Title of Case(s):

Fever, rash and polyarthritis in a 5-year-old boy

Background:

Bacterial arthritis (BA) in children usually presents with acute onset of fever, swelling and pain of joints and limitations of movement. Rapid drainage and antibiotic treatment are keystones to prevent complications. However, overlap exists with systemic juvenile idiopathic arthritis (sJIA), presenting with fever, arthritis and rash. Affection of several joints points towards sJIA, while high neutrophil count in synovial fluid (> 50.000/µl, >70% neutrophils) is indicative for BA. Rat-bite fever, a rare and potentially underdiagnosed bacterial condition, reveals findings of both BA and sJIA.

Case Presentation Summary:

A 5-year-old boy presented with fever of 39°C for 4 days, and a rash on palms and soles for two days. In addition, he reported pain in the left knee for one day. The clinical exam showed arthritis in both wrists, left elbow, left hip, right ankle and a swollen and tender left knee. Inflammatory parameters were elevated (ESR 105 mm/h, blood leucocytes 21.4 G/L, ferritin 407 ng/ml, CrP 90.8 mg/L, S100A8/A9 34’400 ng/ml). Lab findings, rash, fever and involvement of several small and big joints pointed towards sJIA. Knee puncture with intraarticular steroid instillation, however, revealed 181’750/µl leucocytes with 96% neutrophils, suggesting BA and ampicillin/sulbactam was started. Blood and synovial fluid cultures remained sterile after 48 hours and sJIA therapy with steroids and methotrexate was initiated. The patients’ condition improved markedly. On the fourth day, Streptobacillus moniliformis grew in the synovial fluid cultures and repeated medical history revealed close contact to a fancy rat. Immunosuppressive treatment was stopped and sultamicillin was started for 14 days. On follow-up the patient showed full recovery.

Key Learning Points:

- Rat-bite fever caused by Streptobacillus moniliformis is a rare cause of polyarthritis, which untreated may progress to lethal endocarditis or meningitis.

- Taking a careful history of animal contacts is crucial for differential diagnosis.