Development of erythema migrans in spite of treatment with antibiotics after a tick bite

Objectives: The recent information on the appearance of erythema migrans despite prophylaxis with 200 mg of doxycycline was the stimulus for a search among our patients for those who developed the skin lesion regardless of receiving antibiotics after a tick bite. Methods: Data were reviewed for adult patients with erythema migrans diagnosed at our institution from 1994 to July 2001, targeting those who received antibiotics after a tick bite. Results: Seven of 5056 (0.14%) patients, diagnosed with typical erythema migrans, developed the skin lesion despite receiving antibiotics after a tick bite. Antibiotics were prescribed by general physicians: in four cases as prophylaxis of Lyme borreliosis within one day after tick detachment and in three cases because of development of acute respiratory tract infection two, five, and eight days after the bite, respectively. The dosages were as follows: azithromycin in a total dose of 3g in three patients and 1.5 g in the fourth patient, amoxicillin-clavulanic acid 625 mg t.i.d. for ten days in the fifth patient, amoxycillin 500 mg t.i.d. for seven days followed by azithromycin 250 mg o.d. for eight days in the sixth, and amoxycillin 500 mg t.i.d. for eight days in the seventh. The patients (five females and two males, aged 18-61 years) were referred to our Department on average six (1-19) days after the appearance of skin lesions. They had typical solitary (five patients) or multiple (two patients) erythema migrans with the characteristics usually seen in European patients, except for a rather long incubation period (median value 28 days, range 10-40 days). All laboratory tests, including the examination of cerebrospinal fluid in three patients with the disseminated form of the illness, were within normal range. Borrelial antibodies were demonstrated in only one patient. A skin biopsy specimen obtained from the site of the erythema migrans was culture positive for Borrelia in 2/4 patients. Conclusions: Our study did not enable us to assess the frequency of antimicrobial prophylaxis failure or the efficacy of individual antibiotics for the prevention of Lyme borreliosis. However, the seven patients presented demonstrate that antibiotic prophylaxis for Lyme borreliosis after a tick bite, at least in Europe, is not entirely effective.