HOT ZONES IN TIME AND SPACE ON HUMAN HANTAVIRUS INFECTIONS IN NORTHERN SWEDEN

Olsson, Gert E.(1,2); Dalerum, Fredrik (3); Hörnfeldt, Birger (4); Elgh, Fredrik (2); Palo, Thomas R. (1); Juto, Per (2); Ahlm, Clas (2)

(1) Dept. of Animal Ecology, Swedish University of Agricultural Sciences, Umeå, Sweden;
(2) Dept. of Clinical Microbiology, Umeå University, Umeå, Sweden;
(3) Dept. of Zoology, Stockholm University, Stockholm, Sweden;
(4) Ecology and Environmental Science, Umeå University, Umeå, Sweden

Nephropathia epidemica (NE) is the prevalent Hantavirus disease in northern Sweden. It is caused by Puumala virus, which is naturally carried and shed by bank voles (Clethrionomys glareolus). Humans commonly acquire NE by inhalation of aerosolized Puumala virus particles. We used 2,468 notified NE cases from 1991-2001 from within a highly endemic region in northern Sweden to evaluate temporal and spatial patterns on NE occurrence. We correlated the quarterly incidence of NE in respectively four counties included in the study, to bank vole abundance in autumn by use of a long-term survey on small mammals in the region.

In particular middle-aged males living in rural domiciles were infected predominantly when engaged with activities adjacent to or within peridomestic rodent harborage, such as handling firewood, handling hay and cleaning or redecorating dwellings or outhouses. Autumn and winter differed significantly from other seasons on NE incidence, irrespective of county. The correlation between NE incidence in autumn and winter to bank vole abundance in current and immediate preceding autumn revealed a significant relationship.

Kernel density estimates on the spatial distribution of probable sites of Puumala virus exposure to 862 NE cases confident about exposure event, revealed a strong skewness towards the coastal areas of the Gulf of Botnia. Ninety five per cent of these sites of exposure where found within 55% of the study area, the 75% Kernel covers 15% of the study area and the 50% most concentrated sites of human Puumala exposure where found within 5% of the study area. This spatial occurrence of NE is similar to the distribution of the human population per se within the region.

An awareness of the relationships of the frequent activities when exposed to Puumala virus and the temporal patterns, i.e. autumns/winters and bank vole abundance, in concert with the spatial pattern may serve useful in future attempts to forestall NE outbreaks within the region.

Environmental factors potentially facilitating the persistence and circulation of Puumala virus within local bank vole populations need to be further evaluated on the development of precautionary recommendations to the inhabitants potentially at risk within highly endemic areas.

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