Equine Pneumonia: Causes, Symptoms & Treatment

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 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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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 equine pneumonia are:

• 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][19]

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.

Shipping Pleuropneumonia (Shipping Fever)

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 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][20]

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]

Sepsis

Sepsis is defined as life threatening organ dysfunction caused by dysregulated host response to infection. ^[19]

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]

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

There are several causes of aspiration pneumonia such as complication of esophageal obstruction, complications of birth, or laryngeal paralysis.

When a horse experiences esophageal obstruction (choke) liquid and/or food particles can get 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.

This is more likely if they have deformities such as a cleft palate or a mandibular prognathism. Improper bottle feeding can also increase the risk of aspiration. ^[2]

Contact your vet if you notice any mouth abnormalities in the foal, the foal has milk coming out the nose, the foal has trouble suckling, or makes gurgling sounds of respiration or swallowing.

General Anesthesia and Other Iatrogenic Causes

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 in the 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. Horse position and monitoring equipment used in anesthesia will help reduce the risks of complications.

Rarely, horses require mechanical ventilation during anesthesia, which can increase the risk of trauma, inflammation, and bacterial colonization. ^[4][13]

Other iatrogenic (examination or treatment induced) causes of aspiration pneumonia include tracheal intubation with a nasogastric tube, eating, medicating or drinking too soon after sedation and aspiration of particulate during dental procedures.

Veterinarians are trained to look for risk factors and to use techniques that limit these risks.

Granulomatous Interstitial Pneumonia

Granulomatous interstitial pneumonia is a rare form of pneumonia that occurs in older horses. Its known causes include bacteria, fungus, parasites, inhaling silica dust (silicate pneumoconiosis), and tumours which have spread throughout the body (disseminated neoplasia).

Granulomatous lesions on the lungs characterize this form of pneumonia with central necrosis and calcification. The lesions are similar to those reported in human sarcoidosis. ^[14][15]

Mycotic Pneumonia

In some cases, fungi can cause pneumonia in horses. This is known as mycotic pneumonia and it typically affects young horses under 4 months of age. ^[11]

According to studies, several factors are linked with the development of mycotic pneumonia. These include:^[11]

• Prolonged antimicrobial use with certain medications
• Salmonellosis with inflammation of the cecum and colon (ulcerative typhlocolitis)
• Previous hospitalization for displacement of the large colon

Existing conditions such as inflammatory airway disease can also predispose a horse to develop pneumonia, so management of these conditions is crucial. ^[10]

Symptoms of Pneumonia in Horses

The symptoms of bacterial pneumonia vary with the severity of the disease process. They may start out mild and gradually grow worse as the disease progresses. ^[6]

Likewise, different forms of pneumonia can have different symptoms. For example, symptoms of bronchopneumonia may at first be vague and include mild fever or a slight increase in respiratory rate.

However, as this form of pneumonia progresses, symptoms could include rapid breathing (tachypnea), lack of appetite (inappetence), lethargy, and fever. ^[1]

Clinical Signs

Other possible clinical signs of pneumonia may include: ^{[3][6][7][10][14]}

• Bilateral yellow or cream-colored nasal discharge
• Foul-smelling nasal discharge
• Depression
• Exercise intolerance
• Cough
• Unwillingness to eat (anorexia)
• Abnormal airway sounds (tracheal rattling)
• Abnormal lung sounds
• Grunting
• Pawing
• Moving with a stiff forelimb gait
• Weight loss
• Respiratory distress
• Pain on palpation of thoracic wall

In addition to the above signs, horses with shipping pneumonia may stand with their elbows camped out, lie down more frequently than normal, and be reluctant to move. ^[10]

Horses will often stand with a lower and more stretched out head and neck position.

It is critical to call your veterinarian when symptoms are mild and not wait for serious signs to appear. If pneumonia is left untreated it can result in chronic (long lasting) respiratory complications or even death.

Diagnosing Pneumonia in Horses

If you suspect that your horse has pneumonia, a veterinary examination is key to determining the extent and severity of the illness and the best treatment plan.

Pneumonia is often diagnosed based on the horse’s history, physical examination findings, blood work, and other diagnostic tools. However, it may also be diagnosed based on clinical signs alone. ^[1]

Diagnostic tools and tests for equine pneumonia often include:

• Rebreathing examination in which a plastic bag is held over the nose so the horse must breathe the same air in and out, causing the carbon dioxide level to increase. This makes the horse breathe deeper and movement of air can then be heard in the periphery of the lung.
• Blood work (CBC, SAA, and biochemistry panels)
• Thoracic radiographs (X-rays)
• Thoracic ultrasound
• Bronchoalveolar lavage (BAL) to collect cells and mucous from the bronchioles
• Using BAL fluid to determine the exact pathogen(s) responsible in order to prescribe an effective antibiotic ^[3][10]

Thoracocentesis should be considered for horses with pleural effusion—a build-up of excess fluid between the layers of the pleura outside the lungs.

This simple and inexpensive procedure can help differentiate between septic pleural effusion and non-septic effusion which other diseases may cause. ^[1][16]

Treatment of Equine Pneumonia

Early recognition and treatment of pneumonia can help minimize the severity and the chance of mortality. ^[6]

Depending on the results of the culture, your veterinarian may prescribe an appropriate antibiotic or combination of antibiotics to treat any bacterial infection.

Antibiotic resistance is a growing problem; another antimicrobial drug may need to be prescribed if the first one doesn’t work. Resistance has occurred most frequently with the following antibiotics: ^[4]

• gentamicin
• chloramphenicol
• tetracycline
• trimethoprim-sulfamethoxazole

Historically, erythromycin and rifampin were the most common antimicrobial combinations used to treat R equi infections.

Recent years have suggested the use of newer generation macrolide antibiotics. Tulathromycin is one of the newer antibiotics and has the advantage of being injected once every 7 days which improves ease of use in young foals. ^[22]

Other antibiotics used to treat pneumonia include b-lactams, trimethoprim-sulfa, and erythromycin. On occasion, culture reveals another infection and additional antimicrobials may be needed. ^[1]

In addition to antibiotics other important medications such as anti-inflammatories (eg. flunixine), steroids (eg. dexamethasone), mucolytics (eg. dembrexine) may be prescribed at your veterinarians discretion.

Management of Horses with Pneumonia

Along with medications, horses with pneumonia need plenty of rest. Even after symptoms improve, they may need an additional 2-4 weeks to fully recover.

Exercising your horse before they have fully recovered may cause injury to the lungs and respiratory inflammation. ^[5]

Ill horses should be fed a low-dust diet, such as chopped forage or pelleted feed. Hay may need to be moistened or soaked to reduce dust, allergens and irritants.

Feed your horse from floor level to encourage drainage of secretions, but raise feed slightly off the ground to reduce inhalation of dust.

Stall bedding should also be a low-dust material such as wood shavings, cardboard, or newspaper. ^[5]

Additionally, lower limb cryotherapy or ice application can be beneficial to prevent laminitis in horse with signs of systemic inflammation or endotoxemia. ^[7]

Oxygen therapy is likely available at high end equine referral centres. Hyperbolic oxygen therapy chambers can be used to increase the oxygen gradient in the lungs and improve prognosis. ^[23]

Surgical Interventions

Treating mild to moderate pneumonia usually results in a successful outcome, but in more severe cases, treatment can be challenging and permanent damage may occur in the lung tissue. In some cases, pneumonia may affect the horse’s long-term performance. ^[10]

Surgical intervention can be considered with some forms of pneumonia if medical treatment is unsuccessful. This is most helpful in chronic cases with localized pockets of debris.

Thoracotomy is a procedure used to access the pleural space of the chest to remove the cause of pneumonia.

It is usually performed while the horse is standing. This surgical procedure involves an incision through the intercostal region of the ribs or by resection of the ribs. ^[6]

This procedure has some high inherent risks and is only recommended in the hands of an experienced surgeon.

Complications from Pneumonia

Complications aren’t uncommon in horses with pneumonia. The most common complication is thrombophlebitis, inflammation or infection of a vein which results from IV injection and indwelling catheterization. ^[4]

A condition known as polysynovitis (septic arthritis) can also occur in foals with pneumonia. It most commonly affects the tarsocrural, carpal, and fetlock joints. However other synovial structures may be involved as well. ^[7]

Antimicrobial-associated diarrhea and laminitis are other possible complications to be aware of. ^[6]

Prognosis

Your horse’s prognosis will depend on the type and cause of pneumonia, as well as his age and overall health. Pneumonia is often more deadly for foals than it is for older horses, but survival rates depend on the causative factor, how soon treatment is started, possible complications, as well as several other factors.

Survival rates have increased over the years for R. equi pneumonia. Before macrolide treatment was available, survival rates were as low as 20%. Now, survival rates are at least 60%, even with more severe cases. ^[2]

The prognosis for survival in weanlings with pneumonia is around 70-80%. Death is more likely for foals with severe respiratory signs, severe thoracic radiographic changes, lameness, and/or joint effusion. ^[1]

Preventing Equine Pneumonia

Though not always preventable, neonatal pneumonia is often linked with management practices. This includes vaccination of pregnant mares, ensuring transfer of passive immunity through colostrum.

It is also important to minimize risk of inhalation of pathogenic bacteria. This includes ensuring good air quality, clean water bowls and minimal nose to nose contact with horses of unknown health and vaccination status.

Vaccination against respiratory viruses helps prevent immunosuppression due to previous infections.

As the foal matures, using an appropriate deworming and vaccination program can further reduce their susceptibility of developing pneumonia. ^[2]

There are currently no effective vaccines for use in foals against bacterial pneumonia, though there have been attempts to develop one for R. equi pneumonia. ^[2]

Other prevention strategies for equine pneumonia include:

• Minimizing stress, especially when introducing new horses to the herd or transporting horses over long distances;
• Being aware of pre-existing conditions which could make your horse more susceptible to pneumonia. These include PPID or Cushing’s disease, Equine Metabolic Syndrome, Recurrent Airway Obstruction, and Inflammatory Airway Disease; and
• Appropriately managing pre-existing illnesses. ^[10]