Pigeon fever, also known as ”dryland strangles,” is a bacterial infection. It produces typically thick-walled abscesses that contain heavy, creamy pus and resemble tuberculosis lesions.
It was dubbed ”pigeon fever” because over 60% of the infections are on the chest where the abscesses result in a puffed-out appearance that looks like a pigeon breast. The origin of the term ”dryland strangles” comes from the facts that the infection is most common in dry areas with low annual rainfall and the strangles organism also produces abscesses.
For years, pigeon fever was almost unheard of anywhere but the West Coast. However, whether because of better diagnosis or the disease actually spreading, in recent years clusters of cases have been reported in Nevada, Wyoming, Colorado, Oklahoma, Texas and Kentucky.
Pigeon-fever organisms can be found in soil. The theory is that the reason so many horses develop the abscesses on their chest is that the organism gains access through small skin abrasions when the horse is lying down. This fact would also explain why the involvement of the belly, mammary gland or sheath is also common in horses.
Some horses develop generalized leg swelling with this infection and may have small abscesses develop along the course of lymphatic channels in their legs. Because the organism travels along lymphatics, abscesses can form nearly anywhere.
It isn’t an infectious disease from horse to horse in the classical sense that just having contact with an infected horse can cause problems. However, draining abscesses will contaminate the premises and make infection of any herd mates with breaks in their skin likely. And, of course, flies can spread the infection from horse to horse.
Like strangles, an infection in a stalled horse also could be mechanically spread to others in the barn that may have small breaks in their skin. Organisms may be on your hands, shoes or clothing, on wheel barrow tires, pitch forks, grooming equipment, halters, barn animals, ground or fencing in shared paddocks, etc.
Cases tend to peak from late summer into fall. Since the abscesses grow slowly, this trend toward a seasonal pattern suggests that flies play an important role in the infection. It takes a minimum of one to two months after the organism gets into the horse’s body for symptoms to appear.
When swellings are noticed on the chest, they’re often first mistaken for kicks, while swelling on the legs, belly, udder/sheath and elsewhere often baffle veterinarians not familiar with this infection.
As the abscesses continue to grow, it becomes increasingly obvious you’re dealing with abscesses. Their location and the typical thick, creamy pus are highly suggestive of a pigeon-fever infection.
The organism can also be cultured to confirm the diagnosis. Horses with external abscesses usually aren’t sick and have a fever, although variable degrees of lameness and local pain may develop.
Between 60 and 80% of all pigeon-fever cases involve abscesses limited to the body surface. However, the ascesses can form inside the abdomen or chest, or in internal organs. Horses with internal infections will be much sicker than those with external abscesses, usually showing fever at some point, elevated white cell counts, poor appetite, weight loss, depression/lethargy/weakness, possibly elevated liver enzymes, colic and other symptoms.
Prognosis for recovery is much lower in horses with internal abscesses. However, this may be largely because the horses aren’t treated adequately. Internal abscesses are likely to require several months of uninterrupted antibiotic therapy to really clear the infection.
There is an antibody blood test available, and it will always be positive in horses with internal abscesses but may not when the abscesses are all on the body surface.
Antibiotics aren’t routinely used for the external abscesses. Hot packs can help the abscesses mature, and the vet may lance them open if they aren’t bursting spontaneously. The abscess cavity is then thoroughly flushed with a disinfectant and the area treated like an open wound until it heals, which can take up to weeks.
Ideally, abscesses should be opened with the horse in an area that can be easily disinfected, such as a concrete wash stall. The horse should be kept in insect-proof isolation, with all bedding disposed of by burying, double bagging or burning until drainage has ceased.