Objective To estimate current US herd-level and animal-level prevalence of bovine leukemia virus (BLV) in dairy cows and characterize epidemiologic features. Design Cross-sectional observational study design and survey. Animals 4120 dairy cows from 103 commercial dairy herds in 11 states across the US. Procedures Milk samples were collected from dairy cows through routine commercial sampling and tested for anti-BLV antibodies by antibody capture ELISA. Based on the ELISA results of a sample of an average of 40 cows per herd, within-herd apparent prevalence (AP) was estimated by a directly standardized method and by a lactation-weighted method for each herd. Within-herd AP estimates were summarized to give estimates of US herd-level and animal-level AP. Differences in AP by lactation, region, state, breed, and herd size were examined to characterize basic epidemiologic features of BLV infection. Results 94.2% of herds had at least one BLV antibody positive cow detected. The average within-herd standardized AP was 46.5%. Lactation-specific AP increased with increasing lactation number, from 29.7% in first lactation cows to 58.9% in 4th and greater lactation cows. Significant differences were not observed based on region, state, breed, or herd size. Conclusions and Clinical Relevance These results are consistent with a historical trend of increasing prevalence of BLV among US dairy cattle. Given the findings of other studies on the negative impacts of BLV infection on milk production and cow longevity, these findings are clinically relevant for veterinarians counseling dairy clients on the risks of BLV to their herds.

Highlights• Dairy cows were tested for BLV with an ELISA milk test and followed for survival.• BLV-positive cows had shortened lifespans compared with BLV-negative herdmates.• BLV ELISA results were not associated with producer-reported reasons for culling.

Bovine leukemia virus is a deltaretrovirus of cattle and the causitive agent of enzootic bovine leukosis. A small proportion of infected animals develop the clinical manifestation of lymphoma, however the majority remain subclinically infected. Infection is common in the U.S. dairy cattle population and in other major dairy producing countries that have not taken steps to reduce and erradicate infection in their populations. The 1997 and 2008 NAHMS dairy studies examined the prevalence of BLV infection at the cow and herd levels, respectively. There has been no updated estimates of prevalence in the U.S. cattle population in the last decade. Within that time, evidence of the negative impact of subclinical infection has been mounting. Cows exhibit alterations in immune function, decreased longevity, and reduced milk production. The first major objective of this study was to provide an updated estimate of BLV prevalence in a multistate sample of U.S. dairy cattle. The second major objective was to measure herd and cow level associations between BLV prevalence and economically important dairy production outcomes, namely milk production, reproductive efficiency, and somatic cell count.

Bovine leukemia virus (BLV) is now present in almost all US dairy herds, and the prevalence among cattle in our milking herds is now approaching 50%. Most dairy producers know that BLV can cause tumors in a small percentage of cattle, but the associations now being observed with reduced milk production and cow longevity are not yet appreciated by most dairy producers. Over 20 nations have eradicated BLV from their cattle populations by removing all antibody positive animals. This is economically impossible for all but a small percentage of farms that have very low BLV prevalence. Management interventions to reduce transmission have not been promising. New diagnostic methods may be helpful to better target culling and segregation to those animals that are most infectious to herd mates. Producers interested in determining BLV prevalence in their herd are advised to conduct a BLV herd profile, which involves milk or serum ELISA testing of the 10 most recently calved cattle in lactations 1, 2, 3, and 4+. More information is available on our website at www.BLVUSA.com.

Bovine leukemia virus (BLV) is a deltaretrovirus prevalent in approximately 80% of US dairy herds and 30% of US dairy cattle. Previous work by our research group and others has shown that BLV infection leads to decreased milk production, impaired immune response, and decreased longevity among positive dairy cattle. Much of this work was part of a 2010 study of 113 Michigan dairy herds. Our research group is currently undertaking a nationwide study of bovine leukemia virus, in which we will be enrolling 120 herds from 11 states. The objective of this study is to confirm our findings from Michigan on a broader national scope. The descriptive epidemiology of bovine leukemia virus among herds enrolled in this study are presented here.