Hundreds of horses across Eastern Oregon have contracted a disease known as “pigeon fever” over the past two to three months. Grant County has seen dozens of cases.

Corynebacterium pseudotuberculosis, the causative organism of pigeon fever, is a grampositive bacterium that produces a syndrome of lymphadenitis (inflammation of the lymphatic system) or abscess formation. The bacteria survive well in the environment and can persist in the ground for up to 60 days. This hardy bacterium can also infect other species, including man.

Common names for the external abscess form of the disease are “pigeon fever,” “pigeon breast,” “dry-land distemper,” or “Colorado distemper.” There is no breed or sex predilection for the development of the infection. The average age for horses with external abscesses is 5 years, but the range is from as young as 3 months to 28 years. Mares, gelding, and stallions are equally affected by the disease.

Horses become infected by the bacteria through small entry points in the mucous membranes or skin. Badly fitting tack or biting insects can create the necessary portal of entry for the bacteria to infect the skin. Pigeon fever outbreaks usually occur in the late fall and early winter seasons after a hard rain. Once the bacteria have entered the abraded or damaged skin of the horse, the infection may establish itself locally as an external abscess or spread via the lymph or blood. The incubation period of the disease is highly variable and may range from weeks to months after initial exposure of bacterial skin entry.

The disease occurs worldwide in areas where horses are raised. Three forms of Corynebacterium pseudotuberculosis have been described in horses: ulcerative lymphangitis, external, and internal abscesses. In the Western USA the most common form are external abscesses. The external abscesses usually develop in the pectoral (chest area), ventral midline (along the belly), abdominal and inguinal (including the scrotum and mammary gland) areas.

Internal abscesses are much more uncommon than the external abscesses, but can be found in the lungs, mediastinal, bronchial, kidneys, and mesenteric lymph nodes. The most common presenting complaint for horses having internal abscesses is chronic weight loss with a waxing-waning fever.

The majority of the affected horses remain bright, alert, responsive, and continue to eat and drink as normal. Depression, fever, and anorexia may be present but are not common. Internal abscesses may cause chronic weight loss or abnormal pain. Large pectoral abscesses can cause gait abnormalities and pain during ambulation.

Characteristic clinical signs such as large abscesses in the pectoral region, along the belly, or in the mammary glands and scrotum may suggest a C. pseudotuberculosis infection. A definitive diagnosis requires a bacterial culture of the liquid exudate from within the abscess.

The treatment of choice includes surgical incision and open drainage of the abscesses. The abscess must be mature and “ripe” prior to incision in order to drain them. The bacteria are facultative anaerobes, meaning they prefer to live in an environment without oxygen. By lancing or draining the affected abscess outside air is drawn in the internal aspect of the wound and aids in resolution of the infection.

The drainage should be collected and disposed of, and the wound should be rinsed with an antiseptic solution such as 1 percent iodine. Care must be taken to not ingest of inhale the bacteria, as humans can become infected with C. pseudotuberculosis. It is recommended to wear gloves during any surgical abscess intervention.

The effficacy of antibiotic therapy is controversial. The failure of antibiotic therapy may be due to the thick-walled, encapsulated abscess which protects and isolates the organism making antibiotic penetration difficult. When internal abscesses are suspected, extended length antibiotic therapy is recommended for four-eight weeks. Analgesics (pain medication) and anti-inflammatory drugs may be used when pain is present.

Affected horses should be kept isolated and quarantined for at least 30 days. Preventing skin abrasions from tack or harnesses will decrease the likelihood of the disease. Fly control and the use of screened stalls may reduce insect vector spread.

General recommendations to prevent the spread of the infection are isolation of infected animals, fly control, good sanitation, and disinfection of contaminated ground, mud boots, and careful disposal of bedding. C. pseudotuberculosis is susceptible to most disinfectants. There is no effective vaccine available for C. pseudotuberculosis at this time.

Kasey Nash is a veterinarian at the John Day River Veterinary Center.