Ulcerative dermatitis, tendinitis, tenosynovitis, and desmitis caused by chronic infection with Serratia odorifera in a racehorse

We investigated a case of severe Serratia odorifera infection in a racehorse. The horse had developed progressive lameness associated with swelling of the left tarsal region, which progressed to ulcerative dermatitis, tendinitis, tenosynovitis, and desmitis. The plantar side of the superficial digital flexor (SDF) tendon was visible through the skin ulcer, and coalescing nodules were present in the dermis. The horse was euthanized and an autopsy was performed. Grossly, in addition to the changes described above, severe edema of the soft tissues was associated with the ulcer. Microscopically, necrotizing and ulcerative, chronic-active dermatitis with granulation tissue was noted, along with lymphoplasmacytic and suppurative tendinitis (SDF), tenosynovitis (SDF and lateral digital flexor), and desmitis (long plantar ligament and flexor retinaculum [FR]), with collagen degradation and intralesional gram-negative bacteria. Serratia odorifera was isolated in pure culture from the ulcerated skin, SDF tendon, FR, and synovial fluid of the tarsus joint. To our knowledge, soft-tissue inflammation caused by S. odorifera has not been described in horses previously.