Data on reinfections with B. burgdorferi sensu lato in humans are scarce; they are limited to case reports and a few small groups of patients, mostly from the USA and less frequently from Europe. In these reports, the clinical manifestation of reinfection was usually erythema migrans and was most often documented after successful treatment of a previous infection (almost always erythema migrans). The clinical manifestations of Lyme disease in reinfection appear to be similar to those of primary infection, but there are no studies on possible differences in serological or immunological responses.

The current study has 3 primary objectives:

• To determine the frequency of clinically detectable reinfections and to identify the circumstances under which they occur;
• To determine possible differences between primary and reinfection in clinical presentation, disease course and outcome, and specific serum antibody response;
• To determine the aetiology of clinically detectable reinfections by comparing Borrelia isolates recovered from the skin of the same patient at primary and reinfection.

The current study will encompass all patients aged ≥18 years who were treated at the Department of Infectious Diseases, UMC Ljubljana for proven, clinically well-defined Lyme borreliosis in the period 1989 - 2023. Organised data collection in patients with Lyme borreliosis started in 1986 when we established an Outpatient Lyme borreliosis Clinic and introduced uniform approaches to diagnosis, treatment and follow-up of the disease course, thus obtaining a database of high quality clinical and laboratory data in a large number of patients.

The aims:

1. To determine the frequency of clinically detectable reinfections and to identify the circumstances under which they occur (in a cohort of ~15000 European patients with erythema migrans and ~1500 patients with early extradermal or late Lyme disease).
2. To determine possible differences between primary and reinfection in clinical presentation, disease course and outcome, and specific serum antibody response (by comparing clinical findings and serological test results in ~1500 patients who were diagnosed with erythema migrans more than 1 time).
3. To determine the aetiology of clinically detectable reinfections by comparing Borrelia isolates recovered from the skin of the same patient at primary and reinfection (~100 patients and 200 isolates, respectively).

Inclusion Criteria:

• Diagnosis of definitive Lyme borreliosis

Exclusion Criteria:

• /

All patients diagnosed with definitive Lyme borreliosis at LBOC in the period 35 years (1989 to 2023).

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