Though historically uncommon here in the Puget Sound region, Pigeon Fever has started to spread to our area and we want to provide you with some fast facts about this hardy bacterial disease (and no, it doesn't involve the bird).
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What is it?
Commonly called pigeon fever, this disease is caused by the Corynebacterium pseudotuberculosis bacteria and can affect equines, cattle, goats, sheep, buffalo, camelids, and rarely humans. The infection is so named because, in a majority of cases, horses develop a swollen chest which can resemble the protruding breast of a pigeon. It is believed to be transmitted through the bites of contaminated flies as well as introduced through abrasions or wounds in the skin via contaminated insects, direct contact with contaminated soil, or contact with pus from an infected horse's abscess. Ventral midline dermatitis ("sweet itch") is believed to be a predisposing cause of the infection as well.
Why is it here?
While the bacteria thrives in hot, dry environments, and is most commonly seen in California, Colorado, Idaho, Oregon, and other southwestern states, it has started to become more prevalent in the northwest and southeast United States, including Western Washington. It's possible the bacteria is finding more favorable conditions due to changing climate conditions, including warming temperatures and increasing drought, when it is likely being introduced due to the movement of animals from endemic areas. Regardless of the why and how, it is definitely surviving in the Puget Sound region.
What are the symptoms?
There are three forms of C. pseudotuberculosis infection typically seen: external abscesses, internal infection, and ulcerative lymphangitis (limb infection).
External abscesses are the most commonly seen form and frequently are seen in the pectoral (chest) region and along the ventral midline of the abdomen. They can also be present in the prepuce, mammary gland, tricep, axilla, legs, and head. Horses can have one or more abscesses in different areas of their body. The abscess contains a thick, tan, odorless purulent exudate (pus) and requires lancing and draining by a veterinarian when mature. Typically horses with external abscesses do not develop signs of systemic illness, though 25% will develop fever. Antibiotics are usually not indicated unless signs of systemic illness are present or if there are very deep intramuscular abscesses draining through healthy tissue. The fatality rate in horses with external abscesses is very low (<1%).
Internal infection is seen in less than 10% of horses, and typically involves abscesses on the liver, kidney, spleen, and/or lungs. Affected horses typically display inappetence, weight loss, fever, lethargy, abdominal discomfort, and/or signs of respiratory disease. Treatment involves long-term antibiotic therapy, usually for at least 4-6 weeks, and supportive care. Internal infection has a higher rate of fatality (30-40%).
Ulcerative lymphangitis is uncommon in North America and is characterized by severe limb swelling and cellulitis, typically in one or both hind legs. Frequently the horse will be severely lame and have fever, lethargy, and inappetence. Affected horses are treated aggressively with antibiotics and anti-inflammatories to prevent the disease from becoming a chronic issue, as well as hydrotherapy (cold hosing), hand walking, and leg wraps.
How is it diagnosed?
Diagnosis is based on a combination of clinical signs, blood work, diagnostic imaging (abdominal ultrasound), and bacterial cultures of the abscesses.
How can I prevent it?
Biosecurity measures are the best way to limit the spread of Pigeon Fever, focusing on limiting environmental contamination and spreading of the bacteria via insects or objects. Unfortunately the bacteria are hardy and able to survive in hay and shavings for up to two months, and in soil for over eight months, especially if it is contaminated with manure.
Decrease insect exposure by applying insect repellents regularly to all horses
Utilize fly masks, sheets, and feed-thru fly control, especially in sensitive horses
Pick manure out of stalls, paddocks, and trailers on a daily basis to reduce fly activity and locate manure piles and bins at a distance from barns and pastures
Meticulously care for any wounds and abrasions to prevent infection from a contaminated environment
Do not share water buckets, brushes, tack, manure picks, or other similar items
Quarantine new horses and monitor for signs of infection for 3 - 4 weeks
Keep affected horses with actively draining abscesses separated from other horses to minimize spreading of the disease; don't let the affected horse wander the pasture and spread the bacteria via draining abscesses
Clean up pus from lanced abscesses to prevent further environmental contamination
Clean and disinfect non-soil surfaces where an infected horse has been, such as cross ties, wash racks, horse trailers, stall doors, and grooming tools
Practice good biosecurity: wear disposable gloves and protective outerwear when working with infected horses and wash you hands following handling
Previously believed to be a disease of California and the Southwest, Pigeon Fever has been diagnosed in horses throughout the United States and is being seen year-round. There is currently a conditionally licensed vaccine available with further research being done to determine its efficacy. Most importantly, diligent management of your horse, in addition to early veterinarian involvement when symptoms arise, can help to minimize the impact of the disease on your horse and the rest of the herd.
Check out this video from UC Davis Veterinary Hospital of a horse who was successfully treated for an internal abscess
American Association of Equine Practitioners. “Pigeon Fever (Corynebacterium pseudotuberculosis infection).” 2013. Available at: https://aaep.org/sites/default/files/Guidelines/PigeonFeverGuidelines52713.pdf
Center for Equine Health, School of Veterinary Medicine, University of California, Davis. “Pigeon Fever: Varying Forms of Infection in Horses.” CEH Horse Report. [online] Available at: https://ceh.vetmed.ucdavis.edu/health-topics/pigeon-fever
EquiManagement. "Bacterin-Toxoid Vaccine for Corynebacteria pseudotuberculosis." Nancy S. Loving, DVM. 5/25/2019. [online] Accessed 10/22/2020. Available at: https://equimanagement.com/articles/bacterin-toxoid-vaccine-for-corynebacteria-pseudotuberculosis