Ridpath stresses that while vaccination is effective in increasing herd immunity against BVDV, no vaccine is 100% effective. Vaccines sometimes appear to fail, primarily for three reasons:
· A problem with the vaccine’s efficacy.
· A problem with the animal’s ability to respond.
· A problem with management.
As for vaccines, Ridpath says problems arise when there are differences between the strains of viruses circulating in the field and the strains of virus used in producing vaccines. Currently approved vaccines contain BVDV1a and BVDV2a viruses, but another sub-type, BVDV1b has become increasingly prevalent in U.S. herds. When PI calves turn up in vaccinated herds, diagnostic labs frequently have found the 1b subtype, Ridpath says. Vaccine companies currently are working to incorporate the 1b strain into their vaccines.
Problems with the animal being unable to respond are more common than problems with the vaccine, Ridpath says. These issues include animals that are sick at the time of vaccination, interference from maternal antibodies in young calves, inadequate nutrition and animals that are too stressed to respond at the time of vaccination. When calves experience weaning, dehorning, castration and vaccination on the same day, she notes, it should not be a surprise if vaccine response is inadequate. Genetic factors probably also influence vaccine response.
Management problems also play a key role in vaccine efficacy. If cattle are continuously exposed to a PI animal, vaccination will not solve the problem. Other management issues include:
· Insufficient time between vaccination and disease challenge. Allow at least two weeks for a good protective response.
· Failure to follow label directions. Killed vaccines require two doses, and MLV vaccines should be administered within one hour of mixing. Prevent exposure of vaccines to sunlight, and avoid use of detergents or disinfectants to clean multi-use syringes when using MLV vaccines. Do not use vaccines beyond their label expiration date.
Note: This article originally appeared as a part of "BVD Control: Multi-pronged approach" in the March 2017 issue of Bovine Veterinarian.