Risk factors associated with testing positive for brucellosis and occurrence of abortion in high-yielding Holstein heifers

Abstract

This retrospective study aimed to estimate the seroprevalence of bovine brucellosis (bBR) in high-yielding Holstein heifers and identify risk factors associated with seropositivity to this disease. An additional aim was to assess the heifer-related risk factors for abortion. This study was carried out on a commercial dairy farm in northern Mexico (25° N) using 3848 heifers from 2018 to 2021. The seroprevalence of bBR was 30% (95% CI = 29–32%) for lactating heifers (card test). Multiple logistic regression models indicated that heifers with > 680 kg at calving had half the risk of being positive to bBR than heifers with < 680 kg at calving (22.7 vs. 36.1%). Heifers suffering from diarrhea during the growing period were 1.3 times more likely to be seropositive to bBR than heifers not diagnosed with this disease (34.1 vs. 29.4%; P < 0.01). The risk of being seropositive to bBR increased in heifers suffering from retained placenta compared with heifers not suffering from this reproductive disorder (OR = 2.46; 47.5 vs. 26.3%). Heifers vaccinated with two doses of commercial vaccines against brucellosis were associated (P < 0.01) with 2.2 times higher odds of being seropositive to bBR than heifers vaccinated only once. Heifers seropositive to bBR were 3.1 times more likely to abort than heifers seronegative to this disease (38.5 vs. 16.2%; P < 0.01). Heifers with body condition score (BCS) ≥ 3.5 at calving (OR = 0.3) had a protective role for seropositive to bBR. The likelihood of having an abortion was 3.1 times higher in heifers diagnosed with retained placenta than in those not having this reproductive disorder. It was concluded that brucellosis control programs should focus on age at first calving, retained placenta, and the number of brucellosis strain RB51 vaccinations applied to heifers. Increasing BCS was advantageous to achieve lower abortion rates.