Pig-transmitted Taenia spp. in Vietnam: epidemiology and risks to the human population

In many low and middle income countries, Taenia spp. infections are responsible for human suffering and economic losses in livestock. Pigs are the intermediate hosts of zoonotic and non-zoonotic Taenia species. In Vietnam, pork is the most common consumed meat, and appropriate implementation of regulations to ensure safe meat is often lacking. The aim of this thesis was to characterize the pork supply chains and to estimate the occurrence of pig-transmitted Taenia spp. in northern Vietnam. First, a literature review (Chapter 1) of the three pig-transmitted Taenia spp. is given, focusing on their morphology, lifecycle, epidemiology, burden and diagnosis. T. solium is a leading cause of epilepsy and chronic headaches in humans in endemic countries. Porcine cysticercosis can cause condemnations of infected carcasses at slaughter. In contrast, the impacts of T. asiatica and T. hydatigena are less documented. In Vietnam, lack of sanitation, inappropriate pig management practices and consumption of raw/undercooked meat/organs perpetuate the transmission of Taenia spp. In Chapter 2, the pork value chains (PVC) in Vietnam were mapped. A systematic review was performed by consulting 7 databases. A total of 10 main PVC types were identified, among which the one involving producers, slaughter men, retailers and consumers is most common. In cities or export routes, middlemen and/or traders are important additional actors. More industrial forms of PVCs are developing; however, the traditional PVCs are still prevailing in Vietnam. The weak linkages between actors and poor hygienic practices in these chains form a risk to pork safety. Chapter 3 describes a systematic review of the global occurrence of T. hydatigena in pigs and cattle. Results showed a worldwide occurrence of T. hydatigena in these livestock species. In pigs, there was a higher prevalence in Asia (17.2 %) and South America (27.5%), compared to a low prevalence of 3.9% in Africa. Overall, the prevalence of T. hydatigena in cattle was low (mean of 1.1 %). The results show that interpretation of results of sero-diagnostic tests for zoonotic Taenia species in pigs and cattle has to take into account the prevalence of T. hydatigena in different settings. Chapter 4 presents a cross-sectional survey of the apparent prevalence of Taenia spp. in pigs in Phu Tho, northern Vietnam. Carcasses of 399 pigs from 51 small-scale abattoirs were inspected for cysticerci, while predilection sites were sliced and examined. Cysticerci underwent PCR-RFLP and sequencing for species confirmation. Specific antibodies and circulating antigens were measured on blood samples. T. asiatica cysticerci were found in two pigs. Cysticerci of T. solium were found in none of the pigs, although one serum sample was antibody-positive. A high prevalence of T. hydatigena cysticercosis was observed (18.0%). Spirometra erinaceieuropaei was found in one pig liver. The findings confirm the complex epidemiology of Taenia spp. in pigs in Vietnam. The high prevalence of T. hydatigena cysticercosis in pigs necessitated the investigation of its occurrence in dogs, the definite host, in N Vietnam (Chapter 5). A cross-sectional survey was carried out in four dog slaughter-slabs in Phu Tho province, aiming to reveal the occurrence of gastro-intestinal helminths in dogs. A total of 350 dog samples were collected and identified by morphological and molecular techniques. Nine species, including T. hydatigena, S. erinaceieuropaei, Dipylidium caninum, Spirocerca lupi, Ancylostoma ceylanicum, Ancylostoma caninum, Toxocara canis, Toxascaris leonina and Trichuris vulpis, were found. T. hydatigena had a very low prevalence of 0.3%. Six of the recovered helminth species have a zoonotic potential. Finally, the findings presented in the thesis are discussed (Chapter 6) in relation to the supply and the health risks for humans of currently available pork in northern Vietnam, and the role of the non-zoonotic T. hydatigena in parasite control. It can be concluded that the current lack of meat inspection and traceability of a large proportion of pigs and pig products, and the habit of eating undercooked meat and liver form a risk to the consumers. Epidemiological studies and monitoring of T. solium control or elimination programmes are hindered by the non-specificity of currently available serological tests.