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

Pneumonia refers to inflammation of the lung tissue. Upper airways and lining of the lungs (pleura) may also be involved. 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 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, bacterial, and fungal infections, immune-mediated disease and mechanical airway dysfunction. Many factors need to be considered when assessing prognosis and formulating a treatment plan.

Environment and overall health play an important role in mitigating the risk of pneumonia. Horses living in overcrowded conditions or in barns with poor ventilation are more susceptible to developing this illness. [1][2]

Environment and overall health play an important role in mitigating the risk of pneumonia. However, equine pneumonia cannot always be prevented. Once present, this condition requires early and appropriate treatment for a successful outcome. [3]

Types of Equine Pneumonia

The following are classifications based on anatomic areas of the lung that are affected:

  • 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).

Pneumonia can also be described by their underlying pathogenic cause, such as bacterial, viral, or fungal.

Bacterial Causes of Equine Pneumonia

Bacterial pneumonia can be primary meaning it is the original cause of illness.

However, in adult horses, bacterial pneumonia is most commonly secondary, meaning the bacterial infection becomes present after the horses lungs have been compromised by another inciting factor.

The following bacteria have been implicated in adult equine pneumonia:

  • Staphylococcus aureus
  • Klebsiella pneumoniae
  • Streptococcus zooepidemicus
  • Escherichia coli
  • Actinobacillus equuli

These bacteria can be isolated from the upper airway of many horses and do not cause a problem unless they make their way into the lower respiratory tract due to compromised airway immunity.

The most common isolate from adult pneumonia is Strep equi subsp zooepidemicus and it is often isolated in conjunction with equine herpes virus or equine influenza virus (viral pneumonias).

K. pneumoniae is also found in horses that develop pneumonia after strenuous exercise or a history of prolonged travel (shipping pneumonia). [5] Staphylococcus aureus is also opportunistic and is particularly deleterious as it is commonly multi-drug resistant. [21]

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 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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