Equine pneumonia is a common respiratory illness that can be life-threatening in young horses. It can affect different parts of the horse’s lungs and may cause varied symptoms.

Pneumonia refers to an infection in the lungs in which tissues become inflamed, and the air sacs fill with fluid or pus. Horses affected by pneumonia can experience cough, fever, weakness and difficulty breathing.

Pneumonia can affect horses of all ages, but it is most prevalent in 2 and 3-year-old horses. It is also the most common cause of illness and death in foals between 1-6 months of age.

Susceptibility to pneumonia in foals and foals and weanlings may be linked to a decreased transfer of maternal antibodies and a delay in the production of the foal’s own antibodies. [1]

There are multiple causes of equine pneumonia including viral and bacteria infection; some forms are linked with better outcomes than others.

Horses living in overcrowded conditions or in barns with poor ventilation are more susceptible to developing this illness. [1][2] Fortunately, pneumonia is not contagious to other horses.

Equine pneumonia cannot always be prevented. Once present, this condition requires early and aggressive treatment for a successful outcome. [3]

Bacterial Causes of Equine Pneumonia

Equine pneumonia often occurs as a secondary bacterial infection during or directly after another illness when the horse’s immune system is already compromised.

A number of different bacteria can cause pneumonia. Likewise, there are different forms of equine pneumonia, depending on the exact cause.

The following bacteria have been implicated in equine pneumonia:

  • Klepsiella spp.
  • Klebsiella pneumoniae
  • Streptococcus zooepidemicus
  • Escherichia coli
  • Actinobacillus equuli
  • Rhodococcus equi

Klepsiella spp.

Pneumonia in humans can occur after mechanical ventilation when Klepsiella spp. colonizes on the ventilator or other anesthetic equipment.

Horses occasionally develop pneumonia after being on a ventilator as well. Both Klepsiella spp. and K. pneumoniae have been identified as the cause in these cases. [4]

Klebsiella pneumoniae

K. pneumoniae is also found in horses that develop pneumonia after strenuous exercise or a history of prolonged travel (shipping pneumonia). [5]

Streptococcus zooepidemicus

S. zooepidemicus is a common cause of lower respiratory disease in suckling and weanling horses. Additionally, this bacteria has been implicated in the following forms of pneumonia in horses of all ages.

  • Bronchopneumonia: a common type of pneumonia that causes inflammation of the alveoli in the lungs; bronchopneumonia can also be caused by pneumoniae and A. equuli subspecies haemolyticus. In foals under 6 months of age, R. equi may be the cause. [1]
  • Pleuritic pneumonia/Pleurisy: where the layers of tissue that separate the lungs from the chest wall become inflamed.
  • Pleuropneumonia: an infection of the lungs and the cavity between the lungs and underneath the chest wall (pleural space).

Preexisting damage in the lungs is typically needed for S. zooepidemicus to negatively impact a horse.

Respiratory viral infections and environmental stressors such as overcrowding, poor nutrition, preexisting disease, transport, or weaning are often associated with this bacterial cause of pneumonia. [1]

Rhodococcus equi

Rhodococcus equi pneumonia is frequently identified in foals aged 3 weeks to 6 months old. R. equi bacteria inhabit the soil, and a higher stocking density of mares and foals has been associated with an increased risk of this type of pneumonia. [5][6]

R. equi pneumonia occurs almost exclusively in foals. In fact, horses over the age of one year are rarely affected. If older horses are affected by this type of pneumonia, it’s usually because they have a compromised immune system.

R. equi can also cause disease in other parts of the body such as the eyes, bones, joints, GI tract, and intestinal lymph nodes. [7]

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Shipping Pleuropneumonia

Shipping pleuropneumonia is one of the most common forms of equine pneumonia. It occurs when bacteria and environmental irritants such as hay particles, dust, carbon, and exhaust chemicals invade the horse’s lower airway.

Because the horse’s head is often kept above the withers when transported by trailer, there is compromised mucocillary clearance. In other words, the inhaled particles can’t drain like they otherwise might. [9]

The longer a horse is transported, the higher the risk for developing shipping pneumonia. This form of pneumonia can be severe, affecting both lungs and the pleural cavity. [9][10]

Acute Interstitial Pneumonia

Acute interstitial pneumonia is a rare type of bacterial pneumonia that develops when an infection spreads from the pulmonary parenchyma to the pleural space.

Its exact cause has not been determined, but a wide range of bacteria have been detected in foals with acute interstitial pneumonia. It has also been associated with several different viruses. [8]

Acute interstitial pneumonia has a sudden onset of severe respiratory distress and often has a poor outcome.

It is notoriously difficult to resolve because the immune system can’t effectively reach the pleural space. Additionally, inflammatory cells and serous fluid accumulate in this area, which makes further bacterial infection probable. [3][6][8]


Foals that are septic are more prone to developing pneumonia as well. Sepsis is a syndrome defined by the development of a systemic inflammatory response after infection. E. coli is often the causative bacteria in this instance. [5]

Respiratory Viruses and Equine Pneumonia

Viral infections seldom cause equine pneumonia on their own. However, a virus can contribute to the development of pneumonia by making the respiratory system more vulnerable to a secondary bacterial infection.

Three common respiratory viruses associated with pneumonia include:

  • Equine herpes virus
  • Equine influenza
  • Equine viral arthritis

Equine influenza can also lead to bacterial pneumonia secondary to sepsis and severely affect neonatal foals. [2][10]

Acute interstitial pneumonia has been associated with influenza A infection. It often affects horses under 2 months of age. [11] This type of pneumonia is sporadic and rapidly progressive, characterized by acute respiratory distress and a high mortality rate.

Other Causes of Equine Pneumonia

In addition to secondary bacterial infections and associated viruses, there are several other risk factors that can lead to equine pneumonia.

Aspiration pneumonia

Aspiration pneumonia occurs when a horse experiences esophageal obstruction (choke) and liquid gets into the lungs. The longer a horse is obstructed, the greater their chances of developing aspiration pneumonia. [12]

A foal in utero can develop pneumonia from aspiration of fluids or meconium (the first feces that a newborn passes). Aspirating milk after birth can also lead to pneumonia.

Milk aspiration is usually related to a poor suckle reflex, weakness, or dysphagia associated with prematurity or neonatal maladjustment syndrome. However other deformities and conditions, as well as improper bottle feeding, can lead to milk aspiration. [2]

General Anesthesia

General anesthesia is another risk factor for developing pneumonia. In one study, 12.2% of 90 horses with pleuropneumonia had recently undergone anesthesia. [6] Anesthesia increases neutrophils and bronchoalveolar lavage fluid (BALF) while the horse is laying on its side.

Anesthesia can also cause partial or full collapse of a lung (atelectasis) and gas-exchange impairment. Additionally, horses may require mechanical ventilation during anesthesia, which can cause trauma, inflammation, and bacterial colonization. [4][13]

Granulomatous Interstitial Pneumonia

Granulomatous interstitial pneumonia is a rare form of pneumonia that occurs in older horses. Its known causes include bacteria, fungus, pa