Histomoniasis (or histomonosis) is a disease of birds, particularly of chickens and turkeys, due to parasitic infection of a protozoan, Histomonas meleagridis. It is a form of blackhead disease in poultry. The parasite specifically infects the cecum and liver. Symptoms of the infection include depression, reduced appetite, poor growth, increased thirst, sulphur-yellow diarrhoea, listlessness, and dry, ruffled feathers. The head may become cyanotic (bluish in colour), hence the common name of the disease, blackhead; thus the name 'blackhead' is in all possibility a misnomer for discoloration. It is particularly highly fatal in poultry, and less in other birds. Currently, no prescription drug is available to treat this disease.