Preventing Pigeon Fever

Q We have 20 horses that live in various pastures on our property. Last fall we had five cases of pigeon fever--a first for us. For the most part the horses were in different pastures, some areas not contiguous. I've researched pigeon fever, but the one fact I could not find online or by asking vets is the length of the bacterium's incubation period (the time between exposure and the onset of clinical signs of disease). Although I fly spray all the horses, it didn't seem to make a difference. I thought if I knew the incubation period I could do something proactive to prevent more occurrences of this bacterial disease. Any information or prevention suggestions?
--Marsha Adamson, Gainesville, Texas

A Deep intramuscular abscesses in horses caused by Corynebacterium pseudotuberculosis (the causative agent of pigeon fever) were first reported in the San Francisco Bay Area, Calif., in the early 1900s. Since that time, disease caused by these bacteria can be considered one of the most frequently diagnosed infectious diseases in the Western United States, particularly in California. Infections tend to occur both as sporadic cases on a farm involving a few horses or as outbreaks involving hundreds to thousands of horses in a region. Evidence exists that infection is increasing in incidence and spreading easterly, possibly associated with climate change. Unprecedented epidemics in the past decade have affected thousands of horses in Colorado, Florida, Idaho, Kentucky, Louisiana, Nevada, New Mexico, Oklahoma, Oregon, Texas, Utah, and Wyoming--states historically low in prevalence. High temperatures and drought conditions preceded all reported outbreaks.

The most common clinical form of the disease, characterized by external abscesses in the pectoral (chest) region or ventral (lower) abdomen, is often called "pigeon fever," due to the swelling of the horse's pectoral region resembling a pigeon's breast. "Dryland distemper," its other nickname, reflects the disease's prevalence in arid regions of the Western United States. Two other clinical forms of the disease are internal organ involvement, such as liver, kidney, lung, or splenic abscesses, and limb infection.

Researchers believe this soil-dwelling organism gains access to horses' bodies through abrasions or wounds on the skin or mucous membranes. Studies have incriminated many insects as mechanical vectors for transmitting the disease to horses, including the horn fly, the common house fly, and the stable fly. The regional location of abscesses suggests that ventral midline dermatitis (simply, skin inflammation) can predispose horses to infection. Due to variable incubation periods, ventral midline dermatitis may not be present at the time of abscesses' maturation.

Complex statistical outbreak analyses indicated an average incubation period of three to four weeks. Researchers found it took seven to 56 days for the disease to be transmitted over 4.3 to 6.5 km. This strongly suggests the disease can be transmitted through horse-to-horse contact or from infected to susceptible horses via insects, other vectors, or contaminated soil. The organism has been shown to survive for up to two months in hay and shavings and more than eight months in soil. Disease incidence fluctuates considerably from year to year, presumably due to herd immunity and environmental factors such as rainfall and temperature. To date, researchers have not proven that definitive environmental factors contribute to infection spreading. Disease incidence is seasonal, with the highest number of cases occurring during the dry months of the year when insects are numerous, which is late summer and fall in the southwestern United States, although cases can be seen all year. Veterinarians see horses with internal infection more frequently one to two months following the peak number of external abscess cases.

Pigeon fever can affect horses of all ages, although the low disease incidence in foals less than 6 months of age suggests passive transfer of antibodies via the dam's colostrum helps protect foals born in endemic (disease prevalent) areas. A case-control study in an endemic area revealed young horses (less than 5 years old) and those in contact with other horses on summer pasture had increased infection risk. Horses housed outside or with access to an outside paddock appeared to be at higher risk than stabled horses.

Until researchers develop a protective vaccine against C. pseudotuberculosis for horses, I can only suggest that horse owners in endemic areas practice good sanitation and fly control and avoid unnecessary environmental contamination by diseased horses. Proper sanitation, disposal of contaminated bedding, and stall/equipment disinfection may reduce disease incidence. Proper wound care and preventing ventral midline dermatitis can also help horses avoid infection from a contaminated environment. A commercial vaccine is clearly needed to protect horses as the disease becomes endemic in more geographic regions, as witnessed with the recent outbreak in Texas horses.

About the Author

Sharon Spier, DVM, PhD, Dipl. ACVIM

Sharon Spier, DVM, PhD, Dipl. ACVIM, is a professor of the Department of Medicine and Epidemiology at the University of California, Davis. Her areas of interest include Corynebacterium pseudotuberculosis infection (also known as pigeon fever or dryland distemper), hyperkalemic periodic paralysis, and genetic diseases of horses.

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