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.