Bovine leukosis is a contagious disease of cattle caused by Bovine Leukemia Virus (BLV). The disease is characterized by a persistent leukemia which can culminate in malignant lymphosarcoma. The National Animal Health Monitoring System (NAHMS) determined that BLV is present in 89 percent of U.S. dairy operations. Only 30 to 40 percent of BLV carriers will develop leukemia, while less than 5 percent develop malignant lymphosarcoma. A later study determined the incidence of lymphosarcoma was six to seven cases per year per 1,000 cows if the herd prevalence of BLV was 50 percent.
Because BLV seldom causes outward clinical signs of leukemia, the effects of BLV infection on overall bovine health and productivity are believed to be relatively minor when only malignant lymphosarcoma is considered, although this depends on the proportion of cows infected in a herd.
When lymphosarcoma develops, the clinical signs are largely a function of the organs invaded by the malignant cells, often the spinal column, uterus, heart and abomasum.
However, the possibility that the disease can cause less obvious changes in dairy cattle productivity is controversial. Early studies found no influence on milk production, incidence of mastitis or reproductive performance.
In contrast, other studies reported negative effects of BLV infection on reproductive performance, milk production and, in particular, longevity. It is probable the negative effects of BLV on milk production and reproduction are reduced by early culling of poor-performing infected cows, and therefore the major effect of BLV in many herds may be primarily realized by reduced cow longevity.
As part of the USDA-NAHMS 1996 dairy study, the estimated average reduction in productivity was approximately $59 per cow for BLV test-positive herds. A Virginia study suggested a loss of over $400 per case of lymphosarcoma; however, subclinical cost was $64 per cow per year in herds with a BLV prevalence of 50 percent.
To further our understanding of this disease, we surveyed 113 Michigan dairy herds in the summer of 2010 to determine what herd management practices may be associated with herd prevalence of BLV infection. The preliminary results are described below and suggest the herd risk factors for infection are complex.
Field study
We randomly selected 113 dairy herds in Michigan based on the following criteria:
1. Participated in regular DHIA testing
2. Had at least 120 cows on test
3. Willingness to participate
Herds were visited once during the summer of 2010 to complete a survey regarding facilities, history and management practices. Additionally, DHI technicians collected milk samples during one routine test day for submission to the laboratory for ELISA testing of BLV antibodies.
We designed a herd profile as a practical method to estimate the herd prevalence of BLV-infected cows and the relationship of age to infection.
For the profile, we collected 40 milk samples per herd on the study; 10 samples each from first-lactation, second-lactation, third-lactation and fourth-lactation-or-greater cows. We also selected cows that recently calved, based on calving dates recorded from the previous month test date.
To validate the BLV herd profile, we compared BLV milk ELISA with serum ELISA in 142 cows and compared our profile sample size with whole-herd milk ELISA in four herds. We determined that there was a 95 percent agreement between BLV milk and serum ELISA.
Finally, with profile testing, we estimated herd prevalence of BLV-infected cows consistent with that obtained from whole-herd testing.
The percentage of BLV-infected cows in herds ranged from 0 percent to 76 percent. The prevalence of infection in first-lactation cows (18 percent) was less than third-lactation- and-greater cows (42 percent).
This may reflect the slow progression of the disease and/or exposure to transmission risks, e.g., through more frequent injections of adult cattle vs. heifers. Higher risk of BLV infection also was associated with larger herds, especially those that added replacement animals in the last three years.
We determined that higher prevalence of BLV infection in herds was associated with lower milk production per cow (annual rolling herd average basis), which agrees with previous research. Herds with higher prevalence of BLV infection had significantly lower proportions of older cows (third lactation or greater), which suggests increased prevalence of BLV infection is associated with decreased longevity.
Risk factors associated with the transmission of BLV are related to exposure of blood from infected animals. Thus, shared needles, palpation, tattoo pliers, dehorning, and to a lesser extent, nasal discharges, colostrum and flies, all have been incriminated as sources of infection.
Analysis of our survey data could not determine any single management practice significantly associated with increased risk of BLV infection. This suggests the control of this disease is complex and will result from multiple management practices as part of a comprehensive herd plan.
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.
Milk production and longevity have been cited as potential losses from BLV infection. Our study supports this concept.
Older cows and herds that have added replacement animals are at a higher risk of infection. Identifying single herd management practices associated with decreased risk of infection is difficult, and eventual control of this disease within a herd likely will be accomplished from a comprehensive plan of testing and management.
We will continue to evaluate the survey data to update the results at a later date. PD
References omitted due to space but are available upon request to editor@progressivedairy.com .
âÂÂExcerpts from Michigan Dairy Review Vol. 16, No. 2, April 2011.
