ID01 - Castellaniella spp. as a potential emerging pathogen in dogs

Abstract:

Background: Castellaniella is a genus of Gram-negative bacteria within the family Alcaligenaceae, known for its ability to reduce nitrate. These organisms have been isolated from various environments, demonstrating adaptability to highly human-impacted areas. Although generally not considered pathogenic, Castellaniella spp. have been associated with mass mortality events in Daurian pikas and broilers, as well as one case in a harbor porpoise. To date, no disease caused by Castellaniella spp. has been reported in companion animals.

Hypothesis/

Objectives: To report and describe Castellaniella sp. infection in dogs.

Animals: In late 2024, two unrelated client-owned dogs from the same geographic region were presented with pyothorax and suffered a sudden clinical decline, resulting in death within 24 hours.

Methods: Diagnostic evaluations included fluid analysis, bacterial culture, 16S rRNA gene amplification, and necropsy.

Results: Cytologic examination of the exudative effusion identified intracellular short rods, predominantly within macrophages. Bacterial culture yielded pure Gram-negative colonies but failed to determine the organism's species. Amplification of the 16S rRNA gene showed 99-100% similarity to two species within the genus Castellaniella. Necropsy findings did not reveal potential inoculation routes.

Conclusions and Clinical Importance: These are the first documented cases of Castellaniella sp. infection in dogs. The monomorphic bacterial population and its unusual presence within macrophages prompted further investigation. Pending genomic analysis may confirm whether the organisms belong to the same species and help assess the risk of a potential outbreak. The severity and rapid progression of Castellaniella spp. infections underscore the critical need for heightened awareness among veterinary professionals.