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Comparative Behavior of Different Life-Cycle Stages of Ixodesricinus (Acari: Ixodidae) to Human-Produced StimuliAuthor(s): Marie Vassallo and Claudine Pérez-eidSource: Journal of Medical Entomology, 39(1):234-236. 2002.Published By: Entomological Society of AmericaDOI: http://dx.doi.org/10.1603/0022-2585-39.1.234URL: http://www.bioone.org/doi/full/10.1603/0022-2585-39.1.234

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SHORT COMMUNICATION

Comparative Behavior of Different Life-Cycle Stages of Ixodes ricinus(Acari: Ixodidae) to Human-Produced Stimuli

MARIE VASSALLO1 AND CLAUDINE PEREZ-EID

Unite dÕEcologie des Systemes Vectoriels, Institut Pasteur, 25 rue du Dr Roux, 75724 Paris cedex 15. France

J. Med. Entomol. 39(1): 234Ð236 (2002)

ABSTRACT Nymphs of Ixodes ricinus (Linnaeus, 1758), I. scapularis Say, 1821, and I. pacificusCooley&Kohls, 1943 are epidemiologically themost dangerous stage for transmission ofLymediseaseto humans. Many factors play a role in the epidemiological signiÞcance of the nymphs. In this study,we address the question of whether nymphs show a greater tendency than adults to accept humansas their host. To evaluate this, we have compared the host acceptance behavior of nymphs and adults(males and females)with respect to a human inRambouillet forest, a focus of Lymedisease. Individualticks (nymph, male or female) located on a herbaceous stem were exposed to different stimuli (e.g.,approach, stemmovement, breathing), and the response of each individual to these stimuli was noted.Tick responses were categorised into classes (from 0 to 3) according to their intensity. Statisticalanalysis carried out on 22 ticks allowed us to compare the behavior of the nymph stage with respectto males and females. Despite the small sample sizes, it appears that nymphs are more responsive toa human than are the adults.

KEY WORDS Ixodes ricinus, comparative behavior, nymphs, adults

LYME BORRELIOSIS IS an infectious disease, caused bythe spirochete Borrelia burgdorferi which is transmit-ted by ticks of the group Ixodes ricinus. The nymphstages have been recognized as being the stage re-sponsible for pathogen transmission to humans (Rob-ertson et al. 2000) and epidemiological studies haveclearly shown a relationship between nymph abun-dance and incidence of human Lyme disease for I.pacificus Cooley & Kohls, 1943 (Clover and Lane1995), and I. scapularis Say, 1821 (Falco et al. 1999).Possible reasons for this are that nymphs are moreabundant than adults and because of its relativelysmall size, the nymph has a greater chance of passingunnoticedwhile taking its bloodmeal, hence transmit-ting the spirochete, because transmission increaseswith time attached to the host (Sood et al. 1997).Researchers have maintained that the nymphs have amore ubiquitous host-choice than the adults and ananthropophilic host preference has been suggested(Gray 1991). The existence of differences in hostchoice between tick stages could contribute substan-tially to the variation in the epidemiological rolesplayed by adult and nymph ticks respectively.

Materials and Methods

Site and Season. The study was carried out in Ram-bouillet forest, which is situated �60 km southwest ofParis, France, andhasbeenpreviouslydescribed(Vas-sallo et al. 2000a). The study occurred in a zone of theforest characterizedby aherbaceous strata principallycomposed of Molinia coerulea, a grass that by springalready has tall stems upon which ticks can be found.The study was carried out during May and June 1998(total of 16 d), at which time active tick density wasat its maximum. Observations were performed be-tween 0900 and 1100 hours on days when the angle ofthe sun was the same, the ambient temperature wasbetween 18 and 21�C with low to negligible wind.

Locationof IndividualTicks andStimuli.Aftereachindividual tickwas locatedonagrass stem, its responseto different stimuli was assessed by the same observer.In some cases, binoculars were used. The stimuli usedwere (A) host approach with the wind, (M) move-ment of stem, (B) breathing on and (T) touching thetick. For each of these parameters, two variables weredeÞned: A1, approach up to 2Ð3m(observation of tickresponse: 30 s to 1 min); A2, approach up to fewcentimeters about 1 min; M1, light stem movementsimulating wind; M2, brusque stem movement simu-lating physical animal contact; B1, light breathing at�20 cm during 2Ð3 s; B2, continuous breathing at �10cm and up to 20Ð30 s (10 times 2Ð3 s); T1, lightlybrushing and T2, Þrmly touching the tick. Each stim-

The protocol for the use of humans in the research conducted inthis study was approved by the Institut Pasteur Committee on thewelfare of Humans and Animals in research and is on Þle in ourlaboratory.

1 E-mail:vassallo@pasteur.fr.

0022-2585/02/0234Ð0236$02.00/0 � 2002 Entomological Society of America

ulus was carried out for each tick found in the ordergiven above.Each individual tick responsewas grouped into four

classes: (0) no activity, no response; (1) simple activ-ity e.g., gentle movements; (2) speciÞc activity e.g.,the extension of the Þrst pair of legs; (3) markedactivity comprised of both the extension of the Þrstpairof legs anddisplacement toward thehost.Twenty-two ticks (eight nymphs, Þve adult males, and ninefemales) were thus assessed in this study.

Statistical Analyses. Statistical analyses were per-formed inSIMSTAT(N.Peladeau,ProvalisResearch),using only nonparametric tests (Mann-Whitney andKruskall-Wallis).

Results and Discussion

The tick responses are given in the Table 1. Despitethe small sample size, statistical analysesdemonstratedthat a human host elicits different behavioral re-sponses from different tick stages, irrespective of thestimulus (Kruskall-Wallis, 0.00001 � P � 0.02);nymphs were more attracted to a human than wereadult ticks. The results of the comparison between the

different stages with respect to the speciÞc stimuli areshown in the Table 2; with almost all the stimuli used,nymphs were highly signiÞcantly more attracted tothe observer than were the adult ticks (male andfemale), despite the small sample size.This small num-ber of observations was strictly due to the minimiza-tion of sampling bias. Rather than collecting ticks foranalysis in an arena-setting, which would introducebias in the tickquesting state aswell as causeunnaturaldisturbance, by choosing only those ticks (nymphsand adults) visible high on protruding grass stems weimmediately selected for ticks which were host seek-ing (adults in position “head down” were excludedbecause they have been suggested to be in a state ofrest [Lees and Milne 1951]). Secondly, by studyingthe ticks only during days with the same climaticconditions, at the same timeof the day and in the samehomogeneous area any potential effect of daily peri-odicity known to affect nymph activitywas eliminated(Sonenshine, 1993, Vassallo et al. 2000b; M.V., per-sonal communication); adults do not have such cir-cadian activity rhythms (Sonenshine 1993). Finally,interobserver variations did not affect our results be-

Table 1. Responses of I. ricinus to different stimuli carried out by a single experimenter in the natural environment as a function oftick stage (nymphs vs. adult) and sex (male versus female)

A1 A2 M1 M2 B1 B2 T1 T2

Female 0 0 0 0 0 1 1 0Female 0 0 0 0 1 2 0 1Female 1 0 0 1 0 0 0 0Female 0 0 0 1 0 2Female 0 0 0 0 0 0 1Female 1 2 0 1 0 0Female 0 0 0 0 1Female 0 0 0 0 0Female 1 0 0 2 0 1Male 1 2 0 1 1 3 0 1Male 0 0 0 0 0 0 1Male 0 0 0 0 0 0 0 1Male 0 0 0 0 1 2Male 0 0 0 1 1Nymph 1 2 2 2 2 2 2 3Nymph 1 2 2 2 2 2 2 3Nymph 1 2 2 2 2 2 2 3Nymph 1 2 1 1 2 3 2Nymph 2 3 3 1 3 3Nymph 0 2 3 2 2 2Nymph 1 2 2 2 2 2 3 3Nymph 2 3 1 3 3 3 3 3

A1, host approach up to 2Ð3 meters; A2, host approach up to a few centimeters; M1, light stem movement; M2, brusque stem movement,B1, light breathing; B2, continuous breathing, T1, lightly brushing, T2, Þrmly touching the tick. 0, no activity, no response; 1, simple activitye.g. gentle movements; 2, speciÞc activity e.g. the extension of the Þrst pair of legs; 3, marked activity composed of both the extension of theÞrst pair of legs and displacement toward the host.

Table 2. Mann-Whitney P values obtained between the three different data sets (female, male and nymph), with respect to differentstimuli

A1 A2 M1 M2 B1 B2 T1 T2

Females vs males 0,61 0,66 1 0,76 0,76 0,68 0,68 0,18Females vs nymphs 0,01 0,0005 0,0002 0,004 0,004 0,004 0,007 0,009Males vs nymphs 0,02 0,005 0,002 0,005 0,005 0,19 0,01 0,01

A1, approach up to 2Ð3 meters; A2, approach up to a few centimeters; M1, light stem movement; M2, brusque stem movement, B1, lightbreathing; B2, continuous breathing, T1, lightly brushing, T2, Þrmly touching the tick.

January 2002 VASSALLO AND PEREZ-EID: COMPARATIVE BEHAVIOR OF I. ricinus STAGES 235

cause this experience was done with only one exper-imenter.This age-dependant decrease in human acceptance

suggests either the existence of stage- speciÞc differ-ential responses to host stimuli or the possible conse-quence of a learned response. Adults may have agreater tendency to accept familiar host spp. (i.e.,those encountered during larval and nymphal devel-opment), which are more likely to be the local faunathan humans. Clearly both are possible and warrantfurther study especially considering the implicationsfor Lyme disease incidence: if host acceptance behav-ior varies with previous exposure, acceptance of hu-mans as the host will depend on the abundance of thelocal fauna.

Acknowledgments

We thank R.E.L. Paul for reading the manuscript and J. P.Widmer (OfÞce National des Forets) for granting us freeaccess to theRambouillet forest. Thisworkwas supported bya grant from the Fondation Merieux (Lyon, France) to M.V.

References Cited

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Falco, R. C., D. F. McKenna, T. J. Daniels, R. B. Nadelman,J. Nowabowski, D. Fish, and G. P. Wormser. 1999. Tem-poral relation between Ixodes scapularis abundance andrisk for Lyme disease associated with erythema migrans.Am. J. Epidemiol. 149: 771Ð776.

Gray, J. S. 1991. The development and the seasonal activityof the tick Ixodes ricinus: a vector of Lyme borreliosis.Rev. Med. Vet. Entomol. 79: 323Ð333.

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Vassallo,M., B. Pichon, J. Cabaret,C. Figureau, andC. Perez-Eid. 2000a. Methodology for sampling questing nymphsof Ixodes ricinus (Acari: Ixodidae), the principal vector ofLyme disease in Europe. J. Med. Entomol. 37: 335Ð339.

Vassallo, M., R.E.L. Paul, and C. Perez-Eid. 2000b. Tempo-ral distribution of the annual nymphal stock of Ixodesricinus ticks. Exp. Appl. Acarol. 24: 941Ð949.

Received for publication 11 September 2000; accepted 1 June2001.

236 JOURNAL OF MEDICAL ENTOMOLOGY Vol. 39, no. 1