Bovine leukemia and lymphosarcoma is caused by bovine leukemia virus (BLV). BLV is a retrovirus meaning that it is a type of virus that can cause its genetic material to add into a host’s cell. Once an animal is infected with this virus, it is infected for life.
Cattle are the primary natural host of this virus. The virus is very common in cattle in countries where it has not been eradicated. In the United States, more dairy cattle, and dairy cattle herds, are infected than beef cattle or beef cattle herds, but the prevalence of infection in beef cattle is probably increasing.
The virus is spread through infected lymphocytes, a type of white blood cell; therefore, transmission of BLV most commonly occurs through transfer of blood from infected to uninfected cattle. Almost any secretion or excretion can contain lymphocytes, meaning the virus can be present in infected animals in their:
- Nasal discharge
- Semen or urine
Signs & Symptoms
Once a bovine is infected with BLV, they will be infected for life. However, approximately 70% of these infected animals will not ever have any signs of disease. About 2% of cattle infected with BLV will develop cancer, or tumors, from the virus and will die from them. The incubation period of this virus is relatively long, and the vast majority of cattle that develop cancer are between 4 to 8 years old.
The tumors have a tendency to form in one of 5 places or ‘predilection sites’:
- Lymph nodes
- Abomasum (fourth stomach)
- Urogenital system (urinary system and/or uterus)
- Spinal cord
The disease can progress quickly, or over several months. Tumors can form in one or more of these sites, and will cause disease related to that organ system. Specific signs include enlarged lymph nodes, diarrhea, presence of digested blood in the stool, abdominal distention or ‘bloat’, signs of heart failure (brisket edema, exercise intolerance), weakness or paralysis of the hindlimbs, and protrusion of one or both eyes. One or more of these signs can be present in animals with lymphosarcoma or tumors from the BLV virus.
Vague signs of disease include:
- Going off feed
- Weight loss
- Poor milk production
Diagnosis of infection with BLV is relatively simple and is usually done by detecting antibodies to the virus. This process is called serologic testing and is most commonly done by performing an ELISA or an AGID test. It is also possible to detect presence of the virus itself through PCR. Both of these types of tests are blood tests. However, a blood test that is positive for BLV DOES NOT mean the animal has cancer, or even that the animal will get cancer. A positive test just means the animal is infected with the virus.
Diagnosing tumors caused by the virus is usually much more difficult. In order to definitively diagnose lymphosarcoma, the doctor must find the cancer cells and identify them using a microscope. This means taking samples from the tumors themselves (called a biopsy), or in some cases taking a sample of the fluid around the organ system (called a fluid aspirate) in order to find the cancer cells. Veterinarians will often use ultrasound to help locate internal tumors or abnormal fluid in order to collect a sample to look at under the microscope. If the tumor cells are found, and the animal also has the BLV virus, a diagnosis of lymphosarcoma from the BLV virus is made.
Treatment & Aftercare
There is no treatment for either the BLV virus or the tumors it causes. If the virus is present in a herd, it can be managed and most of the infected animals will never get tumors. About two animals out of every 100 that are infected will get cancer and will die from it. If the disease is diagnosed while the animal is still alive, euthanasia should be strongly considered because the animal is going to suffer until it dies.
Prognosis for survival of cattle infected with the BLV virus is good. Prognosis for cattle that have cancer (lymphosarcoma) from BLV is very poor.
Control of BLV
Control of the virus can be done through testing, and either culling infected animals, segregating cattle into infected and uninfected herds, or reducing the number of animals infected by raising BLV negative animals. Spread of the virus from animal to animal can occur through:
- Direct contact
- Transfer of blood through infected surgical instruments, dehorners, needles, ear taggers, tattoo instruments, or repeated use of rectal palpation sleeves
- Via blood-sucking insects
- In-utero from cow to calf and possibly from infected colostrum or milk to calves
Preventing infections in calves involves early separation of the calf from the dam (in dairy animals) and using milk replacer or pasteurized milk. In older calves and adults, rinsing and disinfecting any instruments that get blood on them during processing procedures is very important. Changing needles or palpation sleeves between animals is also very important. Control of blood-sucking insects with ear tags, dust bags, or sprays can also help limit spread of the virus.
Fact Sheet Author:
Katharine M. Simpson, DVM, MS, Diplomate ACVIM (LAIM)