Page 427 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

402 CHAPTER 1

VetBooks.ir worldwide distribution. The mode of infection is the subcutis) has also been described. Lesions develop
slowly and may be single or multiple. They drain a
unknown, but it is postulated that organisms enter
through abrasions in the skin. The organism is trans-
greenish. Pitting oedema, ventral midline dermatitis,
mitted by flies, particularly horn flies (Haematobia creamy to caseous material that may be whitish to
irritans), stable flies (Stomoxys calcitrans) and house- depression, fever and lameness are common.
flies (Musca domestica). The abscesses are caused by The course of the disease may be chronic. Pectoral
haematogenous or lymphatic spread of C. pseudo- abscesses are frequently deep (12.5–20 cm [5–8
tuberculosis. Infections with C. pseudotuberculosis are inches]) and establishing drainage can be a challenge.
on the rise, with infected horses being identified As for any other deep abscesses, there is usually local
throughout the United States and parts of Canada. heat, pain and swelling and there may be associated
The condition was originally reported as seasonal stiffness or lameness in one or more limbs depend-
infections, especially in the Western United States, ing on the location. In one study, over 90% of cases
that occurred most often in dry, dusty conditions of had grade 4/5 lameness. Multiple small nodules and
summer and autumn, when the fly season was at its lymphangitis may be present (Figs. 1.771–1.773).
peak. The condition is now reported from horses of Systemic signs of infection (e.g. fever, anorexia) are
all ages and all months of the year. The bacterium is rare.
soil borne and able to survive and persist in a wide
range of environmental conditions. There may also Differential diagnosis
be a predisposition for horses infested by Habronema Larva migrans; abscesses of other origin, including
spp. or other migrating parasite larvae to develop brucellosis; tumours; reaction to injections, insect
myonecrosis. stings or snake bites.

Clinical presentation Diagnosis
A localised abscess forms within a muscle mass, most Horses with severe lameness caused by C. pseudotu-
frequently in the pectoral region, but also in the limbs, berculosis can present a diagnostic challenge because
inguinal region, ventral abdomen and thorax. A mili- they may not have visible external swelling but often
ary, subcutaneous form (multiple small abscesses in have clinical signs and blood work consistent with

1.771 1.772 1.773

Figs. 1.771–1.773 C. pseudotuberculosis causing diffuse lymphangitis-like cellulitis and multiple muscle
masses in the neck (1.771) and limbs (1.772, 1.773).