Bit & Bridle

Equine Corynebacterium Infection

Other Names

  • Pigeon Fever
  • Dry Land Distemper
  • Dryland Distemper
  • Pigeon Breast

What is Corynebacterium Infection in Horses?

Corynebacterium pseudotuberculosis infection, commonly known as Pigeon Fever or Dry Land Distemper, is a bacterial disease caused by the gram-positive bacterium Corynebacterium pseudotuberculosis. The condition earned its striking common name from the characteristic chest swelling that develops in affected horses, which can resemble the breast of a pigeon.

The disease is most prevalent in the western United States — particularly California — though cases have been reported across many regions. It tends to follow a seasonal pattern, peaking in late summer and autumn when fly populations are at their highest. Horses of all ages and breeds can be affected, though young horses and those in poor condition may be more susceptible.

The infection manifests in three distinct forms: external abscesses (the most common), internal abscesses affecting organs such as the liver, spleen, and lungs, and a less common ulcerative lymphangitis affecting the lower limbs. External abscesses are far more common and carry a much better prognosis than the internal form, which can be life-threatening.

Causes

Corynebacterium pseudotuberculosis is an opportunistic pathogen that enters the horse’s body through breaks in the skin or mucous membranes. The primary transmission routes include:

  • Arthropod vectors: Stable flies, horn flies, and house flies are the main mechanical vectors, carrying bacteria from infected wounds or contaminated soil to new hosts.
  • Contaminated soil and environment: The bacteria can persist in soil for extended periods, particularly in dry, dusty conditions.
  • Direct contact: Nose-to-nose contact with infected horses, or contact with drainage from open abscesses, can spread the infection.
  • Minor wounds and abrasions: Even small cuts, insect bites, or skin irritations provide entry points for the bacteria.

Symptoms and Signs

  • External abscesses: Firm, painful swellings most commonly on the chest, ventral abdomen, and groin. These swellings gradually become fluid-filled and may rupture, discharging thick, creamy to olive-green pus. Affected horses may show mild fever and lethargy.
  • Internal abscesses: Weight loss, intermittent fever, depression, and poor performance. Ventral oedema may also be present.
  • Ulcerative lymphangitis: Painful swelling of one or more lower limbs, with multiple ulcerating nodules along the lymphatic vessels. This form can cause significant lameness.

Diagnosis

  • Culture: Bacterial culture of pus from draining abscesses confirms the diagnosis and identifies antibiotic sensitivities.
  • Serology: A synergistic haemolysis inhibition (SHI) titre blood test detects antibodies. Serial testing every 2–4 weeks helps monitor response to treatment.
  • Ultrasonography: Essential for detecting and monitoring internal abscesses in the liver, spleen, or lungs.
  • Haematology: Blood tests typically show elevated white blood cells in horses with internal involvement.

Treatment

  • External abscesses: Hot-packing to encourage maturation, followed by lancing and thorough flushing of the abscess cavity with antiseptic solution. Always wear gloves — C. pseudotuberculosis can occasionally infect humans.
  • Antimicrobial therapy: Antibiotics are not generally recommended for uncomplicated external abscesses. Internal infections require prolonged antimicrobial therapy — typically penicillin, trimethoprim-sulphonamide, or rifampicin combinations — for 30–90 days or longer.
  • Pain management: NSAIDs such as phenylbutazone or flunixin meglumine help manage discomfort.
  • Rest and supportive care: Affected horses should be rested until abscesses have fully resolved.

Never lance abscesses prematurely — the abscess must be fully mature and fluctuant before drainage is attempted. Premature lancing can drive infection deeper into tissue.

Prevention

  • Implement rigorous fly control throughout summer and autumn using fly sheets, traps, and regular manure removal.
  • Isolate any horse with a draining abscess immediately to prevent environmental contamination.
  • Dispose of abscess drainage carefully — do not allow pus to contaminate pasture or water sources.
  • Keep paddocks clean and minimise dusty conditions where bacteria may persist in soil.
  • Promptly treat any skin wounds or abrasions to reduce entry points for bacteria.

Prognosis

Horses with external abscesses carry an excellent prognosis — the vast majority recover fully following drainage and supportive care, typically within 1–4 months. Some horses develop multiple abscesses sequentially, which can extend the recovery period. Internal infections are considerably more serious; without aggressive and prolonged treatment, the prognosis is guarded to poor. Ulcerative lymphangitis tends to be slow to resolve and may leave permanent scarring of the lymphatic vessels.

When to Call the Vet

Contact your veterinarian if you notice any unusual firm swelling on the chest, underbelly or groin area, a horse running a persistent temperature or losing weight in late summer, or a wound discharging thick discoloured pus. Swelling and pain in one or more lower limbs without obvious traumatic cause also warrants prompt veterinary attention. For related conditions, see our entries on Equine Strangles and equine infectious disease in general.

Frequently Asked Questions

Is Pigeon Fever contagious between horses?

Not directly in the way respiratory diseases spread. The bacteria are shed in abscess drainage material and contaminate the environment, where flies then carry them to healthy horses. Isolating any horse with a draining abscess and practising strict hygiene when handling discharge is essential to prevent spread within a yard.

Can humans catch Pigeon Fever from horses?

In rare cases, yes. Corynebacterium pseudotuberculosis can cause infection in humans through direct contact with abscess drainage. Always wear gloves when handling infected horses or their discharge, and wash your hands thoroughly afterwards. Cases in humans are uncommon but have been documented in people with close occupational contact with affected animals.

How long does a Pigeon Fever abscess take to heal?

External abscesses typically resolve within 1–3 months following proper drainage and wound care. Internal infections take considerably longer — often 3–6 months or more with appropriate antibiotic treatment — and recovery is not guaranteed in all cases.

Should antibiotics be used for Pigeon Fever?

For uncomplicated external abscesses, antibiotics are generally not recommended and may delay maturation and drainage. Your veterinarian should assess whether internal involvement is suspected, which is when antibiotic therapy becomes important. Never start antibiotics without veterinary guidance.