Pigeon Fever as a Lameness Cause (AAEP 2010)

The soil-borne bacterium Corynebacterium pseudotuberculosis can infect horses and cause a condition commonly known as pigeon fever, in which the infected horses often have pectoral swelling, resembling a pigeon's breast. Other clinical signs include fever, lethargy, and lameness. Cases are predominantly found in the western United States, however, the bug is steadily extending eastward, according to Nora Nogradi, DVM, a resident in equine internal medicine at the University of California, Davis, veterinary school. At the 2010 American Association of Equine Practitioners Convention, held Dec. 4-8 in Baltimore, Md., Nogradi presented a retrospective study on the outcome of 35 equine cases featuring lameness attributable to C. pseudotuberculosis.

Cases appear seasonally because they're correlated with fly activity, so pigeon fever diagnoses usually are made in the late summer and early autumn months. Flies inject these bacteria into the ventral abdomen, where lymphatic flow picks up the infection and spreads it to other areas of the body. While most C. pseudotuberculosis infections localize in the pectoral muscles, Nogradi noted that 8% of abscesses localize internally in the liver, spleen, or kidney, and 1% cause ulcerative lymphangitis with profound hind limb swelling. A smaller subgroup of cases with external abscess formation will develop abscesses in the limbs causing a confusing lameness.

Since the abscesses develop slowly, so does the lameness, and a veterinarian's involvement might be delayed for this reason. Most affected horses in the study were Grade 4 (out of 5) lame, which means lameness is noticeable at a walk. Most presented with a mild fever around 102°F, and complete blood count data was consistent with bacterial infection. In nearly all of the horses culturing an abscess yielded a positive diagnosis, and serology titers confirmed this as well.

Of the 35 horses, Nogradi noted that 71% had developed abscesses in the axillary (armpit) and triceps region of the forelimb; abscesses in these areas are the most common source of lameness in pigeon fever cases. Ten horses had no visible swelling, but veterinarians could detect lameness as a decreased cranial (forward) phase of the stride because of discomfort created with limb extension. Ultrasound examination allowed veterinarians to identify an encapsulated abscess beneath the triceps musculature in all cases. Nogradi remarked that the size of abscess did not correlate with a horse's degree of lameness. The depth of these abscesses caused them to take weeks to develop into a systemic illness with fever and blood count changes.

Nogradi mentioned four horses that developed ulcerative lymphangitis during the study period, which persisted as a markedly swollen leg (lymphedema) in each of the horses. The study also included a couple of horses with C. pseudotuberculosis abscesses around the stifle joint, two bone infections requiring surgery, and one other with primary septic arthritis (infection of joints).

To treat C. pseudotuberculosis in the limbs, the veterinarian must localize the abscesses and drain them via ultrasound-guided needle puncture (to avoid inadvertent puncture of vital structures). Timely intervention is the key to prevent secondary complications such as osteomyelitis (bone infection) or support limb laminitis. After establishing drainage, antibiotic treatment with appropriate antimicrobial drugs is important. Non-steroidal anti-inflammatory drugs ease comfort once the abscess is drained. Nogradi noted that all of the cases resolved over 21 to 120 days.

In summary, C. pseudotuberculosis infection of musculoskeletal structures should be considered in horses presenting with severe lameness and signs of systemic illness during the fall months in regions where C. pseudotuberculosis is prevalent. Although diagnosis is challenging, cases with proper and timely treatment carry a good prognosis for recovery.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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