Bit & Bridle

Equine Pigeon Fever (Dryland Distemper)

Other Names

Dryland distemper, dryland strangles, false strangles, pigeon fever. Caused by Corynebacterium pseudotuberculosis.

What is Equine Pigeon Fever?

Pigeon fever is an infectious disease of horses caused by the bacterium Corynebacterium pseudotuberculosis. The condition gets its colloquial name from the characteristic swelling of the pectoral muscles (chest) that causes an affected horse’s chest to protrude like a pigeon’s breast. The condition is endemic in many parts of the western United States — particularly California and other arid western states — and is also seen in South America, Australia, and parts of the Middle East.

In the UK and northern Europe, pigeon fever is uncommon and sporadic rather than endemic, but import of horses from affected regions and global movement of horses means awareness of the condition is important. The disease causes three distinct clinical syndromes: external abscesses (the most common form), internal abscesses (uncommon but the most serious form), and ulcerative lymphangitis affecting the lower limbs.

What Causes Pigeon Fever?

Corynebacterium pseudotuberculosis is a Gram-positive, facultative intracellular bacterium found in soil, particularly in dry, arid environments. It produces a potent exotoxin (phospholipase D) that damages blood vessel walls and facilitates the formation of characteristic caseous (cheese-like) abscesses.

Transmission occurs primarily through skin wounds and insect bites. House flies and horn flies act as mechanical vectors, transmitting bacteria from draining abscesses to small wounds on other horses. Biting flies, contaminated soil, and shared equipment can all facilitate spread. The organism is highly resistant to desiccation and can persist in dry soil for months.

Seasonal incidence peaks in late summer and autumn in endemic areas, corresponding with peak biting fly populations. Warm, dry climates favour both bacterial survival in soil and fly vector activity.

Symptoms and Signs

Three syndromes occur with varying frequency:

External abscesses (approximately 90% of cases):

  • Single or multiple firm, painful swellings — most commonly in the pectoral region (chest), along the ventral midline (belly), or in the axillary region (armpit)
  • Swellings gradually enlarge, soften, and eventually rupture, discharging thick, often odourless, yellowish-green pus
  • Mild fever and reduced appetite during abscess development
  • Variable lameness if abscesses develop near limbs
  • Most horses recover fully once abscesses rupture and drain

Internal abscesses (approximately 8% of cases):

  • Far more serious — abscesses form in the organs (liver, spleen, kidneys, lymph nodes, lung)
  • Significant, often persistent fever
  • Progressive weight loss and deterioration in condition
  • Depression and reluctance to eat
  • Colic or other signs depending on organs affected
  • Carries a high mortality rate and requires prolonged intensive treatment

Ulcerative lymphangitis (approximately 1–3% of cases):

  • Severe, painful swelling of one or more lower limbs
  • Ulceration of the skin over the lower leg with discharge of purulent material
  • Marked lameness
  • May leave permanent lymphatic scarring and chronic leg swelling

Diagnosis

Diagnosis is based on clinical appearance and confirmed by bacterial culture of abscess contents or discharge. Culture of C. pseudotuberculosis from pus is definitive.

Serology (synergistic haemolysis inhibition test — SHI, or ELISA) can detect antibody titres against phospholipase D antigen. Rising titres support active infection and can help identify horses with internal abscesses that may not have obvious external swelling.

Ultrasound examination of the abdomen and chest is essential in horses with suspected internal abscesses or unexplained fever, weight loss, and deterioration. Blood tests typically show leukocytosis, elevated fibrinogen, and elevated SAA.

See our entry on Rhodococcus equi infection for another bacterial infection causing abscess formation in horses.

Treatment

External abscesses:

  • The mainstay is encouraging abscess maturation and facilitating drainage — hot compresses, poulticing, and cleaning the area once the abscess ruptures
  • Systemic antibiotics are controversial for external abscesses — some evidence suggests they may delay abscess maturation and do not improve outcomes. They are generally reserved for horses with fever, systemic signs, or cases where surgery is planned
  • Surgical incision and drainage may be performed by your vet for abscesses that are large, not draining spontaneously, or in problematic locations

Internal abscesses:

  • Require prolonged systemic antibiotic treatment — penicillin, trimethoprim-sulphonamide, or rifampicin-based combinations for weeks to months
  • Response to treatment must be monitored with serial ultrasound and haematology
  • Prognosis is guarded; mortality rates of 30–40% have been reported for internal abscess cases

Ulcerative lymphangitis:

  • Systemic antibiotics combined with local wound management
  • Hydrotherapy and leg bandaging
  • NSAIDs to reduce pain and inflammation

Biosecurity during treatment: All pus from draining abscesses is highly infectious. Wear gloves when handling draining wounds, use dedicated equipment, dispose of contaminated bedding safely, and control fly access to draining lesions as much as possible to prevent spread.

Prevention

  • Aggressive fly control — fly sheets, fly masks, regular application of repellents, fly traps, and proper manure management to reduce fly breeding
  • Promptly isolate any horse with a draining abscess to prevent spread to other horses via fly vectors
  • Disinfect equipment and shared tack regularly
  • Prevent horses from coming into contact with discharge from infected animals
  • Quarantine horses imported from endemic regions

A vaccine is under development but not yet widely available. In endemic regions of the USA, fly control and prompt isolation of affected horses are the primary preventive tools.

When to Call the Vet

  • A horse develops a firm, painful swelling in the chest, belly, or axillary region — particularly if imported from or having travelled through endemic regions
  • Fever, weight loss, or deterioration in condition in a horse with known or suspected pigeon fever history
  • Suspected internal abscesses — this is a serious presentation requiring urgent investigation
  • Ulcerative skin lesions developing on the lower limbs with marked lameness and swelling

Prognosis

The prognosis for external abscesses is excellent — the vast majority of horses recover fully once abscesses drain, with appropriate wound management. Internal abscesses carry a much more guarded prognosis with significant mortality. Ulcerative lymphangitis can cause permanent lymphatic damage and chronic swelling of the affected limb. Early diagnosis and appropriate management substantially improve outcomes for all forms of the disease.

Frequently Asked Questions

Is pigeon fever contagious between horses?

Pigeon fever is not directly contagious in the way a respiratory virus is — it requires a transmission mechanism, typically biting flies or contact with infectious pus. However, on premises with active fly populations and horses with draining abscesses, the disease can spread through a group of horses. Strict isolation of affected horses and aggressive fly control are essential to contain spread during an outbreak.

Why is it called pigeon fever?

The name comes from the characteristic pectoral swelling that develops in affected horses — the chest protrudes forward, resembling the puffed-out breast of a pigeon. This presentation, while visually striking, represents the most common and most treatable form of the disease. Not all horses with pigeon fever show this classic chest swelling — abscesses can develop anywhere along the ventral midline.

Can humans get pigeon fever from horses?

Corynebacterium pseudotuberculosis can rarely infect humans — typically through direct contact with pus from an infected animal through breaks in the skin. Human infection causes lymphadenitis (swollen, infected lymph nodes). Always wear gloves when handling draining abscesses and wash hands thoroughly. The risk is low but real, particularly for immunocompromised individuals.

How long does pigeon fever take to resolve?

External abscesses typically take 1–4 weeks from first appearance to rupture, and another 1–3 weeks to fully heal once draining. The total course of external abscess disease is usually 4–8 weeks. Internal abscesses require much longer treatment — months of antibiotics and monitoring. Horses with internal abscesses may be off work for 6 months or more, and some do not return to full athletic function.