The term BVD, in and of itself, is misleading in that it doesn’t refer to a single clinical presentation associated with diarrhea. Instead, it encompasses a group of syndromes that cause reproductive, respiratory, and enteric diseases that are associated with three different viruses.
It started with one cow
The complex nature of BVD was evident from the initial recorded disease outbreak. The first report of what was subsequently named BVD started with a veterinary call made by Francis Fox to a farm located on the outskirts of Ithaca, N.Y., in March of 1946. The “farm” had a herd that consisted of one animal, a 4-year-old Red Devon cow that was imported two years earlier from England. This cow was off feed and suffered from watery diarrhea, fever, and a marked drop in milk production. Fox’s preliminary diagnosis was classical winter dysentery. The death of that animal, followed by similar clinical presentations on five nearby farms, resulted in the recognition of “an apparently new transmissible disease of cattle.”
The clinical presentations observed in the disease outbreaks on those five farms reflected the mixture of enteric or intestinal, respiratory, and reproductive syndromes that mark BVD outbreaks to this day. The proportion of animals in those herds that became ill ranged from 33 to 88 percent, and 4 to 8 percent of the animals died. Some cows developed pneumonia, while others had watery diarrhea. Fetal abortions occurred 10 days to three months following the initial outbreak.
In addition, a severe drop in disease fighting white blood cells was observed in sick animals, and it was originally thought to be associated with clinical signs of disease. However, when they attempted to treat sick cattle with blood transfusions from “healthy” herdmates, it was noted that the other cows also had severe reductions in white blood cells. This suggested that herdmates had also been infected and that these infections could be subclinical in nature. Further, the loss of white blood cells in BVD-infected animals suggested that immune depletion was a hallmark of BVD regardless of clinical presentation.
In the 70 years that have followed, we have learned that infection of cows in the first trimester of pregnancy can result in the birth of persistently infected (PI) calves. These PI calves have variable fates. Some are born dead (stillbirths). Others are born weak or small (“deer nose”) and die as neonates. Some are born normal in appearance but become unthrifty poor doers that succumb to disease usually in the first year of life. Lastly, some are born normal in appearance and live to adulthood.
It’s lurking in the herd
Even normal appearing PI animals have higher rates of respiratory or enteric disease compared to noninfected herdmates. It was noted that PI animals were very efficient at spreading the virus and that feedlot cattle housed in contact with a PI animal had higher treatment costs and lower rate of gain. The presence of PI cattle in dairies was associated with elevated somatic cell counts and a higher incidence of secondary infections such as metritis and pneumonia.
Variation was observed in the viruses that cause BVD. Originally, it was thought that just one virus, bovine viral diarrhea virus, was responsible for all cases of BVD. It was discovered later that BVD could actually be caused by one of three different viruses, bovine viral diarrhea virus type 1 (BVDV1), bovine viral diarrhea virus type 2 (BVDV2), and HoBi-like virus.
The prevalence of these viruses differs by geographic location. In North America, BVD is caused by BVDV1 and BVDV2. In South America, Asia, and Europe, BVD may also be caused by HoBi-like virus. These viruses belong to the same family tree but are different enough that vaccines and diagnostics that work well for one may not work well for the other two.
Vaccines are available for BVDV1 and BVDV2, but currently there are no vaccines available to immunize animals against HoBi-like virus. In addition, it has been found that these viruses can infect a number of free living ruminants, including deer, antelope, big horn sheep, bison, and mountain goats. Because so little research has been done on these free living animals, it is unclear whether wild ruminants are a source of infection for cattle or if these infections have a negative impact on wildlife populations.
Eradication is the end goal
BVD vaccines have been available in the U.S. market since the 1960s, and a variety of preventative vaccination strategies have been implemented with varying degrees of success. In the absence of the PI cattle within a herd, a BVD vaccine can be very effective in minimizing the impact of this disease. However, vaccination alone will not clear BVD from a herd if PI cattle are still present.
Instead, a three-part approach is necessary to control BVD. In addition to strategic BVD vaccination, biosecurity practices must be in place to prevent PI animals from entering a herd, and PI animals lurking within a herd must be identified and removed for full BVD control.
Several U.S. organizations, including the American Association of Bovine Practitioners, the Academy of Veterinary Consultants, and the National Cattlemen’s Association, have recommended the eradication of BVD from this country. Eradication programs in Europe, based on extensive testing and removal of PI animals with or without vaccination, have been shown to be effective and the resulting disease reduction had significant financial benefits for producers.
Regional U.S. testing programs have been short lived. However, it has been shown that the design and implementation of a BVD control program, including testing, vaccination, and biosecurity, have significant benefits for individual dairy operations. In the absence of a national control program, BVD control programs that utilize vaccination, biosecurity practices, and surveillance must be conscientiously followed year in and year out.
The proceedings from the International Symposium Bovine Viral Diarrhea Virus — “A 50-year review” included this quote from one of the aforementioned founding fathers of BVD research, Francis Fox of Cornell University: “After observing and dealing with this disease for 50-plus years, I conclude that BVD is an interesting, intriguing, unpredictable, and most challenging disease which can quickly bring about financial disaster to the herd owner and apparently is going to be with us for some years to come.”
National efforts to reduce antibiotic use have led to a greater emphasis on disease prevention rather than treatment. Regardless of the severity of a clinical disease episode, BVD results in major damage to the immune system which reduces the ability of cattle to fight off secondary infections or respond to vaccines. Control of BVD in any dairy operation must rely on the implementation of an organized strategy combining biosecurity, surveillance, and bolstered herd resistance.
Next issue: Don't bring BVD home with you
