Various strains of the bovine viral diarrhea virus (BVDV) cause significant losses in cattle herds, not just in the United States, but around the world. And as veterinarians work to control the disease, they can benefit from the research and practical experience taking place in other cattle-producing countries.
That was the general concept behind the first Global BVDZero Web Congress, which took place on March 30th, attracting more than 300 veterinarians from 10 countries. Boehringer Ingelheim hosted the event, with the U.S.portion taking place in Dallas just prior to the Academy of Veterinary Consultants Spring Conference.
The global event featured presentations from three prominent experts in BVD. These were:
¬∑Dr. Julia Ridpath, recently retired after studying BVDV and related pestiviruses at the USDA's National Animal Disease Center (NADC) in Ames, Iowa.
¬∑Dr. Dan Givens, Professor and Associate Dean for Academic Affairs, Auburn University College of Veterinary Medicine.
¬∑Dr. George Caldow, Veterinary Manager, SAC Consulting Veterinary Services, a division of Scotland's Rural College.
Ridpath led off the event with a presentation on "Bovine viral diarrhea: causes, consequences and control." Ridpath stresses that effective BVDV control requires three components: diagnosis, vaccination and biosecurity.
Ridpath says three species of pestiviruses cause BVD disease worldwide; bovine viral diarrhea type 1 BVDV1), BVDV2 and HoBi-like viruses.BVDV1 and BVDV2 are found nearly worldwide, while HoBi-like viruses occur primarily in Asia, South America and somewhat in Europe. Although HoBi-like viruses have not been identified in U.S. herds, Ridpath encourages producers, veterinarians and diagnostic labs to monitor BVD cases for the causative agent.
Field diagnosis of BVD can be challenging, as the presentations can vary. BVDV causes reproductive disease including fetal deaths, congenital defects and persistently infected (PI) calves. The virus also can cause enteric disease and contributes to respiratory disease, usually with involvement of a secondary bacterial pathogen. Immune suppression is one of the most significant effects of the BVD virus, particularly as it contributes to feedyard morbidity.
PI calves occur when the dam is exposed to BVDV1, BVDV2 or HoBi-like viruses 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.
A cow's exposure during the second or third trimester typically results in calf morbidity, poor performance and possibly abortions, but not the creation of PI calves.
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.
Timing of vaccination for cattle arriving in stocker operations or feedyards often is less than ideal, as the cattle experience stress and often have been exposed to BVDV during the weaning, comingling and shipping process. Ideally, vaccination should occur at least two weeks prior to shipping, but in many cases, cattle feeders vaccinate calves against BVD on arrival, since they have no control or records of pre-weaning vaccinations.
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.
While Ridpath stresses there is no such thing as "benign" BVDV, the virulence and pathogenicity of the disease varies widely, depending on the strain of the virus, age of the host, previous exposure, reproductive status and other factors.
BVDV exposure in young calves appears to correspond with reduced immunity and lifetime morbidity risk. Ridpath says the thymus, which plays a key role in producing Tlymphocytesfor the adaptive immune system. The thymus naturally shrinks as cattle age, but exposure to BVDV in neonatal calves speeds the involution process and reduces the production of T cells.
The next article in this series will summarize Dr. George Caldow's presentation titled "Herd level diagnostic approaches for BVDV."
The archived presentations now are available for on-demand viewing on the BVDZero website. Once on the site, click on "Register Here" for a quick registration that will provide access to the presentations.