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Corynebacterium pseudotuberculosis - Pigeon Fever

By: Dr. Jason Grady

Many veterinarians in Oklahoma have recognized an increase in cases of a contagious disease called pigeon fever, also known as dryland distemper.  Historically, this disease has been thought to be most prevalent in the southwestern United States (primarily California), but increased cases in recent years have been reported in other parts of the United States, including Texas, New Mexico, Nevada, Arizona, Colorado, Kentucky, Wyoming, Oregon, Utah, and recently Oklahoma.  The past two years the veterinarians at Sapulpa Equine Hospital (SEH) have diagnosed and treated over 100 cases.

Pigeon fever is caused by a gram-positive, soil-borne bacteria, called Corynebacterium pseudotuberculosis that can survive for months to possibly even years in variable environmental conditions.  Cases or outbreaks of pigeon fever most commonly occur during the fall or early winter, however cases can be seen year round.  In areas where increased cases have been observed, they are commonly seen during the dry months following a season of increased rainfall.  The seasonality of disease has been associated with biting insects such as horn flies, common house flies, and stable flies.  The increased rainfall is thought to provide optimal breeding conditions for the insect vectors.  Other entry points could be through wounds and abrasions in the skin or mucous membranes.  Thus it is possible that other infected horses, humans, and contaminated supplies or tack could potentiate the spread to susceptible horses.  There are no current reports of transmission from horse to human, but precautions should be taken when handling infected horses to minimize human health risks and to minimize exposure to susceptible horses.  There is no breed or gender predilection, however one study performed revealed that horses <5 years of age (range in cases seen at SEH, 5 months to 21 years) and horses that lived outside or had exposure to an outside paddock with other horses were at an increased risk of disease than those that lived indoors.  

Clinical signs associated with pigeon fever are most frequently associated with the development of external abscesses and less frequently internal abscesses and ulcerative lymphangitis.  The external abscesses commonly occur in the pectoral region giving the classic appearance of a pigeon’s breast.  Other locations that external abscesses may develop include ventral midline of the abdomen, mammary gland, prepuce, behind the elbow (axillary region), limbs, flank (inguinal region), and head.  Most involve a single site, but some may have multiple sites affected.  Abscesses are generally encapsulated with a thick capsule, may become quit large, and typically contain a thick, tan, odorless exudate.  The tissue surrounding the abscess often develop a large area of edema, and become firm and painful.  The incubation period is approximately 3 to 4 weeks, and abscess maturation is generally slow and may be difficult to identify if the abscess lies deep to muscle.  Some abscesses will spontaneously rupture, and drain the exudate.  Other clinical signs may include lameness, fever, non-healing wounds, swelling of the mammary gland or prepuce, anorexia (decreased appetite), and depression.  Generally, horses with external abscesses do not show signs of systemic illness.  Internal abscess formation may occur within the thorax (chest) or abdomen with the most common organs affected being the liver and lungs, and kidney and spleen less commonly.  Clinical signs associated with internal abscesses are non-specific and include fever, anorexia, depression, weight loss.  More specific signs would be associated with the organ involved such as respiratory disease or colic.  Death rarely occurs in horses affected with external abscesses however the case-fatality rate is 30% to 40% in horses with internal abscesses.

Presumptive diagnosis can be made based on identification of the classic “pigeon breast” appearance.  The use of ultrasound is helpful in locating abscesses that are located deep to muscle, and identifying organs affected with internal disease.  Confirmatory diagnosis can be achieved through culture of exudate aspirated or collected from draining abscesses.

A blood test (synergistic hemolysis inhibition test) may be utilized to assist in identifying the presence of internal abscesses. If systemic illness develops following external disease, then internal disease should be suspected.  For these horses, ultrasound and blood testing would be advised to help in the diagnosis of internal disease.

Treatment for external abscesses may vary from case-to-case and from one veterinarian to another.  Once the abscess is mature, it is fairly common practice to establish drainage and lavage the wound with some form of an antiseptic solution.  Administration of an anti-inflammatory is also commonly performed to help alleviate the pain and fever that may be associated with the abscesses.  The decision to administer antibiotics or not may be based on the risk of cellulitis forming in healthy tissue after lancing an abscess, or when the horse is showing signs of systemic illness.  However, the type of antibiotic should be determined based on culture and sensitivity, and the duration of antibiotic therapy should be prolonged.  Once drainage of the abscess is achieved resolution typically occurs within 21 days. Treatment for internal abscesses is beyond the scope of this article.

Prevention is currently based on good fly control, sanitation and minimizing exposure of susceptible horses to exudate from infected horses by isolating infected horses and disinfecting tack/supplies.  There is no vaccine currently available for horses.  It is believed that horses develop a long-lasting immunity following infection.  If you suspect your horse may have pigeon fever you should contact your veterinarian.

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