Pigeon Fever (Dryland distemper) in Horses

Key Takeaways

Pigeon fever is a common bacterial infection of horses caused by Corynebacterium pseudotuberculosis

• Even though most affected horses don’t get a fever, it takes its name from the way the associated abscesses shape the horse’s chest to resemble a pigeon breast

• The most common symptoms are limb swelling, usually in one or both hind legs, and abscesses, especially in the pectoral region

Pigeon fever is contagious so quarantine measures are necessary for horses that present symptoms

• Diagnostics include bacterial identification of infected abscesses, blood work, and ultrasound

• Typical treatments include abscess drainage and antimicrobial therapy

• Simple cases of external abscesses usually recover within a month

• The prognosis is guarded for internal and lymphatic infections

A Closer Look: What is Pigeon Fever in Horses?

Symptoms of pigeon fever vary depending on the form. The most widespread form in Canada and the US is characterized by external, pus-filled abscesses and inflamed skin. These abscesses typically develop in the chest area but may also appear along the ventral midline, on the head, and the over the mammary glands.

The ulcerative lymphangitis form presents with swelling and ulcers in the limbs along with lameness and fever. This form is rare in North America occurring in about 1% of all cases. Internal infection is by far the deadliest form. Internal abscesses occur when bacteria find their way into the internal organs, particularly in the liver and the kidneys. The fatality rate for this type of pigeon fever is between 30% and 40%.

Pigeon fever is contagious. The incubation period is around 3 to 4 weeks. Any horse with symptoms must be quarantined. Research strongly suggests that the disease is transmissible through horse-to-horse contact.

Even though pigeon fever isn’t considered contagious to humans, there are a few reports of humans getting sick from working with infected sheep and one of a veterinarian developing pneumonia, which may be associated with exposure to C. pseudotuberculosis. Any signs of pigeon fever should be addressed promptly and people handling potentially infected animals should take additional precautions to avoid exposure until diagnosis is confirmed.

Risk Factors

Pigeon fever may become a medical emergency if early signs are ignored and left untreated. Depending on the form it takes, pigeon fever can lead to chronic dysfunction and recurring infections. The fatality rate is low for external abscesses and ulcerative lymphangitis, but it increases enormously in case of internal infection (mortality of 30 to 40%).   Pigeon fever occurs in horses throughout the whole year with a peak in the summer and fall due to the presence of flying insects. The disease has a worldwide distribution and is found throughout the United States, most predominantly in the Southwest.

Pigeon fever is a contagious disease and horses with symptoms must be quarantined.

Possible Causes

Despite its name, pigeon fever doesn’t come from pigeons or any type of bird. The infection is caused by the bacteria Corynebacterium pseudotuberculosis that enters the body through wounds or mucous membranes. Many insects act as vectors, especially in dry months. The bacteria also thrive in the soil, and manure appears to favor its survival and reproduction.

Main Symptoms

The primary symptoms of pigeon fever include

• Abscesses • Limb swelling (stocking up) • Inflamed skin (Dermatitis) • Lameness

• Fever • Lethargy • Anorexia • Weight loss • Abortion (in pregnant mares)  

Despite the name of the condition, most affected horses do not have a fever.

Testing and Diagnosis

The diagnosis is confirmed by isolating the bacteria Corynebacterium pseudotuberculosis from an abscess. 

Additional diagnostics include: 

• PCR testing of abdominal fluids

• Abdominal ultrasound

• Serologic testing

Steps to Recovery

Treatment is individual to each affected horse, depending on which symptoms it presents and the severity of the condition.  External abscesses are typically treated with lancing and draining.  Long term antimicrobial therapy is indicated for ulcerative lymphangitis and the internal form. Supportive care for limb swelling includes cold hosing and pressure wraps.  

With successful treatment for external abscesses, the swelling is expected to recede visibly within a few days, taking up to a month for a full recovery. Severe cases of ulcerative lymphangitis often become chronic with recurrent abscesses and scarring. Internal infections have a 30-40% mortality rate even when treated appropriately.

Prevention

Fly control measures and good sanitation are helpful for preventing pigeon fever. Insecticides are a useful tool to kill adult flies. Regular application of insect repellents and practicing meticulous wound care with topical insect repellents and antimicrobials can also go a long way in preventing the spreading of the disease.

Always consult a veterinarian before using pesticides on animals. Many available products may be toxic to livestock or other pets, even if advertised as animal grade.

Quarantine of horses with symptoms will help prevent cross infection within the same herd.

Is Pigeon Fever Common in Horses?

Pigeon fever is common in horses, especially in dry seasons when there is peak flying insect activity.

Typical Treatment

• Abscess wound management • Antibiotics • Leg wraps • Supportive care

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