Villagers were selected from Bo Kluea District, Nan Province (n=50), and Chae Hom District, Lampang Province (n=100), northern Thailand. These areas were under Her Royal Highness Princess Maha Chakri Sirindhon's Project for Remote Area Development. Cases with stools previously reported to contain O. viverrini eggs, identified by Kato's thick smear technique, were tracked from hospital records, District Health Office records, or both. Fresh stool samples were collected and re-examined. Praziquantel at 40 mg/kg was given to each patient in a single oral dose. Magnesium sulfate was administered 2 hr later to facilitate stool collection process. Stools were collected up to 4 times consecutively. After simple sedimentation in 0.85% sodium chloride solution, the sediments were examined under a stereomicroscope. Worms were collected, stained with carmine, and morphologically identified. Haplorchis taichui was identified based on the characteristics of spines in the ventro-genital complex, according to Pearson and Ow-Yang [6]. Phaneropsolus bonnei was identified based on the morphology described by Manning et al. [7]. Taenia proglottides were differentiated into T. saginata or T. solium by the India ink injection technique, and lateral uterine branches were counted [8]. Enterobius vermicularis was morphologically identified according to standard textbooks.

Results of the survey are summarized in Table 1. H. taichui was the only fluke found in Nan Province (intensity of infection 5-1520; mean 62 worms per case). In Lampang Province, both H. taichui (intensity of infection 1 to 4,277; mean 326 worms per case) and P. bonnei were discovered. The tapeworm co-infectant was T. saginata, 7 cases in Nan and 10 cases in Lampang, respectively, while the only co-infected nematode was E. vermicularis found in 1 case in Nan.

Surveillance of opisthorchiasis is required by the Ministry of Public Health, Thailand, under the Helminth Control Program. Consequently, diagnostic methods must be specific. Unfortunately, the Kato's cellophane thick smear technique is a method of choice for helminth surveys. It is cheap and permits detection of eggs of several common helminths. However, it is not specific for Opisthorchis eggs in areas where co-infection with MIFs is evident [9]. The present findings indicate that the reported high prevalence of opisthorchiasis in Nan and Lampang Provinces was spurious. In fact, anthelmintic treatment revealed that most of the cases had haplorchiasis. These results are supported by similar findings in a previous report, which showed that H. taichui eggs were found by the formalin-ether concentration technique in up to 48.4% of adult villagers and 14.9% of schoolchildren in Chaloem Phra Kiat District, Nan Province, during a 2002-2004 survey [10]. Infection was confirmed in that case by worm recovery after albendazole treatment [10]. Opisthorchis eggs were not detected in that study. Previously, a cross-sectional survey of schoolchildren in Bo Kluea, Nan Province, revealed O. viverrini eggs in 1.7% of them, but worm recovery was not carried out to confirm the findings [11]. Failure to find adult O. viverrini in our study, however, does not rule out the possibility that infected cases exist, but with low prevalence and intensity. Similarly, haplorchiasis is more prevalent than liver fluke infection in some areas of Laos [12,13]. Although clinically less important than opisthorchiasis, a link between haplorchiasis and irritable bowel syndrome has been suggested [14]. Thus, care must be taken when reporting O. viverrini eggs using Kato's thick smear technique. More accurate diagnostic techniques such as PCR may be essential in the future.