English
Pigeon fever is a bacterial infection in horses caused by the bacterium Corynebacterium pseudotuberculosis. Symptoms vary from lumps on the skin with redness and oozing to systemic signs, such as lethargy, fever, weight loss, and colic. [1][3][4]
Horses contract the infection when the bacterium enters their body through open wounds or broken skin. This transmission typically occurs via flies, contaminated soil, or contaminated objects in the horse’s environment.
Diagnosing pigeon fever requires a bacterial culture to confirm the presence of C. pseudotuberculosis. Blood tests and ultrasounds can support a diagnosis, but they are not definitive for pigeon fever on their own. [1][3][4]
Treatment of C. pseudotuberculosis infection usually includes cleaning the affected area and providing supportive care, which may include pain management. Antibiotic therapy is often necessary, especially for infections that affect internal organs. [1][4][5]
The prognosis for most horses with pigeon fever is good. However, in cases where the infection has spread to internal organs, the mortality rate can be as high as 30-40%. [1][4][6][7]
Pigeon Fever in Horses
Pigeon fever is caused by an infection with Corynebacterium pseudotuberculosis, a gram-positive anaerobic rod bacterium. In horses, this infection results in large swellings on the chest, resembling a pigeon’s breast. [1][3][5]
Pigeon fever is also known by other names, including “dryland distemper,” “false strangles,” or “ulcerative lymphangitis.” Each of these names highlights different aspects of the disease’s symptoms and characteristics. [1][2][3]
While this bacterium can be found globally, it is most prevalent in the southwestern United States. The nickname, “dryland distemper,” reflects its higher prevalence in the drier regions of the country. [4][5]
Prevalence
First identified in California in 1915, Pigeon fever is sporadically diagnosed across the United States, but has an overall low disease prevalence. [5]
Infections can occur at any time of the year. However, outbreaks are more common during late summer and fall, especially after periods of high temperatures and drought. [1][5][8]
The disease does not show a higher prevalence in any particular breed or sex of horses. [5] However, young adult horses tend to contract this infection more frequently. [8]
Types of Pigeon Fever
There are three main forms of pigeon fever, characterized by distinct clinical manifestations: [3][4][5]
- External abscesses: These are abscesses that form on the chest, abdomen, or legs of the horse.
- Internal abscesses: This type involves abscesses developing in internal organs such as the liver, spleen, kidneys, or lungs.
- Limb infections: Also known as lymphangitis, this form leads to infection in the lymphatic system, resulting in oozing sores on the legs.
Mares are more likely than males to develop internal abscesses caused by C. pseudotuberculosis. [4]
Among the three forms, the one affecting the limb lymphatics is the least common manifestation of pigeon fever. [5]
Clinical Signs
Horses with pigeon fever typically display swellings on the chest, abdomen, or lower limbs. These swellings are external abscesses – pockets of infection underneath the skin. [3][4][5]
Swellings can extend to nearby areas like the mammary gland and genital region. [1] External abscesses can grow large, up to 20 cm (8 in) in diameter, often containing a thick, tan fluid that might ooze from the skin. [1][4][6]
Complications of Internal Abscesses
Without timely treatment, lesions and clinical signs may worsen, and skin infections can spread to internal organs. [1] Abscesses inside the body, can result in additional symptoms: [1][3][4]
- Fever
- Weight loss
- Depression/lethargy
- Decreased appetite
- Colic
- Difficulty breathing
Other less common symptoms associated with internal abscesses include: [4]
- Fluid swelling under the skin of the abdomen (potbelly appearance)
- Skin rash
- Uncoordinated, staggering gait
- Bloody urine
- Abortion in pregnant mares
Ulcerative Lymphangitis
When C. pseudotuberculosis infects the lymphatic system, it causes ulcerative lymphangitis. Accompanying symptoms may include: [1][3][4][6]
- Bleeding
- Oozing of the skin
- Soft, fluid swelling of the surrounding tissues
- Rashes or skin lesions (ulcer)
- Red, itchy skin
- Painful limbs
- Crusting of the skin
- Hair loss
- Fever
- Lameness
- Depression
- Anorexia
Ulcerative lymphangitis in horses mainly affects the lower limbs. [1]
Disease Transmission
Infection with C. pseudotuberculosis in horses occurs when the bacterium penetrates the skin, typically through scratches, puncture wounds, or fly bites. Flies such as house flies, stable flies, and horn flies are capable of transmitting the bacteria into open wounds. [1][9]
The bacterium is naturally present in the soil and can thrive in various environmental conditions, including in the absence of oxygen. It can also be found in hay, shavings, manure, and on contaminated farm equipment. [1][3][4][10][11]
Pathogenesis
Upon entering a wound, Corynebacterium pseudotuberculosis is engulfed by white blood cells, which are a critical component of the equine immune system.
Rather than being destroyed by white blood cells, the bacteria can survive and replicate within these cells. The inability of these immune cells to destroy the bacteria leads to the formation of an abscess. [4][12]
Once established in the horse’s body, the bacterium secretes a toxin that causes blood vessels to leak, facilitating local invasion and subsequent spread to the lym