Bit & Bridle

Equine Pneumonia & Foal Pneumonia

Other Names

  • Equine Lung Infection
  • Bronchopneumonia
  • Rhodococcus Pneumonia (specific form in foals)
  • Streptococcal Pneumonia
  • Foal Pneumonia / Pneumonia in Foals
  • Pleuropneumonia (when the pleural space is also involved)

What is Equine Pneumonia?

Equine pneumonia is an infection and inflammation of the lung tissue. It is a significant cause of morbidity and mortality in horses of all ages, but it is particularly dangerous in foals under six months of age, where it remains one of the leading causes of death. In adult horses, pneumonia often develops secondary to other stressors — transport, general anaesthesia, or viral respiratory infections — that impair the lungs’ normal defence mechanisms.

The condition may be confined to the lung parenchyma itself (pneumonia), or may extend to involve the pleural space — the cavity between the lung and the chest wall — resulting in pleuritis or pleuropneumonia. Pleuropneumonia carries a significantly more guarded prognosis and requires more aggressive management.

Pneumonia in horses tends to affect the cranioventral (front-lower) regions of the lung, as bacteria aspirated from the upper airways settle in the most gravity-dependent areas. Recognising the early signs and acting promptly is critical — equine pneumonia can deteriorate rapidly, and delayed treatment significantly worsens outcomes.

Causes

Equine pneumonia can be caused by bacterial, viral, or fungal pathogens — or combinations:

  • Bacterial causes:
    • Streptococcus equi subsp. zooepidemicus — the most common bacterial cause of pneumonia in adult horses; a commensal organism of the upper respiratory tract that becomes pathogenic when lung defences are compromised.
    • Rhodococcus equi — the most important cause of pneumonia in foals aged 1–6 months. A soil-dwelling organism, highly resistant and capable of forming abscesses within the lung. Particularly problematic on stud farms where the organism accumulates in soil over time.
    • Actinobacillus spp., Pasteurella spp., and Klebsiella spp. are other significant bacterial pathogens.
    • Streptococcus equi subsp. equi — the cause of strangles — can occasionally cause “bastard strangles” with internal abscessation including pulmonary involvement.
  • Viral causes: Viral respiratory infections (equine influenza, equine herpesvirus types 1 and 4, equine rhinitis viruses) typically affect the upper airways but can predispose to secondary bacterial pneumonia by damaging mucociliary clearance mechanisms.
  • Aspiration: Inhalation of food, liquid, or gastric content — particularly during or after nasogastric intubation, in horses with choke (oesophageal obstruction), or in weak newborn foals — can lead to aspiration pneumonia.
  • Transport pneumonia (“shipping fever”): Long-distance transport is a well-recognised risk factor. Head elevation during transport (tying the horse’s head up), stress, dust inhalation, and fatigue impair lung clearance. A common cause of pneumonia in horses transported over many hours.
  • Fungal causes: Less common in the UK; seen in immunocompromised horses.

Symptoms and Signs

Adult horses:

  • Fever (often 39°C or above)
  • Depression and reduced appetite
  • Nasal discharge — initially clear or watery, becoming mucopurulent (cloudy, yellow-green) as bacterial infection establishes
  • Cough — may be dry and harsh early on, becoming more productive
  • Increased respiratory rate and effort; flared nostrils; abdominal component to breathing in severe cases
  • Dull breath sounds or crackles/wheezes on auscultation of the chest
  • Exercise intolerance

Foals (Rhodococcus equi pneumonia):

  • Onset is often insidious — foals may appear bright and active until the disease is advanced
  • Cough, nasal discharge, elevated respiratory rate
  • Fever may be intermittent and easily missed
  • Rapid breathing even at rest in advanced cases
  • Reduced nursing frequency, failure to thrive, progressive weight loss
  • Rhodococcus equi can also cause extrapulmonary disease including enteritis, joint infections, and eye disease

Diagnosis

  • Clinical examination and auscultation: Abnormal breath sounds — decreased or absent over areas of consolidation, or crackles and wheezes — help identify affected lung regions. Pleuritis produces a pleural friction rub.
  • Blood work: Elevated white cell count (neutrophilia), raised fibrinogen and serum amyloid A — indicators of systemic inflammation. Hypoxaemia may be present on blood gas analysis in severe cases.
  • Radiography: Chest X-rays are essential for assessing the extent and pattern of lung involvement. Cranioventral consolidation is typical of bacterial pneumonia. Lung abscesses appear as well-defined opacities. Radiography requires specialist equipment and is most readily available at equine hospitals.
  • Ultrasonography: A highly valuable and accessible tool. Ultrasound of the thorax can identify pleural fluid, consolidated lung tissue, and abscesses abutting the chest wall. Widely used in practice as it does not require hospital equipment.
  • Bronchoalveolar lavage (BAL) or tracheal wash: Collection of respiratory secretions for cytology and culture. Provides the definitive bacteriological diagnosis and sensitivity data to guide antibiotic selection.
  • PCR testing: Nasopharyngeal swabs or BAL samples can be tested for viral pathogens and Rhodococcus equi by PCR.

Treatment

Treatment must be prompt and appropriately targeted:

  • Antibiotics: Systemic antibiotics are the cornerstone of treatment. Choice depends on the likely pathogen:
    • For adult bacterial pneumonia: penicillin plus gentamicin, or potentiated sulphonamides, pending culture results.
    • For Rhodococcus equi in foals: the combination of azithromycin (or clarithromycin) plus rifampicin is the current standard of care. Prolonged courses of 4–12 weeks are typically required due to the organism’s intracellular location.
  • NSAIDs: Flunixin meglumine or meloxicam to reduce fever, improve comfort, and address pleural pain if pleuritis is present.
  • Intravenous fluids: For dehydrated or systemically compromised horses.
  • Bronchodilators: Clenbuterol or other bronchodilators may help improve airway clearance in horses with significant airway obstruction.
  • Pleural drainage: For pleuropneumonia with significant fluid accumulation, chest drain placement to drain infected pleural fluid is essential. Repeated flushing of the pleural space (pleural lavage) may be required.
  • Supportive care: Good nutrition, clean fresh water, minimise stress, and provide a well-ventilated, dust-free environment. Head lowering (by removing or lowering hay nets to allow mucus drainage) is beneficial.
  • Rest: All horses with pneumonia require rest from work. Return to exercise should be gradual once the horse has fully recovered.

Prevention

  • Vaccination: Vaccinate against equine influenza and equine herpesvirus (EHV) — while vaccines do not directly prevent bacterial pneumonia, they reduce viral respiratory disease that predisposes horses to secondary bacterial infection. Maintain up-to-date vaccination records.
  • Transport hygiene: Allow horses to lower their heads during transport when possible. Stop and rest horses on long journeys. Maintain good ventilation in the trailer or horsebox.
  • Reduce dust exposure: Use dust-extracted or soaked forage. Clean and ventilate stabling well. Avoid deep mouldy bedding.
  • Rhodococcus equi on stud farms: Test mares’ foals with ultrasound screening at 30–60 days of age. Environmental management (watering dusty areas, manure removal) can reduce soil contamination. Some farms use prophylactic azithromycin in high-risk foals — discuss with your vet. Hyperimmune plasma administration to newborn foals is used preventively on high-risk properties.
  • Manage underlying conditions: Treat respiratory disease (strangles, herpesvirus) promptly and thoroughly to prevent progression to pneumonia.

Prognosis

Prognosis varies considerably:

  • Mild to moderate bacterial pneumonia in adult horses: Generally good with prompt, appropriate treatment. Most horses return to full work.
  • Pleuropneumonia: Guarded; mortality rates of 10–40% are reported in the literature. Survivors may have residual pleural adhesions affecting long-term performance.
  • Rhodococcus equi pneumonia in foals: Guarded; mortality rates vary but can be high in severe or late-detected cases. Foals that survive to adult life generally have a good prognosis for athletic performance.
  • Aspiration pneumonia: Often severe, with a guarded to poor prognosis depending on the extent of pulmonary damage.

When to Call the Vet

Contact your vet urgently if your horse or foal shows:

  • Fever above 38.5°C
  • Increased respiratory rate, laboured breathing, or flared nostrils at rest
  • Nasal discharge that is mucopurulent (coloured and opaque)
  • A persistent cough developing after transport or illness
  • A foal that is coughing, breathing rapidly, or not nursing normally
  • A horse recovering from a respiratory illness that deteriorates suddenly

Frequently Asked Questions

What is shipping fever in horses?

Shipping fever (transport-associated pneumonia) develops when horses are transported over long distances, particularly when their heads are tied up for extended periods. Normally, horses lower their heads to drain mucus and foreign material from the airways. When the head is restrained in an elevated position, this drainage is impaired, allowing bacteria from the nasopharynx to be aspirated into the lungs. Prevention includes allowing the horse to lower its head periodically, maintaining good ventilation in the trailer, taking rest stops on journeys over 12 hours, and monitoring temperature closely in the 24–48 hours after a long journey.

How is Rhodococcus equi pneumonia different from other foal pneumonias?

Rhodococcus equi is uniquely dangerous in foals because it is an intracellular pathogen — it survives and multiplies inside the macrophages (immune cells) of the lung, making it resistant to many standard antibiotics. It causes a suppurative bronchopneumonia with characteristic lung abscesses. The insidious onset means foals can be severely affected before obvious signs are noticed. Diagnosis requires specific testing (culture and PCR), and treatment requires the combination of azithromycin and rifampicin — standard penicillin regimens are not effective.

Can a horse that has had pneumonia return to full performance?

Many horses recover fully from pneumonia and return to their previous level of work, particularly those with straightforward bacterial pneumonia diagnosed and treated early. Horses that develop pleuropneumonia or extensive lung damage may have residual scarring affecting long-term respiratory capacity. Foals that recover from Rhodococcus equi pneumonia generally grow up to have normal athletic potential, though recovery takes months. A veterinary reassessment before returning to competition is always advisable.

Can pneumonia be passed between horses?

Most bacterial causes of equine pneumonia are not highly contagious between horses — the organisms involved (such as Streptococcus zooepidemicus) are commensal bacteria that cause disease in compromised individuals rather than spreading readily between healthy horses. However, the viral infections that predispose to pneumonia — particularly equine influenza — are highly contagious. A horse with pneumonia secondary to influenza should be isolated to prevent spread of the viral infection. Rhodococcus equi is not transmitted directly between horses but accumulates in the environment of contaminated premises.