Ockelbo disease in Sweden: epidemiological, clinical, and virological data from the 1982 outbreak.

An outbreak of Ockelbo disease, a syndrome with rash, arthralgia and moderate fever reactions, which occurred in Sweden 1982 is described. A virus, designated Edsbyn 5/82, isolated from mosquitoes and closely related to Sindbis virus (Togaviridae: Alphavirus) is the probable etiologic agent. Sixty-five patients with typical clinical manifestations of the disease became ill in August 1982 and exhibited antibody titer rises against both Edsbyn 5/82 and Sindbis viruses by indirect immunofluorescence and the mixed hemadsorption technique. With the latter method, but not by the neutralization tests, antibody titers were significantly higher against Edsbyn 5/82 virus than against Sindbis virus. Most cases occurred in central Sweden between the 60th and 63rd parallels. The frequency of antibody in healthy individuals (blood donors) within the endemic area was 2-3% and in foci with high incidence, antibody positivity reached 8%. Antibody frequencies continuously decreased south and north of this area. Antibody decline was followed in a group of 12 patients who had Ockelbo disease in 1981. Mixed hemadsorption titers were 4-6 times higher against Edsbyn 5/82 than against Sindbis virus. A 2- to 4-fold antibody decline took place from early convalescence to 20 months after onset.