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Lois M. Verbrugge, PhD, MPH, Jeanette J. Rainey, MPH, Ronald L. Reimink, MA and Harvey D. Blankespoor, PhD

Lois M. Verbrugge is with the Institute of Gerontology and Jeanette J. Rainey is with the Department of Epidemiology, University of Michigan, Ann Arbor. Ronald L. Reimink is with the Science Department, Hudsonville High School, Hudsonville, Mich. Harvey D. Blankespoor is with the Department of Biology, Hope College, Holland, Mich.

Correspondence: Requests for reprints should be sent to Lois M. Verbrugge, PhD, MPH, Institute of Gerontology, 300 N Ingalls, University of Michigan, Ann Arbor, Michigan 48109–2007 (e-mail: verbrugg{at}umich.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION CONCLUSIONS References

 

Swimmer’s itch (cercarial dermatitis) affects people engaged in open-water activities. We report incidence and risk factors for a US lake. Water exposures and swimmer’s itch experience were reported daily for riparian household residents and guests at Douglas Lake, Michigan, in July 2000. Incidence of swimmer’s itch was 6.8 episodes per 100 water exposure days. Positive risks were (1) exposures in shallow water and in areas with onshore winds and (2) more days of lake use in July. Further epidemiological studies will help public health agencies address this bothersome problem at recreational lakes.

	INTRODUCTION

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Cercarial dermatitis, or swimmer’s itch, is a skin condition that affects people engaged in open-water activities in fresh and salt-water areas around the world. It causes intense discomfort, discourages recreational water use, and leads to economic loss for lake regions if people decide to vacation elsewhere. Since discovery of the parasites responsible for swimmer’s itch,¹ most research has focused on the schistosomes’ life cycle and biological control methods.^2–14 The literature on humans and swimmer’s itch consists largely of clinical and outbreak reports.^15–28 Epidemiological studies are rare.^29,30 We conducted a prospective study at a Michigan lake to obtain an incidence rate and identify risk factors for swimmer’s itch.

The schistosomes for swimmer’s itch have a 2-host life cycle, an avian definitive host and a snail intermediate host. At one point in the cycle, snails release cercariae (free-swimming larval stage of the parasite) into the water, where they may encounter and enter ducks and other birds. If instead they penetrate human skin during recreation or work in the water, an inflammatory response occurs. Mild itching and macular eruptions occur 1 to several hours after a person leaves the water. Intense itching and papules are present 10 to 15 hours later, continuing for about a week. Swimmer’s itch is not communicable. Preventive actions before or after water exposure have been proposed, but no scientific evidence exists on their efficacy.

	METHODS

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A prospective survey of swimmer’s itch among persons with water-based activities was conducted in July 2000 at Douglas Lake, Cheboygan County, Michigan. The lake covers 15.33 sq km and has a maximum depth of 24 to 25 m. Riparian (lake perimeter) households were the target sample. Inclusion criteria were that residents planned to be at the lake for 2 weeks or longer in July and used the lake regularly for swimming, wading, or other activity involving direct contact with the water. Project advertisements were posted around the lake, and households were recruited by personal visit in late June. Participating households filled in a diary for the month of July 2000, a peak period of water use and swimmer’s itch complaints. Water exposures and trouble with swimmer’s itch were recorded for all residents and guests at the household. The diary booklet had a page for each day, plus information sheets (e.g., swimmer’s itch criteria, lake map with numbered areas). Each day, residents and guests were listed by initials, age, and sex.

Water exposure information was as follows: total minutes in water (swimming, wading, other recreation or work directly in Douglas Lake), minutes in shallow water (waist deep or less), lake area where most exposure occurred, time of day when most exposure occurred, and whether preventive action was taken (e.g., oil/lotion/wet suit before exposure, brisk brush/alcohol rub after). These items tap hypothesized risk factors for swimmer’s itch, namely, long duration in water, shallow-water exposure, locations with shallow shorelines or onshore winds, early- or mid-morning exposure, and human practices to avert or remove cercariae. Swimmer’s itch information was as follows: number of new itchy spots that day due to swimmer’s itch, total itchy spots that day, and degree of discomfort. When a resident or guest departed, no data for subsequent days were entered.

We developed an algorithm to detect episodes of swimmer’s itch: each day with new itchy spots was linked to water exposure characteristics the same or prior day. Preference was given to exposure that same day, due to the typically short incubation period for symptoms. New spots without same-day or prior-day water exposure were dropped.

	RESULTS

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Descriptive Statistics
Forty households completed the diary for July. The total number of residents and guests at the households was 313. Almost all residents and guests (301; 96.2%) were exposed to water in Douglas Lake in July. The 301 lake users reported 1300 water exposure days. On exposure days, mean time in the water was 49.0 minutes. Shallow-water use was common (89.2% of the days; mean = 39.8 minutes). Exposures occurred at 36 of the 54 lake areas; most (63.4%) were in the afternoon from 2 to 6 pm. Prevention was used on 29.5% of the days.

The swimmer’s itch incidence rate was 6.8 episodes per 100 water-exposure days (89 episodes; 95% confidence interval = 5.5, 8.2). Fifty-two people acquired swimmer’s itch (17.3% of exposed persons); 58% had 1 episode and 42% had 2 or more.

Swimmer’s Itch Risk Factors: Water-Exposure Days
Significant factors for incidence of swimmer’s itch for water-exposure days (n = 1300) are shown in Table 1. Key bivariate risks were any shallow-water use, location, and preventive action. All episodes occurred on days with shallow-water use. Incidence was highest in south and east lake zones (the 54 lake areas were pooled into 5 zones for analysis; see Table 1, footnote c). Paradoxically, using prevention was positively associated with onset of swimmer’s itch. Although observed incidence rose with total minutes and shallow minutes, and was highest for exposures from 6 to 10 am, those results were not statistically strong (P > .10). Age and sex were unrelated to onset of an episode. Logistic regressions with all covariates showed that location and preventive action remained significant.

	DISCUSSION

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This study found the following risk factors for onset of swimmer’s itch: amount of water exposure, especially shallow-water use; exposure in several lake zones; and taking preventive action. We interpret these results in biologically relevant ways, as follows. (1) Shallow water is where snail beds are typically most dense and where cercariae tend to accumulate, so incidence is higher for people using shallow water. More days of lake use increase a person’s chances of encountering cercariae at any depth. (2) To explain location effects, we assembled existing knowledge about snail densities, duck broods, and weather patterns for Douglas Lake. The most likely reason for elevated incidence in the southern and eastern zones is that persistent winds brought cercariae from the northwestern and western areas, and the sheltered bays prevented the onward movement of incoming and locally produced cercariae. Many factors affect cercarial concentrations from year to year and place to place; our explanation for location differences in 2000 is possible but unproven. (3) Taking preventive action before or after exposure was linked to increased risk of swimmer’s itch. This aligns with evidence that people’s sensitivity to swimmer’s itch increases over time.¹⁴ Past bad experience may prompt people to use prevention, but it is apparently insufficient to avert new episodes.

The low incidence rate of 6.8 episodes per 100 water-exposure days is probably related to weather conditions. Summer 2000 was uncommonly cool in northern Lower Michigan. A warmer summer would probably generate higher incidence because cercarial production and concentrations increase in warm conditions.

For more information about methods and results, contact the lead author. A longer article including complex variance estimations is forthcoming.³¹

	CONCLUSIONS

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The onset of swimmer’s itch depends on how humans interact with the lake. Exposures to shallow water and areas with onshore winds are key risks for swimmer’s itch. The more days a person used the lake in July, the higher his or her chances of having any episode. The underlying biological reasons relate to density of snails, movement of cercariae, and cumulative chances of encountering cercariae. In public health terms, people should avoid using shallow water and areas with persistent onshore winds. If they still choose such places, the less often, the better.

	Acknowledgments

 
This study was a pilot project to evaluate data collection methods and obtain preliminary results. The Douglas Lake Association approved the project and helped advertise it to households. Hope College, Holland, Mich, provided some funds for data analysis.

The project team thanks Dr James M. Lepkowski (Institute for Social Research and Department of Biostatistics, University of Michigan) for consultation on sample size estimation for the project; Jim Laarman, Cliff Graves, and Harry Blecker of the Crystal Lake Association, Benzie County, Michigan, for discussions of swimmer’s itch over several years preceding the project; Aryc W. Mosher for initial design work; and Elmer G. Gilbert for design and analysis advice throughout the project.

Human Participant Protection
This nonfunded pilot project followed contemporary survey practice for household recruitment, retention, and contact. Households could cease diary-keeping at any time (2 of 42 did so). No personal identifiers appear in the data.

	Footnotes

 
Contributors
L. M. Verbrugge was principal investigator, with chief responsibility for project design and analysis. J. J. Rainey performed all data management and computing activities and worked closely with Verbrugge in interpretation and presentation of results. R. L. Reimink managed the project fieldwork, including household recruitment and contacts. H. D. Blankespoor assisted in project design and selection of Douglas Lake as the site. With longtime knowledge of the lake, Reimink and Blankespoor helped interpret location effects.

Peer Reviewed

Accepted for publication May 4, 2003.

	References

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This Article Abstract Full Text (PDF) Submit a response Purchase Article View Shopping Cart Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (5) Citing Articles via Google Scholar Google Scholar Articles by Verbrugge, L. M. Articles by Blankespoor, H. D. Search for Related Content PubMed PubMed Citation Articles by Verbrugge, L. M. Articles by Blankespoor, H. D. Related Collections Social Science Epidemiology Surveys