At the most basic level, you depend on the cow’s own immune system to mount a defense against disease pathogens such as bacteria and viruses. Many things impact how well the immune system is working. Good nutrition, good environments and vaccination are important in making sure the immune system is functioning well. But what if the actual structure of the immune system was compromised? Then what might happen?

In a previous article, How common is the uncommon disease? , from Michigan State University Extension, we looked at the increasing infection prevalence of cattle with the Bovine Leukemia Virus (BLV). Bovine Leukosis, the disease caused by BLV, attacks the cow’s foundation of immunity, with the result that the immune response is suppressed, infected cows are less able to respond to vaccines or immunity from previous exposures and, therefore, the cow is more vulnerable to pathogens. 

A study by Ron Erskine and colleagues at Michigan State University demonstrated a diminished response to a vaccine for E-coli mastitis by cows that were infected with BLV as compared to BLV-negative cows. The BLV-infected cows had lower serum titer levels of antibody after vaccination, at each of 4 vaccinations and at 3 weeks after the 4th vaccination. Therefore, they were less able to respond the challenge initiated by the vaccine. 

It is not too hard to believe that these same cows will mount a weaker immune response to natural exposure to the pathogen. Why is it that BLV-infected cows appear to have a compromised immune system? 

BLV is a disease of the immune system. The virus attacks a type of white blood cell called lymphocytes, incorporating its DNA into the cow’s white blood cells. About one third of BLV-positive cattle show elevated lymphocyte counts. Those animals are called “persistently infected” or “PL” cows. Even though there is a proliferation of lymphocytes, the cow is less protected because of other changes in immune response that also take place. 

These results are very similar to those seen in people infected with human immunodeficiency virus (HIV), the virus that causes AIDS. Their ability to respond with an effective immune defense against pathogens becomes increasingly compromised and eventually, they succumb to pathogens that non-HIV-infected people commonly fight off.

Therefore, one of the most devastating effects of BLV infection in cattle is that the ability of the cows to defend themselves against common pathogens is reduced as the disease progresses.

It makes you want to reduce the transmission of BLV within a herd, doesn’t it? For more information, visit the BLV website