Turkeys give thanks for astrovirus and coronavirus research - By The Biotechnology and Biological Sciences Research Council - Scientists at the Institute for Animal Health discovered that a virus thought only of significance in North America had infected British turkeys. The researchers have now begun a three-year study of the virus to find out the extent of infection in the British turkey flock and look at the effects it has on bird welfare and mortality, reports Andrew McLaughlin.

For turkeys, diarrhoea is a problem. It’s a problem not only for the birds, but also the farmers that raise them - causing (for the birds at least) poor feed conversion, reduced growth rate and increased risk of catching a secondary infection.

“Digestive upsets in turkeys are a world-wide problem that no one has really got a handle on." explains the Dr Cliff Nixey, Chair of the British Turkey Federation’s Research and Development committee.

“Feed is often blamed, but there is no logic to the outbreaks that occur, indicating that infection is a primary cause of the problem." he continues.

Following an outbreak of turkey diarrhoea last summer, scientists at the Veterinary Laboratories Agency (VLA)were given samples of “caecal contents” to find out what exactly could be lurking in turkey intestines and causing the problem. The samples had all come from two-week-old poults (juvenile turkeys)with enteritis -diarrhoea caused by an infected intestine. The birds were also showing signs of stunting and uneven growth and came from flocks with an unusually high mortality rate of 4 percent.

Using an electron microscope, the VLA scientists spotted something resembling avian infectious bronchitis virus (IBV)-a virus usually associated with respiratory disease in chickens. To nvestigate further, the samples were sent to the Institute for Animal Health (IAH)for RT- PCR analysis. The analysis showed that the virus present in the samples had the same gene order as IBV, “But we ’d never seen an IBV like it before." according to Dr Dave Cavanagh from IAH. “When we compared the gene sequences of the virus in the samples to IBV sequences in the databanks we found that it was approximately ten percent different from all of them." he continued.

These findings correlated with studies in the USA of a known virus of significant importance to US turkey production, the coronavirus. North American isolates of the coronavirus also differed from IBV to the same extent. Together with evidence from the VLA, this suggested that the virus in the samples might be responsible for a significant disease in turkeys in Britain. “This was the f rst proof of the existence of coronavirus in turkeys outside North America and its detection matched firmly with the disease symptoms exhibited by the birds." says Cavanagh.

Until this point, very little was known about turkey coronavirus in the UK. Coronaviral infection had only ever appeared in flocks of farm-reared turkeys in USA and Canada, where it was originally known as ‘mud fever ’ or ‘bluecomb disease ’. But, ever since the f rst description of the disease in Washington state in the mid-1940s, and the subsequent attribution of the coronavirus as causative agent in 1971, the disease has rapidly risen in economic importance to US turkey producers.

“Once Dave Cavanagh’s team had indicated that coronavirus is present in the British turkey flock, it was clear that there could be a link between the virus and British turkeys suffering with diarrhoea." says Dr Nixey. With more than 30 million turkeys passing through British slaughterhouses each year, it became clear that the virus could represent a major problem to turkey production throughout Europe. “If we can reduce the risk of diarrhoea it will help improve feed conversion and growth rate and also make individual turkeys fitter." says Nixey.

The major economic threat from coronaviral infection comes through its association with Poult Enteritis and Mortality Syndrome (PEMS). Between March 2000 and April 2001, several large turkey contract growers in the USA were forced to ‘depopulate ’ their turkey houses due to severe PEMS outbreaks. This syndrome mirrors the symptoms of coronaviral infection by weakening the turkey ’s immune system, leaving it highly vulnerable to bacterial and parasitic infections. Outbreaks have cost the US turkey industry millions of dollars in losses with poult mortality ranging between 25 to 96 percent in affected flocks and birds that recover reaching only about 40 percent of typical market weight.

Although specific causative agents have yet to be identified, researchers investigating the syndrome in the USA commonly detect turkey coronavirus in flocks badly affected by PEMS. In addition, another virus found in PEMS test samples, a turkey astrovirus, has also been implicated in the syndrome. “There have been isolated symptoms of PEMS in the UK in places where there has been continuous production for a number of years, but nothing like the scale of what has happened in the USA." says Nixey. “UK producers tend to play much greater attention to basic hygiene and cleaning."

Coronavirus and astrovirus are both found in the epithelium of the intestinal tract and in the bursa of Fabricius -a lymphoid organ that opens into the cloaca of birds and functions in B cell production. Both the viruses are known to suppress the turkey immune system with evidence showing that the viruses cause damage to the bursa of Fabricius, with astrovirus also causing thymic atrophy. Whilst poults are more likely to die, birds of any age can be infected. The f rst symptoms are cold- like, with turkeys huddling together for warmth, stopping eating, losing weight and having wet droppings. Older birds become depressed and their head and skin can become darker. Clinical symptoms last for up to two weeks with recovery of weight, if achieved, taking several weeks.

Given the economic and welfare significance of the viruses in North America, the discovery of the coronavirus over here represents a timely interception. The British Turkey Federation and Merial Animal Health are providing funding for the three-year study. The project will focus on investigating the extent of infection of coronavirus and look for traces of the astrovirus in the British turkey flock, correlating the findings with incidence of disease. “Research in this area could lead to a solution for what is a major economic problem for turkey producers” says Nixey. Research at IAH is underpinned by the Department of Environment, Food and Rural Affairs (DEFRA)and is targeted on the rational manipulation of the IBV genome using recombinant DNA technology, with a view to producing stably attenuated vaccines that are appropriate to new serotypes of the virus.

“The research in the turkeys is mainly directed at investigating how these viruses compromise bird welfare and to consider what can be done to relieve any such problem." says Cavanagh. “We will study the epidemiology of the disease, including whether there is more than one type of each virus, and ultimately look at ways of introducing control ”. One possible control method is vaccination, “We will hopefully be able to develop genetically stable vaccines that humanely reduce the disease." he explains.

Source: The Biotechnology and Biological Sciences Research Council - January 2002