Histomonas meleagridis

Histomonas meleagridis is species of parasitic protozoan that infects chickens, turkeys, peafowl, quail and pheasants, causing blackhead disease, infectious enterohepatitis, or histomoniasis. H. meleagridis can infect many birds but it is most deadly in turkeys.

Etiology
H. meleagridis is most often transmitted in embryonated eggs of Heterakis gallinarum, a cecal nematode, but sometimes it is transmitted directly by contact with infected birds.  H. gallinarum larvae are found inside three species of earthworms, and it is through eating the earthworms that most birds get infected. The earthworm eats infected nematode eggs, which can remain dormant in the soil for years. After a few days of ingestion, Histomonads are released from the Heterakis nematode larvae in the ceca, and begin to replicate rapidly in the cecal tissues. They migrate to the submucosa and muscularis mucosae and cause severe necrosis. The parasites then move to the liver, through the vascular system or through the peritoneal cavity. There they cause more nercotic lesions. Histomonads interact with other gut organisms, such as bacteria and coccidia, and depend on these for full virulence.