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Bovine Leukemia Virus: Final Summary of the 2010 Michigan Study

Ron Erskine and Paul Bartlett
Dept. of Large Animal Clinical Sciences
College of Veterinary Medicine

Todd Byrem
AntelBioSystems, Inc

Bovine Leukemia Virus (BLV), is a retrovirus that infects beef and dairy cattle and targets B lymphocytes, a type of white blood cell that is critical for the immune system.  B lymphocytes make the antibodies that help clear many infections and are the target of many diagnostic tests used in the dairy industry.  In 1996, the National Animal Health Monitoring System (NAHMS) determined that 89% of US dairy operations have BLV [USDA-APHIS, 1997].  Infection with the virus usually does not cause any clinical signs.  However, about 30% of infected animals eventually develop a lymphocytosis, or abnormal increase in lymphocytes in the blood.  This is sometimes referred to as “leukemia”, which does not cause any clinically apparent change in most cows [Schwartz and Levy, 1994].  Fewer than 5% of infected cows will go on to develop a malignant lymphosarcoma, a cancer that most often invades the spinal column, uterus, heart, or abomasum.  A later study estimated that the frequency of lymphosarcoma was between 6 to 7 cases per year/1,000 cows if the herd prevalence of BLV was 50% [Rhodes, et al, 2003].   The effects of BLV infection on overall bovine health and productivity are believed to be relatively minor when only lymphosarcoma is considered, although this depends on the proportion of infected cows in a herd.  Direct losses caused by BLV to the dairy producer include increased replacement costs, loss of income from condemned carcasses of cull cows, and the inability to export cattle, semen and embryos to countries that maintain BLV control programs, such as the E.U. [Ott et al, 2003, Rhodes et al, 2003]. 

Research by investigators at MSU and globally have found that lymphocytes from BLV-infected cattle may have altered immune function as compared to lymphocytes from non-infected cattle.  This change in immune function can alter both B and T lymphocytes and may be present in cows without more advanced clinical signs of infection.  The link between these laboratory findings and possible effects on the host defenses of dairy cattle against other diseases is still largely unknown.  However, a preliminary study at MSU found that BLV-positive cows vaccinated against either J-5 E. coli bacterin or BVD Type-1 virus had lower antibody responses in blood as compared to BLV-negative cows [Erskine et al, 2011a].  In order to further investigate the status of BLV in Michigan, possible impacts, and methods of control, we conducted a field trial in the summer of 2010. The preliminary results were reported earlier [Erskine et al, 2011b].  This paper presents the final analysis and practical considerations of this research. 

Michigan Field Study
Prevalence of BLV in herds-113 dairy herds in Michigan were selected based on the following criteria:  1) regularly DHIA tested, 2) had at least 120 cows on test, and 3) willingness to participate.  Herds were visited once during the summer of 2010 to complete a survey regarding facilities, history, and management practices.  Additionally, DHIA information specialists collected milk samples during one routine test day for ELISA testing of BLV antibodies (AntelBio).  We designed a herd profile as a practical method to estimate the prevalence of BLV infected cows and the relationship of age to infection.  Thus, about 40 milk samples were collected per herd on the trial; 10 samples each from first, second, third, and fourth or greater lactation cows.  Additionally, we selected the most recently calved cows in each of the lactation groups.  To test the accuracy of our sampling, we compared our BLV herd profile (testing 40 cows) with an entire herd test (all milking cows) in 8 herds.  There was excellent agreement between the two test methods.  Thus, our herd profile was verified as a reliable means to estimate the level of BLV infection in a dairy herd.  The percent of BLV-infected cows in each herd ranged from 0% to 76%, with an average of about 33%.  The prevalence of infection increased with the age of cows; 18% in 1st, 29% in 2nd, 39% in 3rd, and 45% in 4thlactation cows, respectively [Figure 1; Erskine et al, 2012a].  Higher percentages of BLV infection in older cows likely reflects increased exposure to transmission risks, e.g., use of common needles (discussed below) as cows increase in age.  The important point from this part of the study is that the level of BLV infection varies greatly between herds and that a herd profile of recently calved cows, with 8 to 10 selected from each of the four lactation groups, will provide a simple means of assessing herd BLV status, and relationships between cow age and infection status.


BLV associated milk losses-Previous research has offered conflicting views on the ability to predict milk production or other losses in individual dairy cattle, based solely on a positive or negative BLV test.  This may in part be due to variable duration of infection at the time of testing, slow progression of the disease, the fact that the majority of BLV infected cows are asymptomatic (no clinical signs), increasing risk of infection with increasing age, and potential immune impairment, especially in cows with leukemia.  However, herd-level associations between the proportion of BLV
infected cows and lost milk production have been more consistent.  Figure 2 is a plot of the herd BLV prevalence (percent of cows infected) and the amount of milk per cow in the herd per year, from two different studies: USDA-NAHMS [Ott et al, 2003], and Michigan [Erskine et al, 2012b].  It is apparent that milk production per cow has increased over the 14 year period between the two studies.  However, what is remarkable is the similarity of the slope between the two lines.  The production loss for each 10 percent increase in BLV-infected cows was similar for the two studies and averaged about 250 lbs in milk per cow per year.  Thus, a herd with 50% of BLV-infected cows would average 1,000 lbs of milk less (per cow in herd) than a herd with 10% of BLV-infected cows.  Herds with a higher prevalence of BLV also had a lower proportion of older cows (third lactation or greater), suggesting that cow longevity was shortened as BLV prevalence increased.  Although predicting milk losses from BLV testing on an individual cow basis may be unreliable, the key point from this phase of the study is that there is a significant linear association between higher BLV prevalence and decreased herd-level milk production, and that BLV may decrease cow longevity.


Transmission of BLV within herds-The bovine leukemia virus does not survive well outside the body.  Thus, in countries such as the U.S., which do not participate in a national BLV eradication program, control of the disease focuses on preventing transmission through hematogenous (blood-borne) routes.  Practices such as gouge dehorning, injections with shared hypodermic needles, control of  biting flies, and tattoo pliers are risks in the transmission of BLV, although the importance of these risks in individual farms may vary depending on the frequency of use or exposure[Thurmond et al, 1983;Sprecher et al, 1991; Kobayashi et al, 2010].  In the previous Michigan Dairy Review article, we identified having an open herd and use of natural service as risks for BLV transmission from herd to herd [Erskine et al, 2011b].  Presently, we are asking the question, “What factors are important for the spread of BLV within a herd, not between herds”. The following table lists, in order of priority (most significant association first), the management practices that are associated with increased risk of BLV infection.

Herd Management Practice that increases BLV risk


Use of selenium injections in dry cows

Herds that happen to use selenium injections are probably also more likely to use other multiple injections.  This is more likely a measure of overall number of routine injections.

Gouge dehorning

Blood contamination between animals; electric or gas de-budding reduces the risk, as would thorough disinfection of equipment betweenanimals 

Use of shared needles

Blood contamination among animals

Lack of a fly control program

Biting flies may spread infected blood among animals

TMR feeding in heifers

A “surrogate risk” that possibly reflects herd size or transmission at shared feed bunks, or more likely herds with loose housing for heifers may also use other intensive practices related to reproduction and vaccination, etc. 

Use of bull breeding for cows or heifers

Unknown if this risk results from infected semen and/or breeding trauma

Average number of routine palpations for heifers and cows

Possible blood contamination among animals.  This risk may be practically reduced by palpating lower prevalence animals (heifers, first lactation cows) separately, or before older cows

The above management practices have been identified as the most critical risks in Michigan dairy cattle, but do not account for all the variables that can lead to BLV transmission within a herd.  Differences in genetics among farms, variation in strains of the BLV virus, the overall prevalence of infection, feed bunk management, etc., may all play a role in reducing the prevalence of BLV within a herd, and were not accounted for in our survey.  Additionally, some practices, such as using non-disinfected ear tag pliers, while not in the final statistical model in our study, could contribute to BLV transmission.  The important message from this part of the study is that the above management practices are the most critical to consider as part of a herd BLV control strategy.

Summary-   Bovine leukosis is a disease that progresses slowly, and is often subclinical in presentation.  Only a small percentage of cows infected with BLV develop lymphosarcoma.  BLV control programs aren’t a “quick fix”, and total eradication may be difficult.  However, depending on the level of infection within a herd and marketing of cattle, herd managers should consider the potential losses from the disease versus the cost of management changes needed to reduce transmission within the herd.  Milk ELISA testing ofa select number of animals is a practical tool to determine age-related prevalence of BLV infection in dairy herds. The BLV profile can assist dairy managers to 1) decide if BLV is a problem in their herds, 2) target which age groups are most infected, 3) use control practices that may be most effective, and 4) provide a means to monitor the progress of herd BLV control.  As with other diseases, effective control of BLV relies on the ability to change management behaviors, other economic priorities on the farm, and facilities.  In many herds the goal may be to reduce the level of infection to an acceptable level, rather than eradication. 

Erskine RJ, PC Bartlett, KM Sabo, LM Sordillo. 2011a. Bovine Leukemia Virus Infection in Dairy Cattle:
Effect of Serologic Response to Immunization against J5 E coli Bacterin. Veterinary Medicine Intl Apr 3;2011:915747.
Erskine RJ, PC Bartlett, LM Sordillo, C Render, C Febvay. 2011b. Elwood Kirkpatrick Dairy Science Research
            Endowment: A Completed Project. Michigan Dairy Review, April; 16(2):6-7.
Erskine RJ, PC Bartlett PC, TM Byrem TM, CL Render CL, C Febvay, JT Houseman. 2012a. Using a
Herd Profile to Determine Age-Specific Prevalence of Bovine Leukemia Virus in Michigan Dairy Herds.Veterinary Medicine Intl doi:10.1155/2012/350374, 5 pages.
Erskine RJ, PC Bartlett, TM Byrem TM, CL Rende, C Febvay C, JT Houseman. 2012b. Association Between Bovine
 Leukemia Virus, Production, and Population Age in Michigan Dairy Herds. J Dairy Science 95(2):727-734.
Kobayashi S, T Tsutsu, T Yamamoto, Y Hayama, K Kameyama, M Konishi, K Murakami. 2010. Risk factors associated
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Health Monitoring Service.1997.1996 Dairy Info Sheet.
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