Coccidioidomycosis, also known as Valley fever, is an infectious fungal disease caused by Coccidioides fungi, most commonly C. immitis. C. immitis resides in soils of the Western Hemisphere, and is present in North, Central, and South America. The fungus is found in the southwestern United States, most commonly Southern California, Arizona, Utah, Nevada, New Mexico, Texas, and areas of Mexico.

Cases of coccidioidomycosis occur sporadically in horses, however it is thought that there is an association between increased outbreaks in horses during periods of drought and dry summers resulting from poor or delayed rainfall. Coccidioidomycosis can occur in horses in three different forms---as a respiratory infection, systemic infection, or as a localized subcutaneous infection. Clinical signs vary depending on the form and severity of the infection; however, common signs include significant weight loss, fever, and current or history of respiratory signs (such as increased respiratory rate, coughing, evidence of pleural fluid, adventitial lung sounds). Other signs observed in horses reflect the specific organ systems infected.

• Respiratory infection: Horses with pulmonary fungal infections often develop pulmonary granulomas, diffuse pneumonia, or pleuropneumonia. Most commonly observed clinical signs include nasal discharge, tachypnea (abnormally rapid breathing), coughing, difficulty breathing, and if a chronic condition, weight loss.
• Systemic infection: Systemic fungal infections can have an insidious progression, often presenting with nonspecific clinical signs. It can affect multiple organ systems and body cavities.
• Localized subcutaneous infection: This is the rarest manifestation of coccidioidomycosis in horses.

Transmission
Coccidioides fungi live in the soil and are transmitted through inhalation of contaminated dust and rarely through skin puncture. Coccidioides infection has been reported in over 64 different animal species, as well as in humans.

Incubation Period
Horses typically can develop signs within 1-4 weeks after exposure, however the majority don't develop signs until months to years after initial exposure.