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Epidemiology of Opisthorchis viverrini
Acta Tropica 88 (2003) 187–194
Review
Epidemiology of Opisthorchis viverrini Paiboon Sithithaworn a,∗ , Melissa Haswell-Elkins b a
Department of Parasitology, Faculty of Medicine, Khon Kaen University, Khon Kaen 40002, Thailand b Department of Community Health, University of Queensland, Brisbane, Australia
Abstract Opisthorchis viverrini is a food borne trematode, important because of the sheer numbers of people infected and its serious morbidities such as hepatobiliary diseases and cholangiocarcinoma (CHCA). Although infections are identified throughout Southeast Asia, the epi-center is northeast Thailand, where high prevalence coexists with a high incidence of CHCA. In this review, we present the basic population features and the factors influencing transmission between the different hosts. © 2003 Elsevier B.V. All rights reserved. Keywords: Epidemiology; Opisthorchis viverrini; CHCA; Liver fluke; Food borne trematodes; Population biology; Northeast Thailand
1. Geographical distribution The liver flukes endemic to Asia and eastern Europe include Opisthorchis viverrini, Clonorchis sinensis and O. felineus: worldwide infections number ∼17 million: 7 million with C. sinensis, 9 million with O. viverrini and 1.2 million O. felineus (Preuksaraj, 1984; Rim, 1986; WHO, 1995). O. viverrini is prevalent in Thailand, Lao PDR and Cambodia, while C. sinensis is widespread in Korea, China, Taiwan, Vietnam and formerly Japan. O. felineus is found in the Russian Federation and eastern Europe. Regional migration and global tourism have in some cases aided parasite distribution or increased the number of people infected. However, establishment of the life cycle is limited to locals with all the intermediate hosts. The geographical pattern of liver fluke infection is not uniform. In Thailand, O. viverrini has ∗ Corresponding author. Tel.: +66-43-348387; fax: +66-43-244417. E-mail address: paib [email protected] (P. Sithithaworn).
marked regional variation and is most prevalent in the northeast (Preuksaraj, 1984; Jongsuksuntigul, 2002). Within the northeast itself, there is variability at the provincial, district and village level such that average prevalence and intensity have large standard deviations. The increased incidence in northern Thailand is likely the result of misdiagnosis of minute intestinal flukes (MIF) due to the limited resolution of the Kato-thick smear technique (Jongsuksuntigul et al., 1992; Jongsuksuntigul, 2002; Jongsuksuntigul and Imsomboon, 1997; Radomyos et al., 1994, 1998).
2. Prevalence of infection In the 2001, national health survey, hookworms were most common (11.4%) of the total 22.5% of helminth infections followed by O. viverrini (9.6%) (Jongsuksuntigul, 2002). Regional liver fluke distribution ranks: the north (19.3%), the northeast (15.7%), the central (3.8%) and the south (0%). The decline in the northeast from 34.6% in 1981 to current
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levels (Jongsuksuntigul et al., 1992; Jongsuksuntigul, 2002) is attributed to intensive and continuous control activities (Jongsuksuntigul and Imsoomboon, the current issue). Significant variation in prevalence (range, 4–33%) follows the pattern of infection among northeast provinces (Jongsuksuntigul, 2002). The parasite is also common to the lowlands of Lao PDR among people with close ethnic ties to northeast Thais. Prevalence and incidence for Lao PDR are unavailable (Giboda et al., 1991; Pholsena et al., 1991), but spot surveys suggest prevalence is higher (range, 36–60%) than previously thought (Kobayashi et al., 1996, 2000). The contribution by MIF, particularly Haplorchis sp., to the putative prevalence of O. viverrini is uncertain.
3. Incidence and re-infection after treatment In contrast to prevalence data, the specific incidence of O. viverrini is frequently not determined, with notable exceptions (Sornmani et al., 1984; Upatham et al., 1988; Saowakontha et al., 1993). In the endemic community, Chonnabot, Khon Kaen, Thailand, incidence of infection per year ranged between 19.4 and 46%. In children under 5, incidence ranged between 2.1 and 6.2%: incidence among males tending to be greater than females (Upatham et al., 1988). In 1993, three villages in Khon Kaen had an incidence range of between 1.7 and 25% per 6 months (Saowakontha et al., 1993). The elevated incidence in some communities explains the high prevalence. Worryingly, an incidence of 40% per year would require only 6 years to advance to a prevalence of 95%. Studies on post-treatment re-infection in the northeast are correlated to the high incidence of infection. A study of irrigated areas in Khon Kaen province revealed a pre-treatment O. viverrini prevalence of 55.1%. Notwithstanding, 1 year after treatment, prevalence returned to 54.8% (Sornmani et al., 1984). Similarly, Upatham et al. (1988) reported that in Chonnabot, Khon Kaen, 97.4% of villagers were infected but 1 year after praziquantel treatment prevalence was back to 94%. Those with high pretreatment intensity tended to have the highest intensity of re-infection suggesting some persons are predisposed to heavy infection. Studies employing a similar approach based on the quantification of worm
load at pre- and post-treatment, evidence a predisposition to heavy infection with parasites such as Ascaris lumbricoides (Elkins et al., 1986), Necator americanus (Schad and Anderson, 1985), Trichuris trichiura (Bundy et al., 1987) and Schistosoma mansoni (Bensted-Smith et al., 1987). Rapid re-infection despite treatment suggest the absence of any protective immunity, but does not rule it out.
4. Age- and sex-related patterns of infection Although the rates of O. viverrini infection vary between villages, patterns of infection are similar. In general, the youngest age groups (i.e. 0–5 years) have the lowest prevalence and intensity, while infections in pre- and early-teens often plateaus by the late-teens (i.e. 15–19). In some areas, intensity of egg excretion tended to increase with age (Upatham et al., 1984), while worm burden declined (Haswell-Elkins et al., 1991; Sithithaworn et al., 1991a), possibly because of a late-developing immune response, lower parasite survival in heavily fibrosed bile ducts, death of parasites in heavily infected persons, or reduced exposure in the elderly. Mothers feeding raw fish to their infants is a possible infection vector among young infants (Sadun, 1955; Harinasuta and Vajrasthira, 1960; Upatham et al., 1982, 1984). However, infection intensity in children under age 4 is low and there is little evidence to suggest young children experience intense exposure. In typical endemic communities, prevalence and intensity differs little, or is slightly higher, among males (Wykoff et al., 1965; Upatham et al., 1982, 1984; Haswell-Elkins et al., 1991). Heavier infections are more common among males (Haswell-Elkins et al., 1991). Since the risk of clinical manifestations, including cancer, increase in a non-linear fashion with infection, sex may be an important disease determinant (Haswell-Elkins et al., 1994; Elkins et al., 1996).
5. Frequency distribution in human As with other helminthes, the population of O. viverrini is aggregated in minority of heavily infected persons. Ramsay et al. (1989) performed expulsion
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chemotherapy in 33 villagers in Khon Kaen, northeast Thailand and found that frequency distribution of O. viverrini was clumped in a small group of subjects. The highest worm load was 565 and the mean number of flukes was 85 (S.D.: 154). Using a larger sample size, Haswell-Elkins et al. (1991) observed 81% of 11,000 worms, recovered after treatment of 246 village residents, were expelled by just 25 individuals (i.e. 10% of the population sample) with burdens over 100 worms. In some egg-positive individuals, even when using expulsion chemotherapy, no worms were found. In an autopsy study in Khon Kaen, where worm burden was meticulously counted, 30 of 181 cadavers contained 66% of all worms recovered and only 7% (13) had a worm burden >400 (Sithithaworn et al., 1991b).
6. Morbidity The frequency and types of clinical disease differ between the three types of liver fluke affecting humans. The Russian literature describes specific signs and symptoms accompanying well defined clinical stages, from acute to chronic (Bronstein, 1986). Acute infection, characterized by high fever, hepatitis-like symptoms and eosinophilia, is frequently reported in O. felineus, but only once for clonorchiasis (Koenigstein, 1949) and never for O. viverrini. This observation may be due to the large number of migrants moving into the area endemic for O. felineus and becoming infected as adults. Much of the published information comes from uncontrolled clinical investigations, e.g. case studies, reviews of hospital records, without inclusion of a control group for comparison (Markell, 1966). Since most studies have been hospital-based, the frequencies with which these diseases occur in infection cannot be inferred. As a result, the tendency has been to overestimate both the frequencies and strengths of association between infections and the various presentations. Some authors recognized this, but have erroneously reported since a majority of infections are clinically unapparent; liver flukes are largely benign, not needing treatment (Markell, 1966; Woolf et al., 1984). Ascending cholangitis and obstructive jaundice are frequently listed as common complications of
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opisthorchiasis. Pungpak et al. (1989) reported only 88 severe manifestations among 15,243 infected persons presenting at a Bangkok hospital: 41 had obstructive jaundice, 26 cholangitis, 16 cholangiocarcinoma and 6 other cancers. Since radiological investigations were not performed, cholangiocarcinoma cannot be ruled out as a cause of these manifestations. Two large studies by Upatham et al. (1982, 1984) within a heavily infected community reported significantly increased frequencies of abdominal pain in the right upper quadrant, flatulence or dyspepsia and weakness associated with increasing intensity of infection. They estimated 5 to 10% of the community had symptoms attributable to the infection. The investigators should have ruled out the presence of cholangiocarcinoma among those with symptoms, controlled for age and sex and performed post-treatment examinations to demonstrate the extent of recovery in the absence of flukes. Recent studies using ultrasonography have demonstrated a relationship between gall bladder enlargement, wall irregularities and sludge enhanced echogenicity of portal vein radicles, and intensity of infection (Dhiensiri et al., 1984; Mairiang et al., 1992). In a larger study of 1,807 individuals, when the intensity of infection was >3000 egg per gram feces (epg), the adjusted odds ratio for distended gall bladder and portal vein radicles was between 14–20 and 10–28, respectively (Elkins et al., 1996). In the same series, a linear trend of the frequency of suspected CHCA and fecal egg count was observed where the odds ratio of 14.1 was found in a group with an epg >6000, which is equivalent to >120 worms (Haswell-Elkins et al., 1994). Reversibility of these abnormalities occurred 11 months after treatment with praziquantel (Dhiensiri et al., 1984; Mairiang et al., 1993). Stones in the gall bladder, liver and bile ducts are frequently correlated to infection; the best evidence being worm fragments in the nidus (Teoh, 1963; Riganti et al., 1988). Hou et al. (1989) reported a consistent increase in gallstone frequency diagnosed by ultrasound with increasing intensity of infection among Hakkanese people in Taiwan, from 4.2% in uninfected subjects to over 14% in those excreting over 5000 epg. It is not clear whether worms killed by the praziquantel contribute to gall stone formation or increase the risk of CHCA with repeated treatment.
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7. Life expectancy C. sinensis can survive up to 26 years (Attwood and Chou, 1978) though this probably represents anecdotal survival in an individual host. By comparison, the life expectancy of O. viverrini has never been published, but based on the age-intensity profile and similarities to Schistosoma mansoni (Anderson and May, 1985), O. viverrini probably survive less than 10 years.
8. Fecundity Approximately 1 month after ingestating metacerariae, adult worms begin releasing eggs, which pass down the bile duct and are excreted in the feces. Eggs can also be found in gall bladder bile. Estimates of daily fecal egg output per worm range between 3000 and 36,000 epg. Based on nine autopsy cases Wykoff and Ariyaprakai (1966) estimated the mean worm burden of 2588 with an average egg output of 3000 epg. Sithithaworn et al. (1991a) autopsied 181 infected cases and found a mean worm burden of 157.5 with an average egg output of 53.3 eggs/200 g feces per day. By expulsion chemotherapy, Elkins et al. (1991) reported an average egg output of 180 eggs/200 g feces per day from a mean worm burden of 42.6. The wide variation suggests a density-dependent fecundity similar to other helminthes (Anderson and May, 1985). Estimated fecundity in infected animals depended on worm burden, and ranged between 80 and 300 eggs/g feces/worm (Flavell et al., 1983; Sripa and Kaewkes, 2000).
9. Infection in intermediate and reservoir hosts When eggs reach a body of freshwater (small ponds, streams and rivers, flooded rice fields and large reservoirs) and are ingested by an appropriate snail they produce sporocysts. Species of Bithynia serve as the intermediate host (Brockelman et al., 1986; Ditrich et al., 1990). The sporocysts produce thousands of redia every daily, approximately 2 months after snail infection. The free-swimming cercaria shed their tail, penetrate the tissue of host fish and encyst; becoming a fully infective metacercariae in 21 days. Over 80 species of the cyprinoid family, and at least 13
species of other families serve as the second intermediate host (Komiya, 1966; Vichasri et al., 1982; Rim, 1986; WHO, 1995). The life cycle requires at least 4 months to complete. In Taiwan, winter hibernation prolongs the cycle (Rim, 1986). Like other trematodes, the prevalence of liver fluke infection in snail intermediate hosts is typically low (range, 0.05–0.07%) in Bithynia sp. (Harinasuta, 1969; Brockelman et al., 1986). Snail population exhibits strong seasonality and is apparently dependent on rainfall being highly abundant in rainy season and distribute extensively shallow water reservoirs and paddy fields, but rapidly disappearing over the dry season (Brockelman et al., 1986). Due to the fluctuation in number, liver fluke control by reducing the snail population is not practicable. Unlike infection in snails, the prevalence of infected fish is much greater. Ninety to 95% of cyprinoid fish species habour O. viverrini metacercaria (Harinasuta and Vajrasthira, 1960; Vichasri et al., 1982). The most common species of cyprinoid fish were Puntius, Cyclocheilichthys and Hampala (Wykoff et al., 1965). The intensity of liver fluke infection in fish varies by season, type of water body, species and individual (Vichasri et al., 1982; Rim, 1986; Sithithaworn et al., 1997). Most metacercariae were found in body muscle (Vichasri et al., 1982) or in the head of the fish (Tesana et al., 1985). The observed discrepancy probably depends on the technique employed for metacercarial extraction. The pepsin digestion technique used by Tesana et al. (1985) is most reliable (Sithithaworn et al., 1991a; WHO, 1995; Waikagul, 1998). Metacercarial burden peaks in winter (between October and February) and dips during the rainy and summer seasons (Vichasri et al., 1982; Sithithaworn et al., 1997). The numbers of metacercariae reported in fish generally range from one to hundreds. However, huge numbers have been reported in some studies. For example, >30,000 per fish and >6000 per gram were found in Pseudorasbora parva both in China and Korea (Rim, 1986; Chen et al., 1994). Recent findings of trematode metacercariae other than O. viverrini in cyprinoid fish in the northeast (Srisawangwong et al., 1997; Waikagul, 1998) and the north (Sukontason et al., 1999) indicate that occurrence of mixed species of trematodes within a given fish species is common. Careful identification of metacercariae from fish is necessary.
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Transmission between hosts in the life cycle of Opisthorchis and Clonorchis are seasonal (Wykoff et al., 1965; Brockelman et al., 1986; Chen et al., 1994). In tropical countries, such as Thailand, peak fecal contamination of water bodies, and relatedly snail infection, occurs at the height of rainy season, when household effluent and defecate in the fields are washed into waterways, ponds and lakes (Wykoff et al., 1965; Brockelman et al., 1986). Transmission to fish and subsequently humans may be highest just after the peak of monsoon flooding, when intermediate hosts and fish are abundant (Vichasri et al., 1982; Haas et al., 1990). In countries with a significant cold season (those endemic for C. sinensis), the infection, particularly the shedding of cercariae from snails, is governed by water temperatures. Parasites may survive over winter as rediae in the snail and erupt in the spring, or new infections may be re-established each year via fecal contamination. In either case, peak transmission occurs in summer months in colder climate (Rim, 1986). The frequency of infection in reservoir hosts like pigs, cats, rats and dogs varies by area and is not closely associated with human infection. Relatively low prevalences of liver fluke infection are observed in dogs in northeast Thailand, Taiwan and parts of Korea, where human infection is common, while similar or higher prevalences are reported in these animals in parts of Thailand and China, even where humans are not infected (Sadun, 1955; Chen et al., 1994). Although fecal contamination from infected animals must contribute to transmission to snails, consideration of their actual importance relative to higher egg excretion from humans, human eating habits and hygiene may be minor (Sadun, 1955; Rim, 1986). However, where fecal contamination from humans is eliminated by mass treatment and hygiene, the consumption of raw fish will maintain re-infection rates.
10. Source of human infection Freshwater fish are an intermediate host in the life cycle of O. viverrini. In Thailand, at least 15 species of fish, able to serve as intermediate hosts, occur naturally (WHO, 1995). Although most published descriptions of social habits regarding consumption of raw fish are anecdotal and careful sociological investiga-
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tion is needed, raw or undercooked cooked fish is the primary vector of infection so too are contaminated utensils, unwashed hands and food preparation surfaces. In the northeast, three types of preparations contain uncooked, small and medium-sized, fish: (1) koi pla, eaten soon after preparation; (2) moderately fermented pla som; stored for a few days to weeks; and, (3) pla ra extensively fermented, highly salted fish, stored for at least 2–3 months (Sadun, 1955). In the past, reported consumption frequencies of koi pla were very high; up to 80% of the population in some communities ate it every week (Migasena, 1982). In a comparison of rural vs. urban dwellers, Kurathong et al. (1987) reported a higher prevalence of liver fluke infection among rural vs. urban northeasterners, and those who reported having eaten koi pla (87%) versus those who did not (61%). A close relationship between koi pla consumption in a heavily infected village, with just 19% of those uninfected reporting consumption, versus 79% of infected and >90% of those heavily-infected persons (Upatham et al., 1984). The frequencies of koi pla consumption have declined and are generally confined to special social occasions, while other under-cooked fish preparations like pla som and other moderately preserved fish are generally eaten several times a week (Elkins et al., unpublished, Jongsuksuntigul, 2001, unpublished). Fully preserved fish (e.g. pla ra and jaewbhong) is an important staple, consumed daily by 60–98% of northeasterners and lowland Laotians (Migasena, 1982; Changbumrung et al., 1989). Although liver fluke infection results from eating these preparations, re-infection is poorly understood. Several studies indicate that survival of infective stages depends on the concentration of salt and degree of fermentation (Vichasri, 1981, Tesana et al., 1986). Koi pla is probably the most infective, followed by fish preserved for <7 days, then pla ra and jaewbhong, in which viable metacercaria are rare. An FAO initiative stressed the need to assess the relative importance of aquaculture versus fisheries as a source of food borne trematode infection (WHO, 1995). Food safety assurance and aquaculture product standards are needed for both domestic consumption and international trade. In addition, food control and inspection techniques such as hazard analysis and critical control point (HACCP) approach are available
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for effective control of food risk at the production stage. Preliminary application of this approach in fresh water carp (Puntius gonionotus) cultures was tested (Khamboonruang et al., 1997) and may result in the identification of “good aquaculture practices” and a means of controlling trematode cross-contamination.
ucation are most effective, however, the type of health education is rarely described. Control campaigns that demand too many changes may backfire by strengthening the cultural significance of food or making people feel powerless. Community participation is a vital element of empowerment (Sornmani, 1987; Keittivuti et al., 1986).
11. Conclusion Acknowledgements O. viverrini and C. sinensis are food borne trematodes that seriously affect public health in several Asian nations. The distribution of live flukes in Thailand is concentrated in the northeast and corresponds with availability of the intermediate hosts, the snails and fish, and people’s eating habits. Transmission to man from the intermediate hosts is seasonal and peaks in winter when fish have their highest metacecarial burden. Several traditional dishes prepared from freshwater fish are the source of live fluke infection. The prevalence of infection occurs early in life and plateaus after the teens and declines in old age. Worm burden increases with age with a slight drop with age suggesting immunity is minimal. A few persons harbor a heavy worm burdens while most people have no, or only a light, infection. In areas of heavy transmission, the high incidence and rapid re-infection after treatment are common. Infection has few clinical manifestations but a considerable proportion of heavily infected people may contract hepatobiliary diseases and even cancer of the bile duct, which is diagnosed by abdominal ultrasonography and endoscopy. The hepatobiliary pathologies can be reversed with chemotherapy. Whether praziquantel treatment is associated with gall stone formation or influences the development of CHCA is unknown. However, begging further investigation, Chernrungroj (2000) found an increased risk of CHCA in those with a history of praziquantel treatment. Predisposition to heavy infections after treatment indicates the possibility of targeted treatment as a strategy for parasite control. Seasonal variation or sampling biases need to be considered when planning further studies. Immediate treatment is required to eliminate the long-lived parasites, while sanitation and personal hygiene interrupts transmission from human feces to snails. Health education encourages people to cook fish. Control programs using treatment plus health ed-
The authors thank Mr. Bryan Roderick Hamman for assistance with the English-language presentation of the manuscript.
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Preuksaraj, S., 1984. Public health aspects of opisthorchiasis in Thailand. Arzneimittelforschung 34, 1119–1120. Pungpak, S., Sornmani, S., Suntharasamai, P., Vivatanasesth, P., 1989. Ultrasonographic study of the biliary system in opisthorchiasis patients after treatment with praziquantel. Southeast Asian J. Trop. Med. Public Health 20, 157–162. Radomyos, P., Radomyos, B., Tungtrongchitr, A., 1994. Multi-infection with helminths in adults from northeast Thailand as determined by post-treatment fecal examination of adult worms. Trop. Med. Parasitol. 45, 133–135. Radomyos, B., Wongsaroj, T., Wilairatana, P., Radomyos, P., Praevanich, R., Meesomboon, V., Jongsuksuntikul, P., 1998. Opisthorchiasis and intestinal fluke infections in northern Thailand. Southeast Asian J. Trop. Med. Public Health 29, 123–127. Ramsay, R.J., Sithithaworn, P., Prociv, P., Moorhouse, D.E., Methaphat, C., 1989. Density-dependent fecundity of Opisthorchis viverrini in humans, based on fecal recovery of flukes. Trans. R. Soc. Trop. Med. Hyg. 83, 241–242. Riganti, M., Pungpak, S., Sachakul, V., Bunnag, D., Harinasuta, T., 1988. Opisthorchis viverrini eggs and adult flukes as nidus and composition of gallstones. Southeast Asian J. Trop. Med. Public Health 19, 633–636. Rim, H.J., 1986. The current pathobiology and chemotherapy of clonorchiasis. Korean J. Parasitol. 24 (1), 1–141. Sadun, E.H., 1955. Studies on Opisthorchis viverrini in Thailand. Am. J. Hyg. 2, 81–115. Saowakontha, S., Pipitgool, V., Pariyanonda, S., Tesana, S., Rojsathaporn, K., Intarakhao, C., 1993. Field trials in the control of Opisthorchis viverrini with an integrated programme in endemic areas of northeast Thailand. Parasitology 106, 283– 288. Schad, G.A., Anderson, R.M., 1985. Predisposition to hookworm infection in man. Science 228, 1537–1540. Sithithaworn, P., Tesana, S., Pipitgoon, V., Kaewkes, S., Pairojkul, C., Sripa, B., Paupairoj, A., Thaiklar, K., 1991a. Relationship between fecal egg count and worm burden of Opisthorchis viverrini in human autopsy cases. Parasitology 102, 277–281. Sithithaworn, P., Tesana, S., Pipitgool, V., Kaewkes, S., Thaiklar, K., Pairojkul, C., Sripa, B., Paupairoj, A., Sanpitak, P., Aranyanat, C., 1991b. Quantitative post-mortem study of Opisthorchis viverrini in man in north-east Thailand. Trans. R. Soc. Trop. Med. Hyg. 85, 765–768. Sithithaworn, P., Pipitgool, V., Srisawangwong, T., Elkins, D.B., Haswell-Elkins, M.R., 1997. Seasonal variation of Opisthorchis viverrini infection in cyprinoid fish in north-east Thailand: implications for parasite control and food safety. Bull. World Health Organ 75, 125–131. Sornmani, S., 1987. Control of opisthorchiasis through community participation. Parasitol. Today 3, 31–33. Sornmani, S., Schelp, F.P., Vivatanasesth, P., Patihatakorn, W., Impand, P., Sitabutra, P., Worasan, P., Preuksaraj, S., 1984. A pilot project for controlling Opisthorchis viverrini infection in Nong Wai, northeast Thailand, by applying praziquantel and other measures. Arzneimittelforschung 34, 1231–1234. Sripa, B., Kaewkes, S., 2000. Relationship between parasitespecific antibody responses and intensity of Opisthorchis
viverrini infection in hamsters. Parasite Immunol. 22, 139– 145. Srisawangwong, T., Sithithaworn, P., Tesana, S., 1997. Metacercariae isolated from cyprinoid fishes in Khon Kaen District by digestion technique. Southeast Asian J. Trop. Med. Public Health 28, 224–226. Sukontason, K., Piangjai, S., Muangyimpong, Y., Methanitikorn, R., Chaithong, U., 1999. Prevalence of trematode metacercariae in cyprinoid fish of Ban Pao district, Chiang Mai Province, northern Thailand. Southeast Asian J. Trop. Med. Public Health 30, 365–370. Teoh, T.B., 1963. A study of gall-stones and included worms in recurrent pyogenic cholangitis. J. Pathol. Bact. 86, 123–129. Tesana, S., Kaewkes, S., Pinlaor, S., 1985. Distribution and density of Opisthorchis viverrini metacerariae in cyprinoid fish from Khon Kaen province. J. Parasitol. Med. Assoc. Thai. 8, 36– 39. Tesana, S., Kaewkes, S., Pinlaor, S., 1986. Infectivity and survivorship of metacercaria of Opisthorchis vivierrini in fermented fish. J. Parasitol. Med. Assoc. Thai. 9, 21–30. Upatham, E.S., Viyanant, V., Kurathong, S., Brockelman, W.Y., Menaruchi, A., Saowakontha, S., Intarakhao, C., Vajrasthira, S., Warren, K.S., 1982. Morbidity in relation to intensity of infection in Opisthorchiasis viverrini: study of a community in Khon Kaen, Thailand. Am. J. Trop. Med. Hyg. 31, 1156–1163. Upatham, E.S., Viyanant, V., Kurathong, S., Rojborwonwitaya, J., Brockelman, W.Y., Ardsungnoen, S., Lee, P., Vajrasthira, S., 1984. Relationship between prevalence and intensity of Opisthorchis viverrini infection, and clinical symptoms and signs in a rural community in north-east Thailand. Bull. World Health Organ. 62, 451–461. Upatham, E.S., Viyanant, V., Brockelman, W.Y., Kurathong, S., Lee, P., Kraengraeng, R., 1988. Rate of re-infection by Opisthorchis viverrini in an endemic northeast Thai community after chemotherapy. Int. J. Parasitol. 18, 643–669. Vichasri, S., 1981. Some biological studies between the liver fluke, Opisthorchis viverrini and its hosts. Master of Science Thesis, Mahidol University, Bangkok, Thailand. Vichasri, S., Viyanant, V., Upatham, E.S., 1982. Opisthorchis viverrini: intensity and rates of infection in cyprinoid fish from an endemic focus in northeast Thailand. Southeast Asian J. Trop. Med. Public Health 13, 138–141. Waikagul, J., 1998. Opisthorchis viverrini metacercaria in Thai freshwater fish. Southeast Asian J. Trop. Med. Public Health 29, 324–326. WHO, 1995. Control of foodborne trematode infection. WHO Technical Report Series 849. Woolf, A., Green, J., Levine, J.A., Estevez, E.G., Weatherly, N., Rosenberg, E., Frothingham, T., 1984. A clinical study of Laotian refugees infected with Clonorchis sinensis or Opisthorchis viverrini. Am. J. Trop. Med. Hyg. 33, 1279–1280. Wykoff, D.E., Ariyaprakai, K., 1966. Opisthorchis viverrini in Thailand—egg production in man and laboratory animals. J. Parasitol. 52, 631. Wykoff, D.E., Harinasuta, C., Juttijudata, P., Winn, M.M., 1965. Opisthorchis viverrini in Thailand—the life cycle and comparison with O. felineus. J. Parasitol. 51, 207–214.
Review
Epidemiology of Opisthorchis viverrini Paiboon Sithithaworn a,∗ , Melissa Haswell-Elkins b a
Department of Parasitology, Faculty of Medicine, Khon Kaen University, Khon Kaen 40002, Thailand b Department of Community Health, University of Queensland, Brisbane, Australia
Abstract Opisthorchis viverrini is a food borne trematode, important because of the sheer numbers of people infected and its serious morbidities such as hepatobiliary diseases and cholangiocarcinoma (CHCA). Although infections are identified throughout Southeast Asia, the epi-center is northeast Thailand, where high prevalence coexists with a high incidence of CHCA. In this review, we present the basic population features and the factors influencing transmission between the different hosts. © 2003 Elsevier B.V. All rights reserved. Keywords: Epidemiology; Opisthorchis viverrini; CHCA; Liver fluke; Food borne trematodes; Population biology; Northeast Thailand
1. Geographical distribution The liver flukes endemic to Asia and eastern Europe include Opisthorchis viverrini, Clonorchis sinensis and O. felineus: worldwide infections number ∼17 million: 7 million with C. sinensis, 9 million with O. viverrini and 1.2 million O. felineus (Preuksaraj, 1984; Rim, 1986; WHO, 1995). O. viverrini is prevalent in Thailand, Lao PDR and Cambodia, while C. sinensis is widespread in Korea, China, Taiwan, Vietnam and formerly Japan. O. felineus is found in the Russian Federation and eastern Europe. Regional migration and global tourism have in some cases aided parasite distribution or increased the number of people infected. However, establishment of the life cycle is limited to locals with all the intermediate hosts. The geographical pattern of liver fluke infection is not uniform. In Thailand, O. viverrini has ∗ Corresponding author. Tel.: +66-43-348387; fax: +66-43-244417. E-mail address: paib [email protected] (P. Sithithaworn).
marked regional variation and is most prevalent in the northeast (Preuksaraj, 1984; Jongsuksuntigul, 2002). Within the northeast itself, there is variability at the provincial, district and village level such that average prevalence and intensity have large standard deviations. The increased incidence in northern Thailand is likely the result of misdiagnosis of minute intestinal flukes (MIF) due to the limited resolution of the Kato-thick smear technique (Jongsuksuntigul et al., 1992; Jongsuksuntigul, 2002; Jongsuksuntigul and Imsomboon, 1997; Radomyos et al., 1994, 1998).
2. Prevalence of infection In the 2001, national health survey, hookworms were most common (11.4%) of the total 22.5% of helminth infections followed by O. viverrini (9.6%) (Jongsuksuntigul, 2002). Regional liver fluke distribution ranks: the north (19.3%), the northeast (15.7%), the central (3.8%) and the south (0%). The decline in the northeast from 34.6% in 1981 to current
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levels (Jongsuksuntigul et al., 1992; Jongsuksuntigul, 2002) is attributed to intensive and continuous control activities (Jongsuksuntigul and Imsoomboon, the current issue). Significant variation in prevalence (range, 4–33%) follows the pattern of infection among northeast provinces (Jongsuksuntigul, 2002). The parasite is also common to the lowlands of Lao PDR among people with close ethnic ties to northeast Thais. Prevalence and incidence for Lao PDR are unavailable (Giboda et al., 1991; Pholsena et al., 1991), but spot surveys suggest prevalence is higher (range, 36–60%) than previously thought (Kobayashi et al., 1996, 2000). The contribution by MIF, particularly Haplorchis sp., to the putative prevalence of O. viverrini is uncertain.
3. Incidence and re-infection after treatment In contrast to prevalence data, the specific incidence of O. viverrini is frequently not determined, with notable exceptions (Sornmani et al., 1984; Upatham et al., 1988; Saowakontha et al., 1993). In the endemic community, Chonnabot, Khon Kaen, Thailand, incidence of infection per year ranged between 19.4 and 46%. In children under 5, incidence ranged between 2.1 and 6.2%: incidence among males tending to be greater than females (Upatham et al., 1988). In 1993, three villages in Khon Kaen had an incidence range of between 1.7 and 25% per 6 months (Saowakontha et al., 1993). The elevated incidence in some communities explains the high prevalence. Worryingly, an incidence of 40% per year would require only 6 years to advance to a prevalence of 95%. Studies on post-treatment re-infection in the northeast are correlated to the high incidence of infection. A study of irrigated areas in Khon Kaen province revealed a pre-treatment O. viverrini prevalence of 55.1%. Notwithstanding, 1 year after treatment, prevalence returned to 54.8% (Sornmani et al., 1984). Similarly, Upatham et al. (1988) reported that in Chonnabot, Khon Kaen, 97.4% of villagers were infected but 1 year after praziquantel treatment prevalence was back to 94%. Those with high pretreatment intensity tended to have the highest intensity of re-infection suggesting some persons are predisposed to heavy infection. Studies employing a similar approach based on the quantification of worm
load at pre- and post-treatment, evidence a predisposition to heavy infection with parasites such as Ascaris lumbricoides (Elkins et al., 1986), Necator americanus (Schad and Anderson, 1985), Trichuris trichiura (Bundy et al., 1987) and Schistosoma mansoni (Bensted-Smith et al., 1987). Rapid re-infection despite treatment suggest the absence of any protective immunity, but does not rule it out.
4. Age- and sex-related patterns of infection Although the rates of O. viverrini infection vary between villages, patterns of infection are similar. In general, the youngest age groups (i.e. 0–5 years) have the lowest prevalence and intensity, while infections in pre- and early-teens often plateaus by the late-teens (i.e. 15–19). In some areas, intensity of egg excretion tended to increase with age (Upatham et al., 1984), while worm burden declined (Haswell-Elkins et al., 1991; Sithithaworn et al., 1991a), possibly because of a late-developing immune response, lower parasite survival in heavily fibrosed bile ducts, death of parasites in heavily infected persons, or reduced exposure in the elderly. Mothers feeding raw fish to their infants is a possible infection vector among young infants (Sadun, 1955; Harinasuta and Vajrasthira, 1960; Upatham et al., 1982, 1984). However, infection intensity in children under age 4 is low and there is little evidence to suggest young children experience intense exposure. In typical endemic communities, prevalence and intensity differs little, or is slightly higher, among males (Wykoff et al., 1965; Upatham et al., 1982, 1984; Haswell-Elkins et al., 1991). Heavier infections are more common among males (Haswell-Elkins et al., 1991). Since the risk of clinical manifestations, including cancer, increase in a non-linear fashion with infection, sex may be an important disease determinant (Haswell-Elkins et al., 1994; Elkins et al., 1996).
5. Frequency distribution in human As with other helminthes, the population of O. viverrini is aggregated in minority of heavily infected persons. Ramsay et al. (1989) performed expulsion
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chemotherapy in 33 villagers in Khon Kaen, northeast Thailand and found that frequency distribution of O. viverrini was clumped in a small group of subjects. The highest worm load was 565 and the mean number of flukes was 85 (S.D.: 154). Using a larger sample size, Haswell-Elkins et al. (1991) observed 81% of 11,000 worms, recovered after treatment of 246 village residents, were expelled by just 25 individuals (i.e. 10% of the population sample) with burdens over 100 worms. In some egg-positive individuals, even when using expulsion chemotherapy, no worms were found. In an autopsy study in Khon Kaen, where worm burden was meticulously counted, 30 of 181 cadavers contained 66% of all worms recovered and only 7% (13) had a worm burden >400 (Sithithaworn et al., 1991b).
6. Morbidity The frequency and types of clinical disease differ between the three types of liver fluke affecting humans. The Russian literature describes specific signs and symptoms accompanying well defined clinical stages, from acute to chronic (Bronstein, 1986). Acute infection, characterized by high fever, hepatitis-like symptoms and eosinophilia, is frequently reported in O. felineus, but only once for clonorchiasis (Koenigstein, 1949) and never for O. viverrini. This observation may be due to the large number of migrants moving into the area endemic for O. felineus and becoming infected as adults. Much of the published information comes from uncontrolled clinical investigations, e.g. case studies, reviews of hospital records, without inclusion of a control group for comparison (Markell, 1966). Since most studies have been hospital-based, the frequencies with which these diseases occur in infection cannot be inferred. As a result, the tendency has been to overestimate both the frequencies and strengths of association between infections and the various presentations. Some authors recognized this, but have erroneously reported since a majority of infections are clinically unapparent; liver flukes are largely benign, not needing treatment (Markell, 1966; Woolf et al., 1984). Ascending cholangitis and obstructive jaundice are frequently listed as common complications of
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opisthorchiasis. Pungpak et al. (1989) reported only 88 severe manifestations among 15,243 infected persons presenting at a Bangkok hospital: 41 had obstructive jaundice, 26 cholangitis, 16 cholangiocarcinoma and 6 other cancers. Since radiological investigations were not performed, cholangiocarcinoma cannot be ruled out as a cause of these manifestations. Two large studies by Upatham et al. (1982, 1984) within a heavily infected community reported significantly increased frequencies of abdominal pain in the right upper quadrant, flatulence or dyspepsia and weakness associated with increasing intensity of infection. They estimated 5 to 10% of the community had symptoms attributable to the infection. The investigators should have ruled out the presence of cholangiocarcinoma among those with symptoms, controlled for age and sex and performed post-treatment examinations to demonstrate the extent of recovery in the absence of flukes. Recent studies using ultrasonography have demonstrated a relationship between gall bladder enlargement, wall irregularities and sludge enhanced echogenicity of portal vein radicles, and intensity of infection (Dhiensiri et al., 1984; Mairiang et al., 1992). In a larger study of 1,807 individuals, when the intensity of infection was >3000 egg per gram feces (epg), the adjusted odds ratio for distended gall bladder and portal vein radicles was between 14–20 and 10–28, respectively (Elkins et al., 1996). In the same series, a linear trend of the frequency of suspected CHCA and fecal egg count was observed where the odds ratio of 14.1 was found in a group with an epg >6000, which is equivalent to >120 worms (Haswell-Elkins et al., 1994). Reversibility of these abnormalities occurred 11 months after treatment with praziquantel (Dhiensiri et al., 1984; Mairiang et al., 1993). Stones in the gall bladder, liver and bile ducts are frequently correlated to infection; the best evidence being worm fragments in the nidus (Teoh, 1963; Riganti et al., 1988). Hou et al. (1989) reported a consistent increase in gallstone frequency diagnosed by ultrasound with increasing intensity of infection among Hakkanese people in Taiwan, from 4.2% in uninfected subjects to over 14% in those excreting over 5000 epg. It is not clear whether worms killed by the praziquantel contribute to gall stone formation or increase the risk of CHCA with repeated treatment.
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7. Life expectancy C. sinensis can survive up to 26 years (Attwood and Chou, 1978) though this probably represents anecdotal survival in an individual host. By comparison, the life expectancy of O. viverrini has never been published, but based on the age-intensity profile and similarities to Schistosoma mansoni (Anderson and May, 1985), O. viverrini probably survive less than 10 years.
8. Fecundity Approximately 1 month after ingestating metacerariae, adult worms begin releasing eggs, which pass down the bile duct and are excreted in the feces. Eggs can also be found in gall bladder bile. Estimates of daily fecal egg output per worm range between 3000 and 36,000 epg. Based on nine autopsy cases Wykoff and Ariyaprakai (1966) estimated the mean worm burden of 2588 with an average egg output of 3000 epg. Sithithaworn et al. (1991a) autopsied 181 infected cases and found a mean worm burden of 157.5 with an average egg output of 53.3 eggs/200 g feces per day. By expulsion chemotherapy, Elkins et al. (1991) reported an average egg output of 180 eggs/200 g feces per day from a mean worm burden of 42.6. The wide variation suggests a density-dependent fecundity similar to other helminthes (Anderson and May, 1985). Estimated fecundity in infected animals depended on worm burden, and ranged between 80 and 300 eggs/g feces/worm (Flavell et al., 1983; Sripa and Kaewkes, 2000).
9. Infection in intermediate and reservoir hosts When eggs reach a body of freshwater (small ponds, streams and rivers, flooded rice fields and large reservoirs) and are ingested by an appropriate snail they produce sporocysts. Species of Bithynia serve as the intermediate host (Brockelman et al., 1986; Ditrich et al., 1990). The sporocysts produce thousands of redia every daily, approximately 2 months after snail infection. The free-swimming cercaria shed their tail, penetrate the tissue of host fish and encyst; becoming a fully infective metacercariae in 21 days. Over 80 species of the cyprinoid family, and at least 13
species of other families serve as the second intermediate host (Komiya, 1966; Vichasri et al., 1982; Rim, 1986; WHO, 1995). The life cycle requires at least 4 months to complete. In Taiwan, winter hibernation prolongs the cycle (Rim, 1986). Like other trematodes, the prevalence of liver fluke infection in snail intermediate hosts is typically low (range, 0.05–0.07%) in Bithynia sp. (Harinasuta, 1969; Brockelman et al., 1986). Snail population exhibits strong seasonality and is apparently dependent on rainfall being highly abundant in rainy season and distribute extensively shallow water reservoirs and paddy fields, but rapidly disappearing over the dry season (Brockelman et al., 1986). Due to the fluctuation in number, liver fluke control by reducing the snail population is not practicable. Unlike infection in snails, the prevalence of infected fish is much greater. Ninety to 95% of cyprinoid fish species habour O. viverrini metacercaria (Harinasuta and Vajrasthira, 1960; Vichasri et al., 1982). The most common species of cyprinoid fish were Puntius, Cyclocheilichthys and Hampala (Wykoff et al., 1965). The intensity of liver fluke infection in fish varies by season, type of water body, species and individual (Vichasri et al., 1982; Rim, 1986; Sithithaworn et al., 1997). Most metacercariae were found in body muscle (Vichasri et al., 1982) or in the head of the fish (Tesana et al., 1985). The observed discrepancy probably depends on the technique employed for metacercarial extraction. The pepsin digestion technique used by Tesana et al. (1985) is most reliable (Sithithaworn et al., 1991a; WHO, 1995; Waikagul, 1998). Metacercarial burden peaks in winter (between October and February) and dips during the rainy and summer seasons (Vichasri et al., 1982; Sithithaworn et al., 1997). The numbers of metacercariae reported in fish generally range from one to hundreds. However, huge numbers have been reported in some studies. For example, >30,000 per fish and >6000 per gram were found in Pseudorasbora parva both in China and Korea (Rim, 1986; Chen et al., 1994). Recent findings of trematode metacercariae other than O. viverrini in cyprinoid fish in the northeast (Srisawangwong et al., 1997; Waikagul, 1998) and the north (Sukontason et al., 1999) indicate that occurrence of mixed species of trematodes within a given fish species is common. Careful identification of metacercariae from fish is necessary.
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Transmission between hosts in the life cycle of Opisthorchis and Clonorchis are seasonal (Wykoff et al., 1965; Brockelman et al., 1986; Chen et al., 1994). In tropical countries, such as Thailand, peak fecal contamination of water bodies, and relatedly snail infection, occurs at the height of rainy season, when household effluent and defecate in the fields are washed into waterways, ponds and lakes (Wykoff et al., 1965; Brockelman et al., 1986). Transmission to fish and subsequently humans may be highest just after the peak of monsoon flooding, when intermediate hosts and fish are abundant (Vichasri et al., 1982; Haas et al., 1990). In countries with a significant cold season (those endemic for C. sinensis), the infection, particularly the shedding of cercariae from snails, is governed by water temperatures. Parasites may survive over winter as rediae in the snail and erupt in the spring, or new infections may be re-established each year via fecal contamination. In either case, peak transmission occurs in summer months in colder climate (Rim, 1986). The frequency of infection in reservoir hosts like pigs, cats, rats and dogs varies by area and is not closely associated with human infection. Relatively low prevalences of liver fluke infection are observed in dogs in northeast Thailand, Taiwan and parts of Korea, where human infection is common, while similar or higher prevalences are reported in these animals in parts of Thailand and China, even where humans are not infected (Sadun, 1955; Chen et al., 1994). Although fecal contamination from infected animals must contribute to transmission to snails, consideration of their actual importance relative to higher egg excretion from humans, human eating habits and hygiene may be minor (Sadun, 1955; Rim, 1986). However, where fecal contamination from humans is eliminated by mass treatment and hygiene, the consumption of raw fish will maintain re-infection rates.
10. Source of human infection Freshwater fish are an intermediate host in the life cycle of O. viverrini. In Thailand, at least 15 species of fish, able to serve as intermediate hosts, occur naturally (WHO, 1995). Although most published descriptions of social habits regarding consumption of raw fish are anecdotal and careful sociological investiga-
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tion is needed, raw or undercooked cooked fish is the primary vector of infection so too are contaminated utensils, unwashed hands and food preparation surfaces. In the northeast, three types of preparations contain uncooked, small and medium-sized, fish: (1) koi pla, eaten soon after preparation; (2) moderately fermented pla som; stored for a few days to weeks; and, (3) pla ra extensively fermented, highly salted fish, stored for at least 2–3 months (Sadun, 1955). In the past, reported consumption frequencies of koi pla were very high; up to 80% of the population in some communities ate it every week (Migasena, 1982). In a comparison of rural vs. urban dwellers, Kurathong et al. (1987) reported a higher prevalence of liver fluke infection among rural vs. urban northeasterners, and those who reported having eaten koi pla (87%) versus those who did not (61%). A close relationship between koi pla consumption in a heavily infected village, with just 19% of those uninfected reporting consumption, versus 79% of infected and >90% of those heavily-infected persons (Upatham et al., 1984). The frequencies of koi pla consumption have declined and are generally confined to special social occasions, while other under-cooked fish preparations like pla som and other moderately preserved fish are generally eaten several times a week (Elkins et al., unpublished, Jongsuksuntigul, 2001, unpublished). Fully preserved fish (e.g. pla ra and jaewbhong) is an important staple, consumed daily by 60–98% of northeasterners and lowland Laotians (Migasena, 1982; Changbumrung et al., 1989). Although liver fluke infection results from eating these preparations, re-infection is poorly understood. Several studies indicate that survival of infective stages depends on the concentration of salt and degree of fermentation (Vichasri, 1981, Tesana et al., 1986). Koi pla is probably the most infective, followed by fish preserved for <7 days, then pla ra and jaewbhong, in which viable metacercaria are rare. An FAO initiative stressed the need to assess the relative importance of aquaculture versus fisheries as a source of food borne trematode infection (WHO, 1995). Food safety assurance and aquaculture product standards are needed for both domestic consumption and international trade. In addition, food control and inspection techniques such as hazard analysis and critical control point (HACCP) approach are available
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for effective control of food risk at the production stage. Preliminary application of this approach in fresh water carp (Puntius gonionotus) cultures was tested (Khamboonruang et al., 1997) and may result in the identification of “good aquaculture practices” and a means of controlling trematode cross-contamination.
ucation are most effective, however, the type of health education is rarely described. Control campaigns that demand too many changes may backfire by strengthening the cultural significance of food or making people feel powerless. Community participation is a vital element of empowerment (Sornmani, 1987; Keittivuti et al., 1986).
11. Conclusion Acknowledgements O. viverrini and C. sinensis are food borne trematodes that seriously affect public health in several Asian nations. The distribution of live flukes in Thailand is concentrated in the northeast and corresponds with availability of the intermediate hosts, the snails and fish, and people’s eating habits. Transmission to man from the intermediate hosts is seasonal and peaks in winter when fish have their highest metacecarial burden. Several traditional dishes prepared from freshwater fish are the source of live fluke infection. The prevalence of infection occurs early in life and plateaus after the teens and declines in old age. Worm burden increases with age with a slight drop with age suggesting immunity is minimal. A few persons harbor a heavy worm burdens while most people have no, or only a light, infection. In areas of heavy transmission, the high incidence and rapid re-infection after treatment are common. Infection has few clinical manifestations but a considerable proportion of heavily infected people may contract hepatobiliary diseases and even cancer of the bile duct, which is diagnosed by abdominal ultrasonography and endoscopy. The hepatobiliary pathologies can be reversed with chemotherapy. Whether praziquantel treatment is associated with gall stone formation or influences the development of CHCA is unknown. However, begging further investigation, Chernrungroj (2000) found an increased risk of CHCA in those with a history of praziquantel treatment. Predisposition to heavy infections after treatment indicates the possibility of targeted treatment as a strategy for parasite control. Seasonal variation or sampling biases need to be considered when planning further studies. Immediate treatment is required to eliminate the long-lived parasites, while sanitation and personal hygiene interrupts transmission from human feces to snails. Health education encourages people to cook fish. Control programs using treatment plus health ed-
The authors thank Mr. Bryan Roderick Hamman for assistance with the English-language presentation of the manuscript.
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