Nocardiosis in Horses: Opportunistic Skin, Lung, and Systemic Infection

Nocardiosis is a chronic, noncontagious bacterial infection caused by organisms in the genus Nocardia. These bacteria live naturally in the environment, especially in soil, compost, decaying vegetation, and organic debris. In horses, infection usually starts when the bacteria enter through a contaminated wound or are inhaled into the respiratory tract.

In many horses, nocardiosis behaves as an opportunistic infection. That means it is more likely to take hold when normal defenses are weakened, such as with pituitary pars intermedia dysfunction (PPID), severe immune problems in foals, or chronic respiratory disease. Some horses develop localized skin and lymph node infections, while others develop pneumonia, pleuritis, oral lesions, abortion, or disseminated disease affecting multiple body systems.

This condition can be frustrating because it often looks like other problems at first. A horse may start with vague signs like fever, poor appetite, lethargy, or weight loss, then later develop draining skin tracts or ongoing respiratory signs. Because the disease can be slow-moving and recurrent, early veterinary evaluation matters.

The outlook varies. Horses with small, localized lesions may do reasonably well with sustained treatment, while pulmonary or systemic nocardiosis carries a more guarded prognosis and often needs longer, more intensive care.

Nocardiosis is caused by environmental bacteria in the genus Nocardia. Reported equine infections have involved organisms historically grouped within the Nocardia asteroides complex, as well as species such as Nocardia nova*, Nocardia farcinica, and *Nocardia brasiliensis. These bacteria are not normal horse pathogens in the way strangles or influenza are. Instead, they tend to cause disease when they gain access to tissue and the horse cannot clear them effectively.

The two main routes of infection are contamination of wounds and inhalation of contaminated dust or organic particles. That is why disease may show up as skin and subcutaneous abscesses, lymph node infection, or pyogranulomatous pneumonia and pleuritis. Oral lesions and mandibular draining tracts have also been described, likely after local tissue contamination.

Risk tends to be higher in horses with immune dysfunction or chronic underlying disease. Merck notes an association with PPID in adult horses and severe combined immunodeficiency in Arabian foals. Published equine reports also describe pulmonary nocardiosis in horses with chronic airway disease or poor respiratory defenses.

Nocardiosis is considered noncontagious. In practical terms, one horse does not usually "catch" it from another horse. Prevention focuses more on wound hygiene, dust control, and managing underlying health problems than on strict isolation.

Your vet will usually start with a physical exam and history, including how long the problem has been present, whether there are draining tracts or respiratory signs, and whether your horse has conditions such as PPID or chronic airway disease. Because nocardiosis can look like other bacterial, fungal, or inflammatory conditions, diagnosis should not rely on appearance alone.

For skin or lymph node disease, your vet may collect pus, tissue, or aspirates for cytology and bacterial culture. For respiratory disease, testing may include transtracheal wash or airway sampling, along with thoracic ultrasound and sometimes radiographs or endoscopy. AAEP respiratory diagnostic guidance supports imaging and airway sampling when evaluating equine respiratory infections.

A key step is culture and susceptibility testing. AAEP antimicrobial guidance recommends submitting samples for culture and susceptibility when possible, and that is especially important here because Nocardia species can be resistant to some commonly used antibiotics. In some cases, molecular identification may be needed to confirm the species.

Additional testing may include CBC/chemistry, fibrinogen or inflammatory markers, and screening for underlying disease that could affect immune function. If lesions are unusual or not responding as expected, your vet may also recommend biopsy or histopathology to help separate nocardiosis from fungal infection, neoplasia, or other causes of pyogranulomatous inflammation.

Because Nocardia organisms are common in the environment, prevention is about reducing opportunity, not eliminating exposure completely. Good wound hygiene matters. Clean cuts promptly, keep bandages and wraps dry when possible, and ask your vet to examine wounds that become swollen, painful, or slow to heal. Chronic draining tracts should never be assumed to be routine.

For horses with respiratory risk, focus on air quality and dust control. Good stall ventilation, reducing moldy or dusty hay exposure, and managing chronic airway disease can help support normal lung defenses. Published equine reports have linked pulmonary nocardiosis with horses that already had chronic airway problems.

It also helps to stay ahead of underlying health issues. Older horses with recurrent infections, poor hair coat changes, muscle loss, or laminitis history may need screening for PPID or other conditions that can affect immune function. A horse that keeps getting unusual infections deserves a broader conversation with your vet.

There is no vaccine for equine nocardiosis, and routine isolation is not the main preventive tool because the disease is not considered contagious. The most practical prevention plan is regular veterinary care, prompt attention to wounds and respiratory signs, and early testing when an infection is not behaving the way a typical case should.