No evidence for a negative association between bovine leukemia virus status and fertility in Kansas beef herds: a cross-sectional study.

OBJECTIVE: Determine the association between bovine leukemia virus (BLV) status and fertility in beef cows. BLV-status was defined using 3 different testing strategies (ELISA-, quantitative polymerase chain reaction- [qPCR], and high proviral load [PVL]-status). Fertility was defined as the overall probability of pregnancy as well as the probability of becoming pregnant in the first 21 days of the breeding season. ANIMALS: Convenience sample of 2,820 cows from 43 beef herds. PROCEDURES: The association of BLV-status with the probability of becoming pregnant was evaluated with a multivariable logistic regression analysis that used pregnancy status as a binary outcome, herd nested within ranch as a random effect, and BLV-status (ELISA-, qPCR-, and PVL-status as separate models) and potential covariates (eg, age, Body Condition Score [BCS] category, and interactions) as fixed effects. RESULTS: Raw data revealed that 55% (1,552/2,820) of cows were classified as BLV-positive by ELISA, and 95.3% (41/43) of herds had a least 1 ELISA-positive cow. Classification as BLV ELISA-positive was positively associated with the probability of being pregnant; however, when qPCR or PVL were used to classify BLV-status, there was no association with the probability of being pregnant. None of the methods of classifying BLV-status were associated with the probability of becoming pregnant in the first 21 days of the breeding season. CLINICAL RELEVANCE: This study did not find evidence that testing beef cows for BLV-status using ELISA, qPCR, or a cut-off of 0.9 PVL and removing test-positive cows will improve cowherd fertility as described by the probability of becoming pregnant during the breeding season or becoming pregnant during the first 21 days of the breeding season.

Cross-sectional study to describe bovine leukemia virus herd and within-herd ELISA prevalence and bovine leukemia virus proviral load of convenience-sampled Kansas beef cow-calf herds.

OBJECTIVE: Determine bovine leukemia virus (BLV) seroprevalence of adult female cattle in Eastern Kansas beef herds and the proviral load (PVL) of those cattle found to be ELISA positive. ANIMALS: Convenience sample of 2,845 cows from 44 beef herds. PROCEDURES: BLV serostatus was determined using an ELISA antibody test (gp-51; IDEXX). BLV quantitative PCR (qPCR) status and PVL were determined utilizing a qPCR test (SS1 qPCR test; CentralStar Laboratories). The association of age, herd size, and body condition score (BCS) category on the probability of being BLV positive was evaluated with a multiple variable logistic regression analysis that used BLV status as a binary outcome, herd nested within ranch as a random effect, and BCS, herd size, and age category as fixed effects. RESULTS: Forty-two of 44 herds had at least 1 BLV ELISA-positive cow (95.5% herd seroprevalence). Overall, 1,564 of the 2,845 cows were BLV ELISA positive (55.0% individual animal prevalence). No association between BLV ELISA status and herd size or BCS was identified. When evaluated by age, the model-adjusted probability of being BLV ELISA positive was lowest for heifers (1 year of age, first parity) and increased until 5 to 6 years of age. Of the 1,564 ELISA-positive animals, 838 were qPCR positive (53.6%). The model-adjusted probability of being qPCR positive was not associated with age, herd size, or BCS category. CLINICAL RELEVANCE: This study indicated that BLV-seropositive status both as a herd classification and individual animal classification was very common in this population. Because the percentage of BLV-seropositive cows varied between herds and by age, this study provides evidence that it is essential for investigators to control for herd and age in any analysis of the association of BLV serostatus and health and production outcomes of interest. Some BLV ELSIA-seropositive cows were classified as BLV negative by qPCR, and risk factors may differ between classification status by ELISA and qPCR.