Options for Controlling BLV
Given the high prevalence of BLV in most US dairy herds, culling of all ELISA-positive cows would be prohibitively expensive. The use of management practices to reduce BLV transmission might eventually decrease the prevalence of BLV-infected cattle on dairy herds to a level low enough that test-and-segregate or test-and-cull programs might be feasible. This would allow the U.S. to follow the example of the many other countries that have eradicated BLV from their cattle populations.
Many European countries, the states of New York and Missouri, and the U.S. Animal Health Association offer BLV-free certification programs. For a herd to become certified BLV-free, it generally must have no cows test positive for BLV on two or more consecutive semi-annual herd tests.
BLV control options for U.S. dairy producers are summarized in Table 1. Some producers might choose to not control BLV transmission (option 1), as the majority of producers have done for decades. The first step to controlling BLV is to conduct a single BLV herd profile – a method for testing a small number of cows to determine estimates of the lactation-specific and within-herd BLV prevalences. If all the tested cattle are negative, the producer might decide to pursue BLV-free herd certification. If the within-herd BLV prevalence is very low, implementation of management practices to minimize BLV transmission in conjunction with culling or segregation of BLV-infected cattle should enable the producer to pursue BLV-free herd certification within a relatively short period of time (option 3 or 4). If the within-herd BLV prevalence is too high to economically pursue eradication, the most cost-effective approach for BLV control might be to implement comprehensive or select management practices to minimize BLV transmission with the aim of lowering BLV prevalence (option 2) before progressing to options 3 or 4.
Table 1: Options for BLV Control on US Dairy Operations56
- No action
- Monitor BLV prevalence with BLV herd profile milk testing or traditional serum testing. Make a comprehensive or selected management changes to reduce transmission and thereby reduce prevalence
- Test all cattle and segregate BLV test-positive cattle. Make selected management changes to reduce transmission. Maintain a closed herd or only add BLV test-negative cattle that retest negative after a quarantine period.
- Test and cull BLV-positive cattle. Maintain a closed herd or only add BLV test negative cattle that retest negative after a quarantine period.
Table 2: Management Practices to Minimize BLV Transmission Within Dairy Herds56
- Use a new single-use disposable hypodermic needle for each cow
- Clean and disinfect blood contaminated equipment for dehorning, tattooing, supernumerary teat removal, and other surgical procedures between animals.
- Use a new or cleaned/disinfected reproductive examination sleeve for each cow.
- Use artificial insemination exclusively for breeding.
- Control biting flies.
- Minimize contact between newborn calves and BLV-positive cattle.
- Avoid feeding fresh colostrum and milk from BLV-positive cows to newborn calves. (Only feed previously frozen or heat-treated colostrum and milk)
- Segregate BLV test-positive cattle from BLV test-negative cattle.
- Cull or segregate BLV-positive cattle with lymphocytosis or high proviral load.
Further studies are needed to determine the proportion of BLV infections attributable to each potential route of transmission. It is likely that the relative importance of each route of transmission will vary widely among herds. For example, most BLV infections in some herds might be attributable to gouge-type dehorning, whereas in other herds, most BLV infections might be attributed to reuse of BLV-contaminated hypodermic needles, the bites of stable files or some other blood borne route of transmission. The extent to which each management practice minimizes or prevents BLV transmission on a particular farm may vary greatly. Some producers will choose to implement a comprehensive program to prevent BLV transmission, whereas others may choose to implement only a few selected control practices and use an annual or semiannual BLV herd profile to assess the effectiveness of those practices. Generally, European countries used the 3rd and 4th options from Table 1 to achieve national BLV eradication; however, their starting prevalence of BLV was typically much lower than that in the United States16,57. It is not known exactly why segregation of infected cattle was effective. It could stop transmission from direct contact among cattle, or possibly may stop transmission during milking. Dairy producers who are reluctant to implement a comprehensive BLV control program (i.e., apply all management practices recommended for BLV control) might develop BLV control protocols based on perceived costs and benefits. For example, producers might ask veterinarians and artificial insemination technicians to change reproductive examination sleeves before reuse for the next cow, or only ask that a new sleeve be used if the last cow was known to be BLV-positive. Producers who continue to reuse hypodermic needles might inject BLV test-negative cattle with a different needle than they use for BLV test-positive cattle. There are many methods for controlling various species of flies, but producers should select fly control strategies that specifically target biting flies. Producers should also make sure that their artificial insemination company uses only BLV-negative bulls. Finally, producers using natural breeding should consider using only BLV-negative bulls.
Producers with low within-herd BLV prevalence might choose to pursue BLV control options 3 or 4 (Table 1) to eradicate the disease from their herds. Once BLV is eradicated from a herd, inexpensive periodic testing can be used to maintain a BLV-free status as prescribed by the certifying organization. In Great Britain, bulk tank milk samples are screened for antibodies against BLV as part of the national program to maintain its BLV-free status58.
Feeding of colostrum from BLV-positive cows may in some cases provide protection through passive antibodies when given to newborn calves59. These passive antibodies are no longer present after the calf is about 6 months old. On the other hand, colostrum and milk from infected cows can transmit BLV if it is not first frozen or pasteurized60,61. A freeze-thaw cycle has been shown to eliminate infectious BLV from colostrum62. Milk that was treated by a simulated high-temperature short-time pasteurization also prevents BLV transmission63.
CONTROL THROUGH GENETIC SELECTION
Results of a study of 114 Holstein herds and 8 Jersey herds64 indicate that the heritability of BLV infection was only 8% for both breeds. However, genetic factors may be more important in determining the degree of immune system degradation when an animal is infected by the virus.
It has been reported that dairy cow longevity is associated with genetic resistance to persistent lymphocytosis39. A major histocompatibility complex class I allele (BoLA-A) appears to be associated with both susceptibility to persistent lymphocytosis and with high milk production potential39. Wu et al.65 reported that BLV-infected cows with high genetic potential for milk and fat yields were more susceptible to developing lymphocytosis than were cows with lower genetic milk and fat yield potentials. Lymphocytotic cows fell short of their predicted milk and fat yields65.
It has been suggested that particular polymorphisms of BoLA-DRB3.2 are associated with high BLV proviral loads (amount of BLV DNA in the lymphocytes), and that cattle with polymorphisms associated with low BLV proviral loads are relatively noninfectious to their herdmates66–68. It may be possible someday that genetic selection could be used to breed cattle that are resistant to developing high BLV proviral loads66; however, in other reports69,70, 40% (21/53) of BLV-infected cows with a high BLV proviral load were not lymphocytotic. Also, 23% (27/119) of cows with the BoLA-DRB3.2 allele, which is thought to confer resistance to BLV, nevertheless had a high BLV proviral load. Thus, genetic selection for BLV-resistant cattle may be more difficult than the simple selection of cattle with one gene.
Vaccination for bovine leukemia virus is being investigated as an ideal management strategy. Unlike some other retroviruses like HIV, BLV has a relatively stable genome. However, previous attempts at developing a vaccine for BLV have been unsuccessful. Field trials for a new vaccine are currently underway in Argentina71.