Bovine leukemia virus infection in neonatal dairy calves: evidence of vertical transmission and management-associated factors.

Bovine leukemia virus (BLV) infection in calves may contribute to early dissemination within dairy herds. This study aimed to determine the BLV infection prevalence in pre-colostral and neonatal calves and its association with management-related factors in dairy farms in Uruguay. Two studies were conducted. Study 1 was a retrospective analysis of archived pre-colostral blood samples from 75 calves from one farm. Samples were analyzed by quantitative real-time PCR (qPCR) and ELISA, and previous ELISA and qPCR results from dams of calves positive by qPCR and/or ELISA were reviewed. Study 2 was a cross-sectional study of blood samples from 542 neonatal calves aged 1-30 days from 30 dairy farms, analyzed by qPCR, with management data collected by questionnaire. In Study 1, 6.7% (5/75) of calves were qPCR-positive, and 4.0% (3/75) were ELISA-positive; one calf was positive by both assays. All seven dams of calves positive by qPCR and/or ELISA were ELISA-positive, while five were also qPCR-positive. In Study 2, 8.9% (48/542; farm-level bootstrap 95% CI 3.7-15.1%) of calves were qPCR-positive, within-herd apparent prevalence ranged from 0% to 54.5%, and 43.3% (13/30) of farms had ≥1 qPCR-positive calf. In Study 1, qPCR positivity before colostrum ingestion was consistent with prenatal transmission. Exploratory multivariable analysis suggested possible farm-level associations between herd size, maternal vaccination against neonatal calf diarrhea, rotation of calf-rearing areas, and type of liquid feed and qPCR-positive calves. These findings suggest that BLV infection can be detected during the neonatal period and support investigation of management and biosecurity practices.