Colorado State University Veterinarians Report Unusual Rise in Cases of Pigeon Fever Among Colorado Horses

Equine veterinarians at Colorado State University’s James L. Voss Veterinary Teaching Hospital report a serious increase in the number of cases of pigeon fever they have treated since early fall and warn horse owners to be alert for signs of the highly contagious disease.

Seventy-six cases from Colorado’s Front Range have been confirmed by the Colorado State University Veterinary Diagnostic Laboratory since early fall, more than six times the number of cases from last year’s total of 12 confirmed cases and far above the seven confirmed cases in 2000.

"What was once considered a disease of California horses is now a growing problem for the Colorado equine population," said Andrea Torres, veterinarian and microbiology resident who conducted a study of the disease in Colorado in 2000-2001. "The increased number of confirmed cases may be due to a more educated horse-owning public and/or to more veterinarians being aware of the disease and testing for it."

Torres and other veterinarians at the hospital point out that the signs of pigeon fever can also initially resemble those of other diseases such as strangles. Sometimes the only initial signs are lameness and a reluctance to move.

Pigeon fever, also called pigeon breast, breastbone fever, false strangles, dryland strangles or dryland distemper, is caused by Corynebacterium pseudotuberculosis and is found worldwide. It can strike a horse of any age, sex or breed, but usually attacks young adult animals. There is a low incidence in foals.

It has also been diagnosed in cattle, and a similar disease affects sheep and goats. The disease is not transmissible to humans, although humans can carry the infectious agent on shoes, clothing, hands or barn tools and transfer it to another animal.

Clinical signs include lameness, fever, lethargy and weight loss and usually is accompanied by very deep abscesses and multiple sores along the chest, midline and groin area and, sometimes, the back. Abscesses also can develop internally.

The disease is called pigeon fever because infected animals often develop abscesses in their pectoral muscles, which swell and resemble a pigeon’s chest. Although the disease is considered seasonal, with most cases occurring in early fall, a number of cases have been confirmed during winter months and other times of the year as well.

The causative bacteria live in the soil and can enter the animal’s body through wounds, broken skin or through mucous membranes. Additionally, some researchers believe pigeon fever may be transmitted by flies.

The disease occurs in three forms: external abscesses, internal abscesses and limb infection, also known as ulcerative lymphangitis. The most common forms are external abscess and lymphangitis, with the prognosis of a full recovery being generally good. Internal abscesses are much more difficult to treat.

"Because this disease is so highly contagious, it is very important that veterinarians accurately diagnose these cases to tailor treatment and control," said Torres.

"Horse owners should be aware of the clinical signs and understand that veterinary care must be timely. Infected horses should be isolated, the abscesses properly treated and the drainage properly disposed of. The area where the infected horse is kept must be properly cleaned and completely disinfected because this is a very hardy bacterium. Pest control is extremely important"

As a service to horse owners, Colorado State’s equine veterinarians have created a fact sheet on pigeon fever (attached) that is posted on the Colorado State University website at www.cvmbs.colostate.edu/vth and www.colostate.edu on the news page.

FACT SHEET: PIGEON FEVER IN EQUINES

Common Names: Pigeon fever, pigeon breast, breastbone fever, dryland distemper, dryland strangles, false strangles, false distemper

Geographic Incidence: Endemic to California, but now found in most Western states in the U.S.

Seasonal: Usually appears in late fall but can appear sporadically at any time of year.

Cause: Corynebacterium pseudotuberculosis

Vaccine: None at this time.

Reservoirs and mode of transmission:

• Can live in the soil and enter the horse’s body through wounds or broken skin and through mucous membranes.
• May possibly be transmitted by flies, including the common housefly and horn flies.
• Disease is usually highly contagious and can easily infect multiple horses on the premises.
• Bacterium in the pus draining from abscesses on infected horses can survive from one to 55 days in the environment. It has also been shown to survive from one to eight days on surface contaminants and from seven to 55 days within feces, hay, straw or wood shavings.
• Lower temperatures prolong the survival time.

Clinical signs:

• Early signs can include lameness, fever, lethargy, depression and weight loss.
• Infections can range from mild, small, localized abscesses to a severe disease with multiple massive abscesses containing liters of liquid, tan-colored pus.
• External, deep abscesses, swelling and multiple sores develop along the chest, midline and groin area, and, occasionally, on the back.

Incubation period: Horses may become infected but not develop abscesses for weeks.

Animals affected:

• The disease usually manifests in younger horses, but can occur in any age, sex, and breed.
• A different biotype of the organism is responsible for a chronic contagious disease of sheet and goats, Caseous lymphadenitis, or CL. Either biotype can occur in cattle.

Disease forms:

• Generally 3 types: external abscesses, internal abscesses or limb infection (ulcerative lymphangitis).
• The ulcerative lymphangitis is the most common form worldwide and rarely involves more than one leg at a time. Usually, multiple small, draining sores develop above the fetlock.
• The most common form of the disease in the United States is external abscessation, which often form deep in the muscles and can be very large. Usually they appear in the pectoral region, the ventral abdomen and the groin area. After spontaneous rupture, or lancing, the wound will exude liquid, light tan-colored, malodorous pus.
• Internal abscesses can occur and are very difficult to treat

Diagnosis: Your veterinarian can easily collect a sample for culture at a diagnostic laboratory. It is important to isolate the bacterium to get a definitive diagnosis since pigeon fever can superficially resemble other diseases.

Treatment:

• Hot packs or poultices should be applied to abscesses to encourage opening. Open abscesses should be drained and regularly flushed with saline.
• Surgical or deep lancing may be required, depending on the depth of the abscess or the thickness of the capsule, and should be done by your veterinarian.
• Ultrasound can aid in locating deep abscesses so that drainage can be accomplished.
• External abscesses can be cleaned with a 0.1 percent povidone-iodine solution
• Antiseptic soaked gauze may be packed into the open wound
• A nonsteroidal anti-inflammatory drug such as phenylbutazone can be used to control swelling and pain
• Antibiotics are controversial. Their use in these cases has sometimes been associated with chronic abscessation and, if inadequately used, may contribute to abscesses, according to one study.
• The most commonly used antibiotic for the treatment of this condition is procaine penicillin G, administered intramuscularly, or trimethoprim-sulfa.
• In the case of internal abscesses, prolonged penicillin therapy is necessary

Care required:

• Buckets or other containers should be used to collect pus from draining abscesses and this infectious material should be disposed of properly.
• Consistent and careful disposal of infected bedding, hay, straw or other material used in the stall is vitally important.
• Thoroughly clean and disinfect stalls, paddocks, all utensils and tack.
• Pest control for insects is also very important.

Recovery time: Usually anywhere from two weeks to 77 days.

Prognosis: Usually good with complete recovery, although some horses may experience recurrence.