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Cercarial Dermatitis Outbreak at a State Park -- Delaware, 1991

On October 28, 1991, an employee of the Division of Public Health (DPH), Delaware Department of Health and Social Services, reported that her son and at least 10 other persons who had recently participated in a high school biology class field trip to Cape Henlopen State Park had contracted pruritic dermatitis. The Delaware Health Monitoring and Program Consultation Section conducted an investigation to confirm the reported presumptive diagnosis of cercarial dermatitis and to assess the extent of this outbreak.

On October 19, 1991, 37 students aged 13-16 years (median: 14.6 years) and their teacher spent 1.2-6.0 hours (median: 2.8 hours) wading at low tide in a shellfishing area. The weather was sunny and unseasonably warm; the air temperature was 80 F (27 C), the highest recorded in October 1991. During October 20-28, 29 students developed pruritic dermatitis; 11 visited their physicians for treatment.

On November 7, all of the students and their teacher were examined and interviewed by staff from the DPH. A case was defined as pruritic papular or papulovesicular dermatitis that developed in a person greater than or equal to 12 hours after seawater exposure. Of 37 (97%) persons who had had contact with seawater, 30 (81%) met the case definition (Table 1). Lesions were restricted to the parts of the body exposed to seawater (i.e., legs, arms, and abdomen); the incubation period ranged from 14 hours to 14 days. Pustules on four persons represented secondary bacterial infections that probably resulted from scratching of vesicles.

Cases occurred among five of 11 persons who wore long pants, compared with 25 (96%) of 26 who wore shorts (relative risk=0.5; 95% confidence interval=0.3-0.9). Promptly drying the skin with a towel did not alter the risk for dermatitis. The only person who was not exposed to seawater did not develop dermatitis.

Two persons were treated with oral prednisone; in one, manifestations recurred after completion of a 1-week course, indicating persistence of cercarial antigen in the skin.

On November 7 and 8, mud-flat snails (Nassarius obsoletus) were collected from the site of the field trip. Snails were isolated in culture dishes and examined after 24 hours for the presence of released cercariae. Two specimens of a brevifurcate apharyngeate cercaria were found in a dish containing two snails. Their morphology was consistent with Microbilharzia variglandis, an avian schistosome implicated as a causative agent of cercarial dermatitis. Shedding of M. variglandis was not observed in 936 snails examined at CDC. An additional 300 snails were then crushed and examined for trematode infections; two (0.7%) were infected with M. variglandis, each with more than 1000 mature cercariae and other cercariae in various stages of development.

Although 10 other groups were identified who had exposure to seawater at the same location during August 8 - October 31, 1991, none reported illness. Two of 18 physicians who were contacted in Kent and Sussex counties reported having diagnosed and/or treated 12 cases during 1991 that were not related to this outbreak; however, the location of exposure to seawater was unknown for these patients.

The DPH recommended to park officials that they advise persons wading in the shellfishing area to wear protective clothing (e.g., long pants, hip boots, and waders) and that this advisory should remain in effect until December 31, when cold water temperatures would reduce unprotected water contacts.

Reported by: R Wiley, Milford High School, Milford; D Wolfe, MPH, R Flahart, PhD, C Konigsberg, MD, PR Silverman, PhD, State Epidemiologist, Div of Public Health, Delaware Dept of Health and Social Svcs. Div of Field Epidemiology, Epidemiology Program Office; Parasitic Diseases Br, Div of Parasitic Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Cercarial dermatitis (i.e., swimmer's itch) is a cutaneous inflammatory response associated with penetration of the skin by cercariae, the free-swimming larval stage of bird schistosomes. Cercarial dermatitis can occur worldwide; in North America, cases have been reported from numerous localities in the United States, Canada, and Mexico. The distinct aspects of cercarial-induced dermatitis (1) that differentiate it from a dermatitis of bacterial etiology were observed in this park-associated outbreak. Symptoms include reddening and itching of exposed skin in the water or immediately after emerging, indicating initial penetration of cercariae through the skin, and delayed onset of pruritic raised papules that may become vesicular.

Typically, hosts of avian schistosomes are migratory waterbirds, including shorebirds, ducks, and geese. Adult worms found in the blood vessels of the avian hosts produce eggs that are passed in feces. On exposure to water, the eggs hatch and liberate a miracidium that infects a suitable molluscan intermediate host. The parasite develops in the snail to produce cercariae that are released under appropriate conditions and penetrate the skin of birds to complete the cycle (Figure 1). Humans are inadvertent hosts; cercariae penetrate the skin but do not develop further. A number of species of dermatitis-producing cercariae have been described from both freshwater and saltwater environments, and exposure to either type of cercariae will sensitize persons to both (2).

Previous contact with cercariae leads to a more immediate and intense immune response. This sensitization phenomenon explains the wide range of incubation periods reported in cercarial dermatitis outbreaks (1). Symptomatic treatment of the dermatitis includes antihistaminic and antipruritic topical medications.

In this investigation, the low infection rate of M. variglandis in the mud-flat snails suggests few snails were infected as a result of the fall bird migration (3). However, trematode life cycles of the type exemplified by M. variglandis include amplification of parasite larvae in the snail host that results in the release of thousands of cercariae from a single snail on a daily basis.

Environmental conditions reported to be associated with shedding of M. variglandis include low tide, calm water, and sunny weather during the colder months of the year (2). The mechanism by which these conditions trigger release of cercariae of M. variglandis from infected snails is not well understood; however, all these conditions were present at the time of the Delaware outbreak.

Focal mollusciciding, an established control measure in freshwater, is not effective in marine settings that are subject to wave action and/or tidal movement. Control and prevention of this condition in marine environments involve recognizing areas in which dermatitis-producing cercariae may be encountered. Although the Delaware outbreak appears to have been restricted to a limited area, outbreaks can occur over larger areas, such as one that occurred on the southwestern shore of Connecticut in September 1983 (Connecticut State Department of Health Services, unpublished data, 1983).

Outbreaks of cercarial dermatitis occur unpredictably, and routine surveillance for cases and outbreaks is impracticable. A timely public health response depends on early recognition of cases by local physicians who diagnose or suspect cercarial dermatitis. Appropriate preventive measures include the posting of signs that warn of the risk for contracting cercarial dermatitis in the affected area and recommendations to curtail water-related activities and wear protective clothing.


  1. Hoeffler DF. Cercarial dermatitis -- its etiology, epidemiology, and clinical aspects. Arch Environ Health 1974;29:225-9.

  2. Olivier L. Schistosome dermatitis, a sensitization phenomenon. Am J Hyg 1949;49:290-302.

  3. Stunkard HW, Hinchliffe MC. The morphology and life-history of Microbilharzia variglandis (Miller and Northrup, 1926) Stunkard and Hinchliffe, 1951, avian blood-flukes whose larvae cause "swimmer's itch" of ocean beaches. J Parasitol 1952;38:242-71.

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