Pigeon fever is a bacterial infection in horses that causes large, painful abscesses, most commonly on the chest, belly, and groin. The swelling on the chest can become so pronounced that it resembles a pigeon’s breast, which is how the disease got its name. Also called “dryland distemper” or “false strangles,” pigeon fever is caused by the bacterium Corynebacterium pseudotuberculosis and occurs most often during hot, dry weather when fly populations are high.
How Horses Get Pigeon Fever
The bacteria live in soil, where they can survive for long periods. Flies are the primary vector, picking up the organism from contaminated soil or from draining abscesses on infected horses and carrying it to healthy ones. The bacteria typically enter through small wounds, abrasions, or breaks in the skin, including areas irritated by insect bites themselves. This is why outbreaks tend to spike in late summer and early fall, when conditions are hot and dry and fly activity is at its peak.
Pigeon fever is not directly contagious from horse to horse through casual contact. The risk comes from shared environments where flies move freely between animals and where soil has been contaminated by pus from ruptured abscesses. A single draining abscess can shed millions of bacteria into the surrounding dirt, creating a reservoir that persists long after the infected horse has recovered.
Three Forms of the Disease
Pigeon fever presents in three distinct forms, and the severity varies dramatically between them.
External Abscesses
This is by far the most common form, accounting for the vast majority of cases. Abscesses develop in the muscles of the chest, along the midline of the belly, in the groin, or in the sheath or udder area. They start as firm, hot swellings that gradually enlarge and soften over days to weeks as pus accumulates. A single abscess can grow to the size of a football and contain several liters of thick, foul-smelling discharge. Some horses develop multiple abscesses at once. The good news is that external abscesses, while dramatic and uncomfortable, rarely threaten the horse’s life. Most horses recover fully once the abscesses drain and heal.
Internal Abscesses
In a small percentage of cases, the infection seeds internally, forming abscesses in the liver, kidneys, lungs, or other organs. This form is far more dangerous. Internal abscesses are harder to detect because the swelling isn’t visible from the outside. Horses with internal infection often show vague signs: persistent fever, weight loss, poor appetite, lethargy, and sometimes colic. Without treatment, internal pigeon fever can be fatal. Even with aggressive treatment, the mortality rate for internal cases is significantly higher than for external abscesses.
Limb Infection (Ulcerative Lymphangitis)
The third and least common form involves infection of the lymphatic vessels in the legs. This causes painful swelling in one or more limbs, along with draining sores along the lymphatic tracts. It can be slow to resolve and sometimes leads to chronic leg swelling if the lymphatic system is damaged.
What It Looks Like in Your Horse
The earliest sign is usually a warm, firm swelling somewhere along the chest, belly, or between the front legs. The horse may seem stiff or reluctant to move, especially if the abscess is in a location that makes walking uncomfortable. Fever is common in the early stages, often reaching 102 to 104°F. As the abscess matures over one to three weeks, it gradually softens at the center as the interior liquefies into pus.
Some horses become noticeably lethargic and go off their feed while the abscess is developing. The area around the swelling is typically very tender to the touch. Once the abscess ruptures or is drained, horses usually improve quickly, with fever dropping and appetite returning within a day or two. But the drainage site can continue oozing for days to weeks, and new abscesses sometimes form nearby as the infection spreads through local tissue.
How Pigeon Fever Is Diagnosed
A veterinarian can often identify pigeon fever based on the location and appearance of the abscesses, particularly in regions where the disease is common. Ultrasound is useful for determining whether an abscess is mature enough to drain by showing the pocket of fluid within the surrounding tissue. For suspected internal abscesses, ultrasound of the abdomen and chest can reveal organ involvement.
Definitive diagnosis comes from culturing the bacteria from abscess fluid. A blood test called the SHI (synergistic hemolysis inhibition) test can measure antibody levels against the bacteria. Rising titers on serial blood tests, taken two to three weeks apart, help confirm active infection. This test is especially valuable for diagnosing internal cases where there’s no external abscess to sample.
Treatment and Recovery
For external abscesses, the primary treatment is allowing the abscess to mature fully and then draining it. Veterinarians sometimes lance abscesses once they’ve softened, or they may apply hot compresses and topical drawing agents to encourage the abscess to come to a head and rupture on its own. Premature lancing, before the abscess has fully walled off, can actually spread the infection into surrounding tissue.
Antibiotics are generally not used for uncomplicated external abscesses. This may seem counterintuitive, but antibiotics can actually slow the maturation process, prolonging the disease. They can also suppress the abscess without fully clearing the bacteria, leading to recurrence. Antibiotics are reserved for cases involving internal abscesses, limb infections, or situations where the horse is severely ill with high fever or secondary complications. When antibiotics are needed, treatment courses can be long, sometimes lasting weeks to months for internal cases.
After an abscess is drained, the cavity needs to be flushed regularly to keep it clean and open so it heals from the inside out. Most horses with external abscesses recover fully within two to four weeks after drainage, though some develop additional abscesses that extend the process. Horses with internal infection face a longer, more uncertain recovery that can stretch over months.
Where Pigeon Fever Occurs
Pigeon fever was first described in California in 1915 and was long considered a disease of the dry western United States, most commonly seen in California, Arizona, and west Texas. That geographic picture has changed substantially. Over the past two decades, cases have expanded eastward, with outbreaks reported across the southern and central United States and into areas that had never previously seen the disease.
Texas has seen a particularly dramatic increase. A study tracking laboratory-confirmed cases from 2005 to 2011 found that the disease expanded steadily across the state, with a major cluster emerging in central Texas. High-risk areas were also identified in the panhandle and northern regions. The trend showed two seasonal peaks and an overall increasing pattern of cases in the horse population. Penn State Extension has published guidance specifically because pigeon fever has been moving into the eastern states, catching horse owners off guard in regions where the disease was previously unknown.
Prevention Strategies
There is no commercially available vaccine for pigeon fever. Prevention centers on reducing fly exposure and limiting environmental contamination.
- Fly control: Use fly spray, fly sheets, fly masks, and fans in barns to reduce contact between flies and horses. Fly traps and parasitic wasp programs can help reduce populations on the property. Since flies are the primary transmission route, this is the single most impactful preventive measure.
- Wound care: Clean and cover any cuts, scrapes, or skin irritation promptly. Open wounds give the bacteria an entry point.
- Isolation of infected horses: Horses with draining abscesses should be separated from the herd. The pus is heavily loaded with bacteria and quickly contaminates soil, bedding, and water sources.
- Environmental cleanup: Contaminated bedding and soil should be removed and replaced. The bacteria thrive in hot, dry conditions, so paying extra attention to hygiene during summer and early fall is especially important.
Horses that recover from pigeon fever do develop some degree of immunity, but it isn’t absolute. Reinfection is possible, though generally less severe than the first episode. In endemic areas where the disease returns every year, consistent fly management throughout the warm months is the most practical line of defense.