A Dark Horse Diagnosis

He slowly improved over several weeks and was discharged to a subacute rehabilitation facility. He completed a 3.5-week course of intravenous antibiotics before leaving against medical advice. He completed eight weeks of oral trimethoprim-sulfamethoxazole and remains without long-term sequelae from the infection.

S. marcescens is a gram-negative rod in the Enterobacteriaceae family known for its red pigment. Primarily, S. marcescens causes nosocomial infections, most commonly of the respiratory and urinary tracts. However, a wide range of manifestations has been documented, including meningitis, ocular infections (conjunctivitis, keratitis, endophthalmitis), endocarditis, skin infections (cellulitis, necrotizing fasciitis), and osteomyelitis.^{1, 2} S. marcescens is often reported as the cause of outbreaks in ICUs;^3-6 infection is thought to occur via contamination of water pipes, hospital equipment, and disinfectants.^{3, 7} Its natural environment includes soil, water, and GI tracts of animals,⁴ and there are published reports of S. marcescens infection in horses.^{8, 9} This patient was most likely exposed to S. marcescens through his work with horses and their environment.

S. marcescens has wide-ranging target organs, and successful treatment can be difficult. S. marcescens can infect the renal, respiratory, gastrointestinal, ocular, cardiovascular, and musculoskeletal systems. S. marcescens, like other “SPACE” organisms (Serratia, Pseudomonas, Acinetobacter, Citrobacter, Enterobacter), expresses inducible AmpC beta-lactamase.¹⁰ At baseline, AmpC beta-lactamase expression is repressed.¹¹ Mutants with stably de-repressed (constitutively expressed) AmpC can be selected during therapy and lead to clinical failure, as has been best described during therapy for Enterobacter infections.¹² Infectious Disease consultation may be helpful when caring for patients with S. marcescens bacteremia given these complexities.

This was an unusual case of S. marcescens infection. It most commonly infects immunocompromised hosts. Reported risk factors include solid organ or hematopoietic stem cell transplant, malignancy, HIV/AIDS, and receipt of immunosuppressive agents. The patient did not have these risk factors, but did have well-controlled type 2 diabetes mellitus. Although diabetes is associated with an increased risk of infection and more severe infections,^{13, 14} there is no evidence in the literature that well-controlled type 2 diabetes mellitus compromises the immune system. A few case reports document cutaneous S. marcescens infection in immunocompetent adults.^15,16 A case report of S. marcescens septic arthritis and adjacent osteomyelitis has also been published, but the patient had poorly controlled diabetes.¹⁷ This case provides a report of systemic S. marcescens infection in an individual without clear risk factors.

S. marcescens osteomyelitis is rare, and there have been only a few prior case reports.^2,18 The presentation of osteomyelitis, regardless of the causative organism, is subtle, often insidious, and can easily be missed. Hospitalists should have a high index of suspicion for the diagnosis as it requires prompt evaluation and treatment for complications, including epidural abscess. Risk factors include diabetes mellitus, rheumatoid arthritis, injection drug use, and other immunocompromising illnesses.¹⁹ Degenerative changes in the spine such as osteoarthritis may be risk factors as well,²⁰ though not well studied or quantified. A hypothesized mechanism involves local inflammation and joint damage, leaving the area susceptible to bacterial seeding. Osteoarthritis and degenerative disc disease, along with exposure to racehorses, likely put this patient at risk for bacterial seeding in the vertebrae, ultimately leading to a “dark horse” diagnosis.