In horses, Corynebacterium pseudotuberculosis causes ulcerative lymphangitis (an infection of the lower limbs) and chronic abscesses in the pectoral region and ventral abdomen. It is one of the most common and economically important infectious diseases of horses in California and Texas and is increasing in prevalence in other western and Midwestern states of the USA. In cattle, the bacteria most commonly cause cutaneous excoriated granulomas. Large, ulcerative skin lesions resembling infected granulation tissue and lymphangitis may occur in 2%–5% of cows. Location on the animal is variable but is often associated with skin trauma. Healing often occurs without treatment or with limited topical treatment in 2–4 wk. Abortion and mastitis may also occur. Rarely, visceral involvement has been reported.
Pathogenesis and Clinical Findings
The onset of ulcerative lymphangitis in horses is variable and usually manifests as painful inflammation, nodules, and ulcers, especially in the region of the lower limb, or lameness and edematous swelling can extend up the entire limb. The exudate is odorless, thick, tan, and blood tinged. Usually, only one leg is involved. If the animal is not treated aggressively with antimicrobials, lesions and swelling usually progress and become chronic with relapses.
In the southwestern USA, C pseudotuberculosis infection in horses is seasonal, with a peak incidence in summer and fall. Infection results in abscessation of the pectoral region or ventral abdominal region with secondary dissemination to internal organs. Clinical signs include diffuse or localized swellings, ventral pitting edema, ventral midline dermatitis, lameness, draining abscesses or tracts, fever, weight loss, and depression. Leukocytosis and neutrophilia may be present. A marked or prolonged fever, anorexia, or weight loss indicates untoward sequelae such as deep or recurring abscesses, internal abscessation, or systemic infection with abortion. Abscesses can be large, up to 20 cm in diameter before rupturing, and take weeks to months to resolve. Weight loss, colic, splinted abdomen, or lethargy may be signs of internal abscesses. Dermatitis lesions are painful and mildly pruritic with alopecia, exudation, crusting, and ulceration.
The bacteria enter via skin wounds by arthropod vectors such as stable flies, horn flies, and house flies, or by contact with contaminated fomites or soil.
Isolation of C pseudotuberculosis from lesions is necessary for confirmation. In all forms of lymphangitis in horses, samples for culture include aspirates of abscesses, swabs of purulent exudate beneath crusts associated with folliculitis, and punch biopsies. Differential diagnoses include pyoderma, abscesses, lymphangitis from other bacteria (eg, Staphylococcus aureus,Rhodococcus equi, Streptococcus spp, or Dermatophilus spp), dermatophytosis, sporotrichosis, equine cryptococcosis, North American blastomycosis, and onchocerciasis.
Abdominal ultrasonography is useful for detection of internal infection of the liver, spleen, or kidneys. Ultrasonography is also useful for detection and drainage of deep abscesses causing lameness, particularly in the triceps musculature. Transtracheal aspirates are required to confirm pneumonia caused by C pseudotuberculosis. Serologic testing with the synergistic hemolysis inhibition test, which detects IgG to the phospholipase D exotoxin, is a useful adjunct for diagnosis of internal infection in the absence of external infection.
Lymphangitis and internal infection should be treated with longterm antimicrobials (1 mo duration or as directed by follow-up ultrasonography). The organism is susceptible to most commonly administered antimicrobials; however, antimicrobial treatment of uncomplicated external abscesses may prolong the disease by delaying abscess maturation. External abscess swellings are treated with hot packs, poultices, or hydrotherapy until they rupture or are drained surgically. Abscesses are lanced and flushed with dilute antiseptic solutions. Deep abscesses in the triceps or quadriceps region require ultrasonography to guide placement of an indwelling drain. Phenylbutazone relieves pain and swelling. General supportive and nursing care is indicated.
If treatment is successful, the swelling gradually recedes over days or weeks. Internal infection has a 30%–40% mortality rate, even with appropriate treatment. Severe or untreated lymphangitis cases often become chronic, and fibrosis and induration of the leg occur. Isolation of infected horses, fly control, and good sanitation are recommended for prevention.
Last full review/revision September 2014 by Sharon J. Spier, DVM, PhD, DACVIM