Corynebacterium pseudotuberculosis is a soil-borne anaerobic bacterium. has a thick lipid coat that lies outside its cell wall. In addition, it produces an exotoxin, phospholipase D (PLD). Both of these bacterial products enhance the survival of the bacterium inside the host. When macrophages engulf the bacterium, the lipid coat protects it from the action of lysosomal enzymes; the bacteria survives and replicates as intracellular pathogen. 3 Once established in the host, Corynebacterium pseudotuberculosis evades the immune system with relative ease, causing chronic infections that persist for the life of the animal. 4 The bacterium can, though rarely, cause lymphadenitis in humans. Infections have been recorded in farm workers and veterinarians exposed to the pathogen. 5,7
In horses Corynebacterium pseudotuberculosis causes infection of lower limbs, internal organs (lungs, liver, and kidneys), abscesses of the pectoral region, and contagious acne. It is closely related to C. diphtheriae and like this species may harbor the diphtheria toxin gene. Nearly all strains produce dermonecrotic toxin. 6,7In the southwestern USA, Corynebacterium pseudotuberculosis infection is seasonal which peaks in the summer and fall. 1 Chest abscesses give a pigeon breast appearance to affected horses, hence the common disease name "pigeon fever," "pigeon breast,", and "breastbone fever." The disease is likely transmitted by ticks, house flies, horn flies, stable flies, and perhaps other insects. In the past, pigeon fever tended to affect only a single horse on a farm. In recent years, thousands of horses have been affected that have extended the geographical regions in which the disease is usually seen. The incidence of the disease is greatly influenced by weather conditions that favor high populations of insects.3
The infection starts slow and results in the abscesses of the pectoral and abdominal regions. Signs include swelling, dermatitis, lameness, draining abscesses, fever, weight loss, and general depression. Abscesses can be large, up to 20 cm in diameter. Typical abscesses contain thick odorless fluid. The diagnosis can be confirmed by ultrasonography of internal organs or a culture of the wound discharge. Corynebacterium pseudotuberculosis seems to be resistant to Β-lactams, tetracyclines, macrolides, chloramphenicol, lincosamies, vancomycin, and sulfonamides. Horses should be kept in clean, dry stalls. Use only clean curry combs and brushes. Remove nails and other sharp objects in stalls and paddocks. Treat skin injuries promptly to prevent infection. A good fly control program is important in preventing the spread of the disease.