![]() ![]() 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. (2) To explain location effects, we assembled existing knowledge about snail densities, duck broods, and weather patterns for Douglas Lake. More days of lake use increase a person’s chances of encountering cercariae at any depth. ![]() (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. We interpret these results in biologically relevant ways, as follows. 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. New spots without same-day or prior-day water exposure were dropped. Preference was given to exposure that same day, due to the typically short incubation period for symptoms. 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. When a resident or guest departed, no data for subsequent days were entered. 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. 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. 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). Each day, residents and guests were listed by initials, age, and sex. The diary booklet had a page for each day, plus information sheets (e.g., swimmer’s itch criteria, lake map with numbered areas). Water exposures and trouble with swimmer’s itch were recorded for all residents and guests at the household. Participating households filled in a diary for the month of July 2000, a peak period of water use and swimmer’s itch complaints. Project advertisements were posted around the lake, and households were recruited by personal visit in late June. 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. Riparian (lake perimeter) households were the target sample. The lake covers 15.33 sq km and has a maximum depth of 24 to 25 m. Preventive actions before or after water exposure have been proposed, but no scientific evidence exists on their efficacy.Ī prospective survey of swimmer’s itch among persons with water-based activities was conducted in July 2000 at Douglas Lake, Cheboygan County, Michigan. Intense itching and papules are present 10 to 15 hours later, continuing for about a week. Mild itching and macular eruptions occur 1 to several hours after a person leaves the water. If instead they penetrate human skin during recreation or work in the water, an inflammatory response occurs. 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. The schistosomes for swimmer’s itch have a 2-host life cycle, an avian definitive host and a snail intermediate host. 29, 30 We conducted a prospective study at a Michigan lake to obtain an incidence rate and identify risk factors for swimmer’s itch. 2– 14 The literature on humans and swimmer’s itch consists largely of clinical and outbreak reports. Since discovery of the parasites responsible for swimmer’s itch, 1 most research has focused on the schistosomes’ life cycle and biological control methods. It causes intense discomfort, discourages recreational water use, and leads to economic loss for lake regions if people decide to vacation elsewhere. 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.
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