Z O O E C O . O R G

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Ticks have preferences in choosing human hosts

DAG NYMAN (1,2), STEN-ANDERS CARLSSON (1,2), AND PETER WAHLBERG (1)
(1) The Åland Borrelia Group, Mariehamn, Åland, Finland
(2) Åland Central Hospital, Mariehamn, Åland, Finland

e-mail sten-anders.carlsson@ahs.aland.fi

Aided by a grant from the Wilhelm and Else Stockmann Foundation

ABSTRACT

This study was done in order to test the common assumption that some individuals are more liable to be bitten by Ixodes ricinus ticks than are others. Among 73 investigated couples, each with similar outdoor and indoor activities, 30 couples (60 persons) were found to have a discordant experience of being bitten, i.e., for the last five years, one spouse in a pair was bitten less than one time per year, but the other more than 5 times per year. The habits and diet preferences of the subjects were studied in detail. No unequivocal differences in dietary or other conditions were found in the two groups. The results show that ticks indeed have preferences when choosing human hosts. The cause for these preferences should be sought for in order to find methods for avoiding tick-bites.
 

INTRODUCTION

Except for the effect of touch, it is not known how ticks are alerted to their mammalian hosts, including humans. People living in tick-infested areas believe that some individuals are more prone to getting tick-bites than are others. Because of the well-defined geographical borders and the easily accessed population (25 000), the Aland Islands in the Baltic have been a site for epidemiological studies. Ixodes ricinus ticks have been a problem in the area for at least a century. Tick-bites are common: in an earlier study (1) it was shown that 85% of the adult population had been bitten by ticks. Several patients with TBE are seen each summer, and Lyme borreliosis is endemic in the area (2).
The present study was undertaken in order to test the validity of the belief that some people are more at risk to be bitten by ticks than are others. Two questions were put forward:
1: May the two members of couples who live together and share the same outdoor activities have marked differences in the respective number of tick-bites?
2: Is there some common factor for those individuals who do, respectively do not, often become bitten by ticks?
 

MATERIAL AND METHODS

Via the local papers, people residing in the Aland Islands were asked to report cases where two spouses have different frequencies of tick-bite. The responders were all asked about the number of tick-bites that they have each summer. Their tick-bite experiences were recorded. They were also asked to fill in a questionnaire with 76 questions containing detailed data on occupation, outdoor activities, diseases and medication, house animals and pets, food habits, alcohol, tobacco, personal hygiene, use of protective clothing and as a free narrative on other points of view. The AB0 Rh blood groups were recorded. The main topics of the questions are shown in Table I.

Table I. Main topics of the questions asked of each participant
 

RESULTS

73 couples, or 146 persons, married or cohabitant couples with identical outdoor activities, reported for the study. All had lived together in the area for more than 5 years, mostly for more than ten years. Those who had been bitten more than 5 times each summer for the last 5 years were considered as bitten "often". Those who were bitten less than once each summer for the last 5 years were considered as bitten "seldom". Bite incidences of 1-5 times per summer were left out as non-conclusive. By this definition, we found 30 couples (60 persons), where one spouse was bitten often, and the other seldom (Table II). The precautions, if any, for avoiding tick-bites were the same within each couple. The distribution of the tick-bite frequencies of these "discordant couples" is shown in Table III. The habits and living conditions of the 30 discordant couples were compared. There were no differences in dietary or other habits between those who had more than 5 tick-bites a year and those who had less than one. Because garlic has recently been suggested to have tick repellent properties (3), the tick-bite statistics for regular consumers vs, non-consumers of garlic are also shown in Table IV. There was some indication that those who took milk regularly had higher bite incidences than those who refrained from milk (Table V). This must be checked in a larger population in order to be ascertained or revoked. There was no correlation between AB0 Rh blood groups and bite incidence.

Table II. Incidence of tick-bites in 73 couples, each with similar life habits

 30 couples (bold) had discordant bite incidences

Table III. Tick-bite incidences of 30 couples with discordant tick-bite experiences (The bite incidence 1-5/year was left out by definition as inconclusive)

Table IV. Tick-bites and garlic consumption

* Calculated as (total row/grand total) x total column

Table V. Tick-bites and milk consumption

* Calculated as (total row/grand total) x total column
a) c²  test = 0,028
b) c²  test = 0,100

DISCUSSION

The tick-bite statistics of this study show that people living together and sharing the same activities may indeed have a different risk of being bitten by ticks or, in other words, that ticks prefer some individuals to others as hosts. The cause or causes of this difference remain unknown. There were no significant differences in tick-bite incidence in people who regularly took garlic from those who did refrain from taking garlic. Because the mechanical stimuli to alert the ticks were the same for the pairs of prospective hosts, it is probable that some olfactory stimulus or repellent is important for the ticks' recognition or avoidance of possible blood meal sources (4). If one or more such factors could be found, this could contribute to developing means of avoiding tick-bites and consequently tick-borne infections.

REFERENCES

1. Wahlberg P. Incidence of tick-bite in Åland Islands: Reference to the spread of Lyme borreliosis, Scand J Infect Dis 1990;22:59-62
2. Carlsson S-A, Granlund H, Nyman D, Wahlberg P. IgG seroprevalence of Lyme borreliosis in the population of the Åland Islands in Finland, Scand J Infect Dis 1998;30:501-3
3. Stjernberg L, Berglund J. Garlic as an insect repellent, JAMA 2000; 284(7)
4. Kelly D. When is a butterfly like an elephant? Chem Biol 1996; 3: 595-602