The surveyed areas were lakeside (the Tonle Sap Lake) villages in Pursat Province (Figure 1) where ≈12,000 persons, including 3,500 schoolchildren, live. For this study, 471 children (237 boys), 10–14 years of age, from 4 primary schools were selected. One fecal sample from each child was collected in June 2007. Samples were transported to the Malaria Station in Pursat within 2–3 days of collection and stored at 4°C until examination. The Kato-Katz thick smear technique was used to detect helminth eggs. Examination of feces and anthelmintic treatment were officially approved by the Ministry of Health, Cambodia, under the agreement of the Korea-Cambodia International Collaboration on Intestinal Parasite Control for Schoolchildren in Cambodia.

Four children who had positive test results for echinostomatid eggs and who had occasional, vague abdominal pain and discomfort were selected for anthelmintic treatment and adult worm recovery at the Malaria Station. After we obtained consent from their parents and the school guardian, the children’s infections were treated with a single oral dose of 10 mg/kg praziquantel (Shinpoong Pharmceutical Co., Seoul, South Korea), and purged with 20 g magnesium sulfate. Whole diarrheic feces were collected 3–4 times and pooled individually. The diarrheic feces were processed as previously described (11). Worms were collected by using a wooden applicator and washed several times in water. They were fixed with 10% formalin under coverslip pressure, stained with acetocarmine, and identified by morphologic features.

A total of 17.4% of samples were positive for helminth eggs. Echinostomatid eggs were found most frequently, followed by hookworm and Trichuris trichiura eggs (Table 1). The percentages of echinostome eggs were significantly higher in school A than in schools B, C, and D (Table 1). However, prevalence did not differ significantly (p<0.01) between boys and girls (data not shown). A total of 20 echinostome adults (12, 3, 3, and 2 worms) were recovered from 4 children who showed 48–120 eggs per gram of feces (Table 2). The worms were leaflike, elongated (Figure 2), and an average of 8.8 mm long (8.0–9.5 mm) and 1.7 mm wide (1.2–2.1 mm) (n = 10). When first passed in the feces, they were pinkish red and coiled in a “c” or “e” shape. The eggs in uteri were an average of 105 μm long (97–117 μm) and 63 μm wide (61–65 μm) (n = 10). On the basis of these characteristics, the worms were identified as E. revolutum (Froelich, 1802) Looss, 1899.

The major sources of E. revolutum infection in humans are freshwater clams (Corbicula producta) in Taiwan and snails (Physa occidentalis or Lymnaea sp.) in Thailand (1,5). According to school personnel, the children were fond of eating undercooked snails or clams of unidentified species sold on the road to their homes after school. They stated that the mollusks are caught near Tonle Sap Lake. Reasons for the higher prevalence in school A than schools B, C and D are unclear.

Of the schoolchildren living near Tonle Sap Lake, Pursat Province, Cambodia, who participated in this study, 7.5%–22.4%, depending on school, were infected with E. revolutum. E. revolutum trematodes are endemic parasites in this area of Cambodia and a likely source of infection is freshwater snails or clams from the lake. The public health significance of echinostomiasis and educational and prevention efforts should be highlighted.

Echinostomiasis is not only an endemic infectious disease in Asian countries, including Cambodia, but also can be imported by overseas travelers from the United States or Europe. An outbreak of echinostomiasis was reported among US travelers returning from Kenya and Tanzania, although the source of infection was uncertain (12). This diagnosis should also be considered in patients with abdominal pain and diarrhea who have traveled to Southeast Asia and eaten snails or clams.

Despite the dangerous nature of echinostomes, the study of echinostomiasis has been neglected for many decades (13,14), possibly because physicians and laboratory personnel lack knowledge about this trematode parasite. In addition, no easy diagnostic technique is available to detect echinostome eggs, except for routine fecal examination. However, some microscopists seem to overlook or misinterpret the presence of echinostome eggs, particularly in Kato-Katz fecal smears. Even if echinostome eggs are detected, the specific diagnosis is not possible unless the adult worm is collected and identified. Thus, both the training of microscopists and emphasis on the clinical significance of echinostomiasis are urgently needed.