Histomonas meleagridis is a flagellate parasite of poultry, particularly turkeys. It affects the caecum, liver and occasionally the kidneys and spleen. It measures between 4 and 30 µm and has no obvious flagellum.
H. meleagridis causes infectious enterohepatitis or ‘blackhead’ in turkeys. The term blackhead refers to the head of an infected turkey that characteristically turns a dark, almost black colour. As the organisms do not form cysts, they do not live long once they are passed out with the host’s faeces. However, these parasites can become enclosed inside the eggshells of a caecal poultry nematode Heterakis gallinarum upon which they are able to reach new hosts when the eggs are ingested by feeding birds.
H. meleagridis is responsible for high mortalities in poultry, especially young birds. When the embryonated eggs of H. gallinarum are fed to young turkeys, a high mortality from blackhead of 60 – 70 % can be recorded. Some infected birds do not show any clinical symptoms but apparently serve as reservoirs of infection to others.
In the invasive stage, Histomonas meleagridis is amoeboid, feeding by phagocytosis. There follows a round, quiescent, vegetative stage, in which the parasite feeds by secreting proteolytic enzymes that diffuse from the parasite and digest host tissues. Digested products are taken into the protozoa by pinocytosis and probably also by diffusion.
Blackhead is one of the most serous diseases of turkeys. Its effects are less severe in chickens, which usually recover after a mild reaction. Resistant chickens and turkeys may harbour the protozoan, Histomonas meleagridis, and the worm, Heterakis gallinarum, at the same time. Such birds are reservoirs of infection for all new chicks. Chickens and turkeys should not be raised together. Even raising young turkeys with older turkeys is bound to expose the young ones to infection.
Symptoms of histomoniasis are similar to those of many other poultry diseases. The birds become listless, their wings droop, while the eyes are fully or partly closed most of the time. Young birds die within two or three days and older birds may survive for one to two weeks before succumbing to death. The birds may recover spontaneously after a few days. Cleanliness and isolation are vital in the prevention of the infection.