Lyme borreliosis in Europe: influences of climate and climate ...

Lyme borreliosis in Europe
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4) with, in some cases, increased exposures of humans to ticks and, in others, decreased
encounters between humans and disease vectors.
5. Observed effects of recent climate variations in Europe
Since 1950, night-time temperatures (i.e. minimum temperatures) have risen proportionally more
in the northern hemisphere than daytime temperatures (Easterling et al., 1997; Beniston & Tol,
1998; IPCC, 2001). Winter temperatures have increased more than other seasons, particularly at
higher latitudes (Easterling et al., 1997; Beniston & Tol 1998). In Europe, the spring now starts
two weeks earlier than it did before the 1980s and the length of the vegetation season has
increased (IPCC, 2001). These are all factors of importance for tick vectors and LB risk.
Early signs of effects from changes in climate are more easily recognized in areas located close
to the geographical distribution limits (latitudinal or altitudinal) of an organism. Latitudinal
changes in tick distribution between the early 1980s and mid-1990s have been reported from
northern Sweden (Jaenson et al., 1994; Tälleklint & Jaenson, 1998), (Fig. 6). Spatial and
temporal analyses showed that these distribution shifts were correlated to changes in daily
seasonal climate (Lindgren et al., 2000). The establishment of new tick populations at the highest
latitudes was related to less severe winter temperatures and increased number of days with
temperatures vital for tick reproduction (i.e. >10 ºC) (Lindgren et al., 2000). Increases in tick
population density in the central parts of Sweden were significantly correlated to “accumulated
temperature days” that represented milder winters, earlier start of the tick activity period in
spring (i.e. >4 ºC), and prolonged autumn seasons over a consecutive number of years (Lindgren
et al., 2000).
Figure 6. Differences in tick prevalence in central and northern Sweden (southern
parts not included).
The left map illustrates conditions before 1980 and the right map illustrates tick distribution after the mid 1990s.
Source: Lindgren et al., 2000.
Mountain studies on I. ricinus populations have been performed in the same locations in the
Czech Republic in 1957, 1979–1980 and 2001–2002. A shift in the upper altitude boundary of
permanent tick population from 700 m to 1100 m a.s.l. has been observed (Daniel et al., 2003).
Specifically, tick surveys (on permanently resident dogs and by flagging) were carried out
between 2001 and 2002 at altitudes between 700 m and 1200 m a.s.l. in the Sumava mountains.