Dyspnea, hyperpnea, somnolence and other signs of nervous system involvement, inappetence, emaciation, and increased thirst may be seen. In chicks or poults up to 6 wk, the lungs are most frequently involved. Airsacculitis in young mature turkeys is a leading cause of postmortem condemnation. Pulmonary lesions are characterized by cream-colored plaques a few mm to several cm in diameter; occasionally, mycelial masses may be seen within the air passages on gross examination. The plaques also may be found in the syrinx, air sacs, liver, intestines, and occasionally the brain. The encephalitic form is most common in turkeys. An ocular form, in which large plaques may be expressed from the medial canthus, has been seen in chickens and turkeys.

The fungus can be demonstrated by culture or by microscopic examination of fresh preparations. One of the plaques is teased apart and placed on a suitable medium, usually resulting in a pure culture of the organism. Histopathologic examination using a special fungus stain reveals granulomas containing mycelia. Pathogenicity of the isolate is confirmed by injecting it into the air sacs of susceptible 3-wk-old chicks.

Differential diagnoses include infectious bronchitis, Newcastle disease, infectious laryngotracheitis, mycobacteriosis, colibacillosis, Dactylaria infection, and nutritional encephalomalacia.