Development of a simulation model to evaluate the effect of vaccination against Tritrichomonas foetus on productive effieciency in beef herds

Objective - To develop a model to evaluate the effect of vaccination against Tritrichomonas foetus on reproductive efficiency in beef herds. Sample Population - A beef herd of 300 cows and 12 bulls (8 bulls ≤ 3 years old and 4 bulls > 3 years old). Procedure - The model was developed by use of data for various risk factors and vaccine efficacy. The reference herd was considered to be one in which T foetus had been diagnosed and bulls were tested for T foetus before the breeding season. Five thousand iterations were run for each of 13 simulations, with each simulation representing a separate combination of risk factors. Results - In all simulations, vaccination resulted in significantly higher calving incidence than nonvaccination. Shared grazing was found to be the most significant risk factor for a decrease in calving incidence attributable to T foetus infection, followed in importance by lack of testing before the breeding season and a higher proportion of old bulls. Combinations of risk factors contributed to a loss of income of up to 22%, some of which could be blunted by vaccination. Conclusions and Clinical Relevance - Highest calving incidence is achieved when all bulls are tested for T foetus before the breeding season and all bulls with positive culture results are culled. Avoiding all risk factors is better than vaccinating, but when this is not feasible for a given herd, the results of this simulation indicate that proper vaccination can decrease economic losses attributable to abortions caused by T foetus.