Climatic, ecological and socioeconomic factors as predictors of Sindbis virus infections in Finland

The clinical SINV infection is known as Pogosta disease in Finland, Ockelbo disease in Central Sweden and Karelian fever in Russian Karelia. The disease has probably only in less than five decades ago been introduced to Finland. Major outbreaks with approximately 7-year cycles (1981, 1988, 1995 and 2002) have caused substantial burden of illness. Forest dwelling grouse are suspected to be amplifying hosts, with the infection transmitted to humans exclusively by mosquito bites. Late summer Culex and Culiseta mosquito species are considered the primary vectors for SINV infection, although the more human adapted Ochlerotatus species may also play a role. The highest seroprevalence has been reported from Eastern Finland.

SINV infection surveillance data for 1984–2010 were used to create a negative binomial hurdle model, with seasonality, long-term cycles, climatic, ecological and socioeconomic variables. Climatic factors during early summer and amount of snow in April described the occurrence and incidence of SINV infections. In summary, the SINV infection dynamics between 1984 and 2011 were characterized by regular annual cycles between late July and October. Regulated water shore and hatch-year black grouse density described the occurrence, while population working in agriculture, agricultural land and income (negative) described the incidence of the disease. The disease most likely spreads between black grouse, mosquitoes and humans when suitable climatic conditions for the reproduction of mosquitoes occur, such as warm temperature and high precipitation with thick snow cover during the previous winter. These conditions are likely to be met in the area of Central-Eastern Finland.

The development of infected mosquitoes is probably expedited by the amount of regulated waters. Hatch-year black grouse may be one of the main amplification hosts for SINV. People are likely to become infected during outdoor activities when exposed to infected mosquitoes. The prediction for 2009 was 85 cases (95% prediction interval: 2-1187), while actual occurrence was 106. As the SINV surveillance data were characterized by an overdispersion of zeros, we were able to model the SINV infections by applying a hurdle model. We identified novel and known risk factors, prevention of SINV infections in regulated water areas by infected mosquito populations should be targeted.

More information:

Katri Jalava, National Institute of Health and Welfare, tel. 029 524 8914, katri.jalava@thl.fiReija Ruuhela, Finnish Meteorological Institute, tel. 0500 424 533, reija.ruuhela@fmi.fi

K.Jalava, J.Sane, J. Ollgren, R.Ruuhela, O.Rätti, S.Kurkela, P. Helle, S.Hartonen, P.Pirinen, O.Vapalahti and M. Kuusi,2013Climatic, ecological and socioeconomic factors as predictorsof Sindbis virus infections in Finland. Epidemiol. Infect. (2013), 141, 1857–1866.