Bovine Leukemia and Lymphosarcoma

What is Bovine Leukemia and Lymphosarcoma?

Leukemia and lymphosarcoma (also called lymphoma) is a form of cancer of one of the cells of the immune system called the lymphocyte. In cattle, a diagnosis of leukemia or lymphosarcoma can be rare but is most commonly caused by bovine leukemia virus (also called bovine leukosis virus or BLV). Disease caused by BLV is referred to as enzootic bovine leukosis. Bovine leukemia virus is a retrovirus meaning that it is a type of virus that can add its genetic material into a host’s cell and use that host cell to make more virus.

Cattle are the natural host of BLV. The virus is very common in cattle in countries where it has not been eradicated. In the United States, dairy cattle are more infected than beef cattle, but the prevalence of infection in beef cattle is probably increasing.

How is BLV spread?

The virus is spread through infected lymphocytes, a type of white blood cell, and so 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:

• Blood
• Colostrum
  
• Milk
  
• Nasal discharge
  
• Semen or urine
  

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
  
• Blood-sucking insects
  
• In-utero from cow to calf and possibly from infected colostrum or milk to calves
  

What are the clinical signs of Bovine Leukosis Virus?

Once a bovine is infected with BLV, they will be infected for life. However, approximately 70% of these infected animals will never have any obvious evidence of disease. About 2% of cattle infected with BLV will develop cancer or tumors and die.  The incubation period of this virus is relatively long, and most cattle that develop cancer are between 4 to 8 years old.

The tumors tend to form in one of 5 places:

• Lymph nodes
• Abomasum (fourth stomach)
  
• Heart
  
• Urogenital system (urinary system and/or uterus)
  
• Spinal cord
  

Tumors can form in one or more of these sites and disease typically reflects the organ system affected. 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
  
• Protrusion of one or both eyes
  

Disease progression is variable, progressing quickly or slowly over several months. Less specific signs of disease include:

• Going off feed
• Weight loss
• Poor milk production
• Depression
• Lethargy
• Fever

How do we diagnose Bovine Leukosis Virus?

Diagnosis of infection with BLV is relatively simple and is usually done by detecting antibodies to the virus through serologic testing by performing an ELISA or an AGID test. It is also possible to detect presence of the virus itself through PCR. Both types of tests are blood tests. However, a test that is positive for BLV DOES NOT mean the animal has or 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 (called a biopsy). In some cases where an obvious tumor is not identified a sample of the fluid (fluid aspirate) from the affected organ system may be taken 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. If cancer cells are found, and the animal also has BLV, a diagnosis of lymphosarcoma from the BLV is made.

Are there any treatments for Bovine Leukosis Virus?

There is no treatment for BLV and the disease it causes. If the virus is present in a herd most of the infected animals will never get lymphosarcoma and it can be managed depending on the goals of the operation. About two animals out of every 100 that are infected will get cancer and will die from it. Due to the poor prognosis of disease and absence of a cure, euthanasia is recommended for animals with clinical signs of lymphosarcoma to prevent suffering.

What is the prognosis for infected cattle?

Prognosis for survival of cattle infected with the BLV virus is good. Prognosis for cattle that have cancer (lymphosarcoma) or enzootic bovine leukosis is very poor.

How do we control Bovine Leukosis Virus?

Planning a control program should involve a detailed cost/benefit analysis in conjunction with working with your veterinarian. Control and eradication of BLV is largely based on reduction of blood transmission between cattle. This can be done through testing, and either culling (removing) infected animals from the herd, segregating cattle herds into infected and uninfected groups, or reducing the number of animals infected by raising BLV negative animals.

The following are some methods that can aid control and eradication in infected herds:

• Early separation of the calf from an infected dam (in dairy animals)
• Feeding pasteurized colostrum and milk or milk replacer to calves
  
• Practicing good hygiene between animals such as rinsing and disinfecting any instruments that get blood on them during processing procedures
  
• Changing needles or palpation sleeves between animals 
  
• Control of blood-sucking insects with ear tags, dust bags, or sprays can also help limit spread of the virus
  

Finally, any new additions to the herd should either be from BLV free herds or should be isolated and tested before adding them to the main herd.