Vertical transmission and epidemiological dynamics of bovine tick fever in beef cattle in a Brazilian feedlot system.

Bovine tick fever is endemic in Brazil and responsible for significant economic losses and reduced animal welfare in beef cattle. Understanding disease dynamics, particularly the role of vertical transmission, is crucial for advancing epidemiological knowledge, especially for a disease for which few studies have been conducted in beef cattle. This study evaluated the frequency of tick fever agents across different categories of beef cattle in a feedlot system in Minas Gerais, Brazil, and investigated the occurrence of vertical transmission in newborn calves. A total of 597 animals were assessed, distributed across five categories: heifers upon arrival (AH), pregnant heifers (pH), newborn calves (NC), weaned calves (WC), and finishing cattle (FC). Diagnostics included blood smears, packed cell volume, indirect immunofluorescence (IFAT) and molecular assays for Anaplasma marginale, Babesia bovis, and Babesia bigemina. Blood smears detected A. marginale across all categories, whereas Babesia spp. were absent. IFAT indicated high seroprevalence (>80%) for A. marginale and B. bovis in adult groups (AH, pH, and FC). The FC and PH groups had serological rates of 64% and 70%, respectively, for B. bigemina, while the WC group had the lowest serological prevalence for all three agents (27%, 28%, and 65% for Anaplasma marginale, Babesia bovis, and B. bigemina, respectively). Passive immunity transfer was classified as good to excellent in over 90% of newborn calves. Molecular analysis of NC group revealed vertical transmission rates of 41.3% for A. marginale, 0.8% for B. bigemina, and no vertical transmission of B. bovis. Additionally, molecular screening of WC showed prevalence rates of 14.4% for A. marginale, 2.3% for B. bigemina, and 6.1% for B. bovis. These findings highlight the complexity of tick fever epidemiology in beef cattle and underscore the role of vertical transmission, particularly of A. marginale, in pathogen maintenance within herds.