Systematic Review of Vaccine Strategies Against Tritrichomonas foetus Infection in Cattle: Insights, Challenges, and Prospects.

Tritrichomonas foetus is a protozoan parasite that causes bovine trichomonosis (also referred to as trichomoniasis) resulting in substantial economic loss in extensive grazing systems. The parasite colonises the reproductive tracts of both male and female cattle, being asymptomatic in males but causing early reproductive failure in infected females. This systematic review aimed to examine research manuscripts describing the development of T. foetus vaccines, the strategies employed, and the immune response associated with T. foetus infection. A systematic review was conducted using indexed sources on Scopus, PubMed, Web of Science, and Embase to search for published vaccine development studies in English utilising either experimental or commercial T. foetus vaccines for bovine trichomonosis. Search terms (cattle, bull, heifer, vaccine, immune response, and Tritrichomonas foetus) were entered into the indexed sources and with no limit was set for the year of publication. Among 374 studies imported for screening, 96 were duplicates, and 255 were excluded for irrelevancy as these studies did not meet the inclusion criteria (report original data, vaccine tests in cattle, and used either commercial or experimental vaccines that incorporated antigens from T. foetus). A further five studies were excluded after full-text review as either the publication described a different target pathogen or the full text was not in English. Eighteen studies met the inclusion criteria and were subjected to data extraction using the Covidence platform. Studies included in the systematic review reported three vaccine strategies, including subunit, cell fraction, and whole-cell killed vaccines. This review considers the design, rationale, and results of each of the 18 studies to provide a comprehensive overview of the current knowledge and to inform future research agendas. While whole-cell killed vaccines are most prevalent, they have shown superior efficacy compared to subunit vaccines studied to date. One study using a purified fractions of T. foetus cell membranes as vaccine demonstrated higher efficacy and higher calving rates compared to the whole-cell killed vaccine. All of these methods require cell culture growth of T. foetus which can be challenging compared to the production of recombinant proteins. At this stage, no T. foetus recombinant antigens have been reported. Advances in understanding the parasite's genome, pathogenesis, host-parasite interactions, and host immune responses to T. foetus will provide opportunities for the development of novel vaccine strategies for bovine trichomonosis.