This is a fungal infection seen occasionally in horses in the arid Southwestern United States and northern Mexico. It is more common in dogs and humans. It earned the name "Valley Fever" after a large outbreak in the San Joaquin Valley, California.

Coccidioides immitus is a soil living fungus found in alkaline desert soils. Inhaled, it can cause widespread nodules in the lungs and lymph nodes. It can also cause disease in other locations. It is likely very dependent on a poorly functioning immune system for establishing disease.

Diagnosis requires blood testing for antibodies to the fungus. Chest x-ray can be helpful and show typical small nodules in the lung field.

It may be responsive to systemic antifungal medications, although reports suggest that once signs are evident, most horses die. A population of Prezwalski's horses stabled in the region was apparently very susceptible to the condition, indicating a lack of appropriate immune response to the organism.