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

Lyme borreliosis in Europe
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Awareness of LB is generally lower in city dwellers than in long-term residents of endemic
areas. Little knowledge of ticks and inaccurate perception of the disease increase the risk of
acquiring infection and of not recognizing the condition (EUCALB).
2.4. High-risk periods
Reports of cases often show distinct seasonality (Fig. 3). Symptoms of disease normally occur
within 2–30 days of a tick bite; hence the seasonal pattern of LB cases lags slightly behind the
seasonal pattern of tick activity (see Section 4.1). The highest risk periods from a public health
point of view take place when peaks in tick activity occur simultaneously with peaks in human
visits to tick infested areas. During the tick activity season there are no “safe” times as ticks may
be active day and night (Mejlon, 1997).
% of annual occurrence
30
25
20
15
10
5
0
Seasonal distribution of Lyme Boreliosis , Estonia
I II III IV V VI VII VIII IX X XI XII
Months
1992-1998 2001-2002
Figure 3. Reported Lyme borreliosis cases per month in Estonia.
Source: Kuulo Kutsar, 2004, personal communication.
In Sweden cases of LB are reported throughout the year but with the majority of affected people
having been infected during July and August (Åsbrink et al., 1986). A study from southern
Sweden showed that tick bites occur most frequently in July, with the highest number of cases
with erythema migrans consequently being reported in August, and other LB manifestations
peaking in September (Berglund et al., 1995). These seasonality patterns are explained by the
fact that more people than usual visit B. burgdorferi s.l. infested areas during these periods since
July is the main summer holiday month in Sweden, and the forest berry and mushroom picking
season as well as the hunting season begins in July/August.
3. Pathogen transmission cycle
3.1. The pathogen
B. burgdorferi s.l., the causative agent of LB, is a gram-negative bacterium, which belongs to the
family Spirochaetaceae. To date, B. burgdorferi s.l. can be divided into eleven genospecies, of
which those with pathogenic significance are B. afzelii, B. burgdorferi sensu stricto, B. garinii
and, possibly, B. valaisiana (Gylfe et al., 2000; Kurtenbach et al., 2002). The only pathogenic
strain found in the United States is B. burgdorferi s.s. (Steere, 2001). In contrast, all four
pathogenic genospecies of B. burgdorferi s.l. are present in Europe, although most of the LB
cases are caused by B. afzelii and B. garinii (Fraenkel et al., 2002; Oehme et al., 2002; Ornstein