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. 
There are multiple causes of equine pneumonia including viral and bacteria infection; some forms are linked with better outcomes than others.
Equine pneumonia cannot always be prevented. Once present, this condition requires early and aggressive treatment for a successful outcome. 
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
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. 
K. pneumoniae is also found in horses that develop pneumonia after strenuous exercise or a history of prolonged travel (shipping pneumonia). 
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. 
- 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. 
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. 
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. 
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. 
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. 
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. 
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. 
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
Acute interstitial pneumonia has been associated with influenza A infection. It often affects horses under 2 months of age.  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 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. 
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. 
General anesthesia is another risk factor for developing pneumonia. In one study, 12.2% of 90 horses with pleuropneumonia had recently undergone anesthesia.  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. 
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).
In some cases, fungi can cause pneumonia in horses as well. This is known as mycotic pneumonia and it typically affects young horses, usually those under 4 months of age. 
According to studies, several factors are linked with the development of mycotic pneumonia. These include:
- Prolonged antimicrobial use with certain medications
- Salmonellosis with inflammation of the cecum and colon (ulcerative typhlocolitis)
- Previous hospitalization for displacement of the large colon
Symptoms of Pneumonia in Horses
The symptoms of bacterial pneumonia reflect the severity of the disease process. They may start out mild and gradually grow worse as the disease progresses. 
Likewise, different forms of pneumonia can have different symptoms. 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. 
- Bilateral yellow or cream-colored nasal discharge
- Foul-smelling nasal discharge
- Exercise intolerance
- Unwillingness to eat (anorexia)
- Abnormal airway sounds (tracheal rattling)
- Abnormal lung sounds
- 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. 
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. 
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 and biochemistry panels)
- Thoracic radiographs (X-rays)
- Trans-tracheal wash to collect sterile fluid sample from the lungs
- Culture and sensitivity to determine exact pathogen responsible in order to prescribe an effective antibiotic 
Thoracocentesis should be considered for horses with pleural effusion—a build-up of excess fluid between the layers of the pleura outside the lungs.
Treatment of Equine Pneumonia
Early recognition and treatment of pneumonia can help minimize the severity and the chance of mortality. 
Depending on the results of the culture, your veterinarian will prescribe an appropriate antibiotic 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: 
Erythromycin and rifampin are the most common antimicrobial combinations used to treat R equi infections. However S. equi is also sensitive to penicillin, chloramphenicol, and tetracycline. 
Other antibiotics used to treat pneumonia include b-lactams, trimethoprim-sulfa combinations, and erythromycin. On occasion, culture reveals another infection and additional antimicrobials may be needed. 
Management of Horses with Pneumonia
Along with antibiotic treatment, 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. 
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. 
Bronchodilators and anti-inflammatories are not usually needed and may actually be counterproductive by masking symptoms or encouraging owners to return the horse to exercise before healing is complete.
However, horses with severely laboured breathing (dyspnea) or airway hyperreactivity may benefit from bronchodilator therapy. 
Many horses may need other medications to stay comfortable. These might include non-steroidal anti-inflammatory medications (NSAIDs). Other pain relievers with opioids may be needed for severe cases. Oxygen supplementation may also be necessary for some horses.
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. 
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 resectioning the ribs. 
Complications from Pneumonia
Complications aren’t uncommon in horses with pneumonia. The most common complication is thrombophlebitis, inflammation of a vein which results from IV injection and indwelling catheterization. 
A condition known as polysynovitis can also occur in foals with pneumonia, affecting the tarsocrural, carpal, and fetlock joints. However other synovial structures may be involved as well. 
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. 
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. 
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, and minimizing risk of inhalation of pathogenic bacteria.
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. 
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. 
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