Epidemiological study of nephropathia epidemica in Finland 1989-96.

This study presents data on 33,000 serum samples studied from July 1989 to June 1996 in Finland, with 6,701 serologically confirmed Puumala virus (PUU) infections. In addition, a PUU serosurvey of 8,000 sera from Finland is presented. On average, 957 PUU infections were detected annually, resulting in an incidence of 19/100,000; mortality was less than 0.1%. The infection was most common in the district of Ita-Savo with an incidence of 90/100,000. The seasonal peak was in November-December; however, the urban population had its incidence peak in August. Local epidemics mirrored bank vole densities, with 3-4-y cycles. Males contracted the disease at a mean age of 40 y, females at 44 y (male:female ratio 2:1). The disease was relatively rare in children and elderly people. The nationwide PUU antibody prevalence for women entering Finnish maternity clinics was 3%, suggesting 5% for the total population. The highest prevalences (7% for young women) were encountered in eastern Finland. In the district with the highest clinical alert, approximately 30% of all PUU infections were estimated to lead to clinical disease with serological confirmation.