A newly detected tick-borne encephalitis (TBE) focus in south-east Sweden: a follow-up study of TBE virus (TBEV) seroprevalence.

In 2002, two cases of tick-borne encephalitis (TBE) were diagnosed in inhabitants of a tick endemic area on the island of Aspö in south-east Sweden. During the previous 25 y, only 2 other cases of TBE had been diagnosed in that region of Sweden. To investigate the presence and evolution of seroprevalence of antibodies to TBE virus (TBEV), we compared inhabitants' anti-TBEV immunoglobulin G (IgG) levels in blood samples drawn in 1991 and 2002. A significant increase in IgG antibody levels was observed in 24/200 (12.0%) 2002 blood samples compared to 7/200 (3.5%) 1991 samples. However, neutralizing antibodies were detected in only 4 of the 2-step enzyme-linked immunosorbent assay (ELISA) IgG positive sera against TBEV, corresponding to a TBEV neutralizing test (NT) prevalence of 2%. Significantly more men than women were seropositive for TBEV antibodies. Compared to other age groups, the greatest increase in TBEV antibody levels was observed in the 20-29-y age group. However, the majority of seropositive samples were from participants >50 y of age. Recommending preventative measures, including vaccination against TBE, to individuals who reside in or regularly visit TBEV endemic areas is suggested.