A horse suffering from pigeon fever usually has extremely large abscesses on its chest. The abscesses have been described as looking like a pigeon’s breast, hence the term. The disease is found throughout the Western United States and is particularly common in California, Colorado and Utah. However, cases it’s been in Kentucky, too.
The culprit organism, corynebacterium pseudotuberculosis (CP), is a bacterium and can live in the soil, manure, bedding, etc. for up to two months in warm weather, longer in cold. The organism enters the horse’s body through wounds, small breaks in the skin or possibly via biting insects that carry the organism on their body. Most cases are found in animals under the age of five.
Recurrences are rare and probably related to inadequate treatment during the first episode. This suggests that once exposed the immunity is long-lasting, and that many young horses are probably naturally “vaccinated” by exposure to small numbers of the organism but not enough to cause the disease.
Pigeon-fever cases can occur at any time of year but are most common in fall and winter. Since these abscesses grow slowly, it’s believed this timing actually reflects infections that were picked up at the peak of insect season, when large numbers of flies increase the odds that some of them will be carrying around the organism when they feed on small wounds or bite the horse.
According to a report from the University of California, Davis, Veterinary School, a survey of 538 cases showed that 57% had chest abscesses. Abscess formation is also common along the belly and in the groin, but can appear virtually anywhere. In addition, 8% had internal abscesses.
Once the huge abscesses start to appear, diagnosis isn’t difficult. The thick, cream-colored pus has a foul odor. The organism is easily cultured from the pus. Before the abscesses become large enough to be visible, a process that may take weeks, the horse may be depressed, lose weight, show unexplained lameness (internal pressure from the abscesses) and may have a fever.
Internal abscesses may form virtually anywhere in the chest or abdomen, including in vital organs like the heart. A history of living in an area where pigeon fever is common, being turned out on pasture, and exposure to horses that had pigeon fever, together with the nonspecific weight loss and fever, will raise the index of suspicion. Ultrasound or X-rays and rectal exams may uncover the abscesses in the horse. There is an antibody test available for diagnosing the internal form.
Treatment of the abscesses involves hot packing to encourage them to rupture and drain, or surgical opening for drainage once the veterinarian thinks the abscess is “mature.” As with strangles, antibiotics usually aren’t prescribed before the abscesses are opened, but may be considered if the horse is very ill or debilitated.
After opening, antibiotics are usually given. The abscess itself is flushed thoroughly, and this flushing procedure will usually be repeated periodically during the healing process. The abscess cavity can be packed with gauze soaked in disinfectant.
Healing may take several weeks but usually proceeds uneventfully. Internal abscesses are treated with prolonged courses of antibiotics (several months). Penicillin is usually the drug of choice, but therapy may be switched to oral trimethoprim sulfa. Pigeon fever is rarely fatal, but the chances of return to full health are lower with the internal form.
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