By Phil Durst
With most dairy herds infected with bovine leukemia virus (BLV), and an average prevalence greater than 40%, we wondered how we could help herd owners eliminate the disease in their herds and what it would take to do that. We also wanted to know if we could inexpensively monitor herds that were not infected.
In the “Moving to Zero” BLV project, funded in 2021 with a Michigan Alliance for Animal Agriculture (M-AAA) grant, we selected herds that, in a previous study, were shown to have either no BLV or low BLV prevalence. Our plan was to use qPCR test results to identify for herd owners and managers, the cows with the highest BLV proviral loads, the supershedders that maintain and spread the disease in their herds. We did not compel anyone to cull animals.
Available funds limited the number of cows enrolled in the project. We recruited 10 herds to participate with an average of 241 cows pe herd (range: 62 – 414). Initially, an ELISA test (milk or blood from nonlactating cows) was conducted on every cow in the herd. Four of the herds tested negative for BLV. The remaining six had prevalence levels ranging from 3 – 22%. Blood was drawn from all ELISA-positive cows for the SS1 qPCR test at CentralStar Labs.
The qPCR test reports the ratio of DNA from BLV to DNA of the host. Therefore, a qPCR result of 2 would indicate twice as much BLV DNA than host DNA. CentralStar Labs categorizes qPCR test reports as high (>1.0), medium (0.5-1.0), low (<0.5) and undetected virus. Test results were reported to herd owners or managers.
Approximately nine months after initial sampling (project was limited to 12 months), we again tested all cows in the herd with an ELISA test, which by then included some heifers that had since freshened. Again, we drew blood from ELISA-positive cows for the qPCR tests. What did we find?
Negative herds remained negative. Although there were occasional ELISA-positive cows in several of these herds, there was never an animal found with detectable virus. These herds had also tested negative over a period of several years in a previous BLV project funded through an M-AAA grant. Thus, we see that negative herds can remain negative if they are not buying animals.
Even low prevalence herds have difficultly eliminating the disease. In this project, the six infected herds remained infected at the end of the project with herd prevalence ranging between 3 – 53%. Though a producer may cull all cows shedding the virus, there are animals, including untested heifers, that were previously exposed and may test positive on a subsequent test. Yet, persevering in culling shedders will eventually eliminate the virus from the herd. The fact that at least one herd increased in prevalence is a reminder that control of transmission pathways is always important.
Farmers differ in the aggressiveness with which they approach BLV. This is not surprising. Farmers that were more aggressive in BLV control (intensive) were those that culled high or moderate BLV cows sooner. Others (extensive) did not cull some of those cows by the time of the subsequent test. The more aggressive farmers were the farms with the lowest BLV herd prevalence rates.
Herds that were more aggressive in controlling BLV had buy-in by employees. It was not an aim of the project to determine the adoption of the priority of control measures by employees, but in observations and interviews with the farmers, it was evident that employees or family members on some farms were more in tune with the control measures. It is logical that those working on the farm would understand high priority issues to the owner or manager.
Culling cows based on BLV results is not an easy decision. It is clear that some infected cows are good cows; they have high milk, low SCC and get pregnant, even though they have a high or moderate proviral load for BLV. Michigan State University (MSU) researchers believe that while these cows are fine today, they are at higher risk for a herdlife-ending episode, whether that is death while on the farm or condemnation at slaughter due to detected tumors.
Buying cattle is a risk. One intensive herd, where infected animals were culled, purchased cattle during the year. At the second whole-herd test, 75% of cattle with detected virus were purchased animals. That addition can set a herd back years in its effort to control BLV prevalence.
Negative herds can be monitored inexpensively. During this project, CentralStar researchers experimented with the milk ELISA test to determine the dilution of bulk tank milk that still enabled them to detect antibodies in the herds with the lowest ELISA prevalence (3%). This is good news for herds that test negative, such as the four in this study, that a single test can confirm their BLV herd status.
The results of this project provide good news for dairy farmers in the battle against BLV. While it is not quick nor easy to eliminate the disease, it is possible to reduce prevalence and to minimize the impact of the disease. Most herds in this project had already implemented transmission control practices of single use needles, single use of sleeves, and improved colostrum practices. These practices are part of good biosecurity on farms that reduce exposure to other pathogens as well. Beyond that, farms were able to use the SS1 qPCR test results to identify the cows that were the greatest risk to their herdmates, and either cull them or be aware of them during herdchecks and other practices.
One of the herds enrolled in the project had been a high prevalence herd, with almost two-thirds of cattle infected. Through transmission control and aggressive culling over a period of seven years, they have brought the prevalence down into the single digits and should be able to eliminate the disease soon. They did it by continually targeting the highest proviral load cows, even as the level of proviral load decreased. Targeting the highest proviral load cows produces the greatest impact on reducing the level of virus within a herd.
Farmers may believe that the disease does not appear to be limiting their herd in any significant way. That is understandable, but the reality is that rampant disease does have an impact and cattle health in general is improved when BLV, a disease that infects the white blood cells, in controlled. Beyond the farm gate, farmers need to consider the impacts of BLV on the dairy industry, an industry that prides itself on marketing healthy products for people from healthy cows. Maintaining that image may become difficult in a world where consumers demand proof of the health and welfare of cows.
Source : msu.edu