Lyme Disease Simulating Septic Arthritis

To the Editor.— We report two recent cases that suggest that the clinical presentation of Lyme disease may simulate bacterial arthritis in children and should be considered in the differential diagnosis of children thought to have septic arthritis. In one patient the Lyme spirochete was demonstrated in tissue obtained by needle biopsy of bone; this patient also had iritis, another possibly septic manifestation of Lyme disease. Report of Cases.— Case1.—A 6-year-old boy was admitted to Babies Hospital because of increasing pain and swelling in the left knee over a two-week period following "minor injury." Aspiration by an orthopedist several days earlier revealed sterile fluid with a white blood cell count of 48 000/mm 3 (48×10 9 /L) (95% [0.95] polymorphonuclear leukocytes). He became febrile with a temperature of 38°C (101°F) one day prior to admission; his erythrocyte sedimentation rate (ESR) was 90 mm/ h. Repeated joint fluid examination yielded