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Olli 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
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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