A 9-year-old boy with septic arthritis and osteomyelitis of the foot

A 9-year-old boy was admitted to the hospital with a 1-week history of fever, swelling, redness and pain to the medial aspect of the proximal portion of his right foot.

His symptoms started after he was playing in a lake near his home and kicked a dead catfish. He saw his primary care provider who gave him an intramuscular dose of ceftriaxone and put him on oral doxycycline. His symptoms worsened, and he presented to the hospital. On admission, he was febrile with a temperature of 101·F but did not appear toxic. Local examination of the right foot revealed swelling, erythema and severe tenderness to the medial proximal region of the foot, especially the first metatarsophalangeal joint.

A magnetic resonance imaging scan of the foot showed cortical irregularity of the medial aspect of the head of the first metatarsal, consistent with osteomyelitis; there was also extensive soft-tissue swelling around the head of the first metatarsal with a small fluid collection with peripheral enhancement suspicious for abscess (Figure 1). The patient had surgical debridement, including arthrotomy of the metatarsophalangeal joint, drainage of the flexor and extensor tendons, and curettage of osteomyelitic bone. He also had removal of a foreign body, which was consistent with a portion of a catfish fin. Cultures yielded a beta-hemolytic, oxidase-positive gram-negative rod (Figure 2).

What is the most likely etiologic agent?

A. Pseudomonas aeruginosa

B. Vibrio vulnificus

C. Aeromonas hydrophila

D. Pasteurella multocida

E. Erysipelothrix rhusiopathiae

The correct answer is Aeromonas hydrophila.

Pseudomonas infection would be unlikely after contact with a catfish, especially with the culture growing a beta-hemolytic organism, although Pseudomonas can be oxidase-positive. Vibrio vulnificus would be high on the differential but is more common in patients with underlying chronic liver disease, and it is a salt-water pathogen and oxidase-negative. Pasteurella infections are mostly seen after animal bites and would be less likely to be a pathogen in the fresh-water setting. Erysipelothrix is a gram-positive organism and is oxidase-negative.

Aeromonas organisms are facultative, anaerobic gram-negative rods that are found in fresh and brackish water. Infections from Aeromonas can present as: (A) gastroenteritis; (B) bacteremia and sepsis, seen mainly in immunocompromised patients, especially patients with hematologic malignancies and chronic liver disease; and (C) water-related skin and soft-tissue infections. Fourteen species of Aeromonas have been identified, of which only five have been reported to be pathogenic to humans: A. hydrophila, A. caviae, A. veronii (formerly A. sorbia), A. jandaei and A. schubertii. A. hydrophila is the most common species found in human infections, followed by A. caviae and A. veronii.

A. hydrophila is usually a pathogen in fish, lizards and frogs, but can also cause human infections, especially after water-related injuries. Infections of the skin and soft tissue have a relatively short incubation period of 1 to 2 days and can progress rapidly, sometimes resembling streptococcal infection. Clinical manifestations include cellulitis, crepitant and necrotizing cellulitis, abscesses, fasciitis, gas gangrene, myonecrosis and ecthyma gangrenosum. Septic arthritis and osteomyelitis caused by Aeromonas are uncommon.

We reviewed eight cases of septic arthritis caused by Aeromonas described in the English literature. Five of these patients had involvement of the knee, and three had involvement of the metacarpophalangeal joints. Most patients had injuries sustained in a fresh-water setting. Five of the patients had A. hydrophila as the only organism identified, whereas three patients had an additional organism grow on culture (K. oxytoca, E. cloacae and Clostridium sp). All of the patients had a combined medical and surgical approach to treatment. Half of the patients received a cephalosporin with an aminoglycoside. The duration of antibiotics varied from 3 to 6 weeks. All three patients who had underlying leukemia died, whereas all of the immunocompetent patients had a good clinical outcome.

In nine cases of osteomyelitis that we reviewed, one patient had acute myelogenous leukemia. All of the patients had infection of the bones of the lower extremity — tibia (six patients); ankle (one patient); hallux (one patient) and femur (one patient). Five of nine patients had water-related injuries. Most patients (six of nine) had at least one additional organism identified on culture (K. oxytoca, E. cloacae, Xanthomonas sp, Clostridium sp and Strep sp). All of the patients underwent surgical debridement besides antibiotic therapy. The antibiotic regimens were a combination of a cephalosporin and an aminoglycoside, an aminoglycoside followed by oral tetracycline, and trimethoprim-sulfamethoxazole. The total duration of antibiotics varied from 6 to 10 weeks.

From the above reviews, it appears that septic arthritis and osteomyelitis caused by Aeromonas are uncommon infections. The joints involved are predominantly of the lower extremities, which is not surprising, given that most of these infections are secondary to water-related injuries. The flora is often polymicrobial, and treatment requires a combination of antibiotics and surgical debridement. The prognosis is fair with early surgical debridement and effective antibiotic use, and poor with underlying hematologic malignancy.

On culture, A. hydrophila is beta-hemolytic on blood agar, catalase and oxidase-positive, and urease-negative. The oxidase positivity of A. hydrophila helps differentiate it from V. vulnificus, which is a salt-water pathogen that can cause invasive soft-tissue infections, especially in patients with chronic liver disease.

The standard of treatment for Aeromonas soft-tissue infections, including septic arthritis and osteomyelitis, is antibiotics with surgical debridement. In a patient with water-related injury that has gram-negative rods present on a gram-stained specimen, empiric therapy for Aeromonas should be strongly considered. A third-generation cephalosporin, TMP-SMX or ciprofloxacin would be appropriate and have the advantage of providing coverage against V. vulnificus as well. An anti-staphylococcal penicillin or vancomycin should be included in the antibiotic regimen until culture results become available.

A. hydrophila is resistant to penicillins and first-generation cephalosporins but sensitive to third-generation cephalosporins, quinolones, aminoglycosides, TMP-SMX, tetracycline and carbapenems. In a review of 11 cases of skin and soft-tissue infections caused by Aeromonas, empiric antibiotics used by the clinicians were effective only in 18% of the cases, as streptococcal and staphylococcal infections were suspected and penicillin-related antibiotics were used.

Most experts recommend third-generation cephalosporins or quinolones as the antibiotics of choice for Aeromonas infections. However, there is a caveat to the use of quinolones — Aeromonas species that are reported by the microbiology laboratory as being resistant to nalidixic acid and susceptible to ciprofloxacin may harbor a gyrA mutation and can quickly develop a second mutation upon exposure to ciprofloxacin, rendering it resistant. Quinolones, therefore, should not be used for species resistant to nalidixic acid.

After surgical debridement, a 3- to 4-week course for septic arthritis and a 4- to 6-week course for osteomyelitis seem reasonable. Quinolones have the advantage of relative low cost and the availability of oral formulations. We treated our patient with a 4-week course of IV ceftriaxone with a good clinical outcome.

In conclusion, A. hydrophila can cause rapidly progressive skin and soft-tissue infections after water-related injuries. Septic arthritis and osteomyelitis caused by Aeromonas are uncommon infections and require surgical debridement with antibiotics. Third-generation cephalosporins and quinolones are considered the antibiotics of choice.

Tasaduq Fazili, MD, is a Clinical Assistant Professor in the Department of Medicine at Oklahoma University Health Sciences Center, Oklahoma City, Okla.

Lubna Wani, MD, is a Clinical Assistant Professor in the Department of Medicine at Oklahoma University Health Sciences Center, Oklahoma City, Okla.

David Bobb, MD, is an Orthopedic Surgeon at Norman Regional Health System, Norman, Okla.

Gautam Dehadrai, MD, is a Radiologist at Norman Regional Health System, Norman, Okla.

Disclosures: Drs. Bobb, Dehadrai, Fazili, and Wani report no relevant financial disclosures.

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