Ticks were collected from 246 study participants. In total 297 ticks were removed ranging from 1 to 18 ticks per individual with an average of 1.2 ticks per individual. All ticks that were identified to species level were Ixodes ricinus. Life stages of 236 ticks could be determined microscopically. Of these ticks, 65 (28%) were adults, 133 (56%) were nymphs and 38 (16%) were larvae. At least 53 participants were bitten by adult ticks, 96 by nymphs and 16 by larvae. Two hundred-ninety-four ticks were tested for Borrelia burgdorferi s.l., Rickettsia spp. and Babesia spp., 286 ticks were also tested for Ehrlichia/Anaplasma spp.. One-hundred-ninety-three (78.5%) participants were reached for a second interview, 51 participants were lost to follow-up. For epidemiological analysis only data of the responding participants were used.
Of all tested ticks 58% were negative for all microorganisms tested for. 16% were positive for B. burgdorferi s.l.
, 19% for Rickettsia
spp., 10% for Babesia
spp. and 12% for Ehrlichia
spp.. The overall infection rate with B. burgdorferi
s.l. was 16% (n = 294, CI 12.1-20.5%), which is significantly lower (p = 0.005) than in the early 90s (24% (n = 521, CI 20.7-28.0%) [32
]. Different sub-species of B. burdorferi
s.l. were found during this study of which B. afzelii
was the most common one. Rickettsiae
that were identified to species level were Rickettsia helvetica
and Rickettsia monacensis
. All but one Babesia species were identified as Babesia microti
. The Ehrlichia/Anaplasma
species identified were mainly Ehrlichia
sp. schotti variant (recently named "Candidatus
Neoehrlichia mikurensis"), one tick contained Anaplasma phagocytophilum
and a last one could not be determined to species level. An overview on the identified microorganisms is given in Table . No larvae were found positive for Babesia
and only one for Borrelia
. Infection rates of the different life stages were calculated and are presented in figure .
Different tick-borne microorganisms found in ticks collected from humans.
Figure 1 Prevalence of microorganisms in different tick stages. Ticks of different life stages were collected from humans and microorganisms were detected and identified using PCR followed by RLB. Bars indicate 95% confidence intervals calculated with Fisher's (more ...)
In total 22 study participants reported symptoms of which 14 reported reddening around the tick bite site and 6 reported systemic symptoms and 2 reported both. This corresponds to an absolute risk of 11.4% for developing symptoms and 8.3% and 4.1%, respectively, for local reddening and systemic symptoms after a tick bite. Reddening at the bite site did not show the pattern of erythema migrans in any of the cases. Systemic symptoms included fever (n = 3), malaise (n = 3), fatigue (n = 3), panic attacks (n = 1), muscle pain (n = 1), joint pain (n = 1) or stiffness of the neck (n = 1). Three study participants reported symptoms but did not specify them further. Local reddening occurred and systemic symptoms occurred most frequently in participants bitten by an adult tick (n = 8 and n = 3, respectively) followed by those bitten by nymphs (n = 4 and n = 2) and larvae (n = 0 and n = 1). For the remaining cases life stages of ticks were not determined. Three 171 participants reported no symptoms. Eighty-four percent of the participants reported that the tick had been attached less than 24 hours while 4.7% reported that the tick had been attached for more than 48 hours. The occurrence of symptoms or reddening at the bite site was not correlated with the infectious state of the ticks (Table , , ). However, a significant positive correlation (at 95% significance) was found between attachment duration of ticks (≥24 h) and symptoms. This was the case for redness (p = 0.08) and systemic symptoms (p = 0.02) analyzed individually and also when symptoms were analyzed conjointly (p = 0.009).
Relative risks of developing any symptom correlated to various risk factors.
Relative risks of developing redness at the bite site correlated to various risk factors.
Relative risks of developing systemic symptoms correlated to various risk factors.
Fisher's exact test were significant (p < 0.30) at the 70% level for local redness with attachment duration of more than 24 h, more than 48 h, more than two tick bites and B. afzelii, for systemic symptoms with >24 h, Ehrlichia spp. and B. garinii infection of the tick and for any symptom with >24 h, >48 h, any infection of the tick, B. afzelii and Borrelia burgdorferi s.l.. Logistic regression with these factors and categorical data yielded no significant result for systemic symptoms. For redness, or any symptoms the following significant regression was found,
where R is the probability of redness at d days of tick presence and A indicates absence ('0') or presence ('1') of B. afzelii. G is the probability of systemic symptoms depending again on the number of days d. Logistic regression indicated the presence of one B. burgdorferi sub-species, B. afzelii, to be associated with local reddening (Figure ). For example, 4 days of B. afzelii presence yields a probability of redness of 66%.
Probability of adverse effects after a tick bite. Described as a function of days of attachment and accounting for the presence/absence of B. afzelii. The formula describing this relationship can be found in the results section.