Targeted Approach Reduces Bovine Leukemia Virus in the Dairy Herd

Ron Erskine says he was taught in veterinary school that bovine leukemia virus (BLV) was of little concern for dairy producers and practitioners.

The reason: BLV was present in fewer than half of U.S. dairy operations 30 years ago (slightly less for beef operations). Plus, then as now, less than 5% of affected animals ever developed malignant lymphosarcoma, which causes an animal to be condemned at slaughter.

But Erskine, DVM and professor emeritus, Michigan State University (MSU) College of Veterinary Medicine, believes veterinarians and dairy producers need to give BLV more attention today, due to its increasing prevalence (now in 90% of herds), herd health implications, societal concerns, and lost milk production at both the individual cow and herd level, per rolling herd average, as illustrated in figure 1. 

He explains that in the past decade, via work in Japan, North America and South America, researchers have come to realize the lymphocytic state is more problematic than just a leukemia that may lead to malignant lymphosarcoma. 

“What it really is signaling to us is that there has been a shift in that cow’s immune function,” he says. “We now realize that the more pronounced the leukemia, the more likely that there are costs to that cow’s immune system, that there may be some dysfunction, and that that can backdoor her ability to respond to vaccines or other infectious agents.”

Identifying and addressing all cattle in a dairy herd with BLV can be timely and expensive. It’s why Erskine encourages veterinarians to target what he calls super shedders in the dairy herd. 

“The term is relative to the distribution of proviral load (PVL) values within each herd,” he says. “In application, the term super shedder is defined as the highest PVL cow, relative to its herd mates.” 

He addressed the topic during the 2021 American Association of Bovine Practitioners (AABP) annual conference.

Find The Troublemakers

High PVL cows are those most likely to spread BLV to other cattle in the herd. The screening process Erskine recommends for identifying those animals was first proposed by his Michigan State colleague, Paul Bartlett, MPH, DVM, PhD, ACVPM-Epidemiology, professor, and further developed by a diverse research team. 

The steps include:

1. Conducting a herd profile test using a series of BLV ELISA screening tests.

2. Depending on the size of the herd, test about 10 cows in the first, second, third, and fourth lactation (40 cows total) as close to calving as possible.

“You are looking to see what proportion of cows are infected, and this process will give you a snapshot of each lactation,” Erskine says. “What you’re trying to do is identify the troublemakers.”

3. Determining the best approach for a specific herd to prevent BLV transmission, based on:
- overall BLV prevalence
- whether the producer is targeting transmission in young stock or the milking herd
- the producer’s ability to cull or segregate positive cattle

Measure The Proviral Load
Along with using a ELISA test, Donald Niles, DVM and owner of Dairy Dreams, Casco, Wisc., uses a PCR test, Super Shedder 1 by Central Star Labs, to confirm those cows that have BLV DNA in their lymphocyctes, as well as to assess the cow’s potential to infect herd mates.

“This is a quantitative PCR assay that detects not only the presence of the provirus, but also the relative levels in the blood stream, which strongly correlates with the animal’s likelihood of cross-infecting herd mates,” Niles says in the proceedings from his presentation at the 2021 AABP conference, Practical BLV Eradication Measures for Commercial Dairy Farms. 

“Results are reported as a ratio of copies of BLV provirus to copies of an endogenous control gene representing the bovine DNA. This test is performed on whole blood, as the BLV provirus is found in the lymphocytes,” Niles adds. 

“The cost of the assay is $10 per sample. It meets the requirement of a good confirmatory test in that it is highly specific and not prone to false positives. All (our) ELISA-positive cows are subjected to the SS1 test.”

Erskine cites work done at the Michigan State Pasture Dairy Center at the W.K. Kellogg Biological Station as one example of what can be accomplished. Staff there opted to start culling cows identified as super shedders. In just a three-year period, the Center reduced BLV prevalence within its herd by nearly half, from 62% to 33%. 

“If we can target these cows, we can really make some headway,” Erskine says. “It’s not just about the money these cows lose that is important to keep in mind – you are helping the producer remove cows that are perpetuating this infection and other problems in the herd."

Work on the issue is ongoing at Michigan State. Learn more here: https://www.canr.msu.edu/blv/basics/