Fatal Pulmonary Phaeohyphomycosis Associated with Large Vessel Thrombosis in a Dog.

Phaeohyphomycosis is an opportunistic fungal infection caused by dematiaceous fungi and is considered uncommon in dogs, particularly when associated with visceral or systemic involvement. Pulmonary disease as a primary site of infection is rarely reported in veterinary medicine and is often associated with an unfavorable outcome. This report describes a fatal case of pulmonary phaeohyphomycosis in a dog, characterized by severe granulomatous pneumonia, vascular invasion by pigmented fungal hyphae, and the development of large vessel thrombosis. Histopathological examination revealed septate, pigmented hyphae consistent with dematiaceous fungi associated with an intense granulomatous inflammatory response. Although molecular analysis by polymerase chain reaction was unsuccessful due to the absence of amplifiable DNA in archived FFPE tissue, the clinicopathological correlation and histopathological findings were sufficient to support a diagnosis consistent with phaeohyphomycosis. Severe pulmonary inflammation likely contributed to vascular endothelial injury, resulting in pulmonary hypertension and thrombosis of major veins. This case highlights the diagnostic and clinical challenges associated with phaeohyphomycosis in dogs and emphasizes the importance of considering this infection in the differential diagnosis of chronic or progressive respiratory diseases accompanied by systemic complications. Furthermore, it reinforces the relevance of histopathology and comprehensive clinicopathological evaluation when molecular confirmation of the etiological agent is not achievable.