Geographic distribution and incidence of Lyme borreliosis in the west of Ireland

A. Vellinga, H. Kilkelly, J. Cullinan, B. Hanahoe, M. Cormican

Lyme borreliosis is caused by Borrelia burgdorferi and is the most common tick-transmitted infection in temperate regions. Infection often presents with erythema migrans and/or other clinical features in early infection. Blood samples are submitted for testing for antibodies to Borrelia burgdorferi by enzyme immunoassay and positive samples are confirmed by a reference laboratory by IgG and IgM line immune assay. A retrospective extraction of all laboratory requests and results for Lyme borreliosis from 2011 to 2014 was performed. Patient addresses were mapped to local electoral area (LEA). The total number of requests was 5049 and 242 (5%) were positive over 5 years. The number of positive and tested samples were 40/748, 45/905, 41/947, 73/1126 and 43/1323 from 2011 to 2014. Even though the number of requests increased over the years, there was no significant increase in the number of positives. Incidences per 100,000 population for requests and positives were calculated at LEA level and showed considerable variation. The highest incidence was shown in one LEA (Connemara) with nearly 500 requests and 43 positives per 100,000 population per year. Increased awareness may explain the increase in requests. There is no indication of an increase in incidence. As many GPs treat suspected Lyme borreliosis empirically without testing and as antibody may be undetectable early in the course of illness, the true incidence of infection is likely to exceed the number of laboratory-confirmed cases.