Free-flying birds also may introduce an infection. Losses usually are low for chickens. Lesions of uncomplicated blackhead are confined to the ceca and liver, thus the reason for the synonymous term, enterohepatitis. The ceca are ballooned and walls may be thickened, necrotic and ulcerated. Peritonitis may be present if ulcers have perforated the ceca walls.
Lesions are yellowish to yellow-green and extend deeply into the underlying liver tissue.
Healing lesions may resemble those seen in visceral leukosis. Lesions observed in chickens frequently are atypical, as the liver lesions may be absent or less pronounced. Blackhead presenting typical lesions is diagnosed readily on the basis of the lesions. Atypical forms, particularly in chickens, must be differentiated from cecal coccidiosis and Salmonella infection in particular. Laboratory tests may be required for positive diagnosis in such cases.