Proof-of-concept trial in mature bulls prophylactically and therapeutically vaccinated with an experimental whole-cell killed <i>Tritrichomonas foetus</i> vaccine

<i>Tritrichomonas foetus</i> causes bovine trichomonosis, a venereal disease that reduces productivity in naturally mated cattle. Its high prevalence in Northern Australian herds underscores the need for a locally made strain-specific vaccine. This study developed and tested a whole-cell killed <i>T. foetus</i> vaccine using the Queensland isolate TfOz5 (vaccine strain) and TfOz-N36 (Northern Territory isolate) as the challenge strain. The heat-inactivated vaccine, adjuvanted with Montanide ISA 61 VG, was administered subcutaneously in 2 doses (5 × 10⁷ cells/dose) at a 1-month interval to mature bulls (<i>n</i> = 6) (4-7 years old), while controls (<i>n</i> = 6) (4-8 years old) received adjuvant with PBS. Bulls were experimentally challenged intrapreputially with live cultures of <i>T. foetus</i> at 2- and 6-months post first vaccination. A therapeutic trial with <i>T. foetus</i>-positive, persistently infected mature bulls (<i>n</i> = 10) (4-7 years old) used the same vaccine regime without the subsequent <i>T. foetus</i> challenges. The vaccine was found to be safe, causing only mild local reactions. The vaccine challenge experiment demonstrated similar duration of <i>T. foetus</i> positivity, confirmed by quantitative polymerase chain reaction (qPCR), compared to controls (94 vs. 106 days, <i>P =</i> 0.73). In the therapeutic experiment, 2/10 treated bulls tested negative for <i>T. foetus</i> at the end of the trial, while the remaining eight remained positive. Vaccinated bulls in both experiments showed significantly elevated serum anti-<i>T. foetus</i> IgG antibody levels, confirming the vaccine's immunogenicity. These findings demonstrate that the experimental vaccine is safe and capable of eliciting a specific immune response in mature bulls.