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Millions of people are infected with Trichomonas vaginalis and some estimates suggest that it is the most common non‐viral sexually transmitted infection in the world (Kissinger 2015). However, prevalences vary considerably between countries, and it is not especially common in the United Kingdom with around 6,000 cases per year (Field et al. 2018). By contrast, a study of trichomoniasis in four African cities found prevalences ranging from 6.5 to 40% (Buvé et al. 2001). Despite its name, T. vaginalis, frequently infects men, but it causes them little harm and they usually clear the infection rapidly. Infected men can transmit the infection to women during sexual intercourse and rectal T. vaginalis infections can occur in men‐who‐have‐sex‐with‐men (Hoffman et al. 2018).

The parasite is ‘tear‐drop’ shaped with five flagellae emerging at the anterior end: four of these flagella are free whilst the fifth curves back to form a short undulating membrane that extends just over half the length of the cell (Figure 3.7). Trichomonas vaginalis expresses only one body shape, but its size varies considerably: the length can be 7–32 μm, whilst the width is 5–12 μm. In common with other diplomonads, there are no mitochondria, but they possess a row of hydrogenosomes alongside the axostyle. This arrangement distinguishes T. vaginalis from other trichomonad species. Hydrogenosomes probably derive from mitochondria but unlike the more degenerate mitosomes, they make ATP, as well as the hydrogen that gives them their name. Interestingly, although T. vaginalis only infects us, molecular evidence indicates that it originated in pigeons and subsequently switched hosts (Peters et al. 2020).

Figure 3.7 Trichomonas vaginalis. This protozoan parasite expresses only one body shape, but its size varies considerably: the length can be 7–32 μm, whilst the width is 5–12 μm. Five flagellae emerge at the anterior end: four of these flagella are free whilst the fifth curves back to form a short undulating membrane that extends just over half the length of the cell.

Although T. vaginalis infects millions of women every year, most of them remain asymptomatic. The protozoa feed on bacteria and sloughed off epithelial cells present in the reproductive tract. However, in some women the parasites induce a severe inflammatory response that manifests as a copious frothy white or greenish vaginal discharge. Infection during pregnancy is often associated with poor outcomes such as premature delivery and below‐average birth weight for the baby, although whether the parasite induces these effects is uncertain.