Trends in Parasitology
OpinionOpisthorchis viverrini: an underestimated parasite in world health
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Opisthorchis viverrini in Southeast Asia
The World Health Organization (WHO) estimates that approximately five million people with HIV live in the Asia–Pacific Region [1]. This can be compared with a conservative estimate of ten million people infected with the liver fluke Opisthorchis viverrini in Thailand and Laos alone [2]. People infected with Opisthorchis parasites are at risk of developing cholangiocarcinoma (CCA), and those who have advanced CCA will almost certainly die. As with HIV, infection with O. viverrini and the
O. viverrini: an underestimated parasite in world health
O. viverrini is an underestimated parasite in world health [4] because it is mistakenly thought to represent a limited, local problem in Southeast Asia, particularly in Thailand [5]. This is predominantly because of the fact that reasonably accurate spatial and temporal morbidity data and CCA incidence are available from Thailand but not from elsewhere within the Mekong region where O. viverrini occurs 6, 7, 8, 9. Although O. viverrini infection in humans is often asymptomatic, this parasite
Prevalence in humans
The most recent estimate of the number of people infected with O. viverrini in the Mekong region is eight million in Thailand and two million in Laos [2]. This is a substantial underestimate of its prevalence because no data are available for Cambodia or Vietnam, although opisthorchiasis in known to be common in parts of these countries 18, 19. In Thailand, an average of 9.6% of the population is infected [20], with the liver fluke being distributed mainly in the North (19.3% prevalence) and
Incidence of CCA
CCA has caused millions of deaths in southeast Asia [25]. The most important risk factor for CCA is O. viverrini infection, with risk being dependent on intensity [26], previous or current exposure to infection and host genetic polymorphism [27]. A recent study detected residual O. viverrini DNA in both cancerous and non-cancerous tissue in CCA patients in Thailand [28]. By contrast, the risk factors for CCA in areas where liver flukes are not endemic include primary sclerosing cholangitis,
Genetic and molecular characterization of O. viverrini
The study of the molecular genetics of O. viverrini has received considerable impetus from the recent publication that provides the first catalogue of expressed genes (∼14% of the entire transcriptome) for this species [34]. This advance will enhance our ability to conduct comprehensive molecular analyses of the systematics and population genetics of O. viverrini in a similar manner to that proven useful for other parasites, as has been the case for the parasitic protozoan Giardia intestinalis
Current and future threats
The movement of people is a major component of globalization in Asia [40]. O. viverrini is regularly carried to regions outside of its normal distribution by tourists and Thai labourers 2, 41. Because infection is often asymptomatic and of a long duration, infected migrants are likely to remain undiagnosed and shed eggs for long time periods. However, the introduction of O. viverrini to human populations outside of the Mekong region is more likely to come from fish imports. Asia is the world’s
Prevention
The availability of praziquantel provides effective chemotherapy for Opisthorchis parasite infection [46]. There is, however, evidence that repeated infection and treatment could increase the risk of CCA 50, 51. Thus, a more basic approach involving prevention by reducing the consumption of infected fish is likely to have better long-term chances of success. It is difficult to achieve this goal, however, because it involves changing traditional patterns of eating behaviour. A control program
Concluding remarks and future directions
Although our knowledge of the biology of O. viverrini is steadily increasing, fundamental studies on epidemiology and ecology, in addition to genomics and proteomics, are still needed. In addition, investigation of the molecular basis of fluke-induced CCA is likely to be of great importance in the development of biomarkers for the diagnosis of this devastating condition.
The scientific challenge of providing a comprehensive epidemiological understanding of the O. viverrini species complex
Acknowledgements
This research was supported by a Wellcome Trust Collaborative Research Initiative Grant, the Thailand Research Fund through the Royal Golden Jubilee PhD. Program, the European Commission through a TREMKIT project, the Faculty of Medicine, Khon Kaen University, Overseas Visiting Professor Program and the Deutsche Forschungsgemeinschaft (PE1611/1–1).
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