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

Overview

• This study reports a case of severe soft-tissue infection caused by the bacterium Serratia odorifera in a racehorse, leading to multiple inflammatory conditions including ulcerative dermatitis, tendinitis, tenosynovitis, and desmitis.
• The infection resulted in progressive lameness, visible tissue damage, and ultimately euthanasia of the horse, making this the first documented case of such an infection in equine soft tissues.

Background and Case Description

• A racehorse developed progressive lameness and swelling localized to the left tarsal (ankle) region, indicating a serious underlying condition.
• The skin over the affected area became ulcerated, revealing the superficial digital flexor (SDF) tendon through the open wound.
• Coalescing nodules were observed in the dermis, suggesting a chronic or severe inflammatory response beneath the skin surface.

Clinical Progression and Findings

• The horse’s condition worsened despite treatment, leading to euthanasia for humane reasons.
• During autopsy, extensive edema (fluid accumulation) was noted in the soft tissues around the ulcerated region, indicating ongoing inflammation and tissue damage.
• Microscopic examination revealed:
  • Necrotizing and ulcerative chronic-active dermatitis characterized by granulation tissue formation, representing the body’s attempt to heal the ulcerated skin.
  • Lymphoplasmacytic (immune cell infiltration) and suppurative (pus-forming) inflammation of:
    • Tendinitis in the SDF tendon
    • Tenosynovitis affecting both the SDF and lateral digital flexor tendons
    • Desmitis involving the long plantar ligament and flexor retinaculum (a fibrous band supporting the tendons)
  • Collagen degradation in the affected tissues, indicating connective tissue breakdown.
  • Presence of gram-negative bacteria within the lesions, signifying bacterial infection.

Bacterial Identification and Significance

• Serratia odorifera, a gram-negative bacterium, was isolated in pure culture from several affected sites, including:
  • The ulcerated skin
  • The SDF tendon
  • The flexor retinaculum
  • Synovial fluid from the tarsal joint
• This identification suggests that S. odorifera was the primary pathogen responsible for the extensive soft tissue and joint infections.
• Importantly, this is the first reported case of S. odorifera causing such soft-tissue inflammation in horses, highlighting a potentially emerging or previously unrecognized equine pathogen.

Implications and Conclusion

• The case underscores the need for awareness of unusual bacterial infections in equine medicine, especially in high-performance animals such as racehorses.
• Early identification and targeted antimicrobial therapy may help prevent progression to severe tissue destruction as seen in this case.
• Further research may be required to understand the pathogenic mechanisms of S. odorifera in horses and to develop effective treatment protocols.