Pigeon Fever in Horses: Signs, Treatment, and Prognosis

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]

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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 lymphatic vessels. ^[4][12]

The time between exposure to the bacteria to the appearance of infection symptoms can range from 7 to 28 days. ^[13]

Diagnosis

If you suspect your horse may be affected by Pigeon fever, consult with your veterinarian to obtain an accurate diagnosis. Your veterinarian will conduct a physical exam and perform laboratory tests to rule out other conditions.

Bacterial Culture

The primary method for diagnosing pigeon fever is by taking a swab from the infected area and sending it to a lab for bacterial culture. Biopsies of infected tissue can also be collected. ^[1]

The sample is cultured to identify the presence of C. pseudotuberculosis. ^[1] Bacterial culture is considered the most definitive way to diagnose a Pigeon fever infection. ^[3]

Bloodwork

Horses showing signs of illness usually have a blood sample collected and submitted for various diagnostic tests to determine the cause of disease.

Horses with pigeon fever typically show decreased red blood cells, increased white blood cells, and elevated protein levels on bloodwork. However, these signs are not unique to pigeon fever and can also be seen in other infections. ^[1]

Blood Tests

A specialized blood test called the Synergistic Hemolysis Inhibition (SHI) test may be performed to support a diagnosis of internal abscesses. The SHI test measures antibodies against the toxin produced by the C. pseudotuberculosis bacterium.

This test may be used if bacterial culture is unavailable. The results of SHI are interpreted in combination with clinical signs, history, and blood work because SHI alone is not specific enough to constitute a standalone diagnosis of Pigeon fever. ^{[1][4][6][14][15]}

Ultrasound

Ultrasound may be performed on horses suspected of having internal organs abscesses. An ultrasound can identify masses present within the liver, spleen, kidneys, or lungs. Ultrasound can also be used for deep infections in limbs. ^[1]

If necessary, biopsy samples can be collected from internal abscesses using ultrasound as a visual guide. Tissue samples are then cultured for the identification of C. pseudotuberculosis. ^[4]

Other Tests

If a lung infection is suspected, a transtracheal wash can be performed to collect and analyze fluids from the trachea and lungs. The bacterium are then cultured and identified in samples. ^[1]

Horses with pigeon fever may develop accumulations of abdominal fluid. Collecting and culturing abdominal fluid can also help identify the bacteria. ^[4][6]

Differential Diagnoses

When diagnosing pigeon fever in horses, it’s important to consider and rule out other infectious agents that can cause similar skin lesions or abscesses.

Several bacterial and fungal species can cause infections that produce skin lumps, mimicking the symptoms of pigeon fever. Other potential infections to rule out include: ^[1][4]

• Staphylococcus aureus
• Rhodococcus equi
• Streptococcus
• Actinomyces
• Nocardia
• Pseudomonas
• Mycobacterium
• Dermatophilus
• Dermatophytosis
• Sporotrichosis
• Cryptococcosis
• Blastomycosis
• Onchocerciasis

Secondary Conditions

Horses with severe cases of Pigeon fever can develop secondary conditions including internal organ damage and systemic infections. These conditions occur because of the spread or prolonged presence of the Corynebacterium pseudotuberculosis infection.

Habronemiasis

Horses with chronic Pigeon fever infections are at increased risk of developing cutaneous habronemiasis, also known as “summer sores.” ^[1]

This condition is caused by the stomach parasite Habronema. Ingestion of these larvae typically leads to asymptomatic stomach inflammation. However, in the summer flies can transfer Habronema larvae to open wounds, causing severe and invasive infections​. ^[16][17]

Osteomyelitis or Septic Arthritis

Untreated Pigeon fever infections can spread to the underlying bone, resulting in osteomyelitis, or inflammation of the bone. If the infection progresses, it can infect joints, leading to septic arthritis.

Both of these conditions are extremely painful and result in a poorer prognosis. ^[3]

Other Conditions

Rarely, horses undergoing treatment for pigeon fever can develop purpura hemorrhagica or infection of the vascular system (vasculitis). ^[6]

Treatment

The primary treatment for pigeon fever in horses involves draining and cleaning infected skin lumps (abscesses) and, if necessary, administering systemic antibiotics. ^[1][5]

External Abscesses

External infections can be treated with a combination of hot packs, hydrotherapy, and poultices (clay-like material to help with pain and swelling) until the masses rupture on their own. Alternatively, abscesses can be surgically drained. ^[1]

Antibiotics may not be required for external abscesses, but can be necessary for prolonged infections to ensure complete resolution. ^[3] Common antibiotics for treating Pigeon fever include: ^[4]

• Trimethoprim sulfa: 30 mg/kg administered by mouth, twice daily
• Procaine penicillin: 20,000 units/kg administered intramuscularly, twice daily

Infections that are deeper within the muscle require a specialized drain to be placed for treatment. An ultrasound is often used to ensure proper placement. ^[1]

Internal Abscesses and Lymphangitis

Antibiotics are recommended for horses with internal abscesses and ulcerative lymphangitis. Early and aggressive treatment is best to prevent serious consequences or long-term issues. ^[1][3][4][6]

Treatment duration with antibiotics can range from 1 to 6 months. Antibiotics used for these conditions include: ^[4][6]

• Procaine penicillin: 20,000 units/kg administered intramuscularly, twice daily
• Potassium penicillin: 20,000-40,000 units/kg administered intravenously, four times daily
• Rifampin/Ceftiofur: 2.5-5 mg/kg rifampin administered by mouth twice daily in combination with 2.5-5 mg/kg ceftiofur administered intravenously or intramuscularly twice daily
• Doxycycline: 10 mg/kg administered by mouth twice daily
• Enrofloxacin: 7.5 mg/kg administered by mouth, one time daily

The horse must be closely monitored to determine the optimal treatment duration. Your veterinarian will use clinical signs and blood tests to determine when the infection has cleared, guiding when to discontinue antibiotics. ^[6]

Other supportive care can be offered to minimize pain. This typically includes anti-inflammatory medications such as phenylbutazone or flunixin meglumine. ^[1]

Prognosis

91% of horses that recover from this disease show no signs of recurrent infection on follow-up years later. ^[4][6]

Infections on the limbs typically respond well to treatment, gradually subsiding over days to weeks. However, if left untreated for an extended period, these lesions can cause significant tissue damage and thickening (fibrosis) during and after healing. ^[1]

For horses with infections of the lymphatic system, a prompt and aggressive treatment approach usually results in a better prognosis. Delayed treatment makes horses more susceptible to developing limb swelling in the future. ^[3]

Unfortunately, horses with internal organ infections face a worse prognosis. Even with appropriate treatment, these cases have a 30-40% mortality rate. Internal abscesses are fatal if left untreated. ^{[1][3][4][6][7]}

Prevention

C. pseudotuberculosis is endemic in the soil and cannot be completely eradicated from the environment. However, good biosecurity policies can reduce exposure to the bacteria and help prevent pigeon fever in horses​: ^[1][3][4][5]

• Isolating horses with the infection to prevent spread to others.
• Practicing good hygiene, such as hand washing after handling infected horses.
• Collecting and properly disposing of fluids from drained abscesses to avoid environmental contamination.
• Quarantining new horses to prevent introducing the infection into the herd.
• Maintaining cleanliness and sanitation in barn areas.
• Disposing of bedding that has been contaminated.
• Regular removal of manure from horse living areas.
• Inspecting barns and paddocks for sharp objects that could injure horses.
• Implementing effective fly control measures, including fly sheets, traps, sprays, or ointments.
• Using feed supplements that contain insect growth regulators to control fly populations.

Proper wound care is also important to prevent infection. Keeping wounds clean and minimizing contamination risk in the environment help to reduce the overall risk of infection. ^[5]

Limiting your horse’s exposure to damp or wet conditions may also help in controlling the risk of infection. ^[2]

Vaccines

There is no licensed vaccine to prevent horses from contracting pigeon fever. However, horses that recover from the disease are less likely to contract it in the future. ^[3]

Summary

• Pigeon fever is a serious infectious disease in horses caused by the bacterium Corynebacterium pseudotuberculosis. ^[1][2][3]
• Clinical signs can include swelling on the chest and limbs, but the bacterium may also cause serious internal infections and lymphangitis, leading to non-specific symptoms like depression, weight loss, and colic. ^[1][3][4]
• Diagnosis of pigeon fever requires bacterial culture to detect the presence of C. pseudotuberculosis. ^[3]
• Antibiotic treatment is often necessary for severe infections, while cleaning the area and pain management may suffice for less complicated skin abscesses. ^[1][4][5]
• With appropriate treatment, most horses have a good prognosis, though severe infections can result in long-term complications and even death. ^{[1][3][4][6][7]}
• Prevention of pigeon fever involves isolating affected horses, adhering to good hygiene and cleaning practices, and ensuring effective fly control. ^[1][3][4][5]