Myth No. 1: Pigeons cause Pigeon Fever

It isn’t pigeons but rather, the soil-inhabiting C. pseudotuberculosis, which produces a toxin called phospholipase D that causes the characteristic severe inflammation and local tissue damage infected horses exhibit. While scientists have speculated about a number of infection routes, they have not yet identified one specific vector (carrier). In all likelihood, horses are infected by the bites of contaminated flies (that have collected the bacteria from soil and/or from an infected horse’s discharge) or via wound contamination with the organism.

Infection in equines may present as:

• External Abscesses: These abscesses can break out anywhere along the body, head, limbs, or trunk. The most common locations are the midline of the belly and beneath the chest muscles. Large lumps in the latter location are the reason for the term pigeon fever; as the chest abscesses develop, the swollen tissue causes a pigeon breast appearance.
• Internal Abscesses: Fewer than 10% of affected horses develop internal abscesses, which can be challenging to diagnose, require prolonged treatment, and are potentially lethal. Veterinarians can diagnose these abscesses using ultrasound or by considering a combination of clinical signs, lab work results, and the synergistic hemolysis inhibition (SHI) blood test outcome.
• Ulcerative Lymphangitis: Horses with this form develop swelling in their lower limbs, as well as oozing sores. This presentation is uncommon but extremely painful.

While sheep and goats can also contract disease related to C. pseudotuberculosis, horses cannot contract the unique bacterial strain that infects sheep and goats. Both cattle and horses, however, can be infected with the same strain. But it is uncommon for horses with no exposure to cattle to become infected.

Veterinarians first isolated C. pseudotuberculosis in California, and cases have been commonly seen during the late summer to late fall months in arid regions of the state. However, this landscape is changing. Owners and veterinarians are reporting infections in Arizona, California, Colorado, Idaho, Kansas, Kentucky, Missouri, Montana, Nevada, New Mexico, Oklahoma, Oregon, Texas, Utah, Washington, and Wyoming, along with Alberta and British Columbia, Canada. Drought and the changing climate may play a role in the disease’s increased geographic spread.

While external abscesses associated with pigeon fever typically mature and rupture spontaneously, involving your veterinarian in the treatment is the best way to ensure a good outcome. Not all cases of C. pseudotuberculosis infection result in "simple" pigeon fever or even the internal organ abscesses described previously. Some abscesses can develop very deep within the muscle causing severe pain, non-weight-bearing lameness, weight loss, and even infection of the bone and nearby joints.

Coordinating treatment with a veterinarian will not only allow for the best outcome for the horse, but will also help prevent the spread of the disease on premises, allow practitioners to track case numbers, geographic spread, and identify any strains developing antibiotic resistance.

In a 2005 retrospective study of 30 horses with internal C. pseudotuberculosis abscesses, Suzanne Pratt, DVM, and co-authors from UC Davis found that 63% of the affected horses also had a concurrent or recent history of external abscesses. This overlap of the internal and external disease might have led some people to believe that treating the external disease causes internal abscess formation. However, no scientific evidence has been found to support this theory.

Veterinarians advocate against a stringent quarantine of individually affected horses. Instead, they recommend using sensible judgment when dealing with C. pseudotuberculosis cases. Minimize soil contamination by collecting pus from lanced abscesses, providing proper wound care, and cleaning and disinfecting nonsoil surfaces that pus can contaminate, such as wash racks, stall doors, tack, or grooming equipment.

(This is a summary of an excellent article published here: http://horseandman.com/medical/tis-the-season-for-pigeon-fever)