Pleuropneumonia is an infection of the lungs and the sac surrounding them (the pleural space). In most instances, it develops as a result of bacterial infection or penetrating chest wounds. Pleuropneumonia is more likely to occur in horses already weakened by previous viral respiratory infection, long-distance transportation (more than 500 miles), general anesthesia, or strenuous exercise. These factors can impair lung defense mechanisms, allowing bacteria to invade. Race and sport horses are particularly at risk. The majority of horses with pleuropneumonia are athletic horses younger than 5 years old.

Signs of pleuropneumonia in horses include fever, depression, lethargy, and poor appetite. Pleural pain is specific to this condition and causes short strides, guarding, flinching on tapping of the chest, and shallow breathing. Horses with pleural pain may have an anxious facial expression and are reluctant to move, cough, or lie down. Their gait may be stiff or stilted, and some horses will grunt in response to chest pressure or examination. Bad breath or foul-smelling nasal discharge indicates bacterial infection. Rapid, shallow breathing is due to pleural pain and restricted ability of the lungs to expand because of secretions accumulated in the pleural cavity.

In horses with a recent onset of pleuropneumonia, laboratory findings reflect the presence of infection in the blood. Horses with more longstanding disease often have anemia. During physical examination, the veterinarian can hear a lack of breath sounds in parts of the chest and abnormal lung sounds (often crackles) in others. Heart sounds may be muffled or absent. Chest ultrasonography is an effective tool for examining secretions in the pleural cavity and should be used to assist with removal of fluid from the pleural cavity, so the veterinarian can determine the best site for maximal drainage. The fluid removal serves 2 purposes: it provides a sample for use in diagnosis, and it relieves the pressure caused by the fluid. Chest x-rays are used after pleurocentesis to evaluate lesions in the lungs as well as to detect air in the pleural cavity (pneumothorax). Bacterial culture is performed to identify the bacteria involved.

Monitoring of affected horses is important to check for fluid in the pleural cavity, determine whether drainage is effective, identify isolated fluid pockets, and assess lung disease. The amount and characteristics of the pleural fluid determine whether continual drainage is needed. Medical therapy includes appropriate broad-spectrum antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), pain relievers, and supportive care. Some horses do not eliminate the pleural infection even after weeks to months of antibiotics and drainage. If needed, an opening in the chest can be created to allow the veterinarian to remove materials and dead lung tissue; however, this technique is used only when the condition has become chronic.

The outlook for horses with pleuropneumonia has greatly improved over the past 20 years due to early recognition, advancements in diagnostic testing, and aggressive therapy. The survival rate is reported to be as high as 90% by some investigators with a 60% chance to return to athletic performance. Even in cases where a chest tube has been necessary, it is still possible for the horse to return to athletic condition. Prompt treatment is important, because if appropriate therapy is delayed by more than 48 hours, infection can worsen and lead to a poor outcome.

Last full review/revision July 2011 by Bonnie R. Rush, DVM, MS, DACVIM; Neil W. Dyer, DVM, MS, DACVP; Joe Hauptman, DVM, MS, DACVS; Ned F. Kuehn, DVM, MS, DACVIM; Stuart M. Taylor, PhD, BVMS, MRCVS, DECVP; Wendy E. Vaala, VMD, DACVIM; Maureen H. Milne, BVMS, MVM, DCHP, MRCVS