Hantavirus infections in Finland

lli Vapalahti *, Antii Vaheri *, Heikki Henttonen **
* Helsinki University, Department of virology, Finland
** Finnish Forestry Research Institute

Thanks to Dr Pertti Arstila for his information concerning the cases on Puumala virus infection diagnosed in the department of Virology at theUniversity of Turku.

Kansanterveys 1995; (5): 9-11 – Finlande

Hantaviruses cause haemorrhagic fever with renal syndrome (HFRS) and hantaviruspulmonary syndrome (HPS), and occur throughout the world (1). The viruses aretransmitted to humans in aerosols of rodent secretions. About 200 000 people,mainly in Eurasia, develop HFRS each year. The most severe forms of the diseaseare caused by the Hantaan virus which is spread by field mice especially in theFar East and in the former Yugoslavia, and has a case fatality ratio of 5% to10%. A less severe form of HFRS is caused by the Seoul virus, which is carriedby rats and typically occurs in cities and harbours. Puumala virus causes amild form of HFRS, nephropathia epidemica (NE), and is found in most of Europealong with its carrier rodent, the bank vole (Clethrionomys glareolus).In Finland about 1000 cases of nephropathia epidemica are diagnosed each year.Puumala virus has recently been isolated also in Hokkaido, Japan, from the greysided vole, closely related to the bank vole. In this report we describe thetemporal and geographical patterns of infection with Puumala virus in Finland.

The surveillance of Puumala virus infections in Finland

Puumala virus infection is usually suspected on the basis of clinical features,but the diagnosis should be confirmed serologically (2). Most serum specimensfrom cases of nephropathia epidemica in Finland are sent to the HelsinkiUniversity Department of Virology for confirmation. In the acute phase IgGantibodies only to the nucleocapsid protein can be detected. The serologicaldiagnosis is based on an immunofluorescence technique: diagnosis of acuteinfection is made on the presence of a typical anti-nucleocapsidpattern or the detection of low avidity IgG. Enzyme immunoassay is also usedto detect IgM. Data on the vole population densities were obtained from theForest Research Institute.

Results

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One thousand one hundred and fifty cases of Puumala virus infection wereserologically confirmed in 1994, suggesting an annual incidence of 22.3 casesper 100 000 population. The incidence was greatest in the winter (figure 1) butvaried widely by place of residence. In the autumn of 1994 it was highest inCentral Finland, Pirkanmaa, and Savo (figure 2). The epidemic occurred mainlyin areas with high populations of bank voles but the regions with the mostcases differ from year to year according to the size of the bank volepopulations. Puumala virus was detected in one in three lung specimens fromvoles analysed in the autumn of 1994 from Central Finland.

Discussion

This study documents a high incidence of nephropathia epidemica in Finland witha marked seasonal and geographical variation. Previous studies have shown thatabout 6% of the human population in Finland has antibodies to Puumala virus,which suggests that on average about 5000 infections occur each year, but only1150 cases were serologically confirmed in 1994 (3). The seasonal andgeographical patterns reflect the epidemiology of the infection in the bankvole population and the interaction of humans with this animal vector.Monitoring of the vole populations by the Forest Research Institute enablesepidemics to be predicted. However, the maps produced are mainly designed tomeet the needs of the forestry economy with respect to field voles, which causethe most destruction to the forests, and have been shown not to carryhantaviruses. The populations of bank and field voles alternate in cycles ofabout three to four years. Numbers of bank voles usually increase one yearbefore the field vole populations. In September 1994, the population of voleswas still quite small in East Savo and Northern Carelia, but it grew rapidlytowards the end of the year (3). This was reflected in the number ofnephropathia epidemica cases, which increased 20 to 30 fold in these areas fromSeptember to December, while in the whole country the increase was only threefold. Historical data show that nephropathica epidemica is endemic in the LakeDistrict of Finland, but the incidence may be affected by the awareness ofclinicians and their interest in the disease. The genetic differences betweenstrains of Puumala virus seem to be correlated with their geographicalseparation. The same strains circulate in the same regions for years with noevidence of change in their pathogenicity in humans. Differences in theseverity of the disease may depend on the viral load and the human immuneresponse. Preliminary evidence shows that the human HLAB8 allele may beassociated with serious symptoms.

References

1. Lundkvist A., Niklasson B., Haemorrhagic fever with renal syndrome and otherhantavirus infections. Reviews in Medical Virology, 1994, 4: 177-184

2. Hedman K., Vaheri A., Brummer Korvenkontio M., Rapid Diagnosis of hantavirusdisease with an IgG-avidity assay, 1991, Lancet, 338: 1353-56

3. Unpublished data

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