are mature and do not have the ability to multiply. Most enteric viruses are able to replicate here and destroy the villi. As a result, when the villi become shorter, the surface of the intestine decreases and nutrients are not absorbed. The bottom of the villi is called the crypt; that is where the enterocytes multiply before they are pushed to the tips. Parvoviruses specifically infect these cells because they need dividing cells to replicate. Thus, parvovirus cause disease by avoiding new enterocytes to repopulate the tip of the villi. From this activity we would like you to remember that there are many viruses that may induce GI tract damage in animals.

The most enteropathogenic viruses are naked viruses, which are extremely resistant to the harsh conditions of the stomach and intestine.