An Unusual Case of Septic Arthritis

Septic or infectious arthritis in healthy adult horses usually develops rapidly, creating significant lameness, but a recent case report found that septic arthritis might appear chronic rather than acute, and that it can be caused by an unusual bacterium.

Septic arthritis is almost always exquisitely painful, and the lameness it causes only gets worse until the infection is treated. Christina Hewes, DVM, treated the first documented case of Mycobacterium avium causing septic arthritis in a horse when she was at Washington State University's (WSU) College of Veterinary Medicine. Other cases probably have been missed since the signs point to chronic, degenerative arthritis rather than septic arthritis. This might be because mycobacteria are notoriously difficult to kill and do not cause acute, severe lameness, as do other infectious causes. The body responds to infection by walling the bacteria off in tumor-like masses of immune cells and bacteria called granulomas. Finding the organism for culture is very difficult, and treatment lasts for months.

When a 12-year-old American Saddlebred gelding was admitted to WSU with chronic lameness of the right carpal (knee) joint, he had all the signs of chronic, degenerative arthritis. He was lame off and on for three years, but the lameness would resolve with cortisone joint injections. The lameness had recently worsened, and an injection was ineffective. Radiographs of the knee identified a large, cyst-like structure at the end of the radius (the upper foreleg bone ending at the knee), which prompted the referral. The case appeared recently in the Journal of the American Veterinary Medical Association.

A thorough workup at the teaching hospital confirmed the need for diagnostic arthroscopy to examine the joint and determine the cause of the lameness. However, when the horse went to surgery for arthroscopy, the synovial surface of the radiocarpal joint was found to be smooth and intact. This was unexpected in a case of chronic arthritis. The cystic structure in the radius, which was debrided, was full of soft material surrounded by a thickened synovial layer. After surgery, careful examination of the removed synovial tissue revealed the mass to be a granuloma, which was positive for Mycobacterium avium. The diagnosis was changed to "septic arthritis and granulomatous synovitis."

How Mycobacterium infected this horse's radius will never be known, and due to the poor prognosis for soundness, the gelding was ultimately euthanatized when a relapse of severe lameness occurred.

About the Author

Susan Piscopo, DVM, PhD

Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.

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