Public Health Importance and Risk Factors for Cercarial Dermatitis Associated with Swimming in Lake Leman at Geneva, Switzerland

Epidemiol. Infect. (1998), 120, 305–314. Printed in the United Kingdom # 1998 Cambridge University Press

Public health importance and risk factors for cercarial dermatitis associated with swimming in Lake Leman at Geneva, Switzerland

" # " E.CHAMOT *, L.TOSCANI  A.ROUGEMONT

"Institute of Social and Preventive Medicine, University Medical Center, PO Box, 1211 Geneva 4, Switzerland # Geneva Department of Social Welfare and Public Health, Geneva, Switzerland

(Accepted 2 December 1997)

SUMMARY Fifteen cohorts of healthy bathers were recruited at four Lake Leman beaches between 3 July and 23 August 1996 to assess the public health importance of cercarial dermatitis in Geneva, Switzerland. Telephone follow-up interviews were carried out after 2–7 days. Overall, 153 bathers out of 555 reported probable cercarial dermatitis at follow-up. Median daily attack rate was 27n7% (2n2–57n7%). Of the cases, 11n1% noticed more than 30 skin lesions, 19n6% described severe itching, 50n3% used a drug treatment, 3n9% visited a doctor and 15% claimed they would reduce their bathing activities. History of cercarial dermatitis, time spent in the water, hour of the day, barometric pressure and maximum daily atmospheric temperature predicted disease occurrence in multivariate analysis. While a benign disease, cercarial dermatitis may have a negative impact on the local water recreation industry. The identiﬁcation of risk factors for the disease may help produce better preventive recommendations for the bathers.

and swim freely into the surrounding water. They die INTRODUCTION within a day if they are unable to locate a deﬁnitive Cercarial dermatitis (‘swimmers’ itch’) is a common, vertebrate host and penetrate its skin. The life cycle is neglected, and non-communicable cutaneous disease completed when the organisms mature to adult males caused by the penetration of the skin by cercariae of and females in the new deﬁnitive host [1, 2]. Matu- some non-human parasitic trematodes of the family ration is impossible in humans. Nevertheless, persons Schistosomatidae [1]. exposed to contaminated waters may become dead- The life cycle of these worms is similar to those of end hosts when cercariae accidentally penetrate the human schistosomiasis. Generally male and female skin and die. adults live in the portal vessels of aquatic birds. The The invasion of the exposed skin by cercariae may gravid females produce eggs that are excreted in the produce an immediate prickling sensation that lasts faeces. If the eggs reach a body of water, they hatch for about 1 h. The precocity and intensity of the and liberate a miracidium that swims about seeking following hypersensitivity response depends on the an appropriate intermediate snail host. In the mollusc, number of previous contacts with cercariae. In a the miracidium develops and gives rise to thousands primary infection, the skin reaction is either inap- of cercariae. Under favorable microconditions of parent or mild. Small and transient macules or temperature and light, the cercariae leave the snail maculopapules develop after 5–14 days. In re-exposed persons erythematous papules or papulovesicules * Author for correspondence. typically appear within 12–24 h. Urticarial-like lesions

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are common. Itching is severe and may cause between 1100 and 1700 hours on a trial day. insomnia. The eruption resolves over 1–3 weeks [3–6]. Enrolment usually took place on Tuesdays and Rarely, massive infections cause fever, limb swelling, Thursdays. The schedule had often to be adapted, nausea and diarrhoea [7]. Diagnosis relies on clinical however, as a result of poor weather. Interviewers and epidemiologic data. The treatment of the der- distributed a letter of information signed by the matitis is symptomatic and relies on antipruritic Geneva health authority. The letter addressed the topical preparations [1, 2]. Systemic antihistamines issue of cercarial dermatitis, explained that the study and corticosteroids are useful in the case of a massive was performed as a response to public concern and infestation. stressed the confidentiality of the information that Cercarial dermatitis has a widespread distribution would be collected. Visitors were invited to complete in temperate and tropical areas of the world [1, 2, a baseline self-administered questionnaire. Children 8–11]. In Asia, it is sometimes a debilitating oc- less than 16 years old were excluded if no legal cupational disease among rice farmers [12–15]. In representative was present. When appropriate, a Europe and the USA, it is seen as an emerging contact person was designated among the members of problem associated with swimming and other aquatic a same household. activities [1, 2, 7, 9, 11, 16]. In Switzerland, cercarial The baseline questionnaire focused on bathing dermatitis is caused by Trichobilharzia ocellata [6, 7] during the summers 1994 and 1995, bathing during and has been reported to the Swiss health authorities the 7 days prior to the enrolment, intention to swim increasingly during the past 15 years [7, 16]. In on the day of the survey, and history of skin lesions Geneva, cercarial dermatitis has received considerable resembling insect bites after bathing in a lake. attention from the public and the media during the Additional information was collected on age, sex, last few years. Local newspapers have repeatedly residential area code, phone number and best time of warned the population about the risk of contracting the day to make telephone contact. Bathing was ‘duck flea disease’, as the infection is called there. The defined as any contact of a body part with the water Geneva Department of Social Welfare and Public of the lake. At the end of the questionnaire, a Health has carried out the present study to better reminder encouraged participants to remember the answer the questions of the public, to assess the public timing and mode of their bathing. health importance of cercarial dermatitis in Geneva, and to identify behavioural and meteorologic risk factors for the disease. Follow-up interview and case definition Follow-up information was solicited by phone 2–7 days after the baseline interview. The follow-up survey MATERIALS AND METHODS was restricted to individuals who gave a phone Study sites number in Switzerland or in the nearby French area. Six attempts were made to contact each subject. Studies were conducted at four Geneva beaches: In close agreement with the clinical case definition Ce! ligny and Versoix on the right bank of the Lake proposed by the CDC [10], a probable cercarial Leman, and Savonnie' re and Hermance on the left dermatitis was defined as one or several skin lesions bank. These locations were selected out of 17 Geneva reported by the respondent that met the following public beaches that the General Office of the En- characteristics: (1) they resembled skin lesions due to vironment reported as adequate for swimming in 1996 insect bites, (2) no actual insect bite was observed, and on the basis of coliform counts. The beaches were at (3) the lesions appeared during or after the night least 5 km from each other. following the beach visit. Follow-up questions included inquiry on time, duration and type of swim on the enrolment day, on Study design and baseline questionnaire bathing during the following days, on prickling The design consisted of a series of short prospective sensations while bathing or during the first hour after cohort studies that were carried out in July and bathing, as well as on itching, and insomnia related to August 1996. For each discrete study, interviewers the skin eruption. An estimate of disability was contacted all individuals who visited a selected beach obtained by asking whether the respondent felt too

sick to go back to the beach, too sick to work, too sick Table 1. Characteristics of the respondents to the to leave home; whether he or she had to visit a baseline questionnaire, by follow-up status pharmacy, to see a doctor or to go to the hospital. Attitude toward future bathing in the lake was With Without follow-up follow-up assessed by asking whether the subject was planning (n l 681) (n l 493) to swim in the lake as usual, less often or never again. Characteristics % % P* Regional meteorological data collected at the Geneva airport were obtained from the Swiss Age (years) 0–20 39n126n1 Meteorological Institute. The following daily variable 21–40 34n543n6 were considered: lower, mean and higher atmospheric 41 25n829n1 temperatures (mC), barometric pressure (kPa), rainfalls Unknown 0n61n2!0n0001 (mm), average and maximal wind speeds (m\s), and Sex # solar energy (kW h\m ). Male 41n442n5 Female 58n657n50n71 Country of residence Switzerland 87n366n3 Statistical analysis France 9n019n5 Since the number of observed skin lesions was Unknown, other 3n714n2!0n0001 reported on an ordinal scale, we used ordinal logistic No. baths in a lake regression modelling to examine the associations per summer (1994–5) 07n313n6 between bathing characteristics and self-reported 1–5 25n727n6 cercarial dermatitis after adjusting for other potential 6–20 30n838n1 risk factors [17]. Using BMDP LR [18], we built "20 36n220n7!0n0001 continuation ratio models that considered four out- Bathing in a lake during come categories (0, 1–5, 6–10, and & 11 skin lesions) the 7 days prior to the [19]. The proportion of subjects with at least one skin baseline interview lesion among all subjects was modelled in a first Yes 47n838n7 No 52n261n30n002 outcome stratum; the proportion of subjects with at History of skin lesions least 6 skin lesions among individuals with at least 1 after bathing in a lake skin lesion was modelled in a second outcome stratum, Yes 37n027n2 and the proportion of subjects with at least 11 skin No 62n872n6 lesions among individuals with at least 6 skin lesions Unknown 0n20n20n0005 was modelled in a third outcome stratum. Considered bathing on The overall goodness of fit of the models was the day of the baseline assessed by using the Hosmer–Lemeshow goodness of interview Yes 83 4696 fit test [20]. n n No 16n630n4!0n0001 # * Pearson’s χ test of independence

RESULTS obtained from 681 of them (86%) 2–7 days (median: Between 3 July 1996 and 23 August 1996, the 3 days) after the initial beach interview. Table 1 interviewers spent 15 days recruiting subjects at the compares baseline data for subjects with and without beaches. Of the 1426 eligible individuals contacted, follow-up. 252 (18%) refused to answer the initial questionnaire. Five hundred and fifty-five subjects (81n5%) con- Of these 152 persons (56%) refused because they firmed they swam on the day of the enrollment. Of already had contributed to the study on a previous them 30n3% reported one swim, 27n6% two swims, occasion and 23 subject (9%) since they did not 20n9% three swims, and 21n2% from 4–10 swims. One intend to swim that day. For 77 individuals (35%), hundred and seventy-seven persons (31n9%) swam at the reason for refusal remained unknown. Seven least once between 1000 and 1400 hours, 485 (87n4%) hundred and eighty-nine subjects agreed to be between 1400 and 1700 hours, and 172 (40n0%) after recontacted by phone. Follow-up information was 1700 hours. Also, 42n1% of the bathers declared that,

Table 2. Reported skin lesions, associated symptoms, and consequences of probable cercarial dermatitis among 153 swimmers in Geneva, Switzerland (1996)

Reported manifestations and consequences No. % 95% CI*

No. skin lesions 1–5 75 49n040n9, 57n2 6–10 39 25n518n8, 33n2 11–30 22 14n49n2, 21n0 " 30 (max. 100) 17 11n16n6, 17n2 Subjects with skin lesions Face 8 5n22n3, 10n0 Head and neck 19 12n47n6, 18n7 Upper limbs 72 47n138n9, 55n3 Hands 6 3n91n5, 8n3 Chest and back 84 54n94n4, 6n8 Under the swimming suit 29 19n013n1, 26n1 Lower limbs 116 75n868n2, 82n4 Feet 21 13n78n7, 19n9 Itching None 22 14n39n2, 21n0 Negligible 55 36n028n4, 44n1 Uncomfortable 46 30n122n9, 38n0 Distressing 23 15n09n8, 21n7 Unbearable 7 4n61n9, 9n2 Insomnia Slept as usual 126 82n475n4, 88n0 Slept less well because of the skin lesions 20 13n18n2, 19n5 Did not sleep for at least one night because 63n91n5, 8n3 of the skin lesions Disability Too sick to go back to the beach 10 6n53n2, 11n7 Too sick to work 1 0n70n0, 3n6 Too sick to leave home 1 0n70n0, 3n6 Used a drug treatment 77 50n342n1, 58n5 Visited a chemist 15 9n85n6, 15n7 Visited a doctor 6 3n91n5, 8n3 Was hospitalized 1 0n70n0, 3n6

* CI, conﬁdence interval.

during each swim, they spent less than 10 min in the It was similar at Ce! ligny (17n0%; 95% CIl9n9, water, 47n2% 10–30 min and 10n7% more than 26n6), Hermance (12n9%; 95% CIl8n0, 19n4) and 30 min. Finally, 34n5% reported that they mainly Savonnie' re (17n4%; 95% CIl11n1, 25n3), but lower swam in water less than 1 m deep, 23n3% in water at Versoix (1n5%; 95% CIl0n3–4n3). 1–2 m deep and 42n2% in water more than 2 m deep. One hundred and fifty-three bathers (27n6%) Overall, 10n5% of the bathers (95% confidence reported probable cercarial dermatitis as compared interval [CI] l 8n0, 13n3) reported prickling sensations with only four non-bathers (3n2%). No non-bather during or immediately after bathing. The attack rate had more than five new skin lesions. These data suggest of prickling sensations was similar in men (9n3%) and that about 90% or more of the cases of dermatitis women (11n3%; relative risk [RR] l 0n83; 95% CI l reported by bathers were attributable to bathing. The 0n50, 1n36; P l 0n49). It was higher among subjects characteristics and the consequences of the skin 11–20 years old than among any other age group (RR eruptions are described in Table 2. l 2n56 after collapsing the age groups 0–10 and " 20 Among bathers, the median daily attack rate of into a single category; 95% CI l 1n54, 4n28; P ln01). probable cercarial dermatitis was equal to 27n7%

(Table 3) As shown in Figure 1, attack rates varied age, number and type of baths, bathing in a lake in considerably from day to day throughout the entire 1994–5, bathing in a lake during the 7 days prior to season (range l 2n2–57n7) and strongly correlated with the enrolment, bathing in a lake during follow-up, and daily attack rate of prickling sensations while bathing time to follow-up did not contribute to the final (Pearson’s coefficient of correlation: R l 0n65; P l model. 0n008). All except four subjects who reported prickling sensations while bathing were probable cases of cercarial dermatitis (RR l 11n61%; 95% CI l 4n51, DISCUSSION 29n92, P ! 0n0001). Figure 1 also indicates that the daily attack rate of To our knowledge, this study is the first epidemiologic probable cercarial dermatitis and regional barometric investigation intended to prospectively estimate attack pressure covaried consistently throughout the entire rates of cercarial dermatitis in swimmers throughout a study period except on 15 August and 16 August. bathing season. Several important issues were clari- Visual comparisons of curves suggested that maximal fied. First, cercarial dermatitis is a very common and regional atmospheric temperature was an effect widespread problem in Geneva although attack rates modifier of this relationship (data not shown). No vary considerably from one day to another. Second, other simple association was observed between the given a similar exposure to contaminated water, both incidence of probable cercarial dermatitis and any sexes and all age groups are equally affected. Third, other regional meteorologic factor listed in the method while a benign disease, cercarial dermatitis never- section (figures available from the authors). theless inconveniences some bathers to the point Drug treatment for skin lesions included a systemic where they reduce or stop their water activities. antihistamine in 10 subjects, a topical antihistamine in Finally, attack rates of self-reported cercarial der- 26 subjects, and topical steroid in 10 subjects. Twenty- matitis are strongly associated with factors related to nine persons used various other topical pharmaceu- the biology of the parasite, to the behaviour of the tical preparations, 9 used topical alcohol, vinegar or bathers, to previous history of cercarial dermatitis, sodium hypochlorite, and 2 used a homeopathic and possibly to individual susceptibility to the preparation. Expenditures for drug treatment were cercariae. Provided they are corroborated, these $4n0–30n0 (average: $7n0). results could lead to the development of improved Of the subjects who reported probable cercarial prevention messages to the swimmers. dermatitis after bathing, 2n6% (95% CI l 0n7, 6n6) In multivariate analysis, swim duration, period of claimed that, in the future, they would never swim in the day when swimming, barometric pressure and a lake anymore, 12n4% (95% CI l 7n6, 18n7) that they maximal atmospheric temperature strongly predicted would swim in a lake less often than before, and not only cercarial dermatitis but also the number of 85n0% (95% CI l 78n3, 90n2) that they would swim as reported skin lesions. In contrast, history of cercarial often as before. dermatitis was strongly associated with cercarial After adjusting for calendar time to enrolment dermatitis but only weakly with the number of skin (Mantel–Haenszel procedure [21]), bathing at Versoix lesions. was associated with a lower risk of self-reported These results are consistent with the fact that past cercarial dermatitis than bathing at Ce! ligny (RR l cercarial dermatitis is a risk factor for early and strong 0n41; 95% CI l 0n25, 0n69; P l 0n0002), at Savonnie' re skin reaction after a new infection [3, 5, 22], while (RR l 0n48; 95% CI l 0n33, 0n70; P l 0n0009), and swim duration, period of the day when swimming, at Hermance (RR l 0n61; 95% CI l 0n41, 0n91, P l barometric pressure and maximal atmospheric tem- 0n02). perature reflect the likelihood that cercariea will reach In multivariate analysis, the risk of cercarial the bather’s skin. The probability of contact of dermatitis was strongly associated with history of cercariae with the skin increases with the amount of cercarial dermatitis, time that bathers spent in the time spent in the water [23]. Cercariae of T. ocellata water, period of the day when bathing, beach, regional are mainly released during the morning hours and barometric pressure and regional maximum atmos- their activity decrease progressively with time [24]. It pheric temperature (Table 4). All factors except is also likely that meteorologic conditions somehow history of cercarial dermatitis were also strong affect cercarial behaviour by modifying the physical predictors of the number of reported skin lesions. Sex, characteristics of the water.

Table 3. Crude attack rates of probable cercarial dermatitis among bathers in Geneva Switzerland (1996), by baseline, behavioral, and meteorological characteristics

No. Attack rate, Relative risk Characteristic No. cases Non-cases % (95% CI*)

Age, years 0–10 48 124 27n91n0† 11–20 30 47 39n01n40 (0n97, 2n02) 21–30 19 63 23n20n83 (0n52, 1n32) 31–40 20 78 20n40n73 (0n46, 1n16) 41 36 88 29n01n04 (0n72, 1n50) Sex Male 69 177 28n01n0† Female 84 225 27n20n97 (0n74, 1n27) Number of baths in a lake per summer (1994–5) 0 5 26 16n11n0† 1–5 31 105 22n81n41 (0n60, 3n34) 6–20 45 129 25n91n60 (0n69, 3n72) " 20 72 141 33n82n10 (0n92, 4n78) Bathing in a lake during the past 7 days No 62 201 23n61n0† Yes 91 200 31n31n33 (1n01, 1n75) History of skin lesions after bathing in a lake Never 61 289 17n41n0† 1–5 times 65 94 40n92n35 (1n75, 3n15) & 6 times 27 19 58n73n37 (2n41, 4n70) No. of baths (recruitment day) 1 40 128 23n81n0† 2 48 105 31n41n32 (0n92, 1n88) 3315635n61n50 (1n01, 2n21) & 4 34 113 23n10n97 (0n65, 1n45) Bathing time (recruitment day) 1000–1400 hours 63 114 35n61n0† 1400–1700 hours 133 352 27n40n77 (0n60, 0n98) After 1700 hours 40 132 23n30n65 (0n47, 0n91) Average bath duration (recruitment day, min) ! 10 39 194 16n71n0† 10–30 88 174 33n62n01 (1n44, 2n80) " 30 26 33 44n12n63 (1n75, 3n95) Type of bath (recruitment day) In ! 1 m of water 49 142 25n71n0† In 1–2 m of water 41 88 31n81n24 (0n87, 1n76) In " 2 m of water 63 171 26n91n05 (0n76, 1n45) Beach (recruitment day) Versoix 39 160 19n61n0† Ce! ligny 29 59 33n01n68 (1n12, 2n53) Hermance 44 103 29n91n53 (1n05, 2n22) Savonnie' re 41 80 33n91n73 (1n19, 2n52) Recruited in July 116 270 30n11n0† August 37 132 21n90n73 (0n53, 1n01)

Table 3.—cont.

No. Attack rate, Relative risk Characteristic No. cases Non-cases % (95% CI*)

Time to follow-up (days) 2–3 63 218 22n41n0† 4–7 90 184 32n81n47 (1n11, 1n93) Additional bath in a lake during follow-up No 83 238 25n91n0† Yes 70 164 29n91n16 (0n88, 1n52) Max. atmospheric temperature (mC) (Barometric pressure, kPa) ! 24n5 ! 969 2 18 10n01n0† &969 19 31 38n03n80 (0n97, 14n83) & 24n5 ! 969 28 93 23n12n31 (0n60, 8n96) & 969 104 260 28n62n86 (0n76, 10n75)

* CI, conﬁdence interval. † Referent category.

80 975

60 970

40 965 Attack rate (%) 30 Barometric pressure (kPa)

20 960

0 955 07/03 07/11 07/12 07/16 07/18 07/23 07/25 07/30 07/31 08/06 08/08 08/15 08/16 08/20 08/23 Calendar time Fig. 1. Attack rates of probable cercarial dermatitis in summer 1996. , Prickling sensation; *, cercarial dermatitis; 4, barometer pressure.

Table 4. Results of ordinal logistic regression analysis (continuation ratio model) for the relation between probable cercarial dermatitis and various risk factors among swimmers in Geneva, Switzerland, 1996

Adjusted

Characteristic No. subjects Odd ratio* 95% CI† P

History of skin lesions after bathing in a lake (outcome stratum 1)‡ Negative 325 1n0§ 1–5 times 136 3n09 2n24, 4n28 ! 0n0001 ! 5 times 46 9n58 6n94, 13n24 ! 0n0001 History of skin lesions after bathing in a lake (outcome stratum 2)‡ Negative 61 1n0§ 1–5 times 57 1n16 0n66, 2n04 0n60 ! 5 times 27 3n60 2n05, 6n30 ! 0n0001 History of skin lesions after bathing in a lake (outcome stratum 3)‡ Negative 31 1n0§ 1–5 times 32 0n64 0n45, 1n34 0n12 " 5 times 15 1n97 0n93, 4n16 0n06 Average bath duration (recruitment day, minR) ! 10 283 1n0§ 10–30 344 1n82 1n40, 2n37 ! 0n0001 " 30 103 3n32 2n56, 4n32 ! 0n0001 Bathing timeR Exclusively ! 1400 hours 251 1n0§ Between 1400 and 1700 hours 302 0n73 0n58, 0n92 0n007 After 1700 hours 177 0n53 0n42, 0n92 ! 0n0001 BeachR Versoix 258 1n0§ All others 472 1n63 1n10, 2n41 0n014 Max. atmospheric temperature (mC) (Barometric pressure, (kPa) ! 24n5R ! 969 23 1n0§ & 969 80 7n53 1n86, 30n40n005 & 24n5 ! 969 152 4n09 1n07, 15n70n04 & 969 475 3n58 1n89, 6n73 ! 0n0001

Fifty-six observations representing five covariate pattens were excluded from the analysis on the basis of regression diagnostics [20]. Goodness of fit test for the final model: P l 0n463. * Odds ratios were adjusted for history of skin lesions after bathing in a lake, average bath duration, hour of the day when bathing, beach, regional maximum atmospheric temperature and regional barometric pressure. † CI, confidence interval. ‡ In outcome stratum 1, the odds estimates the risk of reporting skin lesions among all swimmers. In outcome stratum 2, the odds estimates the risk of reporting 6–10 skin lesions among swimmers who reported at least one skin lesion. In outcome stratum 3, the odds estimates the risk of reporting more than 10 skin lesions among swimmers who reported at least 6 skin lesions. § Referent category. R Odds ratio estimates did not vary significantly across outcome strata 1, 2 and 3.

There are, however, alternative explanations to ceptibility to infection. Under experimental condi- these ﬁndings. A ﬁrst possibility is that selection of the tions, cercariae penetrate the skin of some persons but bathers occurred on the basis of their relative sus- avoid others [3]. If the risk of infection given a similar

exposure to cercariae were higher in some individuals likely to consider bathing on the day of the interview than in others, the former would be more likely to and to have a reported history of skin lesions after repeatedly develop numerous skin lesions. Thus, they bathing in a lake. As a result, the overall attack rate may selectively avoid swimming in the lake. Detection of cercarial dermatitis was probably overestimated bias is another issue. Subjects who had already since participants were more likely to be exposed to experienced episodes of cercarial dermatitis may have cercariae, to have a strong skin reaction after been more likely to identify and report one or a few infection, and to report skin lesions. lesions than subjects without such experience. The role of regional meteorologic factors remains Overall, our study presents several limitations. The unclear. It is hard to imagine a plausible biological strategy of case identification was neither very explanation that would support a direct causal sensitive nor specific. It is likely that light and primary relationship between barometric pressure and cer- infections were underreported since both types of carial dermatitis. Most likely the observed association infection lead to trivial symptoms. However cercarial is due to the confounding effect of one or several dermatitis is not a transmitted disease. Therefore, unmeasured factors jointly associated with the re- cases that remained unnoticed were of no public health gional meteorologic conditions and the risk of significance. More importantly, self-reported cases of cercarial dermatitis. Because many exploratory com- cercarial dermatitis may have been confused with parisons were made, the complex relationship dermatitis of bacterial etiology, contact dermatitis, or observed among regional barometric pressure, re- insect bites from chiggers, fleas and mosquitoes [2, 10]. gional maximum atmospheric temperature, and attack No confirmatory procedure was used. Serologic tests rate of cercarial dermatitis may also be due to chance and skin biopsies are of limited practical value [12, 25] only. and physical examination was considered to be too In conclusion, the frequency of cercarial dermatitis obtrusive. Nevertheless, authors and interviewers in Geneva and its interference with popular leisure observed many typical cases of cercarial dermatitis activities are causes of concern to the population. Our among the study site visitors. Moreover, the wide- study indicated that cercarial dermatitis is usually a spread distribution of T. ocellata in the Lake Leman benign disease among swimmers. It also showed that is well established [7]. A study performed in 1983–4 the incurred direct costs are low. Nevertheless, bathing reported confirmed cases at 34 beaches around the in the lake is very popular in Geneva and economic lake including Versoix and Hermance [7]. Since then, activities related to the lake are substantial. Therefore, infested snail hosts have repeatedly been collected at indirect costs to the community may be significant. various locations. In our study, few new skin lesions Overall, this situation suggests that cercarial der- were reported at follow-up by subjects who did not matitis is a public health issue that deserves more swim on the date of enrollment. As already mentioned, attention. around 90% or more of the probable cercarial dermatitis reported by bathers were attributable to ACKNOWLEDGEMENTS bathing. Finally, according to bacteriologic tests the study beaches were suitable for swimming and no This research was supported by a grant from the cause of dermatitis related to bathing in Lake Leman Geneva Department of Health and Social Welfare. other than T. ocellata has yet been described. The authors would like to thank Lucienne Beney, Another weakness of our study was the difficulty of Suzanne Espejo, Benito Perez, Ahmed Sharafat, maintaining a high participation rate throughout the Robert Klein, Philippe Sudre, Jean-François entire investigation. Individually, the return rate to Donnard, Jean-Paul Dubois, Jean-Bernard Lacha- the baseline questionnaire was high, the proportion of vanne, Claude Vaucher, Patrick Jacot and The! re' se subjects who accepted to be recontacted by phone was Barale for assistance with the project. in the range expected for such a survey in Geneva [26], and follow-up was obtained for most subjects who REFERENCES communicated their telephone number. Overall, however, a substantial proportion of eligible individuals 1. Baird JK, Wear DJ. Cercarial dermatitis: the swimmer’s itch. Clin Dermatol 1987; 5: 88–91. either did not participate or were lost to follow-up. As 2. Hoeffler DR. ‘Swimmers’ itch’ (cercarial dermatitis). shown in Table 1, they were more likely to be adults Cutis 1977; 19: 461–7. and to be occasional bathers. They were also less 3. Cort WW. Studies on schistosome dermatitis: status of

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