Characterisation of factors influencing trichinellosis in humans and pigs in Nan Province, Northern Thailand
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The work presented in this thesis focuses on trichinellosis, a food-borne zoonosis caused by a nematode of the genus Trichinella, in Thailand. The main objectives were to characterise human trichinellosis, determine its endemic foci and characterise factors influencing infection in humans and in the pig, which is an important domestic animal reservoir host of Trichinella. The investigations comprised (i) a retrospective review to characterise human trichinellosis in Thailand, (ii) a survey to characterise factors influencing trichinellosis in pigs and (iii) quantitative and qualitative studies to characterise the factors influencing trichinellosis in humans. An analysis of Thai national trichinellosis surveillance data between 1981 and 2008 highlighted the five northernmost provinces in the Northern Region as particularly affected with trichinellosis. The incidence of human trichinellosis in Thailand decreased significantly during this period. Until recently, trichinellosis was found to cluster significantly in these provinces. Domestic pigs and wild boar appeared to be the major sources of infection. Field studies focused on populations in Nan Province in the Northern Region, where specific foci of human trichinellosis have been observed. A survey of pig production in both lowland and highland areas in five districts of Nan Province found small-scale pig production to be predominant. Production and management practices differed significantly between lowland and highland areas. In the highland areas, pig production was mainly for the owners’ own use. The holdings were poorly managed compared with those in lowland areas. A survey of trichinellosis seroprevalence in these five districts showed the disease to be associated with scavenging pigs (OR = 2.96, p = 0.02) and older pigs (OR = 1.02, p = 0.02). Seroprevalence was estimated with 95% confidence and was in the range 0 - 0.36% in lowland areas and 0.46 - 1.48% in highland areas. A pig acquired from a Thai-Laotian border market was among the sero-positive pigs identified in the survey. A survey for trichinellosis in wild animals (n = 97), mostly wild boar (n = 53; 55%), using the digestion method did not detect Trichinella in any of the animals screened. A retrospective gender-matched and age-matched case-control study in four trichinellosis-affected communities during 2003 – 2006 showed that consumption of raw wild boar (OR = 2.66, p = 0.005) and consumption of raw meat at social gatherings (OR = 3.89, p = 0.008) were risk factors, and the belief that alcohol can kill the parasite in raw meat (OR = 0.36, p = 0.03) was a protective factor associated with individual trichinellosis cases. Qualitative studies on communities’ knowledge, attitudes and practices relating to trichinellosis in 12 villages indicated that the communities received information about food-borne diseases. However, the practice of raw food consumption continued because of individual taste preferences and the belief that consuming raw food infrequently and in small amounts lessened the risk of eating infected meat. In terms of pig management, although there had been improvements in the practices within Nan Province, with the majority of pigs kept in pens, due to insufficient pig feed and poor survival rates of piglets in inappropriately designed pens, free-range scavenging still occurred. For control of trichinellosis to be improved, the factors identified as influencing its maintenance in the study areas must be communicated to the local administrative organizations and veterinary and public health offices. This will enable them to construct and implement guidelines for good management practices in pig farms in the highland areas. Additionally, there is a need to specify a requirement for Trichinella-free certification of pigs and meat products sold at the border markets. There is also a requirement for the continued education of the general public regarding the safe consumption of adequately-cooked meat.