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 a common and economically important infectious diseases of horses and cattle worldwide. 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 weeks. Abortion and mastitis may also occur. In cattle, visceral involvement has been reported but appears much less commonly than in horses There have been reports of disease in camels, alpacas, llamas, and buffalo.
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.
C pseudotuberculosis infection in horses occurs at any time of the year. However, peak incidence of disease is during the summer and fall, when flying insects are present. 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
depression
Also, anemia, leukocytosis, neutrophilia, hyperfibrinogenemia, and increased serum amyloid A (SAA) and hyperglobulinemia (indicative of inflammation) are usually 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. Nonhealing abscesses or wounds may be concurrently affected with cutaneous habronemiasis Cutaneous Habronemiasis in Animals Habronemiasis on the penis of a Quarter horse stallion. This intensely pruritic lesion should be differentiated from squamous cell carcinoma and other conditions via biopsy. Cutaneous habronemiasis... read more 
(“summer sores”).
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.
Diagnosis
Bacterial culture of pus from external abscesses
Abdominal ultrasound, clinicopathologic evidence of inflammation, and serology for internal infection
PCR testing of abdominal fluid for Corynebacterium pseudotuberculosis
Isolation of C pseudotuberculosis from lesions is necessary to confirm diagnosis of infection. 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 of C pseudotuberculosisinfection include:
pyoderma
abscesses
lymphangitis from other bacteria (eg, Staphylococcus aureus, Rhodococcus equi, Streptococcus spp, or Dermatophilus spp)
dermatophytosis
sporotrichosis
equine cryptococcosis
North American blastomycosis
onchocerciasis
Ultrasonography of the abdomen and thorax is useful for detection of internal infection of the liver, spleen, kidneys, or lungs. 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. In horses, 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. Serologic testing should not be used alone for diagnosis of infection.
Treatment
Lance and drain external abscesses
Longterm antimicrobial therapy for ulcerative lymphangitis, limb infection, and internal infection
Fly control
Lymphangitis and internal infection should be treated with longterm antimicrobials (a minimum of 1 month 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 or flunixin meglumine 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 in horses, even with appropriate treatment. Severe or untreated lymphangitis cases often become chronic, and fibrosis and induration of the leg occur. Isolation of infected animals, comprehensive fly control including insect growth regulators, and good sanitation are recommended for prevention. A conditionally licensed bacterin/toxoid is currently available for horses.
Key Points
Corynebacterium pseudotuberculosis is a gram-positive, soil-dwelling bacterium.
Infection occurs worldwide and is increasing in frequency in North America.
In horses, external abscesses involving the ventral abdomen and pectoral region are the most common presentation (“pigeon fever”).
In horses, internal infection and ulcerative lymphangitis require longterm antimicrobial therapy.
Fly control and vaccination are recommended for prevention of disease.
