Final answer:
The most common cause of maxillary sinusitis in a horse is a tooth root abscess. Other possible causes include foreign bodies, guttural pouch mycosis, and brachygnathia.
Step-by-step explanation:
The most common cause of maxillary sinusitis in a horse is a tooth root abscess. When a tooth becomes infected, bacteria can spread to the maxillary sinus, causing inflammation and infection. This can occur when there is decay or trauma to the tooth, allowing bacteria to enter the tooth root and surrounding tissues.
For example, if a horse has a broken tooth or a tooth with deep decay, bacteria can invade the tooth root and lead to an abscess. The abscess can then put pressure on the maxillary sinus, leading to sinusitis.
Other possible causes of maxillary sinusitis in a horse include foreign bodies that become lodged in the sinus, guttural pouch mycosis (a fungal infection), and brachygnathia (a condition where the upper and lower jaws do not align properly).