Pigeon fever, also known as dryland distemper or false strangles, is caused by the bacterium Corynebacterium pseudotuberculosis. The infection typically leads to large, pus-filled abscesses in a horse’s pectoral (chest) muscles, making them resemble a pigeon’s breast, along the midline, or on the limbs. While most common in the summer and fall, horses can become infected with this highly contagious disease any time of the year.

“The (C. pseudotuberculosis) bacterium is viable for months in the soil, as well as on hay or in shavings,” says Cassandra Olds, PhD, veterinary entomologist and assistant professor at Kansas State University, in Manhattan. “There can also be a long time between infection and when they get sick,” she adds, which can make it more challenging for owners and veterinarians to determine the source.

“The place where the bacteria enter the body is not always the same place that the disease symptoms are observed,” she explains. “The bacteria may enter a scratch on the leg or hock sore contacting the ground, but a pus-filled swelling is observed on the chest.”

Signs of Pigeon Fever in Horses

Again, C. pseudotuberculosis can persist in the environment for prolonged periods of time, says Robin Knight, DVM, of Idaho Equine Hospital in Nampa. “It has been found to survive for two months in hay and bedding and eight months in the soil. When horses are forming these abscesses, they are typically lethargic and may have a fever. They often have a visible swelling that is very painful to the touch. Horses that have abscesses deep in the triceps (the muscles that extends the elbows and are essential for weight bearing) or thigh (between the stifle and hip joint) may just present for lameness because the abscess is not visible externally.”

Veterinarians occasionally see internal abscesses on the liver, kidney, spleen, and lungs in horses with pigeon fever, says Knight. Another manifestation of the disease, ulcerative lymphangitis, causes oozing sores along the lower limbs, which is extremely painful for horses and challenging for veterinarians to resolve.

If you suspect pigeon fever in your horse, contact your veterinarian immediately. Typically, veterinarians culture some of the abscess fluid, which helps them arrive at an accurate diagnosis and develop a treatment plan, says Knight.

Managing and Treating Horses With Pigeon Fever

When treating horses with pigeon fever the veterinarian’s main goal is to identify the abscess(es) and drain them, says Knight. “We often use ultrasound to help us identify the actual abscess and the thick capsule around it so that we can actually incise into the pus-filled area itself.” As the pus drains, be sure to minimize soil or bedding contamination with C. pseudotuberculosis by covering the wound and having handlers follow safe sanitation practices (i.e., washing hands and changing clothing after caring for affected horses). Having the abscess drained also means your veterinarian can safely dispose of the abscess contents and any contaminated supplies.

“Affected horses as well as nearby horses should have good fly control measures to help limit spread to additional horses,” says Knight. Cover wounds when possible, she adds, and for wounds that can’t be bandaged use fly ointment around the area to help limit transmission.

Most horses recover from pigeon fever and, clinically, many recovered horses appear to develop some immunity, says Knight; however, more research is needed about how long protective immunity persists and what percentage of infected horses develop it.

Preventing Pigeon Fever in Horses

Veterinarians tend to observe pigeon fever more in dry, warm conditions, but C. pseudotuberculosis can cause infections anytime horses have open wounds. “Both biting and nonbiting flies are indicated in the transmission of this disease as they move it from horse to horse,” Olds says. “A fly picks up the bacteria on their mouthparts then mechanically delivers (them) from horse to horse.” However, because the bacteria can live in soil, flies are not a necessary part of the transmission process.

Fly sheets, fly boots, and other coverings reduce the surface area on horses where flies can land. While fly sprays might be effective for just a few minutes, covering your horse with sheets and boots can protect them for much longer. If your horse has any open wounds, keep flies away from those areas with a petroleum-based gel repellent, says Olds.

Follow good sanitation practices by picking up manure regularly and composting when possible. Reduce mud by fixing leaks, improving drainage around the farm, and keeping areas around water troughs dry. Minimize hay wastage on the ground, which also attracts flies, by removing spent hay before it mixes with manure, urine, or water.  

Take-Home Message

Pigeon fever is an infection caused by the bacterium Corynebacterium pseudotuberculosis. It usually results in large, pus-filled abscesses in a horse’s pectoral muscles, along the midline, or internally in organs such as the liver, kidneys, spleen, or lungs. If you suspect your horse has pigeon fever, contact your veterinarian immediately to allow for an accurate diagnosis and prompt treatment. Prevent the spread of disease by removing affected horses from the herd and with good fly management practices. While pigeon fever is most common in the summer and fall, it can infect horses at any time of year.