Comparison of Coccidioides immitis serological antibody titres between forms of clinical coccidioidomycosis in horses

A retrospective study was performed to determine if there is an association between serological Coccidioides immitis antibody titres (IgG) and form/severity of coccidioidal disease in horses, and to identify trends in survival and treatment success based on the form of the disease. Data were obtained on horses with positive serological titres tested at the Coccidioidomycosis Serology Laboratory, School of Medicine, University of California, Davis from 1981 to 2004. Thirty-nine cases in which a diagnosis of coccidioidomycosis had been made were selected for inclusion. Six distinct categories were identified including abortion (n = 6), miliary/interstitial pneumonia (n = 6), pneumonia with thoracic effusion (pleural or pericardial) (n = 11), disseminated (n = 10), osteomyelitis (n = 3) and external abscessation (n = 3) both without pulmonary disease.Statistical differences in titre distribution were found between the abortion category and the pulmonary category (P = 0.003), the abortion category and pneumonia with thoracic effusion (P = 0.001), the abortion category and disseminated disease (P = 0.001), and the pulmonary form and pneumonia with effusion (P = 0.001). The other categories had overlapping titre results. Higher serological antibody titres seemed to be associated with a poorer prognosis for survival. Categories with the highest titres, disseminated (geometric mean titre = 104) and pneumonia with thoracic effusion (geometric mean titre = 226), were overwhelmingly fatal (19/21 known deaths) due to severe clinical disease. The categories with lower titres, abortion (geometric mean titre = 4), bone involvement only (geometric mean titre = 13) and cutaneous (geometric mean titre = 5), had a better survival rate (10/12 known survivors) and less severe clinical disease. Measurement of serological titre may be a useful diagnostic aid in establishing form and severity of disease and thus inform prognosis.