Pyothorax is caused by penetrating thoracic injuries

Etiology and effusion characteristics in 29 cats and 60 dogs with pyothorax (2010-2020)

Johnson, Epstein, Reagan

Background
Pyothorax, a collection of purulent fluid in the pleural cavity, arises from diverse etiologies in cats and dogs, influenced by species, environment, and geography. The study aimed to compare clinical, microbiologic, and etiologic findings between cats and dogs with pyothorax, hypothesizing that penetrating thoracic injuries (e.g., bite wounds, foreign bodies) are the primary cause and that pleuropneumonia is uncommon.

Methods
This retrospective study analyzed medical records from 2010–2020 at the University of California-Davis. Inclusion required contemporaneous cytology and microbiological cultures of pleural fluid from cats (n=29) and dogs (n=60) diagnosed with pyothorax. Clinical, cytologic, microbiologic, and imaging data were evaluated to determine probable causes. Antimicrobial susceptibility testing was also reviewed.

Results
Clinical Findings: Median ages were similar between species (cats 8 years; dogs 5.25 years). Pleural fluid in cats had higher cell counts than dogs but similar protein concentrations. Intracellular bacteria were more frequently detected in cats (93%) than in dogs (73%).

Etiology: Penetrating thoracic injuries were implicated in most cases (cats 76%; dogs 77%). Foreign body migration was more common in dogs, while bite wounds were more frequent in cats. Pneumonia was an uncommon cause in both species.

Microbiology: Cats had a higher median number of bacterial isolates per case (3) compared to dogs (1), with more frequent anaerobes in cats (73% vs. 45%). Polymicrobial infections were also more common in cats (76% vs. 50% in dogs).

Antimicrobial Resistance: Multidrug-resistant (MDR) bacteria were isolated in 6 cases (5 dogs, 1 cat). Aerobic isolates were generally susceptible to vancomycin and doxycycline, while anaerobes showed broader susceptibility to various drugs.

Limitations
Retrospective design constrained the study to available records, with variability in diagnostic approaches and limited histopathology data for definitive etiologic assignment. Exclusions due to incomplete cytology and culture data reduced the study population. Precise antibiotic treatment histories were not consistently available.

Conclusions
Penetrating injuries, primarily bite wounds in cats and foreign body migration in dogs, were the leading causes of pyothorax. Despite pre-treatment with antibiotics, microbial cultures often yielded actionable results, highlighting the importance of combined cytology and culture. The findings underline the need for both aerobic and anaerobic cultures and susceptibility testing to guide effective therapy. Prospective studies are recommended to refine etiologic assignments and explore preventive measures.

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