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.
The disease was initially discovered in Rhode Island in the year 1893. Soon after, it was shown to have devastating effects on the turkey industry, especially in New England, dropping production from 11 million birds in 1890 to 6.6 million in 1900. However, improvements in turkey management have curbed the effects of this disease. It has since spread across the globe. It has been found in turkeys, chickens, guinea fowl, and other game birds. Bobwhite quail can also be infected.
Transmission and Pathology
The disease causing agent, Histomonas meliagridis, is transmitted in the eggs of the worm Heterakis gallinarum. Once in the environment, the eggs are carried by earthworms. When the worms are eaten and the eggs hatch in the ceca, the pathogen is released. Bird to bird transmission can also occur from cloacal drinking 
Visible signs of this disease are cyanosis of the head (hence, “blackhead”) and sulfur-yellow diarrhea. The pathogen causes lesions on the ceca and the liver. The ceca experience ulcerations, enlargement, and caseous masses start to form inside of them. The liver develops round, haemorrhagic, 1-2 centimeter oci that have caseous cores.
This disease can only be prevented by quarantining sick birds and preventing migration of birds around the house, causing them to spread the disease.
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