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Trichomonas gallinae has a worldwide distribution and is a common parasite of poultry, pigeons, and many other birds. In doves and pigeons, it causes a condition called ‘canker’ whilst in birds of prey the condition is known as ‘frounce’. It lives predominantly in the upper digestive tract and particularly the crop. In contrast, the related species Tetratrichomonas gallinarum tends to live in the lower digestive tract, caeca, and sometimes the liver.

The trophozoites of T. gallinae are usually ovoid in shape, 7–11 μm in length, and have four free flagella with a fifth recurving to form an undulating membrane (Figure 3.8). There is no cyst stage. Strains differ in their pathogenicity and pigeons, doves, and other members of the colubriformes are more badly affected than most other birds. In addition, raptors such as peregrine falcons (Falco peregrinus) and sparrowhawks (Accipiter nisus) that consume pigeons often suffer badly from T. tenax infections, and this can have consequences for their conservation (Dudek et al. 2018).

Avian trichomoniasis (trichomonosis) is an emerging disease in finches and other passerine birds in the UK, Europe, and Canada (Chi et al. 2013; Forzán et al. 2010). The first report of the problem was from the United Kingdom in 2005. Since then, it has become an ongoing epidemic as migrating birds spread the parasite across the country and throughout Europe. In the United Kingdom, this has resulted in a catastrophic decline in the number of greenfinches (Chloris chloris). In Europe, the epidemic is associated with a single clonal strain of T. tenax that is distinct from the strain causing extensive bird mortalities in America (Alrefaei et al. 2019).

Figure 3.8 Trichomonad parasites of animals. (a): Tritrichomonas foetus; (b): Trichomonas gallinae.

Source: Reproduced from Chandler and Read, (1961), © Wiley‐Blackwell.

The pathogenic strains of T. tenax induce lesions in the wall of the intestine and the parasites, then spread around the bird’s body and cause liver pathology similar to that of Histomonas meleagridis. Young birds are the worst affected, and whilst adults are often infected, they do not show evidence of disease – although they act as carriers of infection. In badly affected birds, necrotic lesions to the intestine and mouth can extend to the bones.

Pigeons often clash bills during social interactions and until they are 10 days old; young squabs feed by pushing their beaks into their parent’s mouth to feed on ‘milk’ held within the parent’s crop. This provides many opportunities for parasite transmission to occur. Trichomonas tenax probably spread from pigeons to finches through infected pigeons contaminating garden birdbaths. In addition, in the United Kingdom, there has been a marked rise in the population of wood pigeons, and this has probably increased the levels of contamination. The parasites can survive for up to 1 hour in water, so a single infected bird can potentially infect many others after using a popular birdbath. They can also survive on moist but not dry bird seed, so it is possible that in some circumstances bird feeders may act as an additional source of infection (McBurney et al. 2017).