Isolation and characterization of Buchananella hordeovulneris from feline pyothorax.

A 1-year-old female mixed-breed cat was admitted to a veterinary hospital in Southern Brazil with tachypnea, low thoracic amplitude, restrictive breathing pattern, and cyanotic mucous membranes 2 days after elective castration surgery. Radiography revealed pleural effusion, and approximately 100-200 mL of fluid was collected by thoracocentesis. The reddish purulent exudate contained large numbers of yellowish-white granules with branched filamentous structures on cytological examination. The fluid was plated on blood agar and incubated under aerobiosis at 37 °C. On the third day of incubation, circular, dry, and opaque colonies, measuring < 0.5 mm in diameter, were observed. Their phenotypic and molecular characteristics were compatible with Buchananella hordeovulneris (basonym: Actinomyces hordeovulneris), a pathogenic actinomycete rarely detected in cats. Our findings indicate that B. hordeovulneris should be included in the differential diagnosis of pyothorax in cats together with Actinomyces spp. and Nocardia spp. Taxonomic confirmation of disease-causing microorganisms in animals is important to understand the course of infection and its association with disease epidemiology.