Pigeon fever, scientifically known as an infection caused by the bacterium Corynebacterium pseudotuberculosis, is a common concern for horse owners. While particularly prevalent in dry, arid regions, its geographic range is expanding across the United States. This article addresses the risk of this zoonotic transmission—whether the infection can cross the species barrier from horses to humans—and outlines steps people can take to protect themselves.
Understanding Pigeon Fever in Horses
The infection in horses is caused by the Gram-positive bacterium Corynebacterium pseudotuberculosis, a soil-dwelling organism that can survive in the environment for months. It earned the name “Pigeon Fever” because of the large, painful abscesses that frequently develop in the pectoral (chest) region, resembling a pigeon’s puffed-out breast. These external abscesses are the most common presentation and generally carry a very low fatality rate.
The bacteria typically gain entry through small breaks in the skin, such as scrapes or minor wounds. The disease can also manifest as ulcerative lymphangitis, causing severe swelling and lameness, or as an internal infection. The internal form affects organs like the liver, spleen, or kidneys, is less common, but carries a higher mortality rate, sometimes reaching 30 to 40 percent. Flies, such as stable flies and house flies, act as mechanical vectors, picking up the bacteria from draining abscesses or contaminated soil and depositing it on open wounds.
The Risk of Human Infection
While Corynebacterium pseudotuberculosis is an established pathogen in horses, the risk of transmission to humans is rare. Human infection is classified as a zoonosis, but documented cases remain infrequent globally. The bacterium is found primarily in the soil and contaminated environments, surviving in dirt for up to eight months and on materials like wood or hay for weeks.
Transmission to people requires direct inoculation of the bacteria into a wound, not casual contact with a horse. This most often happens when handling draining abscesses or cleaning stalls contaminated with pus and soil. The bacteria enter the human body through a cut, scratch, or abrasion on the skin. This makes it an occupational risk for veterinarians, farriers, and horse handlers.
The equine biotype of C. pseudotuberculosis can cause disease in humans, but person-to-person spread is virtually non-existent. The risk remains tied solely to direct contact with the infected horse or its environment.
Symptoms and Disease Progression in Humans
When a human becomes infected with C. pseudotuberculosis, the clinical presentation is generally localized, manifesting as cutaneous lymphadenitis. This involves the swelling and inflammation of the lymph nodes nearest to the site of entry. The affected lymph nodes become painful, firm, and may form an abscess that can eventually rupture and drain pus.
The infection often presents with systemic signs such as fever, chills, body aches, and malaise. Lymph nodes in the neck, armpit (axillary), or groin region are commonly affected, depending on the location of the initial skin break. Although the infection is typically confined to the lymph nodes and adjacent skin, delayed diagnosis can lead to a prolonged or recurrent clinical course.
In rare instances, particularly in immunocompromised individuals, the bacteria can spread beyond the localized lymph nodes. This systemic infection can lead to serious conditions like pneumonia or abscesses in internal organs such as the liver or spleen. For most healthy individuals, the disease remains localized, and treatment involves surgical removal of affected lymph nodes, often supplemented with antibiotics.
Preventing Transmission
Preventing the transmission of Corynebacterium pseudotuberculosis relies on strict hygiene and environmental control when dealing with an infected horse. Individuals should always wear disposable gloves when handling a horse with an active, draining abscess or when cleaning contaminated equipment and stalls. Any open cuts or abrasions on the hands and arms must be thoroughly covered before working in a contaminated environment.
Thorough handwashing with soap and water immediately after contact with an infected horse or contaminated materials is an effective measure. Environmental management is also crucial, as the bacteria thrive in soil and are spread by flies. Implementing aggressive fly control, such as fly sprays and manure management, helps reduce the mechanical transfer of the bacteria between the horse and the environment.
Proper disposal of contaminated bedding, soil, and drainage from abscesses prevents the pathogen from persisting. Isolating horses with active infections and using dedicated cleaning supplies limits the spread of bacteria to other horses and minimizes human exposure.