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This parasite infects a wide range of birds but for some reason it is particularly pathogenic in young turkeys, in which untreated infections are usually fatal. It lives within the lumen of the caecum and the liver parenchyma and causes the disease histomoniasis. Infected birds lose condition, become listless and suffer from anorexia, poor growth and sulphur‐yellow diarrhoea. The neck and head often become black – and hence the infection is commonly known as ‘blackhead disease’. The pathogenicity often links to concurrent infections with other parasitic protozoa, such as Coccidia spp., and pathogenic bacteria such as Escherichia coli and Salmonella typhimurium.

The morphology of H. meleagradis is variable (i.e., it is pleomorphic) and depends upon the organ that it infects and the stage of the disease. For example, the form found free within the lumen of the caecum (and in culture) is amoeboid, 5–30 μm in diameter, with a clear outer ectoplasm a more granular endoplasm and with one or two flagella emerging from close to the nucleus. The invasive form that lives within tissues is also amoeboid, but it is smaller (8–15 μm) and the flagellum is absent.

Histomonas meleagradis has an unusual means of transmission that involves becoming incorporated within the eggs of Heterakis gallinarum. Heterakis gallinarum is a common nematode parasite that lives in the caecum of many wild and domestic birds. The nematode has a direct life cycle in which its eggs pass out in the faeces of its host and, after embryonation, are ingested by another bird in which they hatch and initiate an infection. Earthworms can act as paratenic hosts for the nematode. Interestingly, both H. meleagradis and H. gallinarum both require interactions with bacteria to establish themselves in their bird host. The infection and subsequent pathology associated with H. meleagradis therefore depends upon a complex interplay between the protozoan, a nematode, and microbial flora (Bilic and Hess 2020).