Relationship between the serological status towards enzootic bovine leukosis and reproductive parameters in specialized dairy herds in Costa Rica

An analytical longitudinal study was conducted to determine the cumulative incidence, and the relationship of the serostatus towards enzootic bovine leukosis virus (EBLV) and reproductive parameters, in 26 dairy farms in Costa Rica using the VAMPP Bovine information system. A first sampling was carried out in 2006- 2007 in animals older than 6 months old; then, in 2009, a second sampling was performed in all cows seronegative in 2006-2007. The overall and farm- specific cumulative incidence were calculated; then, comparisons of these were performed by race and parity number, using the Chi-square test. The relationship of EBLV status with the calving interval (CI), services per conception (SPC) and days open (DO) by parity number and race groups, was determined using the Kruskal-Wallis test. The overall incidence was 21.8% (173/794), with 38.5% of farms with incidences higher to 40%. The seroconversion rate in cows with 1-2 parities (23.5%) was different than those with 6 or more parities (p=0.03). Holstein x Jersey cows had the highest incidence (33.3%), followed by Holstein (27.1%). According to the farmers, in the 26 farms (100%) they used an individual needle and a single rectal palpation glove per animal; however, 53.8% of farms showed incidences above 15%. In general, there was a tendency to lower CI in seronegative cows with regard to seropositives and seroconverters, especially in seropoisitives of 1-2 lactations (p=0.017) and 3-5 lactations (p=0.038). Regardless parity number or race, the seropositives and seroconverters required more SPC (p=0.031); this same tendency (not significant) was observed in DO. Thus, it was found some relation of the status about EBLV with CI and SPC, but not with DO, although there was no statistical significance in some of the differences found, with better parameters in seronegative cows.