Immunogenicity and efficacy of an Australian whole‑cell killed Tritrichomonas foetus vaccine in experimentally challenged young bulls.

Tritrichomonas foetus is a protozoan parasite responsible for bovine trichomonosis, a venereal disease that causes reproductive failure in cows while leaving bulls asymptomatic. Currently, no vaccine is available in Australia. This study aimed to develop and assess the safety, efficacy, and immunogenicity of a locally sourced T. foetus whole-cell killed vaccine in young bulls. The TfOz5 Queensland isolate was used as the vaccine strain, and the TfOz-N36 Northern Territory isolate served as the challenge strain. The heat-inactivated vaccine, adjuvanted with Montanide ISA 61 VG, was administered subcutaneously to 30 tropical composite T. foetus-naïve bulls in two doses (5 × 10⁷ cells/dose) one month apart. Control bulls (n = 30) received a mock formulation of adjuvant and phosphate-buffered saline. Bulls were challenged with live T. foetus cells directly into the prepuce at two and six weeks post-second vaccination with 1.37 × 10⁷ and 2.70 × 10⁷ cells, respectively. The vaccine was well tolerated causing only mild local reactions with no significant differences in weight or average daily gain between groups. Vaccinated bulls exhibited a significantly shorter infection duration (14 vs. 40 days, p = 0.0044) and lower parasite DNA burden compared to controls. Vaccination induced a strong systemic IgG response, whereas mucosal IgA responses remained low in both groups. These findings demonstrate that the Australian whole-cell killed T. foetus vaccine was well tolerated under the monitored study conditions and was effective in reducing infection duration and parasite DNA burden in young bulls.