A silent animal sickness: Bovine leukemia virus
For many years, the U.S. dairy industry has primarily focused its attention on a few costly diseases, including mastitis (an infection of the udder), bovine viral diarrhea and lameness. But that lens is slowly shifting to a lesser-known virus that is a likely contributor to many other cattle illnesses. Researchers, including a team at Michigan State University (MSU), are looking at bovine leukemia virus (BLV) – a retrovirus that causes infection in dairy and beef cattle, and can lead to cancerous disease.
In the 1970s, less than 10 percent of U.S. dairy cows were affected by the virus. Today, MSU experts estimate that more than 40 percent are BLV-positive. Surveys by the U.S. Department of Agriculture (USDA) suggest that 83 percent of dairy herds in the country have at least one BLV-infected cow.
Experts say the large escalation in occurrence is primarily because not much attention has been paid to the virus, plus it is extremely difficult to detect. About 5 percent of BLV cases result in a cancerous tumor known as lymphoma. Tumors can appear in numerous places from easily visible lymph nodes in the neck to ones hidden inside organs. In fact, the USDA reports that BLV-induced lymphoma is the chief reason for U.S. cattle condemnation at slaughter, accounting for nearly 14 percent of beef and 27 percent of dairy rejections.
While the tumors are relatively rare, about 30 percent of BLV infections result in persistent lymphocytosis — a very high number of blood lymphocytes that are associated with immune system dysfunction.
“This is a tricky disease because there is no obvious physical manifestation of BLV infection in most cases,” said Paul Bartlett, a professor in the Department of Large Animal Clinical Sciences (LCS) within the MSU College of Veterinary Medicine. “If BLV disrupts the immune system, the animal becomes susceptible to many kinds of opportunistic pathogens. Most producers don’t realize that BLV may be playing a role in making their cows more susceptible to other diseases.”
A complicating factor in controlling BLV is that transmission from one cow to another is possible in many ways. Infected white blood cells can appear in several bodily fluids, such as blood, semen and colostrum, meaning that some routine on-farm practices can lead to new infections. The most common sources of transmission include reusing hypodermic needles and obstetrical sleeves, as well as dehorning, tattooing, ingesting contaminated colostrum and milk by calves, blood exchange during breeding and fly bites.
With any retrovirus such as BLV, there is the potential for spread across multiple species. Current research suggests that BLV is not a health threat to humans, but studies are ongoing. More than 20 countries in Europe, Asia and Africa have eradicated the illness through testing and culling positive animals. The size of the U.S. cattle herd, coupled with the pervasiveness of BLV, makes that method largely unrealistic. In most instances, producers wanting to address BLV in the U.S. must consider management strategies to reduce transmission. As researchers learn more about BLV, the urgency behind solving the problem heightens. Since 2010, MSU has received nearly $2.5 million to study BLV, mostly from USDA’s National Institute of Food and Agriculture (NIFA).
A DAIRY DILEMMA
Bartlett is leading the MSU charge against BLV along with LCS professor Ronald Erskine. While crunching some numbers for a separate mastitis study in 2010, Bartlett and Erskine noticed a trend.The duo saw that cattle with BLV responded less strongly to the vaccine used to prevent bacterial mastitis. Bartlett and Erskine believed this was evidence that the immune system was disrupted.
“Once Ron and I noticed that BLV was affecting the immune response, it became apparent that we needed to look into it further,” Bartlett said. “Before 2010, the literature on BLV’s impact on cattle was very minimal. We received some funding, and that’s when we started to really find some interesting things. There was some conflicting information published about milk production and longevity, and we wanted accurate measures of any damage caused by BLV.”
A 1996 USDA study concluded that as the occurrence of BLV rises in a herd, milk production goes down. Bartlett and Erskine endeavored to test those findings in Michigan. The MSU duo performed an analysis of 104 herds around the state and found a decrease in milk production similar to that of the USDA project. For every 10 percent increase in prevalence, producers lost an average of 209 pounds of milk annually for each cow. Bartlett and Erskine also determined that herds with higher prevalence rates of BLV tended to be younger. The revelation led to a second study aimed at cow longevity.
The initiative utilized a test called ELISA that gives researchers a count of antibodies against BLV. The presence of these antibodies indicates that an animal has the infection. Tracking 3,849 animals from 112 herds for more than a year and a half, Bartlett and Erskine found that cows with antibodies against BLV were 23 percent more likely to be removed from the herd. This could be from dying naturally or through culling. Each cow was classified into one of four categories based on the amount of antibodies: negative, low positive, medium positive or high positive. As the number of antibodies increased, the survival probability decreased.
“Not so long ago, we used to think that BLV didn’t have any real impact on the farm beyond the occasional tumor,” Bartlett said. “But we’re showing that infected cows don’t live as long, suggesting that the welfare of the animals could also be adversely affected. Milk production suffers as well, so there are a host of issues caused by this infection that haven’t always been apparent. That’s why we’re tackling the issue now and devoting resources toward educating our producers, as well as developing new management techniques to control BLV transmission.”
REACHING OUT TO PRODUCERS
The incidence of BLV infecting at least one animal in a beef herd is 39 percent, slightly lower than in dairy herds. Daniel Grooms, a professor and chair of LCS, received NIFA funding to examine the longevity of beef cattle in Michigan and beyond. Grooms and his team are measuring the prevalence and impact of BLV in an ongoing two-year project sampling 3,500 cows from the Great Lakes region. Each cow has been screened for the disease and is being closely evaluated to see if the virus develops. Grooms said it is likely that BLV will negatively affect beef cattle longevity, as shown in previous dairy herd studies.
“There hasn’t been as big of a focus on beef operations, so there are a lot of unknowns,” Grooms said. “We’ve done such great research with dairy, and that has guided some of our initial work. In addition to overall beef research, I’m very interested in how bulls factor into the spread of BLV, so we’ve moved into that area as well.”
Each spring prior to the breeding season, Grooms conducts a series of bull fertility exams throughout Michigan. He has collected blood samples from bulls in three of the last four years and determined the frequency of BLV to be between 30 and 40 percent. Grooms indicated that semen or reproductive secretions from an infected bull could be a potential source of transmission, which could create further disease development in the U.S. and potentially impact trade with other countries that have adopted strict BLV-free policies. Grooms and his team are continuing to evaluate the risk of bulls in transmitting BLV during breeding.
Funding from the Michigan Alliance for Animal Agriculture, a joint venture between MSU and animal agriculture industry groups, has aided researchers and MSU Extension educators in providing significant outreach efforts.
“We have a ways to go in research and implementation of management strategies, but producers have been very responsive to new information,” Grooms said. “They obviously want to do what makes the most economic sense, but they also really care about the animals. They want to do what they can to improve health and welfare of their cattle. As we generate more information and awareness, BLV will get the attention it deserves.”
For more information on BLV and to learn more about MSU’s research, visit BLVUSA.com.
Click to subscribe to our e-publications:Subscribe