Wildlife a concern in BVDV control
Lessons from tuberculosis eradication attempts could improve bovine viral diarrhea virus control programs.
Dr. Paul Walz, an associate professor at Auburn University, said BVD can infect white-tailed deer and, if they are pregnant, their infections can result in persistently infected offspring. It is not as clear whether the virus is capable of maintaining itself in deer populations.
However, his and other research groups have found free-ranging white-tailed deer that have tested positive for BVDV through immunohistochemistry of skin samples, which is also used to identify persistently infected cattle.
"We may not be able to completely eradicate BVDV in the United States if we have a reservoir in white-tailed deer, because pastured beef cattle and white-tailed deer make contact," Dr. Walz said. "With BVDV, it is not known if contact between cattle and white-tailed deer is sufficient to result in BVDV transmission, but we have another disease present in cattle where white-tailed deer are important in transmission, which is bovine tuberculosis."
Dr. Walz said wild ruminants such as deer can serve as reservoirs for bovine tuberculosis and transmit the agent back into cattle populations after it has been eradicated from those cattle herds.
"As we execute control and eradication programs, it's very important to know where all the potential reservoirs are, and that's why studies on white-tailed deer as a potential reservoir of BVDV are very important," Dr. Walz said. But it is premature to say deer are responsible for infections with the virus in cattle populations.
Dr. Walz was one of about two dozen speakers and nearly 200 attendees at a BVDV symposium Jan. 25-27 in Phoenix. He was also one of several speakers who talked about the connections between BVDV infections in livestock and wildlife.
The symposium was the fourth since 2002, and the most recent meeting was titled "BVDV Variability: Impact on Virulence, Host Range and Control."
Dr. Christopher Chase, conference chair and a professor at South Dakota State University, said presentations on wildlife reservoirs of BVDV may have helped give attendees perspective on the difficulties of eradicating the virus. Other presentation topics included the virus' ability to subvert immune systems, infection in alpacas, impacts on cattle and industry, and examples of what producers have done to successfully control outbreaks.
Dr. Robert Fulton, a professor at Oklahoma State University, said in his presentation that groups trying to combat BVDV should collaborate and provide a consistent message to livestock producers about control programs. He recommended giving serious consideration to establishment of a national committee for BVDV control that would unite groups already committed to control of the virus.
Dr. Fulton also outlined research priorities for BVDV, including assessment of the virus' economic impact; design and evaluation of surveillance and control programs; surveillance, identification and determination of impact of emerging subtypes; development and evaluation of diagnostic tests; evaluation of vaccines; development of new vaccines; and investigation of the pathogenesis of BVDV in cattle.
Dr. Julia Ridpath, a microbiologist for the Department of Agriculture's Agricultural Research Service and an organizer for the symposium, also spoke about BVDV in wild ruminants. She said researchers have found wild ruminants with antibodies against the virus as well as others with ongoing infections.
In deer herds, which are primarily composed of females and young deer, between 30 percent and 60 percent of the animals had antibodies, a similar rate to that seen in unvaccinated cattle, she said. Studies involving hunter-collected deer had lower rates, but they included higher proportions of bucks, which do not have much contact with herds outside mating season.
"We want to look at studies that look at populations that run in herds rather than (those that) are living singularly," Dr. Ridpath said.
It's not clear how prevalent BVDV is in wild ruminants, but persistently infected bison, mule deer, and white-tailed deer have all been captured in the U.S., Dr. Ridpath said.
It is not clear whether those animals pose a substantial risk to others in the wild, but they can infect herds of other deer when they gather at cattle feeding areas and use livestock salt blocks, she said. Some white-tailed deer also graze with cattle.
Dr. Ridpath said study results reported at the symposium indicated researchers at Auburn University transferred BVDV back-and-forth between cattle and deer. She said there is no way of knowing whether deer commonly serve as a source of infection in cattle.
Infection among domestic species may be a problem during reintroduction of protected wild ruminants, Dr. Ridpath said. BVD was isolated in carcasses of some bighorn sheep following an attempt to reintroduce the sheep in Colorado and Wyoming, and the virus may have contributed to the animals' deaths, Dr. Ridpath said.
Although the principal goal is to limit damage of BVD infection in cattle, Dr. Ridpath said research needs to address wildlife species and answer questions about controlling the virus. She said the big question involves how, in free-ranging ruminants, to manage pathogens that have an impact on domestic species.