Sindbis virus infection in resident birds, migratory birds, and humans, Finland

Abstract

Sindbis virus (SINV), a mosquito-borne virus that causes rash and arthritis, has been causing outbreaks in humans every seventh year in northern Europe. To gain a better understanding of SINV epidemiology in Finland, we searched for SINV antibodies in 621 resident grouse, whose population declines have coincided with human SINV outbreaks, and in 836 migratory birds. We used hemagglutination-inhibition and neutralization tests for the bird samples and enzyme immunoassays and hemagglutination-inhibition for the human samples. SINV antibodies were first found in 3 birds (red-backed shrike, robin, song thrush) during their spring migration to northern Europe. Of the grouse, 27.4% were seropositive in 2003 (1 year after a human outbreak), but only 1.4% were seropositive in 2004. Among 2,529 persons, the age-standardized seroprevalence (1999-2003) was 5.2%; seroprevalence and incidence (1995-2003) were highest in North Karelia (eastern Finland). Grouse may contribute to the epidemiology of SINV in humans.

Figure 1

A) Map of Finland with the hospital district divisions. Migratory bird samples were collected from Lågskär, Tauvo, Jurmo, and Kokkola. B) Mean seroprevalence (1999–2003) of Sindbis virus (SINV) infection in human population (according to place of treatment) in the hospital districts of Finland. N/A, not available. C) Average annualized incidence (1995–2003) of SINV infection in human population (according to place of treatment) in the hospital districts of Finland. D) Prevalence of SINV hemagluttination-inhibition antibodies in resident grouse in Finland. The left bars represent the year 2003 and right bars 2004. The bars represent the total number of samples available; the black areas, the number of seropositive samples. *In 2003 in East Savo, the only sample collected was seropositive.

Figure 2

Mean seroprevalence (1999–2003) and average annualized incidence (1995–2003) of Sindbis virus infection in the human population, Finland, according to age groups. *Standardized according to the age distribution of the Finnish population in the respective period.

Figure 3

Average annualized incidence of Sindbis virus infection in human population, Finland, 1995–2003.