Pleuropneumonia in Horses: Signs, Causes, and Emergency Treatment

Pleuropneumonia is a severe infection that affects both the lungs and the pleural space, which is the thin area between the lungs and the chest wall. In horses, it usually starts as bacterial pneumonia and then spreads to the pleura, causing painful inflammation and fluid buildup around the lungs. That fluid can become infected, making breathing harder and recovery more complicated.

This condition is often called shipping fever when it develops after transport, but travel is not the only trigger. Horses of many ages and uses can be affected. The disease can progress quickly, and some horses become systemically ill with dehydration, endotoxemia, or inflammatory complications.

Because the pleura is sensitive, horses with pleuropneumonia may show signs of chest pain as well as respiratory distress. They may stand stiffly, resist turning, grunt when moving, or seem anxious and uncomfortable. Even a horse that only looked mildly off after a trip can worsen over a day or two.

Pleuropneumonia is an emergency because delayed treatment lowers the chance of a smooth recovery. Early recognition, imaging, drainage when needed, and targeted antibiotics have improved outcomes, but this is still a serious disease that needs prompt veterinary care.

Most equine pleuropneumonia cases are caused by bacteria that normally live in the horse's upper airway and then gain access to the lower respiratory tract. A common isolate is Streptococcus equi subsp. zooepidemicus, but mixed infections can occur, including aerobic and anaerobic bacteria. Anaerobic involvement may be associated with a more guarded outlook in some horses.

Long-distance transport is one of the best-known risk factors. During travel, horses often keep their heads elevated for long periods, which reduces normal airway drainage and clearance. Stress, dehydration, dust exposure, poor ventilation, and fatigue can further weaken respiratory defenses and make bacterial infection more likely.

Other risk factors include recent viral respiratory disease, strenuous exercise, general anesthesia, and anything that interferes with normal mucociliary clearance. Some horses aspirate material into the lungs, while others develop infection after a period of immune suppression or airway irritation.

Pleuropneumonia is usually not thought of as a highly contagious disease spreading horse-to-horse in the way some viral respiratory infections do. Instead, it is more often a severe bacterial complication that develops when normal airway defenses fail. That said, any horse with fever or respiratory signs still needs prompt isolation guidance and examination from your vet until the cause is clearer.

Your vet will start with a careful history and physical exam, including temperature, heart rate, breathing rate, lung sounds, and signs of chest pain. A recent history of transport is an important clue, but it is not required for diagnosis. Horses may have reduced or absent breath sounds in areas where fluid has collected, and some show obvious pleural pain during movement or palpation.

Thoracic ultrasound is one of the most useful tests because it can identify pleural fluid, fibrin strands, adhesions, lung consolidation, abscesses, and the safest site for thoracocentesis. Chest radiographs may also help, especially after some fluid has been drained, because large effusions can hide deeper lung changes on x-rays.

Bloodwork helps your vet assess inflammation, hydration, and complications. Depending on the stage of disease, horses may have leukopenia or neutropenia early, then later show leukocytosis, hyperfibrinogenemia, increased serum amyloid A, low albumin, or anemia of chronic inflammation.

Definitive diagnosis and treatment planning often include sampling pleural fluid and collecting a tracheal aspirate for cytology and bacterial culture. These tests help confirm sepsis, identify the organisms involved, and guide antibiotic selection. In many cases, both diagnosis and treatment happen at the same time because draining infected fluid can immediately improve comfort and breathing.

Prevention focuses on protecting normal airway defenses, especially during transport. If your horse is traveling, allow regular opportunities to lower the head so mucus and debris can clear from the airway. Good trailer ventilation, reduced dust exposure, sensible trip planning, and attention to hydration also matter.

Long trips deserve extra management. Work with your vet on travel plans for horses with a recent respiratory illness, poor fitness, or a history of shipping fever. Monitoring temperature before travel and for several days after arrival can help catch problems early, when signs may still be mild.

Keeping horses current on respiratory vaccinations may reduce the risk of viral infections that can set the stage for secondary bacterial pneumonia. Clean transport equipment, avoid overcrowding, and minimize unnecessary stress when possible.

Even with good prevention, some cases still happen. The most practical step for pet parents is early recognition: if a horse seems dull, febrile, off feed, or uncomfortable after transport, contact your vet promptly rather than waiting for obvious respiratory distress.