The impact of intestinal viruses

Intestinal viruses affect the intestinal epithelium in the chicken’s gut, impacting the absorption rate of nutrients and feed intake. As feed is by far the largest cost item at the poultry holding, feed efficiency is of utmost importance for cost-efficient production. Anything that affects the intestines affects the technical data. Intestinal viruses result in an impaired feed utilisation, deficient litter quality and often treatment with antibiotics. The latter already is, or will be, restricted more and more on a global scale. In addition, the immune system is largely linked to the intestines. Immune responses and the repair of tissue does cost energy and ingredients, so it will affect feed conversion.

Profile

Dr Sjaak de Wit (54)
Deventer
Senior researcher at GD Animal Health. Dr De Wit has worked as a poultry
veterinarian and immunologist at 
GD Animal Health in the Netherlands for 27 years.

Mixed infections

The intestines are very complex. They contain enormous amounts of bacteria, which have to be in balance, but the guts also contain several viruses and it plays an important role in the digestion process. Viruses, gut flora and digestion, continuously interact with one another. “This interplay makes the intestines very complex, and in my view there is still a lot unknown,” says Dr Sjaak de Wit. He was part of the team of researchers from GD Animal Health in the Netherlands which delved into viruses and their impact on intestinal health. The study was paid for by Dutch poultry farmers and led by Christian ter Veen. “In our study we wanted to discover which intestinal viruses occur and when they do so. This would enable us to estimate their relevance and determine what can be done to prevent or combat the viruses. When we initiated the study we hoped we could map the intestinal viruses.” But in practice, this proved to be problematic.

The research team worked with 98 Dutch broiler flocks at 86 farms during the study. “On a weekly basis we sampled 5 birds per flock and tested them for viruses such as Rotavirus A and Rotavirus D, Reovirus, Astrovirus and Avian Nephritis 3. We found many mixed infections. In some cases we found only 1 virus present in the guts at the moment of testing, but on average we found 2 or 3 and sometimes even 4 viruses simultaneously. This made it very hard to map the damage per individual virus. Moreover, it turned out to be difficult to work with a negative control group, as we also found several viruses in healthy flocks. These viruses didn’t necessarily cause obvious clinical problems, but it did however cause histological damage, as a certain level of intestinal inflammation was visible under microscopic inspection. This applied to all flocks during the entire research period.” The intestinal damage – despite the lack of clinical problems – resulted in less opportune technical results. The correlation between the technical performance and specific pathogens remained unclear due to mixed infections and the impact of the mixture on technical performance remained unclear due to the lack of negative control flocks.

“More research is necessary in order to determine whether or not it is useful to invest in combating these viruses. Breeders can be vaccinated against reoviruses in order to prevent early infection, but there are no vaccines for other viruses. This makes it difficult to determine whether developing vaccines is an economically efficient option. Particularly because of the many mixed infections,” explains Dr De Wit.

Eliminating intestinal viruses

The problem with intestinal viruses is that they are very resistant, which often occurs in early age; sometimes in the first and otherwise in the second week. A large part of the flock are carriers of intestinal viruses and this makes it very hard to ban them from the house. Good hygiene can lower infection pressure, but eliminating it is difficult, particularly as infection of the viruses spreads vertically, via the egg. Breeders with a virus can infect offspring after several weeks. In practice broilers are often a mixture of several breeding flocks, which makes it possible for them to produce infected chicks for several months. “Controlling intestinal viruses will have to be via maternally derived antibodies. A well protected breeder transmits less and produces safer chicks. Breeders with a field infection will produce antibodies, which they pass on to their offspring.”

When these chicks enter a house where the virus is still present, antibodies provide increased protection during the first weeks. “Vaccinating the breeder may be beneficial. However, in order to find out whether it is economically efficient to develop a vaccine, thorough research is necessary. And it is difficult to obtain these data. Until then there is not much a poultry farmer can do besides controlling infection rates.”

Ideally nutrients would take over the damage caused by intestinal viruses after the first weeks. “Certain nutrients have a strong anti-inflammatory effect. Other nutrients are easily digestible and therefore have less impact on the chickens’ guts. This may add to a swifter recovery of intestinal damage due to viruses. Poultry farmers aim to offer their birds the cheapest feed they develop well on. It would be extremely beneficial when the poultry farmer knew how a specific virus mix affects the feed conversion, allowing him to decide whether investing in additional feed additives is favourable. Yet, as long as we do not exactly know the impact of individual viruses and virus mixtures, this remains a distant prospect. In order to be able to do this we need knowledge of what damage individual viruses cause and how this can be influenced by nutrients.” Up until that moment early detection of infections is no priority. “Early detection is not beneficial yet, as we currently don’t know how to combat them. A quick PCR is not very useful just yet. But once we know how we can use feed to help birds recover from intestinal viruses, early detection will be great.”