Puumala hantavirus infection (nephropathia epidemica) causes a mild
form of haemorrhagic fever with renal syndrome (1–5), and is prevalent
throughout Europe, particularly in Finland and Scandinavia. Usually,
onset of symptoms (such as fever, abdominal pain, headache, back pain
and/or gastrointestinal symptoms) is sudden (5). In endemic regions
it may be the most common cause of acute renal failure and 5% of hospitalised
patients may have renal function impaired enough to need dialysis. Diagnosis
is based on serology and no specific treatment exists. Some patients
can develop severe disease requiring hospitalisation (with an average
stay of 11 days) (1), and some may suffer longterm sequelae such as
hypertension (3) or impaired hypophyseal function (4). In Finland the
reported case fatality ratio of 0.03% (1) may be an underestimate (5),
and during epidemic periods the burden on public health services can
Puumala virus is transmitted to humans by inhalation of aerosolised
excreta of the bank vole Clethrionomys glareolus, which is the reservoir
and vector. Activities which bring humans into close contact with bank
voles or their excreta, such as living in endemic areas (6,7), farming
(2,4,6,8), exposure to the forest (e.g. through forest work (6,8) or
other activities which involve exposure to forest dust/earth (9)), raising
dust during cleaning (9) or military field exercises (10), are all associated
with increased risk of disease. Infection is related to prevalence of
the virus in the vole population, as peaks in the incidence of human
infection occur every 3-4 years and have correlated with peaks in the
vole population (5). In Finland most cases in humans occur during the
To evaluate trends in incidence of human Puumala virus infections,
we analysed national surveillance data from 1995 to 2002. The aim was
to shed light on the pattern of infection in Finland, so as to provide
information for a more detailed study to model epidemic occurrence.
Since 1995, all Finnish microbiology laboratories notify to the National
Infectious Disease Register (NIDR) at the National Public Health Institute
in Helsinki all laboratory-confirmed diagnoses of Puumala virus infection
(mostly by antibody detection). Each notification includes information
on date of specimen, date of birth, sex and place of treatment. Multiple
notifications for persons with the same information received within
a 12 month period are combined as one case.
As the incidence of human infections peaks during the winter months,
we investigated 12-month periods from March to the following February.
This method ensured that the entire winter season for peak infection
was included within each period. The periods under study could then
be investigated as epidemic or non-epidemic periods.
Surveillance data reported to the NIDR from March 1995 to February
2002 were analysed by age, sex and month of onset as well as by health
district (HD) of treatment. There are 21 HDs in Finland, a country of
337 030 km2 with a population of 5.5 million. For each HD, the incidence
for each 12-month period was calculated, as well as the mean incidence
during non-epidemic and epidemic periods. The HDs were categorised as
low, intermediate or high according to the range of their mean incidence
for both epidemic and non-epidemic periods. The geographic distribution
of mean incidence was examined.
Annual data from the National Population Register (1995–2000) were
used as denominators for calculation of age- and sex-specific incidence,
as well as incidence by HD. For each 12-month period, the population
for the calendar year starting the period was used. Data from 2000 were
also used for 2001–02.
Between March 1995 and February 2002, 8184 laboratory-confirmed cases
of Puumala infections were notified to the NIDR. Of these, 63% were
male and 81% were between 25 and 64 years old. Male tended to be slightly
younger than female (median: 42 years vs 46 years old; figure 1). The
mean incidence among males was almost twice as high as among females,
both for epidemic (43 vs 24 per 100 000/yr) and non-epidemic (19 vs
10 per 100 000/yr) periods.
During the time under study, three epidemic periods were identified
(Figure 2), during which the total number of cases was over 1400 (range:
1481–2100). The average number for non-epidemic periods was 756 (range:
624-895). Incidence during non-epidemic periods ranged from 12 to 17
per 100 000 (mean 15 per 100 000), and this was more than twice as high
during epidemic periods (mean 33 per 100 000). There was a small overall
increase in the number of reported cases over the study period, mostly
due to an increase in the eastern part of the country (data not shown).
Figure 2 shows the distribution of cases during the study period by
month of diagnosis. Except for 2000/2001, when numbers were exceptionally
low, non-epidemic periods contained two peak months (August and December).
All three epidemic periods had December as the peak month. Nonetheless,
there were at least three other months during epidemic periods in which
the number of cases was greater than even the peak December month in
any non-epidemic period.
The mean number of notified cases by HD was variable, from 0 to 96
during non-epidemic periods and from 3 to 206 during epidemic periods
(data not shown). In addition, some HDs had epidemic periods at times
that were different from those observed overall.
Helsinki was one of the five lowest HDs for mean incidence of disease.
Other HDs with low incidence were also situated in southern Finland
(Table 1 and map). The HDs with the highest incidence were located in
the east central area of Finland, and the remaining (intermediate incidence)
HDs were spread throughout the country.
Health districts by mean incidence of reported Puumala virus infection
per 100 000 population, Finland, 19952002
N° de districts
Nr of HDs
Localisation géographique Geographical
Incidence moyenne (hors épidémie)
Mean range in incidence (non-epidemic period)
Incidence moyenne (période épidémique)
Mean range in incidence (epidemic period
North, east, central, southwest
There is a clear geographical variation in the incidence of reported
Puumala virus infections in Finland. The rates are highest in the east
central and northern areas of the country and lowest in the south. The
lower incidence in the primarily urban and less forested south could
be because this area has a smaller vole population, and therefore less
likelihood of exposure (1). Cases reported in persons residing in these
areas could have occurred after travel to endemic areas in the north
or east. Typically, many Finns holiday in forest and lakeside cottages
during the summer and autumn months, and forest trekking is common,
especially in Finnish Lapland in the north.
The small increase in reported cases overall was partly due to increases
in the eastern part of the country. This may, however, also be an artefact
of the time under study. Epidemics of Puumala virus infections occur
every few years, and the three epidemic periods happened to occur at
the latter end of the timeframe. More years of data will be needed to
more accurately estimate the trend.
The bank vole population in rural Finland peaks during the autumn months.
As the temperature drops, and the ground freezes, rodents (including
voles) seek shelter in or near human habitats (11), increasing the potential
for exposure in humans. Hence the main peak in incidence of reported
human cases in early winter, which then drops to a low point by the
following spring (as the vole population density declines) (12,13).
A previous study has suggested that, in humans, the smaller August peak
is in the urban population (1).
The relation between vole population density and incidence of human
infection is complex and poorly understood (14,15). However, since the
early 1980s, bank vole populations have been reported to follow a 3-4-year
cycle (1,15,16), often with two consecutive high density years, giving
potential for epidemics in humans. Different parts of Finland have been
in different phases of the cycle. These variations in seasonality and
geographical distribution of the vole population in Finland are believed
to have led to the irregular pattern of Puumala virus infection in humans.
As only microbiologically confirmed cases are reported nationally,
the true extent of this disease is difficult to estimate. Both under-diagnosis
and under-notification exist. In some HDs, the disease is so rare that
it may not be recognised; and in all areas there may be persons with
mild disease, who never visit a physician. In epidemic HDs, the disease
may be so well recognised that some cases will be treated on clinical
suspicion without microbiological confirmation, and district-to-district
variation in the ratio of serological to clinical diagnoses has been
reported (1). In addition, although for specialist physicians, clinical
diagnosis may correlate well with serodiagnosis (17), this may not be
the case in general. For patients with severe disease, differential
diagnoses can include systemic inflammatory response syndrome or sepsis
Puumala virus infection is an emerging disease, with a potentially
increasing trend in Finland, for which there are no current official
prevention guidelines. The main general recommendation, to avoid contact
with bank voles or their excreta, is difficult both to implement and
to monitor. In order to develop effective guidelines, it might be helpful
if knowledge of high bank vole populations (preceding potential human
epidemic periods) could be predicted. Further study of longer-term data,
including investigation of the correlation between human incidence of
disease and bank vole population cycles and/or seasonal weather patterns,
might enable creation of a model to predict future epidemics. In addition,
it is possible that any increase in number of cases is due to increasing
bank vole population density, changes in the pattern of vole epizootics
and/or changing human behaviour patterns. Identification of changes
in risk factors will help in the design of interventions to reduce risk
We thank Pekka Holmström and Jaana Heino for their assistance
during the investigation.