The Sindbis virus uses birds as vertebrate hosts in the summer amplification cycle, and the virus is transmitted by ornithophilic Culex species. Previous field and experimental studies have shown that mainly passerine birds are involved in the amplification. To delineate the pattern of Sindbis virus infections among passerines, we collected and sampled birds for blood at five study sites located in northern, central, and southern Sweden. All study sites were lowland forested wetlands and humid forests. The blood samples were assayed for Sindbis neutralizing antibodies, and we tested if the prevalence of Sindbis antibodies varied in relation to bird characteristics (i.e., species, body-mass, sex, and age), and environmental factors (i.e., year, month, and location). We found that Sindbis virus infections occurred in almost all passerine species sampled, but that the infection prevalence was unequally distributed among species. The fieldfare, the redwing, and the songthrush each had significantly higher prevalence than the average for all species. Large passerine species had higher infection prevalence than small species. The infection was less prevalent in hatching-year birds than in older birds during June and July, but not in August. Males and females had the same infection prevalence. The prevalence of Sindbis antibodies was higher in central than in southern Sweden, which coincided with a higher proportion of fieldfare-redwing-songthrush samples in the central region of the country. Thus, it is possible that regional and annual variations in the prevalence of Sindbis antibodies in Swedish passerine species depend on the number of fieldfares, redwings, and songthrushes available for feeding by vector mosquitoes.

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