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This is a sexually transmitted parasite of cattle, and although it is also found as a sexually transmitted infection in other animals (e.g., horses), it is not usually pathogenic in them. The trophozoite of T. foetus is a pear‐shaped organism 10–25 μm long and 3–15 μm wide with four anterior flagella, three of which are free and one flagellum curves backwards to form an undulating membrane that extends the length of the body and then projects freely from the posterior apex (Figure 3.8). There is no cyst stage although they form pseudocysts in response to iron depletion. It is uncertain whether the pseudocyst stage plays an important role in parasite transmission.

In bulls, the parasites are usually found in the preputial cavity and cause little harm although there is sometimes inflammation that causes painful urination and unwillingness to copulate. In cows, the parasite causes more serious pathology. The infection begins with vaginitis and then spreads to the uterus where they can cause early abortion and permanent sterility. The parasite remains in the lumen and does not penetrate the underlying tissues.

There are increasing reports of T. foetus causing large bowel diarrhoea in domestic cats in the UK, USA, and parts of Europe. The parasites isolated from cats are morphologically identical to those from cattle, and there are only very minor differences in their DNA sequences (Yao and Köster 2015). Therefore, the current consensus is that they represent two isolates of a single species. However, in cats, transmission of the parasite occurs through faecal–oral contamination.