Resumen de Options for the control of bovine leukemia virus in dairy cattle
The subclinical impact of bovine leukemia virus (BLV) on the sustainability of the US dairy industry is only now being fully recognized. Findings of recent longitudinal studies conducted in Michigan dairy herds were consistent with the results of previous studies in showing that within-herd prevalence of BLV–infected cattle was negatively associated with milk production and cow longevity. Risk factors relating to routes of hematogenous transmission such as the use of shared hypodermic needles, shared reproductive examination sleeves, and natural breeding were associated with BLV within-herd prevalence. Few US dairy producers know the prevalence of BLV-infected cattle in their herds or are aware of the insidious economic impact of BLV or the options for BLV control. As an increasing number of countries eradicate BLV from their cattle populations, restrictions on the movement of US cattle and cattle products will likely increase. Veterinarians should be aware of recent developments for screening serum and milk samples for antibodies against BLV and the results of research regarding the economic impact of BLV so they can advise their dairy clients of available alternatives for monitoring and controlling BLV infection.
Enzootic bovine leukosis is a contagious retroviral disease of cattle caused by BLV Nearly 40% of BLV-infected cattle will develop persistent lymphocytosis within a few years of infection (Figure 1)1; however, < 5% of infected cattle develop malignant lymphosarcoma, the most common neoplastic disease identified in cattle slaughtered in the United States.1–3 In the 1960s and 1970s, BLV prevalence rates in cattle in the United States and Canada were approximately 10%, and the only recognized impact of infection was BLV-induced lymphosarcoma.3–7 Consequently, the US and Canadian cattle industries decided that BLV was not sufficiently problematic to warrant control. In contrast, 21 countries and Western Australia have now successfully eradicated BLV from their cattle herds.8,9 However, in the United States, approximately 40% of dairy cattle are now infected with BLV, and costs associated with subclinical BLV infection have now become apparent.4,10–12 Results of multiple studies13–16 indicate that BLV infection degrades lymphocyte function in cattle, which diminishes the immune response to vaccinations and opportunistic infections. Investigators of multiple studies4,10–12 report that BLV infection impairs milk production and cow longevity. Additionally, export of US dairy cattle and cattle products may become increasingly difficult as more countries attempt to maintain their BLV-free status or implement BLV eradication programs.17 View larger version(35K) Figure 1— Absolute lymphocyte concentration for 208 BLV-positive (gray bars) and 103 BLV-negative (black bars) cows in 5 dairy herds in Michigan (n = 2), Wisconsin (1), Iowa (1), and Pennsylvania (1) during 2008 through 2010. (From Swenson CL, Erskine RJ, Bartlett PC. Impact of bovine leukemia virus infection on neutrophil and lymphocyte concentrations in dairy cattle. J Am Vet Med Assoc 2013;243:131–135. Reprinted with permission.) For these reasons, US dairy producers need to make informed decisions about whether BLV control is feasible for their operations. Because BLV is a retrovirus, prospects for an effective vaccine are problematic. Genetic selection for BLV-resistant cattle might be beneficial for minimizing economic losses, but it currently appears that complete resistance to BLV cannot be obtained by selection of cattle with BLV-resistant alleles of the DRB3.2 gene that has been the subject of recent research.18,19 Hence, the US cattle industry may need to use management interventions to reduce BLV prevalence to a sufficiently low level so that test-and-segregate and test-and-removal control programs become economically feasible. In a recent longitudinal study4,10 conducted by our field epidemiology group, 15 of 113 Michigan dairy herds had no evidence of cattle with antibodies against BLV, which suggests that BLV-negative herds can be maintained, even in a state where most dairy herds contain BLV-infected cattle. Traditionally, management practices recommended for the control of BLV transmission have included single use of hypodermic needles and reproductive examination sleeves, use of artificial insemination instead of bulls for breeding purposes, control of stable flies and other biting insects, segregation of infected cattle, feeding calves only heat-treated colostrum or colostrum replacers, and cleaning and disinfecting blood-contaminated equipment between animals during routine procedures such as application of ear tags, tattooing, and dehorning.5,11,17 Accurate estimates of the cost-effectiveness of BLV control measures are required, as are methods to identify and overcome educational, behavioral, and attitudinal barriers to the additional expenditures, labor, and inconvenience necessary for the control of BLV. Because the importance of different routes of BLV transmission will likely vary by farm, each dairy farm may need a customized BLV control program and have to routinely monitor its BLV prevalence to determine the program's effectiveness.
Implementation of ELISA screening of milk samples for anti-BLV antibodies to detect cows subclinically infected with BLV is somewhat analogous to the adoption of individual-cow SCC to screen cows for subclinical mastitis in previous decades. When the capability to determine individual-cow SCC first became available to US dairy operations, producers did not realize the economic impact of subclinical mastitis or know how to interpret the SCC data. Consequently, they rarely requested individual-cow SCC testing from their DHI organizations. Wide-scale usage of SCC data was greatly augmented by educational and extension programs to teach dairy producers about the economic impact of subclinical mastitis and ways that SCC data can be used as an effective monitoring tool. Although an ELISA to test milk from individual cows for antibodies against BLV (ie, BLV milk ELISA) is now available to US dairy herds through their DHI testing service, only approximately 0.5% of US dairy cows are tested for BLV annually.a Many producers do not yet appreciate the costs incurred to their operations because of subclinical BLV infection or know how to use diagnostic test results to monitor BLV transmission within their herds.11 The BLV milk ELISA has been proposed as a science-based tool that can be used by dairy producers to monitor within-herd BLV prevalence, and may become used more frequently as was the individual-cow SCC. Herd veterinarians can help producers use new BLV-screening technologies to reduce BLV in their herds and perhaps eventually qualify for BLV-free certification. Currently, the United States does not have a national eradication program for BLV, but individual dairy producers can qualify for state and privately certified BLV-free programs.
