Wikipedia
Blackhead disease (also known simply as blackhead) is a commercially important avian disease that affects chickens, turkeys and other poultry birds. The disease carries a high mortality rate and primarily affects the liver and cecum. It is a form of histomoniasis which is transmitted by the protozoan parasite Histomonas meleagridis. The protozoan is in turn transmitted by the nematode parasite Heterakis gallinarum. H. meleagridis resides within the eggs of H. gallinarum, so birds ingest the parasites along with contaminated soil or food. Earthworms can also act as a paratenic host. A characteristic symptom of the infection is the development of cyanotic (bluish) discoloration on the head, giving rise to the common name of the disease, "blackhead".
Poultry (especially free-ranging) and wild birds commonly harbor a number of parasitic worms with only mild health problems from them. Turkeys are much more susceptible to getting blackhead than are chickens. Thus, chickens can be infected carriers for a long time because they are not removed or medicated by their owners, and they do not die or stop eating/defecating. H. gallinarum eggs can remain infective in soil for four years, a high risk of transmitting blackhead to turkeys remains if they graze areas with chicken feces in this time frame.
The most common symptom of blackhead disease is yellow, watery bird droppings. To reduce the spreading of the disease, sick birds must be removed and their litter changed.