Listeriosis in Iceland, 1978-2000: a description of cases and molecular epidemiology

The purpose of our study was to review all cases of listeriosis in Iceland during the period 1978-2000 and to analyse the genetic relatedness of their isolates. Case records of all patients in Iceland with listeriosis during the period were reviewed and the isolates compared using serotyping and pulsed-field gel electrophoresis (PFGE) using SmaI, AseI and ApaI restriction enzymes. Forty cases of listeriosis were diagnosed during the period, resulting in a mean annual incidence of 6.9 cases per million and a case fatality rate of 33%. In the first 5 y of the study only serotype 4b was observed; subsequently serotypes 1/2a and 1/2b appeared and serotype 4b declined in prevalence. PFGE yielded 24 different genotypes with 7 clusters of indistinguishable genotypes, each comprising 2-6 cases. During 1992-95 the annual incidence of listeriosis in Iceland rose to 15 cases per million. This was largely due to 2 clusters, 1 of 3 cases and the other of 6. No cases of listeriosis were diagnosed during 1998-2000. Our data show an increased number of cases within clusters in the latter half of the period. At the same time, food processing and distribution has become increasingly centralized in Iceland, suggesting an increased risk of listeriosis outbreaks.