Serological and molecular screening of arenaviruses in suspected tick-borne encephalitis cases in Finland

Lymphocytic Choriomeningitis virus (LCMV) is one of the arenaviruses infecting humans. LCMV infections have been reported worldwide in humans with varying severity. To detect arenavirus RNA and LCMV-reactive antibodies in different geographical regions of Finland, we screened human serum and cerebrospinal fluid (CSF) samples, originated from suspected tick-borne encephalitis (TBE) cases, using reverse transcriptase polymerase chain reaction (RT-PCR) and immunofluorescence assay (IFA). No arenavirus nucleic acids were detected, and the overall LCMV seroprevalence was 4.5%. No seroconversions were detected in paired serum samples. The highest seroprevalence was detected among individuals of age group III (40-59 years) (5.2%), followed by age group I (under 20-year-olds) (4.9%), while in age group IV (60 years or older) the seroprevalence was the lowest (3.8%). Lower LCMV seroprevalence in older age groups may suggest waning of the immunity over time. The observation of higher seroprevalence in younger age group and the decreasing population size of the main reservoir host, house mouse, may suggest exposure to another LCMV-like virus in Finland.