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 27±7% (2±2–57±7%). Of the cases, 11±1% noticed more than 30 skin lesions, 19±6% described severe itching, 50±3% used a drug treatment, 3±9% 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 Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.40, on 27 Sep 2021 at 01:15:56, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0950268898008826 306 E. Chamot, L. Toscani and A. Rougemont 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 Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.40, on 27 Sep 2021 at 01:15:56, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0950268898008826 Cercarial dermatitis in Geneva, Switzerland 307 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 ¯ 681) (n ¯ 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 39±126±1 Meteorological Institute. The following daily variable 21–40 34±543±6 were considered: lower, mean and higher atmospheric 041 25±829±1 temperatures (mC), barometric pressure (kPa), rainfalls Unknown 0±61±2!0±0001 (mm), average and maximal wind speeds (m}s), and Sex # solar energy (kW h}m ). Male 41±442±5 Female 58±657±50±71 Country of residence Switzerland 87±366±3 Statistical analysis France 9±019±5 Since the number of observed skin lesions was Unknown, other 3±714±2!0±0001 reported on an ordinal scale, we used ordinal logistic No. baths in a lake regression modelling to examine the associations per summer (1994–5) 07±313±6 between bathing characteristics and self-reported 1–5 25±727±6 cercarial dermatitis after adjusting for other potential 6–20 30±838±1 risk factors [17]. Using BMDP LR [18], we built "20 36±220±7!0±0001 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].