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

Tritrichomonas foetus 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 T. foetus 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 (n = 6) (4-7 years old), while controls (n = 6) (4-8 years old) received adjuvant with PBS. Bulls were experimentally challenged intrapreputially with live cultures of T. foetus at 2- and 6-months post first vaccination. A therapeutic trial with T. foetus-positive, persistently infected mature bulls (n = 10) (4-7 years old) used the same vaccine regime without the subsequent T. foetus challenges. The vaccine was found to be safe, causing only mild local reactions. The vaccine challenge experiment demonstrated similar duration of T. foetus positivity, confirmed by quantitative polymerase chain reaction (qPCR), compared to controls (94 vs. 106 days, P = 0.73). In the therapeutic experiment, 2/10 treated bulls tested negative for T. foetus at the end of the trial, while the remaining eight remained positive. Vaccinated bulls in both experiments showed significantly elevated serum anti-T. foetus 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.