Histomonas meleagridis is a parasitic flagellate that infects birds such as turkeys and chickens. It causes black head disease, which involves tissue damage to the bird's liver and cecum and can be fatal.
Histomonas has no cyst stage, and survives in the outside environment within the eggs of the cecal nematode Heterakis gallinarum. The flagellate is ingested by the nematode within the cecum of the bird, and migrates to the reprodutive organs to enter the nematode's egg. These eggs are then released into the envronment through the bird's feces, where they can survive for up to two years. The eggs can be either eaten directly by other birds through accidental consumption of soil or can be eaten by an earthworm intermediate host, which can then is consumed by a bird. Once inside another bird, the flagellate will hatch from the egg, multiply rapidly through binary fission, and invade the tissues of the new host, causing necrosis.
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