A Look at Bovine Leukosis.

 By Jaclyn Krymowski.

 Enzootic bovine leukosis (EBL) is a difficult disease for farms to handle. Not only can it be fatal, but it is also contagious and can significantly impact milk production with no known curative treatment. This disease tends to peak in mature cattle, usually in the six to eight-year-old range.

The virus survives in lymphocytes (white blood cells) making it a truly blood-borne disease. Cattle with a clinical form of the disease will have tumors, some of them often visible, located in and around the uterus, abomasum, heart, spinal canal, and/or lymphoid tissue behind the eye, according to Cornell University’s Animal Health Diagnostic Center. Sometimes the lymph nodes will also become so enlarged that they noticeably protrude from under the skin. Only about 5% of infected animals will become clinical, meaning EBL can easily go undetected in a herd.

Mindfulness of the condition and proper sanitary procedures to mitigate transmission is essential to avoiding a widespread outbreak.

About Enzootic Bovine Leukosis
Due to its blood-borne nature, EBL can be transmitted between animals through bodily fluids containing infected cells (primarily from milk and blood). However, the root cause of EBL is due to a retrovirus, bovine leukemia virus (BLV).

Lymphocytes that are infected with the virus will either create a benign tumor called lymphocytosis or become neoplastic, which is cancerous, and attack other organs and bodily systems. Tumor development may also impact the immune system and cause an animal to be more susceptible to other diseases.

According to USDA’s Animal and Plant Health Inspection Services, surveys indicated that as many as 89% of dairies had at least some animals that were seropositive for BLV. This means they showed a presence of antibodies in their blood. After any BLV infection, animals will produce antibodies throughout their lifetimes.

Even if they are not clinically ill, seropositive animals can still infect other cattle through blood and other bodily fluids.

According to a Merck Veterinary Manual article by Dusty Nagy DVM, many different fluids such as urine, feces, saliva, respiratory secretions, semen, uterine fluids, and embryos, have been examined for their ability to transmit BLV. The consensus is that these are all noninfectious for EBL. However, while blood is the main source, even colostrum from BLV-positive cows also contains the virus and therefore also infectious to calves.

Herd impact
Even a few BLV infected (and improperly managed) animals can be a problem for the entire herd. Testing to identify the seropositive animals can be a worthwhile strategy to mitigate herd risks. However, not all farms are on the same level for susceptibility and disease transfer.

For example, certain managerial practices such as tattooing, dehorning, rectal palpation, injections, blood collection and routinely sharing needles (such as when giving oxytocin in the parlor) provide more opportunities for BLV spread.

Establishing and familiarizing employees with a sanitation plan for equipment and movement between animals can greatly help reduce the likelihood of infection to new animals. Keep in mind that blood and other fluids, present on many tools, equipment and instruments used, will spread BLV. Think about things like needles, syringes, shared vaccine bottles, palpation gloves, saw and gouge dehorners, nose grips, hoof knives, ear tags, tattooers and much more are all ready BLV vectors if not possibly sanitized, according to the Animal Health Diagnostic Center.

In the University of Missouri Extension online article Bovine Leukosis, Jeanette Floss DVM and Richard Randle note that this also extends to calves via colostrum:
“Seropositive cows secrete BLV antibodies into colostrum. Non-infected calves nursing these cows will acquire detectable antibody levels in their blood. The BLV-antibody levels in these calves should decline during the first six to eight months of life. Therefore, calves less than six months of age from seropositive cows are not routinely tested, because the presence of antibodies does not indicate infection.”

Solutions in prevention and management
Prevention is always one of the best protocols to incorporate into the operation, but it’s not always an option. The second best thing is to manage any infected cattle properly.

There is no treatment for viral infection or for lymphosarcoma, but there are programs that can be implemented on-farm that are aimed at eventual eradication. This may include protocols like identifying infected animals via routine testing, culling positive animals, retesting after a period of time and testing young replacement heifers.

Having a biosecurity protocol in place can also be beneficial to ensure that any new animals are negative.

“Eliminating the movement of blood from infected animals to naive animals is the cornerstone of prevention protocols,” reads Nagy’s article. “In calves, feeding colostrum from seronegative cows is often advocated. The replacement of whole milk feeding with high-quality milk replacer may also be considered.”

Other practical ways to decrease transmission are changing rectal sleeves in between cows, using clean needles for every single injection, keeping shared medications separate from positive and negative animals and disinfecting equipment for all procedures where blood or blood-containing bodily tissue can easily spread. Due to its sometimes quite nature and ease of transfer, EBL is a disease requiring vigilance from all dairies, even those with closed herds. While prevention does involve a bit of investment both laboriously and financially, a long-term strategy can save years’ worth of losses.