The bull is the important link in the transmission of the disease in the herd. The parasite is present on the mucosa of the penis and in the crypts of the prepuce of the bull. These crypts are downfoldings of the epithelium which forms the lining of the prepuce and they offer an environment that is conducive to the proliferation of the trichomonas organisms. An important fact in the prevention and control of the disease is the poor development of prepucial crypts in bulls less than four years of age. Trichomoniasis causes a very mild inflammation of the affected tissues in the bull with virtually no clinical signs of disease.
Transmission then occurs from sexual contact between animals.
Bulls can passively transmit the organism when a noninfected bull serves an infected cow and, soon after, serves a noninfected cow, however infection rates by this means of transmission are low. A more common scenario occurs with the introduction of a mature bull in the herd, who is infected by a cow who actively transmits the organism which establishes itself in the prepucial crypts where it evidently stimulates little immune response on the part of the bull. This animal can then infect many cows in the herd and subsequently other herd bulls. The organism can be found in the cervix, vagina and uterus of cows after both experimental and natural infection. The part of the tract where long-lasting infections occur has not yet been identified.