Lyme borreliosis in Europe: influences of climate and climate ...
Lyme borreliosis in Europe: influences of climate and climate ...
Lyme borreliosis in Europe: influences of climate and climate ...
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<strong>Lyme</strong> <strong>borreliosis</strong> <strong>in</strong> <strong>Europe</strong><br />
Page 11<br />
Awareness <strong>of</strong> LB is generally lower <strong>in</strong> city dwellers than <strong>in</strong> long-term residents <strong>of</strong> endemic<br />
areas. Little knowledge <strong>of</strong> ticks <strong>and</strong> <strong>in</strong>accurate perception <strong>of</strong> the disease <strong>in</strong>crease the risk <strong>of</strong><br />
acquir<strong>in</strong>g <strong>in</strong>fection <strong>and</strong> <strong>of</strong> not recogniz<strong>in</strong>g the condition (EUCALB).<br />
2.4. High-risk periods<br />
Reports <strong>of</strong> cases <strong>of</strong>ten show dist<strong>in</strong>ct seasonality (Fig. 3). Symptoms <strong>of</strong> disease normally occur<br />
with<strong>in</strong> 2–30 days <strong>of</strong> a tick bite; hence the seasonal pattern <strong>of</strong> LB cases lags slightly beh<strong>in</strong>d the<br />
seasonal pattern <strong>of</strong> tick activity (see Section 4.1). The highest risk periods from a public health<br />
po<strong>in</strong>t <strong>of</strong> view take place when peaks <strong>in</strong> tick activity occur simultaneously with peaks <strong>in</strong> human<br />
visits to tick <strong>in</strong>fested areas. Dur<strong>in</strong>g the tick activity season there are no “safe” times as ticks may<br />
be active day <strong>and</strong> night (Mejlon, 1997).<br />
% <strong>of</strong> annual occurrence<br />
30<br />
25<br />
20<br />
15<br />
10<br />
5<br />
0<br />
Seasonal distribution <strong>of</strong> <strong>Lyme</strong> Boreliosis , Estonia<br />
I II III IV V VI VII VIII IX X XI XII<br />
Months<br />
1992-1998 2001-2002<br />
Figure 3. Reported <strong>Lyme</strong> <strong>borreliosis</strong> cases per month <strong>in</strong> Estonia.<br />
Source: Kuulo Kutsar, 2004, personal communication.<br />
In Sweden cases <strong>of</strong> LB are reported throughout the year but with the majority <strong>of</strong> affected people<br />
hav<strong>in</strong>g been <strong>in</strong>fected dur<strong>in</strong>g July <strong>and</strong> August (Åsbr<strong>in</strong>k et al., 1986). A study from southern<br />
Sweden showed that tick bites occur most frequently <strong>in</strong> July, with the highest number <strong>of</strong> cases<br />
with erythema migrans consequently be<strong>in</strong>g reported <strong>in</strong> August, <strong>and</strong> other LB manifestations<br />
peak<strong>in</strong>g <strong>in</strong> September (Berglund et al., 1995). These seasonality patterns are expla<strong>in</strong>ed by the<br />
fact that more people than usual visit B. burgdorferi s.l. <strong>in</strong>fested areas dur<strong>in</strong>g these periods s<strong>in</strong>ce<br />
July is the ma<strong>in</strong> summer holiday month <strong>in</strong> Sweden, <strong>and</strong> the forest berry <strong>and</strong> mushroom pick<strong>in</strong>g<br />
season as well as the hunt<strong>in</strong>g season beg<strong>in</strong>s <strong>in</strong> July/August.<br />
3. Pathogen transmission cycle<br />
3.1. The pathogen<br />
B. burgdorferi s.l., the causative agent <strong>of</strong> LB, is a gram-negative bacterium, which belongs to the<br />
family Spirochaetaceae. To date, B. burgdorferi s.l. can be divided <strong>in</strong>to eleven genospecies, <strong>of</strong><br />
which those with pathogenic significance are B. afzelii, B. burgdorferi sensu stricto, B. gar<strong>in</strong>ii<br />
<strong>and</strong>, possibly, B. valaisiana (Gylfe et al., 2000; Kurtenbach et al., 2002). The only pathogenic<br />
stra<strong>in</strong> found <strong>in</strong> the United States is B. burgdorferi s.s. (Steere, 2001). In contrast, all four<br />
pathogenic genospecies <strong>of</strong> B. burgdorferi s.l. are present <strong>in</strong> <strong>Europe</strong>, although most <strong>of</strong> the LB<br />
cases are caused by B. afzelii <strong>and</strong> B. gar<strong>in</strong>ii (Fraenkel et al., 2002; Oehme et al., 2002; Ornste<strong>in</strong>