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BLV causes one of the most relevant viral diseases of dairy cattle although the most of the infected animals are asymptomatic. There are controversies regarding reproductive losses on infected herds. Our purpose was to evaluate the horizontal transmission of the virus in a group of Holstein heifers confined in a rearing system and to determine the impact of infection on reproductive performance. Three hundred eighty-nine animals were sampled and seroconversion was determined by ELISA. The occurrence of estrous behavior, number of services/pregnancy, conception rate, percentage of pregnant animals by artificial insemination and by bull service during two breeding periods (June-July and November-December 2012) were recorded. Herd BLV initial seroprevalence was 45% and annual conception rate was 39.8% (CI 30.5%-49%). During the 18 month time course two seroconversion peaks were detected. The first one happened during the admission period, most likely due to quarantine and intensive health management of the animals. The second peak occurred during the first gestational period. At the end of the study, 73.6% of the heifers were BLV seropositive and there was reduction of 27% in the herd conception rate in the second breeding period (p=0.005). In conclusion, BLV infection and reproductive performance were negatively associated and high seroconversion was observed during the gestational period.

HORIZONTAL TRANSMISSION DYNAMICS OF BOVINE LEUKEMIA VIRUS (BLV) AND NEGATIVE EFFECT ON REPRODUCTIVE PERFORMANCE IN NATURALLY INFECTED HOLSTEIN HEIFERS