Overwintering of Culex mosquitoes in Sweden and their potential as reservoirs of human pathogens.

1. Culex pipiens L. and Cx torrentium Martini are suspected enzootic vectors in Sweden of Sindbis virus causing Ockelbo disease in man. To estimate their potential as virus reservoirs over the winter, mosquitoes were collected from August to May from a cellar in a rural area, central Sweden. The mosquitoes were dissected and examined for blood content, gonotrophic stage, parity and insemination, and tested for fructose. 2. Cx pipiens, Cx torrentium and Cx territans Walker constituted 94% of 887 females caught. A few Culiseta annulata (Schrank), Cs. alaskaensis (Ludlow) and An. maculipennis Meigen s.l. were also collected. 3. Data suggested that all blood-fed, parous Culex females disappeared during the winter and that only non-bloodfed, nulliparous females survived. 4. Fructose, detected in some of these females, and other plant sugars may be the main energy sources for winter survival. 5. Unless transovarially or venerally infected, Culex females are presumably of no or very limited importance in rural, central Sweden as winter reservoirs for blood-borne pathogens of vertebrates.