Endemic Persistence of Bovine Leukemia Virus in aHigh‐Producing Brazilian Dairy Herd: Seroprevalence Across Production Strata, Vertical Transmission, and Mortality Risk

Abstract

Bovine leukemia virus (BLV) is globally distributed and remains difficult to control. The infection is associated with substantial economic losses resulting from reduced productivity and early culling. Seroepidemiological studies conducted in Brazil have demonstrated that BLV prevalence rates range from 18.30% to 46.76%, with prevalence estimates reaching up to 90% in Goiás State, where the present study was performed. This study investigates the silent epidemiological persistence of BLV in a high‐producing dairy herd, highlighting category‐specific seroprevalence patterns, serological evidence suggestive of possible vertical exposure, and the association between BLV seropositivity and mortality. A total of 570 Girolando females were divided into the following groups for analysis of anti‐BLV antibodies: multiparous (I), primiparous (II), bred nulliparous heifers (III), and nonbred heifers (IV). We performed the analysis using a commercial enzyme‐linked immunosorbent assay. Our research group evaluated the impact of BLV seropositivity on milk yield (MY), somatic cell count (SCC), services per conception (SC), and calving interval (CI). In addition, serological evidence suggestive of possible vertical exposure and herd incidence rates were assessed during a 3‐month follow‐up period. Prevalence of BLV seropositivity was 59.82% (95% CI: 55.75%–63.77%). Cows in Group I showed a significantly higher likelihood of seropositivity than those in Group IV, with an approximately 8.6‐fold (95% CI: 4.96–15.03) increased risk of BLV seropositivity. Serological findings suggestive of possible vertical exposure were detected in 13.59% of the evaluated neonates, and the BLV incidence rate in the herd was 34.06 new seropositive cases over the 3‐month follow‐up period. Despite the high seroprevalence observed in the herd, no adverse effects were associated with productive or reproductive parameters. No significant association was observed between BLV seropositivity and lymphocytosis (p > 0.05). In contrast, mortality was significantly higher among BLV‐seropositive cattle than among seronegative cattle (RR = 1.41; 95% CI: 1.19–1.67). The impact of this disease on the lifespan of the herd should not be underestimated, particularly when considering mortality rates, incidence, and vertical transmission, which underscore the progressive nature of the virus.