What is the most common cause of pyothorax in dogs?

The most common cause of pyothorax in dogs is indeed associated with Pasteurella species, particularly following bite wounds or trauma. Pyothorax, characterized by the presence of pus in the thoracic cavity, often arises when bacteria enter through the pleura, commonly after penetrating injuries or from infections that accompany pneumonia. Pasteurella is a normal inhabitant of the oropharynx in dogs and cats, and it is frequently implicated in infections caused by bites from other animals, especially when there is an open wound.

This organism's ability to cause significant infections stems from its virulence factors, which can lead to a rapid inflammatory response and subsequent formation of pus in the thoracic cavity. The fact that it is often encountered in bite-related injuries contributes to its role as a prime pathogen in cases of pyothorax.

In contrast, while Bordetella, Escherichia coli, and Streptococcus can also cause respiratory and systemic infections, they are less commonly associated with pyothorax in the context of canine populations. Bordetella primarily causes kennel cough, E. coli is more often linked to gastrointestinal issues, and Streptococcus is generally involved in a range of other infections but doesn't have the same strong association