Modified-live versions implicated in repro failures
Growing evidence suggests that modified-live virus (MLV) vaccines for infectious bovine rhinotracheitis (IBR) do more harm than good when given to pregnant cattle.
IBR is a herpes virus. Like all herpes viruses, it can go into a latency phase within the animal’s brain. But when the animal is stressed with another shot of the live virus, the virus is reactivated, enters the bloodstream and migrates to the ovaries and fast-growing tissue such as embryos and fetuses.
Once there, it replicates and does its dirty work, says Chris Chase, a veterinarian with South Dakota State University. "Reproduction is a big problem with this virus," he says.
"Diagnosticians now have enough experience with these episodes to say that post-vaccination abortion can occur following on-label use of vaccines with modified-live vaccine IBR virus," adds Donald O’Toole, a veterinarian with the University of Wyoming. "It is not just a problem when MLV vaccines are used off-label, as has been known for years."
Abortion storms and poor reproduction can result, says Doug Scholz, director of veterinary services for Novartis Animal Health. He points to a number of large western dairies that experienced high numbers of abortions and poor conception rates when MLV IBR vaccines were given in the dry period and close-up pens.
One Colorado dairy was seeing an abortion rate of nearly 25% and a conception rate of just 30%. When the dairy stopped using the MLV IBR vaccine, abortions dropped to 8% and the conception rate climbed to 45%. On a second dairy, pregnancy losses dropped from nearly 30% to less than 10% and the conception rate jumped by 15% when the ML IBR vaccine was no longer used.
At the University of Wyoming, researchers found one or two aborted fetuses per day some 30 to 40 days after an MLV IBR vaccine was given to Angus heifers that were seven months pregnant. Total losses were 25% in the 55 heifers studied. "There was no evidence of infection other than IBR," O’Toole says. "My recommendation: Use inactivated IBR vaccines on breeding females."
The tendency for the IBR virus to go into latency is also a problem when young calves are vaccinated with a modified-live vaccine. When they’re later stressed, the virus can then be reactivated.
While this may cause only a slight fever for a day or two, the vaccinated animal can also shed live, virulent virus to its pen mates. If those animals are not vaccinated, an IBR outbreak can occur, says Clinton Jones, a veterinarian at the University of Nebraska. His bottom line: "Do not use an MLV IBR vaccine on young animals and do not mix unvaccinated calves."
Inactivated IBR vaccines for both cows and calves are the preferred method of vaccination, say these veterinarians. "Inactivated vaccines will decrease symptoms during clinical outbreaks, and they eliminate the possibility of virus spread to the fetus," Jones says.