A dog with pleural effusion and fever shows non-acid fast filamentous organisms in pleural fluid cytology. What is the likely cause?

The presence of non-acid fast filamentous organisms in the pleural fluid cytology of a dog with pleural effusion and fever strongly suggests an infection caused by Actinomyces. This genus is known for producing filamentous, branching rods that can be observed in cytological specimens. Actinomyces species are typically classified as anaerobic and are often associated with chronic infections that can result in pleural effusion, especially in cases where they spread from sites like the oral cavity or gastrointestinal tract.

In this scenario, the clinical signs of fever and the cytological finding reinforce the likelihood of Actinomyces infection. The characteristics of the organism—non-acid fast and filamentous—are key identifiers that distinguish Actinomyces from other filamentous organisms, as they do not retain the acid-fast stain used to identify mycobacterial organisms, such as Mycobacterium, which would be a consideration if acid-fast rods were present.

Understanding the nature of Actinomyces infections can aid in treatment decisions, as they typically require prolonged antibiotic therapy, often with agents like penicillin. In contrast, the characteristics and typical presentation of other possible organisms, such as Nocardia or Staphylococcus, would not align with the non