ORIGINAL ARTICLE Geographic distribution and incidence of Lyme borreliosis in the west of Ireland A. Vellinga 1,2 & H. Kilkelly 3 & J. Cullinan 4 & B. Hanahoe 5 & M. Cormican 1,5 Received: 20 April 2017 /Accepted: 16 October 2017 # Royal Academy of Medicine in Ireland 2017 Abstract Background Lyme borreliosis is caused by Borrelia burgdorferi and is the most common tick-transmitted infec- tion in temperate regions. Infection often presents with er- ythema migrans and/or other clinical features in early infection. Methods Blood samples are submitted for testing for antibod- ies to Borrelia burgdorferi by enzyme immunoassay and pos- itive 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). Results 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 in- creased over the years, there was no significant increase in the number of positives. Incidences per 100,000 population for requests and posi- tives 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. Conclusions Increased awareness may explain the increase in requests. There is no indication of an increase in incidence. As Key messages: • The incidence of Lyme borreliosis is high in certain parts of the west of Ireland. • Even though the awareness of Lyme borreliosis is increasing, as measured in the number of test requested, the actual number of patients testing positive for antibodies to Borrelia burgdorferi remains stable over the years. • The true incidence of infection is likely to exceed the number of laboratory-confirmed cases because laboratory testing is not always per- formed and testing may not detect early infection. * A. Vellinga akke.vellinga@nuigalway.ie H. Kilkelly hazelkilkelly@hotmail.com J. Cullinan john.cullinan@nuigalway.ie B. Hanahoe belinda.hanahoe@hse.ie M. Cormican martin.cormican@nuigalway.ie 1 Discipline of Bacteriology, School of Medicine, NUI Galway, Galway, Ireland 2 Discipline of General Practice, School of Medicine, NUI Galway, 1 Distillery Road, Galway, Ireland 3 Discipline of Pharmacology, NUI Galway, Galway, Ireland 4 School of Business and Economics, NUI Galway, Galway, Ireland 5 Department of Medical Microbiology, University Hospital, Galway, Ireland Ir J Med Sci https://doi.org/10.1007/s11845-017-1700-2