While vital research continues, veterinarians know quite a bit about the bovine viral diarrhea virus (BVDV), how it spreads and how to prevent or control it in beef and dairy herds. And yet the virus continues to cause substantial losses across the production chain.
Julia Ridpath, PhD, studies BVDV and related pestiviruses at the USDA’s National Animal Disease Center (NADC) in Ames, Iowa. She has authored or co-authored over 200 research papers, mostly on BVDV and related viruses in cattle. Ridpath co-edited the book "BVDV: Diagnosis, Management, and Control", and she currently leads a research project at NADC titled "Intervention Strategies to Control Viral Diseases of Cattle."
Ridpath recently presented a webinar on BVDV control, which was hosted by the American Association of Extension Veterinarians (AAEV). In a previous webinar, she outlined the basic biology of BVDV1, BVDV2 and related HoBi-like viruses. Recordings of both webinars are now available on the AAEV website.
Ridpath says three species of pestiviruses cause BVD disease worldwide; BVDV1, BCDV2 and HoBi-like virus, which occurs primarily in Asia, South America and somewhat in Europe.
Effective BVDV control, Ridpath says, requires three components: diagnosis, vaccination and biosecurity.
Diagnosis presents challenges because there is no single “typical” BVD presentation. Cases can be from mild to severe, and veterinarians cannot rely on herd history, clinical presentations or post-mortem exams alone to diagnose the disease. Accurate diagnosis requires laboratory testing, ideally on two samples submitted about three weeks apart.
Disease syndromes associated with BVDV include immunosuppression, reproductive disease, mucosal disease, enteric disease and persistently infected (PI) calves, which sometimes show little or no clinical sign of disease.
PI calves occur when the dam is exposed to BVDV during the first trimester of gestation. Many are sickly and do not survive long, but some others show little or no signs of disease and shed the virus continuously, exposing the rest of the herd. Diagnosis, removal and isolation of PI cattle serves as a critical step in a BVDV-control program.
Current vaccines available for protection against BVDV include modified-live virus (MLV) and killed-virus vaccines. MLV vaccines typically require a single dose while killed vaccines need two doses. Ridpath points out that some vaccines are labeled for preventing clinical disease following acute infection while some others are labeled for prevention of the birth of PI calves. Also, some are labeled for use in pregnant cattle.
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 field strains of BVDV and BVDV serotypes used in producing vaccines. Currently approved vaccines protect against BVDV1a and BVDV2a, but another sub-type, BVDV1b has become increasingly prevalent in U.S. herds.
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.
Part 2 of this series will outline Dr. Ridpath’s coverage of BVDV control points and testing, disposition of PI calves and opportunities for regional or national programs for control of BVDV.