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Pneumonia is an infection of the lungs and is generally classified as to cause: bacterial, viral, fungal, parasitic, or aspiration in type. Stressed, old, malnourished, or debilitated horses, along with very young foals are the horses most likely to be affected by this disease. The bacteria S zooepidemicus is the most common cause of pneumonia, but many other bacteria can produce adult pneumonia.

Pneumonia in foals is very serious and often fatal in foals up to eight months of age. Failure to receive colostrum, overcrowding, cold and damp quarters, inadequate vaccination and deworming programs, recent viral respiratory illness, or any physical condition that weakens the foal may lead to reduced resistance to pneumonia.

As in humans, one of the first indications that a horse has pneumonia are abnormal lung sounds of a moist crackling nature that can be heard through a stethoscope.


  • Weakness and lethargic behavior
  • Lack of appetite
  • Respiratory distress
  • Depression
  • High fever
  • Diarrhea
  • Rapid pulse
  • Cough
  • Nasal discharge
  • Watery eyes
  • Loss of gum color due to poor oxygen intake


In adult horses, the most common cause of pneumonia is the bacteria Streptococcus zooepidemicus. In young foals, the most common bacterial cause is also S Zooepidemicus with Rhodococcus equi causing an especially severe illness that is associated with contagious outbreaks.

Bacterial pneumonia sometimes follows viral infections or stressful events that lower the natural lung defense mechanisms. Bacteria are carried into damaged airways or dust may be inhaled from feed, bedding, or working surfaces.

Viral pneumonia is usually caused by Equine herpes virus 2, which is common in the general horse population. Foals usually acquire viral pneumonia in utero and become weak and lethargic shortly after birth.

Once weakened, bacterial infection can occur and many foals succumb during the first seven days. Fortunately, a vaccination program during the mare's pregnancy will prevent most cases of foal pneumonia.

Parasitic pneumonia is not common among horses, but the typical case involves Dictyocaulus arnfieldi or Parascaris equorum. Diagnosing parasitic pneumonia is difficult because of the necessity of identifying larvae in the trachea.

Fungal pneumonia usually occurs when the horse's respiratory system is weakened and the horse inhales fungal spores from moldy, poor quality hay or other sources.


Prevention of pneumonia in foals is best achieved by making sure the pregnant mare is up-to-date on vaccinations, including Rhinopneumonitis vaccines containing EHV-4. Brood mares should continue to be vaccinated with DHV-1. Seek advice from a knowledgeable veterinarian as to how often vaccines need to be administered.

Keeping horses in the best possible physical condition is important since pneumonia strikes horses that are older, run-down, weak, malnourished, or stressed.

Clean, dry, well-ventilated stables and barns are important to the horse's well-being and will help prevent pneumonia. In colder areas, making sure that horses have proper shelter and, in some cases, heating the stall area may help in preventing respiratory problems.


Treatment of pneumonia should begin as soon as early symptoms appear since severe infection can cause scarring of the lungs and loss of performance ability.

The horse should be moved to a warm, dry area and given plenty of fresh water. Humidifying the air will help with breathing.

Cough suppressants should not be given because the horse needs to cough to clear the airways of accumulated secretions.

Injectable, oral, or nebulized antibiotics and the use of bronchodilators in the feed or via an equine mask system will aid in getting rid of accumulated secretions.

Most, if not all, common bacteria are sensitive to penicillin, which may be started while waiting the results of antibiotic sensitivity tests. The antibiotic treatment should continue for one to two months, depending on the horse's condition.

Immune stimulants may be prescribed to decrease recovery time and protect against a relapse. The horse will need to be rested for several weeks after recovery. Advice from a veterinarian is important in all cases of pneumonia.

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EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.