Final answer:
The most common cause of maxillary sinusitis in a horse is a tooth root abscess, likely caused by bacterial infection close to the maxillary sinus.
Step-by-step explanation:
The most common cause of maxillary sinusitis in a horse is a tooth root abscess:
Maxillary sinusitis in horses usually occurs when a tooth root abscess develops. The horse's upper cheek teeth have roots that are located very close to the maxillary sinus, and infection from these teeth can easily spread to the sinus. When a tooth root becomes infected due to bacterial invasion, the infection can lead to an abscess, causing inflammation and fluid accumulation within the sinus.
This scenario is much more likely than other options such as foreign bodies, brachygnathia, dentigerous cysts, or guttural pouch mycosis, which are less commonly implicated in sinusitis. It should be noted that while bacterial pathogens such as S. pneumoniae, H. influenzae, and M. catarrhalis are known to cause bacterial rhinosinusitis in general, the direct attribution of maxillary sinusitis in horses to these specific bacteria would require further specific diagnostic procedures.