Veterinary Medicine Extension, School of Veterinary Medicine,

University of California Davis, Tulare, CA.

Bovine Leukemia is a viral disease of cattle. This disease goes by several names such a BLV, Bovine Leukosis, Lymphosarcoma or Malignant Lymphoma. Within BLV infected cows, the virus is associated with the white blood cells call lymphocytes. BLV is transferred from cow to cow or cow to calf in blood that contains the virus-laden lymphocytes. Very small amounts of blood have been experimentally shown to be capable of transmitting the virus. Many cows are infected with BLV; however, only 2-5% of infected cows eventually develop tumors of the lymph nodes after a prolonged incubation period.

The sarcoma tumors that develop are the most common malignant tumor found in cattle. About 1% of the cattle shipped to market in California are condemned due to BLV. This amounts to roughly 5200 cows per year. The numbers being condemned is increasing and is three times that noted in the 1970s (Picture 1). Keep in mind that when a lymphosarcoma is found in any location, the cow must be condemned (Picture 2).

When exposed to the virus, not all cows become infected. Those that do become infected remain infected for the remainder of their life. The virus seems to be able to hide within the lymphocytes from the cow’s immune system. Even though the cow develops antibody titers that can be measured in a blood sample, the virus is not inactivated by the immune system. The titers make it easy to detect infected cows, but it cannot be used to predict which cows will develop tumors. BLV can readily be transmitted though milk from a cow to her calf. Bloody milk may be particularly loaded with BLV.

Most infections (90%) take place early in the life of a calf when virus is transmitted in the milk. Some calves are born infected. The mechanism for these in utero infections is not known. Infections can also take place by movement of blood from one cow to another through routine husbandry practices. There is a risk of transmission from ear tagging, gouge dehorning, tattooing, multiple animal needle use and rectal palpation. In some areas of the US, large biting flies are thought to be able to transmit BLV.

Colostrum presents an interesting dilemma. Colostrum from BLV infected cows, those with BLV positive blood titers, may contain both the virus and protective BLV-antibodies. The trick here is to take advantage of the protective antibodies but to remove the BLV virus by freezing or heating above 146 F. Studies have shown that at 90 days of age, calves without BLV-antibody from colostrum are 2.0 to 2.75 times more like to be BLV infected compared to calves with antibody.

If only a small percentage of cows develop the tumors, why all the fuss about BLV? For one, purebred breeders are very concerned, as BLV-positive cows cannot be exported to many foreign countries. There is also the cost of treatment of BLV-cow before a diagnosis is made or suspected. Over all there is lost milk production, premature marketing, increased death loss and condemnation at the slaughterhouse due to the tumors. After 36 months of age, BLV titer-positive cows are marketed sooner than BLV-negative cows. Tumor development is not associated with early marketing.

BLV may be diagnosed on routine reproductive palpations as the tumors often develop in the uterus. Enlarged internal lymph nodes can sometimes be palpated rectally as well. BLV should always be suspected in older cows with enlarged external lymph nodes (Picture 3). Blood testing will confirm the diagnosis in cows with signs of illness. The blood test is very accurate so that positive cows are almost always positive and test negative cows are BLV-free.

About 30-40% of US adult cows are blood test positive for BLV. The percentage varies with the management and husbandry practices of the dairy. In herds with lots of BLV tumor cows, the prevalence is usually high. The classic, sick BLV cows is over 4 years old, is loosing body condition with a decreasing appetite. They are usually dropping in milk production with enlarge lymph nodes somewhere in the body. The clinical signs will be related to the location of the tumors. For instance, a common location is the wall of abomasum. These cows would be probably showing some signs related to the GI tract.

BLV has been eradicated from some countries using a test and slaughter system. This can be used on US dairies and completed in 1-3 blood tests of the entire herd. Using this approach, the dairyman must be willing to market all positive cows and therefore it is a costly approach. All new cows entering the herd will then be tested to keep the herd BLV-free. Incidentally, these are the countries that will not allow entry of BLV-positive cows.

Another eradication strategy is to test and segregate the BLV-positive cows. The blood test positive and negative cows are housed separately at least 10 feet apart. Slowly the BLV-positive cows will be marketed and the herd will become BLV-negative. This is a slower method that removes BLV-positive cows by natural attrition. Testing of herd additions is necessary to prevent re-introduction.

Often dairymen prefer to use a strategy combining testing and management changes. With this method, cows are blood tested frequently and BLV-positive cows are identified. Some dairymen separate the BLV-positive cows. Calves are removed from BLV-positive cows before they can suckle. Colostrum is heated or frozen to inactive the virus. No bloody milk is fed to calves. Steps are taken to prevent transfer of BLV in blood by high-risk procedures. Only single-use needles are used for injections and all instruments such as dehorners, taggers, tatooers or surgical items must be disinfected between uses. While not of particularly high risk, palpation sleeves can be changed or cleaned between cows. Over time, this system will surely reduce the number of infected cows.

In summary, Bovine Leukemia is a viral disease of cattle that is transferred in blood and causes some cows to develop lymphosarcoma tumors. Most infections take place during calfhood. Many adults are blood test positive but few develop the tumors. BLV can be eradicated from a dairy herd. It can be controlled in a dairy herd by modifying high-risk management procedures that might carry blood with virus from one cow to another.

Picture 1. Slaughterhouse inspectors looking for signs of BLV tumors or other food safety conditions that might cause a cow to be condemned.

Picture 2. BLV induced tumors on the heart of a cow. Tumors from BLV are commonly found in the uterus, abomasum, heart and external lymph nodes.

Picture 3. Enlarged external lymph nodes on a cow with Bovine Leukemia.