In Lao People's Democratic Republic (PDR), we detected 7 patients infected with Centrocestus formosanus (1–122 adult specimens) after praziquantel treatment and purgation, together with several other trematode species including Opisthorchis viverrini and Haplorchis taichui. The patients were all men, 23–42 yr-of-age. Three subjects were from Vientiane Municipality and 1 each were from Khammouane, Saravane, Champassak, and Xiengkhouang Province. The patients had frequently eaten raw freshwater fish and were experiencing variable degrees of epigastric pain and indigestion accompanied by occasional diarrhea, although the relationship of these symptoms with C. formosanus infection was unclear. Centrocestus formosanus specimens were ovoid, 0.46 mm (0.41–0.52 mm) long, and 0.18 mm (0.16–0.20 mm) wide (n = 10) and were equipped with 32 circumoral spines on the oral sucker. The uterine eggs were 33.2 μm long (31.8–34.9 μm) and 18.5 μm wide (17.4–19.8 μm) (n = 20). Analysis of the nucleotide sequence of the 18S ribosomal RNA gene of our specimens (Laotian isolate) revealed 100% homology with that of an isolate from the United States reported in GenBank. Several species of freshwater fish collected from Xiengkhouang Province revealed a 17.0% prevalence (9 of 53 fish examined) for C. formosanus metacercariae. The results suggest that human C. formosanus infections have been masked by other trematode infections.

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