With the expansion of herds and uncontrolled transmission of the causative virus, bovine leukosis has slowly but surely become a disease we have to deal with. And the good news is … it is possible to get rid of it.

Bovine leukosis: What is the cause?

Bovine leukosis is a disease, a tumor of the white blood cells, specifically in a subgroup of B-cells. These cells get infected with a virus called bovine leukemia virus (BLV). The most important characteristic of these types of viruses is: They make copies of themselves, which then integrate into the cow’s genome. Therefore, once a cow gets infected with BLV, it can never get rid of this virus. The B-cells that get infected are a component of the immune system; when infection with BLV occurs, these cells will change and will not function properly. We therefore expect that BLV-infected cows will have a higher chance of developing other diseases.

How do I know I have BLV in my herd?

Only a small fraction of the infected animals will develop tumors; this means most of the infected animals will not easily be recognized. To find out if BLV is present, laboratory testing on milk or blood is the only reliable option. Once antibodies can be detected in the cow’s blood, she will be infected for life. Testing is an important measure to understand what is going on in the herd and is an integral part of any control program. We estimate that greater than 90 percent of all herds in Canada and the U.S. have at least one BLV-infected animal.

Will a BLV-positive animal always die?

Generally, less than 5 percent of BLV-infected animals will eventually develop tumors. Once the animal has tumors, it will die very quickly. Up to 50 percent of infected animals will have abnormal B-cells, and the cow will suffer more often from other infections and diseases. These conditions will lead to decreased milk production and reduced longevity, and will impact animal welfare.

Many countries are free of BLV; why is it still here in North America?

The simple answer to the question posed above is: We did not care and did not pay attention. When the prevalence was still low in dairy herds, nobody recognized the need to control this virus. Changes in management, such as expansion of herds, increased use of injections and other herd practices, have facilitated BLV transmission.

BLV control: Is it financially worth it?

The main question asked when BLV control is discussed is whether it is economically worthwhile.

During an economic modeling we compared four different control strategies in a Canadian context:

1. All proposed management strategies are implemented, and the reduction of BLV prevalence is quick.

2. Some of the management strategies are implemented, and a slower reduction of BLV prevalence will be achieved. This is a more realistic scenario on many farms.

3. Animals are tested, and the positive animals are removed from the farm.

4. Animals are segregated on-farm based on their BLV status.

We compared those options financially to a situation where no control was implemented, whereby the prevalence remains the same.

We calculated the costs and benefits of BLV control over a 10-year period; a positive net benefit was obtained after at least four years of implementation. However, these model-based calculations must always be interpreted with caution – no two farms are the same, and “the average farm in Alberta” doesn’t exist.

What measures can be taken to control BLV on a dairy farm?

Unfortunately, vaccination against BLV is not an option. Therefore, we have to rely on other measures to control BLV. The most likely transmission routes are blood, colostrum and milk. Blood can be transmitted through dehorning methods, reuse of needles/syringes, reuse of examination gloves, breeding with a bull, hoof knives and other equipment that are not cleaned and disinfected between uses.

One of the most important methods of transmission to a very susceptible part of the herd is using colostrum from BLV-positive animals. Especially when colostrum from different cows is combined, the risk of transmission is high. The best and most reliable control option is pasteurization of milk and colostrum.

An advantage of pasteurization is: It also reduces the risk of transmission of the bacterium that causes Johne’s disease. A second method that can be considered is freezing to -4ºF. BLV is not a very strong virus, and the idea is that freezing milk or colostrum will disintegrate all the cells, therefore reducing the chance BLV can be passed on to the calf. The use of dried milk or colostrum is a very safe option. Last but not least, in 10 to 15 percent of the pregnancies the virus is passed to the calf during pregnancy; this percentage increases to 40 percent in animals with a high proviral load.

All these options need to be considered when a complete control program will be rolled out on-farm. Not everything can be implemented at the same time; therefore, it is important farmers and veterinarians, together with other stakeholders, discuss the priorities of BLV control. To aid farmers and veterinarians during the discussions about BLV control, the University of Calgary (Dr. Alessa Kuczewski and Molly Kavanagh) has developed a poster (Figure 1) and a fact sheet (Figure 2).

We can send a digital version of the original fact sheet when requested through Email Frank van der Meer.

BLV control is possible for farmers motivated to maintain these control measures for a long time. Farmers should not expect that BLV eradication is easy to reach and have to anticipate setbacks. They are in it for the long term.  

Alessa Kuczewski is also with the University of Calgary.

Frank van der Meer is on the faculty of veterinary medicine at the University of Calgary. Email Frank van der Meer.