New studies by Ron Erskine and Loraine Sordillo at Michigan State University (MSU) and others are showing that cows infected with bovine leukemia virus (BLV) may have impairments to their immune system. BLV can be fairly common. The percentage of infected dairy herds was 89% in the National Animal Health Monitoring System (NAHMS) study published in 1997. However, within herd prevalence can vary widely. In a study of 113 Michigan dairy herds in 2010, the prevalence of infected cows within herds ranged from 0% to 76%. The question is, what is the prevalence of BLV-infected cows in your herd, and how can you know that without testing every animal?
In the Michigan study, milk samples from about 40 cows in each herd were collected determine the herd BLV prevalence. These herds had at least 120 cows and used DHI testing. Of those 40 cows sampled, 10 were 1st lactation cows, 10 were 2nd lactation, 10 were 3rd lactation and 10 were from cows in lactations beyond the 3rd. In each of those groups, the cows sampled were those most recently fresh.
In order to validate the sampling method, milk ELISA test results, from a subset of 142 cows were also diagnosed by ELISA analysis of blood serum samples. There was a 95% agreement between BLV milk and serum ELISA. This gave them confidence that the milk ELISA test could accurately diagnose BLV-infected cows as compared to the serum ELISA test.
The method of analyzing 40 cows to determine herd prevalence of BLV infection was checked by whole-herd testing in five herds. Again, the results of the two methods were well correlated with each other. So you can adequately characterize your herd BLV prevalence by sampling 40 cows. At $6 per sample, testing 40 cows would cost $240.
One last comment; a concern has come up about the ability of current testing methods (blood and milk ELISA) to detect all infected animals. The ELISA test is based on binding antibodies produced by the animal’s immune system in response to the infectious agent. If the animal’s immune system has not responded to the infection, or has responded weakly, so that little antibody has been produced, then the ELISA test may not correctly diagnose the disease state.
This may be more likely the case in younger animals, for diseases that are more slowly developed in the animal, or for which the immune response is weak for one of a variety of reasons. Therefore, work with your veterinarian to evaluate the interpretation of the test results.
Understanding the prevalence of BLV in your herd can help you determine a control program that makes the most sense economically for your herd and will help in understanding what you may see in breakdown of cows in the herd.
Sampling 40 cows in your herd, distributed across the lactations, can characterize the disease prevalence and give you a baseline level from which you can evaluate if a control program is needed, or the success of the control program that you implement.