Vertical transmission, infection dynamics and risk factors for the occurrence of tick fever agents in beef cattle from a tropical region.

Abstract

This study investigated the vertical transmission, infection dynamics, and risk factors associated with tick fever agents in beef calves raised in a tropical region over a two-year period. A total of 98 cow-calf pairs were monitored from birth until slaughter (males) or first calving (females). Blood samples were analyzed by qPCR and iELISA for Anaplasma marginale, Babesia bovis, and Babesia bigemina. Vertical transmission was detected only for A. marginale (6.5%). At 20 days of age, qPCR detected A. marginale DNA in 85% of calves and Babesia spp. DNA in 60-80%, indicating early exposure and active infection. Tick burden was the only variable independently associated with both blood smear positivity and bacteremia, providing observational evidence of a potential epidemiological role of R. microplus in A. marginale infection under field conditions. Low Babesia parasitemia precluded comparative analyses between sexes. Serological prevalence exceeded 75%, indicating widespread exposure within the herd. Nevertheless, clinical cases of anaplasmosis occurred in 20-day-old calves, 8-9-month-old steers, and 13-14-month-old heifers, suggesting that seropositivity alone was not sufficient to prevent clinical disease. These findings reveal significant shifts in the epidemiology of tick fever under tropical conditions, characterized by early infection, widespread pathogen exposure, and the persistence of clinical disease, emphasizing the need to reassess current control strategies for tick fever in beef herds.