Center for Infectious Diseases, Centers for Disease Control, Uppsala University, National Bacteriological Laboratory, Karolinska Institute, Swedish Defense Research Establishment, Ft. Collins, Colorado, Sweden
Field studies were conducted in central Sweden from 1983 through 1985 to obtain information on the etiologic agent of Ockelbo disease, described in Sweden in the 1960s and probably identical to Pogosta disease in Finland and to Karelian fever in the western USSR. Mosquitoes (63,644) collected during this 3 year period yielded 21 virus strains. Ockelbo virus isolations were from Culiseta morsitans (5 strains), Culex pipiens and/or Cx. torrentium (6 strains), and Aedes cinereus (3 strains). Inkoo (6 strains) and Batai (1 strain) viruses were recovered from Ae. communis. Blood samples collected March–May from migrating birds on the southeast and east coast of Sweden and in July and August from resident birds in east-central Sweden were tested for neutralizing antibody to Ockelbo virus. Antibody was not detected in 328 birds sampled during spring migrations. Two of 58 (3.4%) birds bled in July and 8 of 78 birds (10%) bled in August had antibody to Ockelbo virus.
Ockelbo virus circulates in a mosquito-bird-mosquito cycle, with Cs. morsitans and Cx. pipiens and/or Cx. torrentium as enzootic vectors. Antibody was detected in passerine birds. Other classes of birds or other vertebrates were not sampled. Aedes cinereus may serve primarily to transmit virus to people. The role of other mosquito species as vectors for people is unknown.