Animal Health Matters: Bovine leukosis, the secret cattle killer

Russ Daly
Special to the Farm Forum
Russ Daly

Dairy and beef producers face a long list of infectious diseases that could threaten the livelihood of their animals and operations. Of the diseases on that list, some are quite familiar to producers while others are virtually unheard of.

We know this thanks to the U.S. Department of Agriculture’s NAHMS (National Animal Health Monitoring System) surveys conducted periodically among the various animal commodity groups. Producers rate high awareness of “newsy” yet highly-unlikely-to-affect-their-cattle diseases: Bovine Spongiform Encephalopathy (“mad cow” disease) or Foot and Mouth Disease, for example. Other, more common diseases, on the other hand, score much lower on the awareness scale. Bovine leukosis is one of them.

In fact, most dairy herds have bovine leukosis-infected cattle whether their operators realize it or not. According to some recent estimates, 94% of dairy herds house at least one infected cow, with 50% or more infected cows in some of those herds. Leukosis doesn’t affect quite as many beef herds, but still more than most would expect.

An inconspicuous infection

If bovine leukosis is so prevalent, why don’t more producers know about it?

Bovine leukosis almost entirely flies under the radar screen, with its effects very inapparent in most animals in most herds. Leukosis is caused by a virus called Bovine Leukemia Virus (BLV).

Just like other animal viruses, BLV prefers to invade very specific types of cells within the body, in this case the lymphocytes: white blood cells critical to the animal’s immune system. Lymphocytes typically accumulate in lymph nodes – those lumpy little immune organs spread throughout the body – and in accumulations of lymphoid tissue within other organs.

In the vast majority of these infected lymphocytes, BLV just hangs out, causing no problems. In a small percentage (5-10%) of infected cows, however, BLV sets off lymphosarcoma, an aggressive form of cancer. This first shows up as abnormally enlarged lymph nodes – swellings emerging in various parts of the body. It’s an eventually fatal condition, with severely affected cows condemned at slaughter.

Managing the spread

As with any infectious disease, prevention is always preferable to dealing with the disease after it’s established itself. This requires knowing how the disease is transmitted. For bovine leukosis, transmission takes place in an unconventional way – through the bloodstream. How might blood make its way from one animal to another? Mostly through the things we – people – do to the animals.

While blood-feeding insects can move infected blood cells between animals, in heavily managed herds such as today’s dairy and beef operations, we are more efficient than the insects. When we inject animals one after the other with vaccines, antibiotics, or other treatments, infected blood cells hop a ride on the needle to the next animal.

Significant transmission can also occur through dehorning, tattooing, or castrating calves. Spend any time working calves in a dairy or beef operation and you’ll see plenty of opportunities for bloodborne pathogen spread. It’s therefore not hard to imagine how, once in a herd, BLV can easily make its way from one animal to another.

If the disease only harms a small percentage of the cattle infected with the virus, is it worth even worrying about? This is a fair question, with a different answer for different herds depending on their production goals. While commercial operators may be fine with taking the risk of having a few cows go by the wayside, others (herds with valuable seedstock or cattle being exported) will be less tolerable, as a simple BLV-positive blood test may drastically drop an animal’s value, even if they aren’t sick.

There is also evidence that those infected cows that don’t develop lymphosarcoma – the subclinical cows -- may be adversely affected by BLV, with subtle but important declines in immunity and production parameters.

Whatever your production goal is, the consequences of having a BLV-positive herd are excellent topics of discussion with your veterinarian. Blood tests that detect antibodies against BLV are readily available, and can help you determine whether – and how profoundly -- your herd is affected. Bovine leukosis control in a positive herd is admittedly inconvenient – changing needles and palpation sleeves between animals, and bleaching tattoo pliers and dehorners can be cumbersome to pull off.

Understanding your herd status – or whether it’s even of importance to understand it in the first place – is a good first step.

Russ Daly, DVM, is the Extension Veterinarian at South Dakota State University.  He can be reached via e-mail at russell.daly@sdstate.edu or at 605-688-5171.