Pneumonia is not a common disease in horses but there are several predisposing factors that can cause pneumonia to develop. Dr. Harold McKenzie from Virginia indicates most pneumonias develop secondary to upper respiratory viral infections such as influenza and herpes virus, and this is one reason to vaccinate your horse for these diseases. Transporting horses long distances is also a risk factor for pneumonia if the horses are not allowed to lower their heads in the trailer. Tying their heads up for long periods decreases clearance of bacteria organisms so if horses are hauled long distances, they should be taken out of the trailer every 3 hours at least to allow them to lower their heads. If possible, not tying their heads at all is better.
Horses that undergo anesthesia are also susceptible to pneumonia as are those that have recovered from an episode of choke. A more severe form of pneumonia can also develop if the infection escapes the lung and enters the space outside the lung; this is called pleuropneumonia. The diagnosis of pneumonia is based on finding coughing, labored breathing, fever, abnormal breath sounds and wheezing. Examining cells from the lungs and culturing lung fluid is helpful in making the diagnosis and determining correct treatment. Ultrasound of the chest is helpful to check for lung abscesses or fluid outside the lung in the case of pleuropneumonia. Certainly exercise restriction is required as are long-term antibiotics. Horses that are having a really difficult time breathing may respond to bronchodilators and some horses may need non steroidal anti-inflammatory drugs (NSAIDs) for pain and inflammation. These cases are difficult to treat so consult your vet before starting antibiotics if you feel your horse may have pneumonia.