Epidemiology of opisthorchiasis and national control program in Thailand.

Opisthorchiasis is a disease caused by opisthorchis trematode commonly known as liver fluke. In Thailand Opisthorchis viverrini is the only parasite of opisthorchiasis, the first case of opisthorchiasis was reported in 1911 by Leiper from the autopsy of corpse in Chiang Mai. Later on Sadun in 1953, Harinasuta and Vajjarasthira in 1961, and Wykoff in 1965 had demonstrated a complete life cycle of O. viverrini. History of opisthorchiasis control has dated back to 1950 as a small scale helminthiosis control program in some high risk areas. Following a number of studies and trial projects of the Faculty of Tropical Medicine of Mahidol University, the national liver fluke control program has been developed and operated under different National Public Health Development Plans. Presently, the program is being operated in some central provinces, and all northeastern and northern provinces of Thailand. The main strategies for liver fluke control are comprised of 3 interrelated approaches, namely stool examination and treatment of positive cases with praziquantel for eliminating human host reservoir; health education for a promotion of cooked fish consumption for preventing infection; and the improvement of hygienic defecation for transmission interruption. Data obtained from liver fluke control operation in 1996 show the countrywide prevalence of opisthorchiasis at 21.5% with a variation in all age groups. Differences of infection are also found among geographical regions. A large percentage of infection is mild infection. Comparing figures of 1981 and 1991, there is a considerable increase in the central and northern regions, while the northeastern region has significant decrease.

The impact of a decade long opisthorchiasis control program in northeastern Thailand.

Based on the figures obtained from a regional survey in 1981, the overall prevalence of opisthorchiasis in northeastern Thailand was as high as 34.6%. Traditional consumption of improperly cooked fish dishes and unhygienic defecating habits among rural inhabitants have been recognized as significant factors determining such high prevalence of this liver fluke in that area. Following experience of Mahidol University's Faculty of Tropical Medicine in Thailand of treating liver fluke infected individuals with praziquantel, the Ministry of Public Health started a liver fluke control program in the northeast as special service units in 1984 in 4 provinces. In 1988, the program was expanded to cover all 19 northeastern provinces, services being rendered at a community level covering stool examination and treatment of positive cases. Intensive health education was integrated as one element of liver fluke control program. During the early phase of operation (attack phase) a mobile stool examination team was the crucial strategy for active service. Following the attack phase, passive service was operated at each health service facility provided that results of population based program evaluation in each area showed a low prevalence according to the set target. Regionwide assessments in 1991 and 1992 indicated a declining trend of prevalence from 34.6% to 30.19% and 24.01%, respectively. In 1994, a population-based sampling survey was carried out to measure the effect of the liver fluke control program over 10 years operation (1984-1994) on both behavioral outcomes and prevalence impact. Sample for the assessment of behavior were 1,268 individuals, while another 1,912 samples were used for determining disease prevalence. Health related behaviors regarding frequent consumption of uncooked fish dishes decreased from 14% in 1990 to 7% in 1994 while occasional consumption remains as high as 42%. The prevalence of opisthorchiasis is 18.5% with a large variation in infection rate (5.20-56.25%).