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PhD Defence Dao Thi Ha Thanh

Epidemiology of Opisthorchis spp. in Central Vietnam

23 nov. 2017

Supervisors

  • Prof. Dr. Pierre Dorny (ITM/Ghent University) 
  • Prof. Dr. Sarah Gabriël (Ghent University)
  • Dr. Thanh Thi Giang Nguyen (Ministry of Health, Vietnam)

Summary

The Southeast Asian liver fluke Opisthorchis  viverrini causes serious morbidity and mortality in the  greater Mekong region. People infected with O.  viverrini are at risk of developing cholangiocarcinoma.  While O. viverrini is endemic in Central and South  Vietnam, the epidemiology and the burden of disease  that it causes have received little attention. Recently,  our research group found a O. viverrini-like species in  domestic ducks in a O. viverrini endemic area in Binh  Dinh Province, Central Vietnam. This thesis aimed to  study the occurrence and the life cycle of O. viverrini  and the O. viverrini-like fluke in Central Vietnam.  In the first chapter a general review of the  literature of the Opisthorchis genus and opisthorchiasis  is given, focusing on the morphology and the molecular  biology, and on the epidemiology and the diagnosis of  opisthorchiasis in SE Asia, including Vietnam. The  chapter also gives a description on the finding of an  Opisthorchis sp. in domestic ducks in Central Vietnam  (further referred to as Opisthorchis sp. BD2013).  Following this chapter, the rationale and  objectives of the thesis are given.  The second chapter describes the current  status of opisthorchiasis in a Central Vietnamese  community. The apparent prevalence of O. viverrini in  the population determined by stool examination was  11.4%. The mean number of worms recovered after  treatment was 14.5. Male gender and the consumption  of raw freshwater fish were found to be significant risk  factors associated with opisthorchiasis in the area.  In the third chapter, morphological and  molecular identification was performed of an  Opisthorchis sp. found in the bile ducts of domestic  ducks in Binh Dinh Province. Morphological  characteristics of the bird flukes were compatible with  O. viverrini, although some characteristics differed from  those described in specimens collected from mammal  hosts. Computation of the phylogenetic trees on the  partial sequences of ITS2 of the ribosomal (rb) DNA  and COI markers of the mitochondrial (mt) DNA  showed close similarity of Opisthorchis sp. BD2013  with O. viverrini. We speculated that these bird flukes  were O. viverrini that showed intra-species  morphological and molecular variability compared to  isolates from mammals.  In the fourth chapter, we provide new  sequence data from the mitochondrial genome and the  nuclear ribosmal transcription unit of Opisthorchis sp.  BD2013. A phylogenetic analysis was conducted to  clarify the basal taxonomic position of this species from  ducks within the genus Opisthorchis. From four  developmental life stages, the complete cytochrome b  (cob), nicotinamide dehydrogenase subunit 1 (nad1)  and cytochrome oxidase subunit 1 (cox1) genes; and  near-complete 18S and partial 28S rb DNA sequences  were obtained by PCR-coupled sequencing.  Phylogenetic trees were inferred from concatenated  (cob+nad1+cox1) nucleotide sequences and from  combined 18S+28SrDNA nucleotide sequences of five  Opisthorchis sp. BD2013 samples and additional  reference taxa. Both trees demonstrated the  anticipated clustering of taxa within the  Opisthorchioidea, the paraphyly of the genus  Opisthorchis and the sister-species relationship of  Opisthorchis sp. BD2013 with O. viverrini.  In the fifth chapter, we determined the  infection rate and intensity of infection with  Opisthorchis sp. BD2013 in ducks in 4 districts of Binh  Dinh province. An infection rate of 34.3% was found;  the intensity of infection was 13.8 worms per infected  duck.  The sixth chapter describes a study on the  occurrence of O. viverrini and Opisthorchis sp. BD2013  in snails and fish intermediate hosts in the endemic  area. A total of 12,000 snails belonging to six families,  as well as 754 fish representing 12 species were  examined. Shedding of O. viverrini cercariae was  observed only in Bithynia s. goniomphalos and B.  funiculata, at infection rates of 0.86% and 0.14%,  respectively. O. viverrini metacercariae were found in  10 fish species. Carassius auratus, a fish species  commonly eaten raw, had the highest prevalence of  74.0%. Sharing of the same snail and fish intermediate  host species was found for O. viverrini and  Opisthorchis sp. BD2013.  In the seventh chapter, the life cycle of  Opisthorchis sp. BD2013 was developed.  In the eighth chapter, we discuss the findings  of our research on Opisthorchis sp. BD2013 in the  context of its co-existence with O. viverrini in Central  Vietnam and of the sharing of snail and fish  intermediate hosts. There is a need to perform studies  to define the host range of Opisthorchis sp. BD2013.  Meanwhile, a One Health control program should be  built for prevention and control of the neglected O.  viverrini in the region.