External abscesses may arise anywhere on the body but develop most frequently in the pectoral region and along the ventral midline of the abdomen. This form of infection is commonly known as pigeon fever, because of the large size of the pectoral abscesses giving the contour or appearance of a pigeon’s breast, or dryland distemper, because of its prevalence in arid geographic regions. Abscesses contain tan, odor-free, purulent exudate and are usually well encapsulated. Additional sites with a predilection for abscess formation include the prepuce, mammary gland, axilla, triceps musculature, limbs, and head. Less common areas are the thorax, neck, parotid gland, guttural pouches, larynx, flanks, umbilicus, tail, and rectum. Septic joints and osteomyelitis have been reported. Horses may have an abscess involving a single site or involving multiple regions of the body. It is common to observe multiple subcutaneous abscesses coursing along a suspected lymphatic vessel.

The clinical signs most frequently associated with external abscesses are regional edema, fever, and nonhealing wounds. Other clinical signs include lameness, ventral dermatitis, weight loss, depression, anorexia, and mammary gland or preputial swelling. Generally, horses with external abscesses do not develop signs of systemic illness, although one fourth will develop fever. If signs of systemic illness are present, further diagnostic testing to rule out internal infection is warranted. In horses with external abscess formation, a large area of edema is often observed in the region. As the abscess matures, the area becomes hard and painful. Abscesses can become quite large, particularly in the pectoral region. Abscesses typically have a thick capsule and can cause severe lameness if located in the axillary, triceps, or inguinal region. Maturation can be slow and drainage difficult to establish if the abscess lies deep to muscle.

After drainage is established, either by spontaneous rupture or lancing, most horses recover within 10 to 14 days without complications. The abscesses may contain 5 to 400 mL of thick, tan, purulent exudate. Ultrasonography may aid in determining the best location for establishing drainage in external abscesses. The case fatality rate for horses with external abscesses is very low (0.8%); recovery generally is complete within 2 to 4 weeks, although rarely horses develop persistent or recurrent infections lasting for more than 1 year.

Horses with severe acute or chronic lameness usually have abscesses deep in musculoskeletal structures associated with the limbs that should be surgically drained, rather than being managed conservatively by waiting for the abscess to mature. Affected horses typically have an inflammatory leukogram, with anemia and hyperglobulinemia. High synergistic hemolysis inhibition (SHI) titers are common, and ultrasonography may be useful in assisting with localization of lesions and facilitating surgical drainage to alleviate lameness. Rarely, osteomyelitis or septic arthritis develops, carrying a poor prognosis.

Regardless of the form of C pseudotuberculosis infection, clinicopathologic abnormalities that may be observed include anemia of chronic disease, leukocytosis with neutrophilia, hyperfibrinogenemia, and hyperproteinemia. These hematologic changes can occur with both internal and external abscesses but are more consistently observed with internal abscesses.