Successful management of pneumonia and pyogranulomatous cranial mediastinitis due to Coccidioides species infection with fluconazole in a 3‐year‐old horse

Summary A 3‐year‐old Quarter Horse gelding originating from Texas presented for fever and inappetence. An encapsulated, mixed echogenic cranial mediastinal mass visible ultrasonographically in the left and right fourth intercostal space that displaced the heart caudally was identified. Histological evaluation of needle biopsies obtained under general anaesthesia revealed a severe pyogranulomatous inflammatory process associated with intralesional round, 15–25 μm in diameter, thick double‐contoured cell wall fungal structures. Sequencing of products obtained from DNA extracted from the paraffin‐embedded biopsy sample and a panfungal PCR assay targeting the large ribosomal unit region had a 99.1% identity to either Coccidioides immitis or C. posadessii. Bronchoscopy revealed purulent exudate in the right primary bronchus. Cytological evaluation of bronchial alveolar lavage fluid (BALF) contained 96% moderately degenerative neutrophils and 4% macrophages. Fungal structures were not appreciated in the BALF. However, the mediastinitis could have resulted from inhalation and dissemination of arthroconidia through lymphatics or directly from the lung parenchyma. Quantitative immunodiffusion of serum for the presence of coccidioidal IgG antibodies yielded a titre of 1:256 suggestive of disseminated coccidioidomycosis. After oral fluconazole for 24 months, the horse's general health and athletic performance had returned to normal and the mediastinal mass was not appreciated sonographically.