First report of Castellaniella spp. infection in dogs and the genomic evidence of a novel species

AIMS: This study reports the first documented cases of Castellaniella spp. infection in dogs, describing associated clinical and pathological findings and characterizing a novel species within this genus. METHODS AND RESULTS: Pleural effusions from two dogs presenting with acute respiratory distress and systemic illness were evaluated via cytology, bacterial culture, and next-generation sequencing. Both cases exhibited neutrophilic-macrophagic inflammation with intracellular Gram-negative rods, primarily within macrophages. Bacterial culture failed to identify the organisms. Metagenomic analysis identified organisms belonging to the genus Castellaniella in both cases. In case 2, an unclassified Castellaniella species was detected, suggesting the presence of a previously undescribed species within the genus. Both dogs died shortly after presentation, and necropsy and histopathology findings were described. CONCLUSIONS: Castellaniella spp. warrant consideration as potential emerging pathogens in domestic animals, challenging their previous classification as non-pathogenic environmental bacteria. The identification of a novel species also underscores the genus's genetic diversity and adaptive potential.