Lesions of uncomplicated blackhead are confined to the cecae and liver, thus the reason for the synonymous term, enterohepatitis. The cecae are ballooned and walls may be thickened, necrotic and ulcerated. Peritonitis may be present if ulcers have perforated the ceca walls. Livers are swollen and display circular depressed areas of necrosis about one-half inch in diameter. Smaller lesions coalesce to form larger ones.
Lesions are yellowish to yellow-green and extend deeply into the underlying liver tissue.
Healing lesions may resemble those seen in visceral leukosis. Blackhead diagnosis is made readily on the basis of the lesions. Atypical forms, particularly in chickens, must be differentiated from cecal coccidiosis and Salmonella infections in particular. Medications may interfere with atypical lesions. Laboratory tests may be required for positive diagnosis in such cases.