The incubation period of this disease is generally 15 to 20 days. Affected turkeys are listless and have a poor appetite; they hold their heads down and have ruffled feathers. The scalp often appears purplish blue or black, hence the name blackhead disease. As the disease progresses, affected turkeys become depressed and stand alone in a corner. When standing, their wings droop, their eyes are closed, and their heads are drawn into their bodies and tucked beneath their wings. They walk with a high-stepping gait, as if walking on stilts.
The disease usually has two forms: one is the peracute form, commonly characterized by blood-stained feces or entirely bloody feces. The other is the chronic form, in which affected turkeys pass sulfur-colored feces. This is rarely seen in chickens, although bloody cecal discharge has been observed. Chronic cases in larger turkeys generally involve emaciation. Around the twelfth day after infection, turkeys begin to lose weight; chickens rarely show clinical signs. Following infection with Histomonas, the total white blood cell count increases on the first day and peaks on the tenth day, reaching 7 ten thousand per cubic millimeter. The increase is mainly in heterophils, but during recovery, monocytes and eosinophils increase markedly. The total counts of lymphocytes, basophils, and red blood cells remain unchanged. The blood cell count returns to normal 21 days after infection.