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Bovine Lymphosarcoma

Bovine Lymphosarcoma

Clinical signs.
Diarrhea is one of many clinical signs associated with adult multicentric lymphosarcoma, and it is not a consistent sign. Invasion of the lamina propria with neoplastic cells can cause malabsorption and diarrhea. Masses associated with the wall of the intestine also can cause diarrhea by interfering with motility, much like fat necrosis. Other common signs of lymphoid neoplasia in cattle include weight loss, decreased milk production, abomasal ulceration, infertility, cardiac tumorization, enlarged lymph nodes(external and internal), exosthalmos, and recumbancy. The disease has a higher prevalence in dairy cattle than beef cattle.

The adult enzootic form of bovine lymphosarcoma is caused by a retrovirus called the bovine leukosis virus (BLV). The virus is species-specific for cattle. The virus is highly associated with lymphocytes. As little as 0.1 ml of blood from a BLV-positive animal injected into a seronegative bovine can cause seroconversion. The bovine leukosis virus is spread hematogenously by blood-sucking insects, by blood-contaminated surgical equipment, ear-taggers, needles, dehorners, rectal sleeves, etc., and during blood transfusions. Transmission also can occur (to a lesser degree) through BLV-contaminated colostrum. In utero infection can occur in BLV positive dams; the incidence is probably less than 10%. Transmission through semen is possible through natural cover, providing a reproductive inflammation is present and the bull is BLV positive. Transmission by this route is unlikely however, especially if artificial is insemination is used (semen with high lymphocyte counts is discarded, and the ejaculate is routinely diluted).
As in the case of most retroviral infections, infected animals are infected for life despite a strong humeral response. Only a small percentage of BLV-infected bovines actually develop tumors (1 to 5%) but some herds have an unusually high incidence of tumor development; genetics might play a role in this phenomenon. Clinical signs (tumors) rarely appear before 2 years of age, and are most commonly noted in 5-8 year old cattle. Tumors most commonly develop in the abomasum, heart, spleen, lumbar spinal cord, uterus, peripheral lymph nodes, and retrobulbar tissues. Economic losses arise from carcass condemnation. Furthermore, BLV positive cows produce less milk. Indirect economic losses occur from loss of sales to foreign countries (BLV positive animals, semen, embryos restricted from import).

In adult cattle, a positive BLV AGID indicates the animal has anti-BLV antibodies and is therefore infected with the retrovirus. A negative BLV test allows you to take lymphosarcoma off your list of rule outs for chronic diarrhea because the test is sensitive and specific. A positive test only means that this disease remains an etiologic consideration as a contributor to the diarrhea.
A calf less than 8 months of age might test positive if it ingested colostral antibodies from a BLV positive cow. Retest the calf after it reaches 8 months of age in order to determine if the calf is truly infected.
Persistent lymphocytosis develops in approximately one third of BLV-positve cattle even in the absence of other clinical signs. However, approximately 75% of the cattle with tumors have peripheral lymphocytosis. Abnormal lymphocytes can occasionally be noted in the peripheral blood. Moderate-to-severe anemia can be present in animals with bleeding abomasal lesions secondary to tumorization.
Findings of enlarged peripheral lymph nodes on physical (and rectal) exam, and a biopsy (preferred to an aspirate) of an abnormal node make the diagnosis more definitive. Conclusive evidence that the diarrhea is caused by lymphosarcoma comes from gross and microscopic findings of lymphoid tumors involving the digestive tract during laparotomy or on a postmortem examination.

No vaccines are available, so control must be made through management changes. Positive animals can be culled but this is an expensive strategy and not very practical in the United States (at this time). It is most applicable in herds where valuable calves are being raised for sale in foreign markets. A more practical approach is to segregate positive animals from negative animals, and to take measures to decrease transmission. However, segregation is not possible for all producers. The good news is that the incidence of seropositivity can be significantly reduced over time by simply following the guidelines below.

Guidelines for decreasing transmission of the virus:
1. Use individual sterile needles
2. Replace rectal sleeves between animals
3. Rinse and disinfect tattoo dehorning equipment, and other surgical equipment between animals. Chlorhexidine and dilute sodium hypochlorite are good disinfectants.
4. Feed calves only pasteurized colostrum
5. Insect control
6. Use BLV negative bulls
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