What is 'Dryland Distemper / Pigeon Fever', and how does a horse get it?

Dry Land Distemper, or Pigeon Fever, is caused by a bacterium called Corynebacterium Pseudotuberculosis. The disease is found in mares, geldings and stallions of all ages and breeds. It can occur in foals, though it rarely occurs in foals less than six months of age. Dry Land falls into one or more of three forms. The first form, called ulcerative lymphangitis, presents itself as swelling in one or more limbs with draining sores on the legs. These horses may develop severe lameness, decrease in appetite, fever and lethargy. The second form, external abscesses, usually involves the pectoral muscles (chest muscles), muscles along the bottom of the abdomen, the mammary glands, inguinal region, limbs, head, and axilla. The abscess are seen as firm to soft bumps that eventually rupture and drain externally. In a small number of cases, internal abscesses develop (the third form of Dryland). These are within the body of horses, usually in the liver, but sometimes in other organs. Some horses with internal abscesses have external abscesses, and some do not. Signs of internal abscesses include lethargy, weight loss, lack of feed intake, colic, fever, blood in the urine, coughing and abnormal results on blood work. Internal abscesses occur in less than 10% of horses with Dryland.

Corynebacterium Pseudotuberculosis can survive for long periods of time (as long as months or years), in the soil. The disease seems to occur most frequently dry months of the year, following large amounts of rainfall. Biting flies are thought to spread the disease, though it is speculated that the disease can also be spread through contact with inanimate objects, horse to horse contact, and through contaminated soil. The exact incubation period of Dryland in horses is not known, but has been documented to range from weeks to months.