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Schistosomiasis among River Rafters -- Ethiopia

On October 20, 1982, 4 days after returning from a rafting trip down the Omo River in Ethiopia, a Colorado resident noted the onset of a low-grade, intermittent fever and myalgias. His symptoms persisted, but he did not seek medical attention. However, in early February 1983, a call from another rafter, in whom schistosomiasis had just been diagnosed, prompted him to visit his physician. Stool examination revealed eggs of Schistosoma mansoni, and his serum was positive by the indirect immunofluorescent (IIF) test for schistosomiasis.

CDC and the Colorado Department of Health were independently informed of several other possible cases of acute schistosomiasis mansoni among other participants. Three tour guides had schistosomiasis mansoni diagnosed by stool examination after the trip, and they were symptomatic, with intermittent fever, cough, lethargy, and myalgias associated with an absolute eosinophilia ( greater than or equal to 600/mm((3))). A fourth guide had developed a 1-cm diameter, nontender, firm mass on her left buttock but had an absolute eosinophila count of 408/mm((3)) and was otherwise asymptomatic. Her stool examination revealed eggs of S. mansoni. The mass was not biopsied but resolved 6 days after treatment with praziquantel.

There were two separate raft trips; the first from September 19 to October 16, and the second from October 27 to December 4. Both followed the same route down the Omo River as another trip that led to an outbreak of schistosomiasis a year earlier (1). Participants had been made aware of the possibility of contracting schistosomiasis by the tour organization. Thirty people participated in one or both trips, and all four U.S.-based river guides were on both trips. Attempts were made to contact all participants except five staff from the Ethiopian office. Four non-U.S. citizens and one U.S. citizen could not be located. Of 20 participants contacted, 19 had stool and serologic examinations performed and completed a questionnaire concerning exposure; one with a positive IIF test but four negative stool examinations and a negative rectal biopsy is excluded from further analysis. Seven (39%) of the 18 participants had S. mansoni infection confirmed by stool examinations, and 11 (61%) were negative on testing three or more stools and had negative IIF tests.

All seven infected and two uninfected rafters developed an illness during or after the raft trips. Six of the seven infected persons had onset of a febrile illness, with eosinophilia consistent with acute schistosomiasis, within 3 weeks after a trip; the seventh was the guide who developed the soft tissue mass. Quantitative stool examinations of three infected participants revealed relatively light infections with 26, 24, and 18 eggs per gram.

Two persons with schistosomiasis also had confirmed Plasmodium vivax malaria. Of the two uninfected participants who became ill, one developed diarrhea, which resolved with metronidazole during the trip. The second developed confirmed P. vivax malaria in March 1983. Finally, stool from one asymptomatic participant had cysts of Entameba histolytica.

No specific site of exposure could be identified. Most participants took few or no precautions other than toweling-off, despite an awareness of the risk of acquiring schistosomiasis. However, those who towel-dried most of the time after water exposure had a significantly reduced likelihood of infection; eight of 11 of the noninfected and one of seven of the ill towel-dried after water exposure during the last third of either trip (p = 0.02). Reported by J Sisson, MD, Salt Lake City, C Nichols, MPA, Utah State Dept of Health; R Hopkins, MD, State Epidemiologist, Colorado State Dept of Health; Div of Field Svcs, Epidemiology Program Office, Helminthic Diseases Br, Div of Parasitic Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: This outbreak of acute schistosomiasis mansoni resembles an outbreak that occurred in 1981 (1). The unusual presentation of schistosomiasis as a subcutaneous mass has been previously described with S. japonicum, S. mansoni, and S. hematobium. While the present case was not parasitologically confirmed, the rapid resolution of the mass after praziquantel therapy suggests that it was schistosomal.

With multiple potential sites of exposure along the route, the exact mode and site of exposure could not be distinguished. For such an activity, exposure would be difficult to avoid, and such measures as chemically treating or heating water before bathing would not always be possible. Nevertheless, epidemiologic data gathered on these trips imply that towel-drying after exposure to infested water can markedly reduce the risk of infection supporting the recommendations that this is a useful protective measure (2).


  1. CDC. Cercarial dermatitis among bathers in California; Katayama syndrome among travelers in Ethiopia. MMWR:31:435-8.

  2. Schistosomiasis. In: Control of communicable diseases in man. Twelfth edition. Benenson AS, ed. American Public Health Association, 1975:282-4.

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