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Bacterial Pneumonia in the Horse

By: Dr. Melissa R. Mazan

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Pneumonia refers to inflammation and consolidation of the lungs. When areas of the lung that are normally filled with air become filled with inflammatory secretions like mucous and pus, this is called consolidation.

In horses, pneumonia is usually characterized by colonization and infection of the bronchioles, air sacs, and parenchyma by bacteria. Rarely, fungi and parasites may also cause pneumonia, as well.

The most commonly involved bacteria are streptococcus species that are gram positive, which refers to the type of cell wall. We often find a variety of gram negative bacteria, which are bacteria that have a thin cell wall and carry endotoxins as well.

The most common route of infection is inhalation (meaning that the horse breathes in the bacteria). Horses can also, less commonly, develop pneumonia through a primary infection in the blood.

Pneumonia in horses is generally not contagious; that is, there is no rapid spread from individual to individual. However, the same risk factors that apply to one horse in the barn may well apply to others, so occasionally you may have multiple horses with pneumonia.

Horses have a tremendous respiratory reserve, so pneumonia may be well established by the time you see signs such as an increased respiratory rate, cough, or fever. Severe heaves, a non-infectious cause of respiratory difficulty, may have many of the same signs as pneumonia, but the treatments are very different. It is important to pursue a diagnosis and treatment promptly so that your horse has the best chance of recovery.

Risk Factors

  • A common misconception for many years is that pneumonia in horses arises from a head cold. This led to the use of vaporized oils for the treatment of the nasal passages and throat. However, viral upper respiratory infection – Equine Influenza Virus and Equine Herpes Virus, in particular – can impair the respiratory system's ability to clear bacteria, thus making it easier for pneumonia to become established.

  • Physical stress – especially endurance rides and racing. This type of exercise has been shown to suppress the ability of some of the white blood cells to fight infection.

  • General anesthesia. Horses are so heavy that even with the best of padding, their lungs tend to experience partial collapse (atelectasis) due to the weight of their own bodies pressing on their lungs when they are down for long periods of time. Lungs with atelectasis are more susceptible to bacterial infection. When horses are anesthetized, they often have an orotracheal tube placed. This tube goes from the mouth to the trachea, and allows the anesthetist to regulate the horse's breathing. Bacteria from the mouth can contaminate the trachea during intubation. The pressure of the tube can cause inflammation of the trachea, and consequent difficulty in clearing the respiratory system. Finally, after anesthesia, horses sometimes have difficulties swallowing properly, which increases the risk of aspiration pneumonia.

  • Exercise induced pulmonary hemorrhage (EIPH, or bleeding) may also contribute to pneumonia. The blood that ends up in the lungs is a good growth medium for bacteria, so some veterinarians will routinely treat horses that have bled with antibiotics. There is no proof that this prevents pneumonia.

  • Other systemic illnesses that may contribute to immune system suppression, for example, colic, blood parasites, and gastrointestinal parasites.

  • Long-distance transport. The physical stress of transport can suppress the immune system. Although there is not a definition of what constitutes long distance, most veterinarians consider trips greater than 6 to 8 hours to be potentially stressful. Studies have also shown that when horses' heads are tied up for even a few hours, the bacterial load in the trachea and bronchi increases. This is because the horse is not physically able to drain his airways. Also, the common practice of hanging hay bags in trailers or vans actually serves as a good vehicle for respirable particles of dust and hay – which bring bacteria with them.

  • Overcrowded barns and poor barn hygiene increase the contact between horses and subsequently the spread of disease. Poor barn hygiene causes increased levels of ammonia, which can impair the lung's ability to clear itself of bacteria.

  • Esophageal obstruction (choke) can cause horses to inhale particles of food and saliva, along with their accompanying bacteria, as they try to swallow.

  • Indwelling intravenous catheters may become infected, and the infection can spread to the lungs.

  • In some horses, pneumonia can develop into pulmonary abscesses, or pleuropneumonia. With pleuropneumonia, the infection spreads to the pleural space, which surrounds the lung within the chest cavity. Lack of response to treatment may make your veterinarian suspect that one of these complications has arisen.

  • Syringing medications through the mouth may cause the horse to inhale some of the medication along with his own saliva, thus contributing to an aspiration pneumonia.

  • Smoke inhalation

  • Prolonged cold stress, such as exposure to cold rain, snow, and wind.

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