We appreciate Dr Karstorp's firsthand account of the trip, with her reemphasis on the surprisingly short incubation period for acute schistosomiasis and elaboration on some of the possible protective measures. We were not sufficiently precise in referring to concentrations of chlorine or iodine needed to kill cercariae. The figure of 1 ppm in our report refers to a concentration of free residual chlorine, correctly stated by Dr Karstorp. In general, water purification preparations used according to general instructions should achieve cercariacidal concentrations of free residual chlorine or iodine.1The risk of contracting schistosomiasis is far greater from bathing, swimming, wading, or similar activities with a relatively long exposure to large volumes of cercaria-infested water, than from brief oral exposure to small volumes of the same water. Filtration, heating, or chemical treatment, while recommended for drinking water and for situations in which bathing water can be pretreated, do
Fontaine RE, Istre GR. Prevention of Schistosomiasis-Reply. JAMA. 1984;252(22):3129–3130. doi:10.1001/jama.1984.03350220037019
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