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Bovine lymphosarcoma (Proceedings)
Bovine leukemia virus (BLV) is an oncogenic retrovirus associated with lymphosarcoma in cattle.
Bovine leukemia virus (BLV) is an oncogenic retrovirus associated with lymphosarcoma in cattle. The virus was first identified in 19691 and transmissibility was confirmed in 1972.2
The virus infects lymphocytes and is rarely found free in the animal. Transmission, therefore, is by transfer of BLV infected lymphocytes from one animal to another. Blood, milk and colostrum would be the main fluids one would expect to be infective, however, other bodily fluids could be infective if exudative processes are occurring. Infection with bovine leukemia virus is for life.
Numerous reports from all regions of the world indicate various prevalence of BLV in both beef and dairy herds. In general, the prevalence is higher in North America than in Europe probably due to the participation in eradication programs in various regions. Also, in general, the prevalence is higher in dairy cattle than beef cattle. Prevalence also varies from herd to herd due to establishment of eradication programs and differences in management practices. It is reported that in 1997, 43.5% of dairy cattle and 89% of dairy herds were infected with BLV.3 A statewide prevalence of 8.5% was reported in purebred bulls for sale in Kansas.
One factor influencing the prevalence of disease is susceptibility to BLV infection. Genetic factors influence the susceptibility of cattle to BLV. One study reported that the heritability of acquisition of BLV was 0.48.5 In comparison, the milk yield of cattle has a heritability value of approximately 0.25.6 Bovine major histocompatibility complex (BoLA) allele types have also been correlated with risk of BLV infection7 and have been shown to vary by breed. These alleles confer either resistance or susceptibility. Age is another factor in acquiring BLV infection. Studies indicate BLV infection rates increase with age,5 and relatively few new infections occur in cattle older than 3 to 5 years.8 Cattle in herds with a high prevalence of BLV infection appear to become infected with BLV at an earlier age.9 In general, no breed or sex predilection appears to be reported.
Finding that an animal is positive for BLV is not necessarily a death sentence. Approximately 70% of cattle positive for BLV will not display any clinical signs of disease during their lifetime. Twenty-five to twenty-eight percent of those positive for BLV will not display clinical signs, but will develop a persistent lymphocytosis. Of these, roughly 5% of them will go ahead and develop solid tumors. Finally, only 2 to 5% of animals positive for BLV will develop lymphosarcoma. Only approximately 1 in 10,000 cows have lymhosarcoma without BLV infection.10
The economic impact to the cattle producer is most directly and indirectly realized by death of animals with lymphosarcoma and restrictions placed on trade of seropositive cattle, respectively. Subclinical infections may be associated with reduced milk production in dairy herds and premature culling. As for production, one study reported that herds with seropositive cows produced 3% less milk than herds with negative cows and that the average reduction in value of production was $59.00 per cow. In addition, for the US dairy industry, BLV seropositivity was associated with loss to producers of $285 million and loss for consumers of $240 million. Therefore, the costs of BLV infection can be divided into direct and indirect losses to the producer.
Various modes of transmission of BLV have been investigated. As stated earlier, the transmission depends on the transfer of BLV infected lymphocytes. Routine procedures performed most commonly in calves such as dehorning, castrating, tail docking, insertion of growth implants, tattooing for brucellosis vaccination and removal of supernumerary teats with instruments not disinfected between animals could result in the transmission of the virus. More specifically, gouge dehorning has been associated with high rates of transmission
Theoretically and from the literature it is clear that perenteral injection with blood- contaminated needles, from BLV positive animals is a highly efficient mode of transmission. The number of infected lymphocytes in the blood has an effect on the amount of blood required for transmission, therefore, Animals with persistent lymphocytosis are more infective than those that have normal lymphocyte counts.
Bovine leukemia virus has been identified in the milk and colostrum of positive cows. The ability of milk and colostrum to transmit the virus has, however, been demonstrated primarily in sheep. In addition, the presence of antibodies to BLV in colostrum decreases the susceptibility of the calf to infection, however, while the potential exists for transmission of BLV via milk and/or colostrum, the importance in a natural setting is unclear.
Rectal palpation, and the occasional accidental irritation of the mucosa associated with it, can create a means of blood transmission from one animal to the next if palpation sleeves are not changed or washed between each animal. It also seems likely that blood obtained per rectal examination from cows with persistent lymphocytosis would be more infective.
Breeding by natural service or artificial insemination has not been shown to be a significant mode of transmission of BLV. However, if common AI materials are used, or common sleeves are used among sereopositive and seronegative cows, transmission could occur. In addition, BLV positive bulls with exudative processes occurring in the reproductive tract could provide a source of infected lymphocytes. Also, breeding injuries to the cow via trauma induced by the bull may lead to bleeding and transfer of infected blood. However, authors of one study concluded that semen alone from BVL positive bulls was unlikely to serve as a mode of infection due to the low numbers of lymphocytes in the semen.11 The risk of transmitting BLV to recipient cows via embryos harvested from BLV positive cows is practically nonexistent if normal procedures are used. Embryos from infected cows have failed to induce BLV infection in recipients in many studies.
Bovine leukemia virus and antibody have been demonstrated in newborn calves prior to the ingestion of colostrum indicating in utero transmission.12 A study in 120 cows reported that 4% of calves born to BLV positive dams were infected.13 Factors that have shown to have an influence on the risk of in utero transmission include infection of the cow during pregnancy, age of the cow and parity.
We all are aware of the every day battle of dairy and beef operations to control blood feeding flies and ticks. It is also accepted that they are vectors for many blood-borne diseases. But what about BLV? Researchers have studied the risks of acquiring BLV via flies and ticks in three basic designs: allowing insects to feed on BLV infected animals or blood and then inoculating susceptible animals with mouthparts harvested from the insects, allowing insects to feed naturally on BLV infected animals co-mingled with susceptible animals and using serological examinations of populations to determine if seasonality exists in acquiring the virus.
Mosquitoes, horn flies, deer flies, horse flies and ticks have all been evaluated to various degrees. It is important to note that in some of these research trials sheep, who are much more susceptible with infection than cattle, are used to determine if transmission has occurred. The results are often quite more alarming in sheep than for cattle. The lymphocyte counts of the cattle infected with BLV also play a role in the likelihood of transmission via blood feeding insects. In one study, inoculation of almost 4,000 stable fly mouthparts were required to transmit BLV from a cow with lymphocyte count of 10,500/µL.14 In another study, 0 of 5 sheep inoculated with 10 mouthparts of horse flies, and only 1 of 5 sheep inoculated with 20 mouthparts seroconverted following feeding on a BLV infected cow.15 Experiments in which insects are allowed to naturally feed on cattle have demonstrated the potential for BLV transmission. Horse flies allowed to feed on two BLV infected cows with lymphocytosis transmitted BLV to susceptible calves after 100 to 150 flies were manually transferred to the calves.16 However, another study found no BLV transmission following the feeding of stable flies on lymphocytotic BLV infected cow and manual transfer to calves.17
Contact transmission as a result of prolonged physical contact has been implicated in the transmission of BLV. Due to the fact that BLV is very rarely free of cells, the likelihood of transmission of this nature would be slim without a means of cell associated fluid transfer. T
Diagnosing the presence of BLV is generally not complicated and can be done routinely by most diagnostic laboratories on serum. Calves less than 3 months of age should be tested by evaluating whole blood due to the possible presence of maternal antibodies that interfere with results. Due to the nature of the virus (retrovirus, once infected, always infected).. The standard test is an agar gel immunodiffusion or ELISA test for the gp-51 antigen.
Lymphosarcoma is primarily recognized in cattle from 3-6 years of age. Clinical signs depend very much upon the particular organ(s) where solid tumors develop. The most common organs involved in lymphosarcoma include the right atrium, abomasum, peripheral or internal lymph nodes (sublumbar), uterus, kidneys and retrobulbar lymph nodes. Most cases of lymphoma present with varying degrees of anorexia and weight loss depending upon duration of disease. Cattle with right atrial involvement present with signs of congestive heart disease which may include increased heart rate, distended jugular pulse, tachypnea and muffled heart sounds. Signs of abomasal involvement include distended abdomen, melena, diarrhea and anorexia with weight loss. Peripheral lymph node involvement is usually readily apparent. Internal lymph node enlargement is sometimes palpable per rectum. In cases of sublumbar lymph node involvement, the animal may display hind limb ataxia to recumbency as the disease progresses. Retrobulbar lymph node enlargement is characterized by exophthalmia. Uterine involvement can often be appreciated by rectal palpation of a thickened uterus potentially containing multiple masses. Other organ involvement is less noticeable on physical examination.
Complete blood counts are generally unremarkable unless cattle have persistent lymphocytosis. Less than half of cattle with lymphosarcoma (malignant lymphoma) have elevated lymphocyte counts.10 A mild, non-regenerative anemia may be noted. On serum chemistry, LDH may be very elevated. Several studies have shown that one or more of the isoenzymes of LDH are significantly elevated in solid tumors. However, findings on routine blood work are merely pieces of evidence and must be combined with history and physical examination to arrive at even a tentative diagnosis.
A definitive diagnosis of lymphosarcoma may be difficult in some cases. Many of the clinical signs, depending upon organ involvement may mimic other diseases. Due to the grave prognosis and herd ramifications of a positive BLV animal, it is imperative, especially in genetically valuable animals that clinicians are confident in antemortem diagnosis of lymphosarcoma before rendering a death sentence.
As stated earlier, serologic testing for the gp-51 antigen can provide one piece of evidence for the definitive diagnosis of lymphoma. Additional testing for the p-24 antigen, performed in some laboratories may also provide information of value. If this test is negative, the likelihood of the animal having solid tumors due to BLV is diminished. However, a positive p-24 antigen test only further builds a case for lymphoma and is not alone enough for definitive diagnosis. Finding bizarre lymphocytes in peripheral blood smears with concurrently elevated counts is suggestive of malignant lymphoma.
Fine needle aspirates on enlarged lymph nodes in the bovine are often unreliable means of definitive diagnosis. In small animals, fine needle aspiration with subsequent cytological examination of enlarged peripheral lymph nodes can be diagnostic. Aspirates composed of a predominance of large lymphoblastic cells with large, prominent nucleoli with a high nuclear-to-cytoplasmic ratio are indicative of lymphoma. In the bovine clinicians are not afforded this luxury due to similarities between lymph nodes that are reacting to infection and those infiltrated with neoplasia. Histopathologic examination of biopsied tumors or lymph nodes may be more beneficial. In one of our studies, we had pathologists look at tru-cut biopsy specimens of cattle presented with enlarged peripheral lymph nodes that went to necropsy due to diagnosis of terminal disease. The pathologists were unaware of the final necropsy and histopathologic results. It appears that if a good sample is obtained, a pathologic diagnosis of lymphosarcoma from a biopsied lymph node is diagnostic and correctly diagnoses the disease.
Prevention and control programs for BLV infection should be based on management plans and tools designed to address the aforementioned means of transmission of the virus. As for preventing the spread of BLV, any change in procedure that would decrease the likelihood of transfer of infected lymphocytes from one animal to the next would be beneficial.
Three basic strategies for the control of BLV infection are: test and slaughter, test and segregation, and test and implementation of corrective management.18,19,20 The selection of one of these strategies is dependent upon many factors including the cost of the program, costs associated with testing, goals and personal preferences of the producer, the producers attitude toward risk and the amount of monetary and labor resources available to carry out the program. The cost of a program varies from farm to farm but generally depends upon the prevalence of infection and the value of the animals at the start of the program. In commercial herds where the cost of animals is less, eradication of test positive animals may be feasible. In addition, animals may be removed gradually from the herd to avoid sudden changes in cash flow. Therefore, test and removal is not likely to be an economically feasible program where the prevalence is high.(dairy?) Test and removal in beef operations may be a reasonable means of control because generally the herd prevalence is much lower.
Test and segregation programs depend on the availability of housing, confinement and/or pastures for those animals that test positive. Testing and implementing control strategies to decrease the transmission of the disease are viable in most incidences. The willingness of the producer to accept risk comes into play with the implementation of some or all of the recommended control measures.
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2. Miller LD, Miller JM, Olson C: Inoculation of calves with particles in phytohemagglutinin-stimulated lymphocyte cultures of bovine lymphosarcoma. J Natl Cancer Inst 48:423, 1972.
3. USDA-Veterinary Services. High prevalence of BLV in U.S. dairy herds. 1997.
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