Turkey parvovirus ‘widespread’ in US flocks

Turkey parvovirus (TuPV), a recently discovered pathogen, is not only widespread in US commercial flocks, it appears to be an important cause of gut disease, says Dr. Laszlo Zsak, a researcher with the United States Department of Agriculture, Agricultural Research Service.

“Our data show that, in fact, most if not all major turkey-producing states have flocks where parvovirus infection has been detected, and there is a clear understanding that the infection plays a major role in the etiology of enteric diseases of turkey,” says Zsak, research leader for the ARS Endemic Poultry Viral Diseases Unit, Southeast Poultry Research Laboratory in Athens, Georgia.

Using genetic sequencing techniques, Zsak and colleagues identified TuPV in the gut contents of turkeys with enteric disease and determined that it represents a new member of the parvovirus family that’s closely related to chicken parvovirus.

To get a handle on the prevalence of TuPV, they tested enteric samples from 29 turkey farms in five states with a polymerase chain reaction assay and found that 78% of the samples were positive. Of samples from North Carolina, California and Minnesota, 95%, 83% and 42%, respectively, were positive, and the parvoviruses found in samples from different states were closely related to each other, Zsak and colleagues reported in the March 2009 issue of Avian Diseases.

Further evidence that TuPV causes enteric disease now comes from more recent studies in which the researchers orally challenged 2-day-old specific-pathogen-free (SPF) turkeys with the virus.

“It’s important to note that these turkeys had no maternal antibodies to TuPV, nor did they have any detectable astrovirus, reovirus or rotavirus. We had a clean experimental environment and SPF birds to study the effect of parvovirus infection alone,” Zsak told Turkey Health.

ENTERITIS, STUNTED GROWTH

After challenge, birds developed characteristic signs of enteric disease such as diarrhea, and starting about 14 days after challenge, approximately 75% displayed significant growth retardation. Turkeys with stunted growth averaged 15% behind that of uninfected control birds and they never achieved normal weight, says Zsak, who reported the findings at the Western Poultry Disease Conference earlier this year.

Parvovirus shedding was also observed in cloacal swabs 4 days after challenge and lasted until 32 days post-challenge. In addition, 80% of the turkeys infected with TuPV developed viremia that peaked at 14 days after challenge. At the end of the 35-day study, TuPV was still detectable in the intestine, spleen and bursa of Fabricius.

Like other parvovirus infections, turkey parvoviruses are present in birds as young as 4 days of age, confirming the possibility that the virus may spread vertically — from parent to progeny. However, “considering the rapid growth characteristics of parvoviruses and their efficient spread via the fecal-oral route, horizontal transmission of the virus from infected birds to naïve individuals may also play an important role in the spread of virus within flocks,” Zsak says.

Asked if TuPV might be a more important cause of enteritis in turkeys than other pathogens, the researcher says there is no real scientific evidence that one pathogen would be more important in the etiology of turkey enteric diseases than any other factor.

“However, our results suggest that infection with parvovirus, which can happen at a very early age, may result in a rapid and robust virus replication in the gut, which is followed by diarrhea and stunting of the birds.” In these cases, there is an increased chance for other, sometimes opportunistic, pathogens to further contribute to the clinical and pathological course of the disease.

“While TuPV infection may not cause the most severe [enteric] disease by itself, we believe that early infection with this virus can induce a series of major, primary events in the gut, which, in conjunction with other factors, will turn eventually into a complex enteric syndrome,” he says.

CONTROL OPTIONS

The key to control of TuPV, Zsak says, may be immunizing parents to provide offspring with maternal immunity sufficient enough to protect against TuPV during the first few weeks of life. This approach has been used successfully to control Derzsy’s disease in geese, which is caused by a parvovirus, he notes.

Because technology to propagate and attenuate live turkey parvoviruses in cell culture is lacking at this time, the most realistic approach for now may be using an inactivated vaccine to immunize breeders, then boosting them with a subunit vaccine to provide a high level of maternal antibodies for prevention of parvovirus infection or to at least eliminate the primary clinical signs. It is very likely that a high level of maternal antibodies in turkeys would successfully protect against parvovirusinduced enteric disease, the veterinarian predicts, noting that he and his team are working to develop an in-house ELISA that could detect TuPV-specific antibodies in layer flocks and maternal antibodies in young poults.

“Since enteric disease in turkeys is a very significant health problem, these novel research findings should be taken into consideration when we try to improve the health status of commercial US turkey flocks,” Zsak says, noting that he is confident that researchers, working collaboratively with the turkey industry, will ultimately find an efficient strategy for controlling TuPV.

Zsak: 78% of samples were positive for TuPV.

Parvovirus