Epidemiological transmission patterns of Taenia solium cysticercosis in endemic areas: the case of Ecuador.

Taenia solium taeniasis/cysticercosis is a neglected zoonotic parasitic disease complex that causes public health and socio-economic problems in many developing countries. Despite the increasing knowledge about the parasite and the risk factors for infection, control of T. solium has received little attention from governments of endemic countries. To date, no national control program has been implemented in Ecuador although the country is endemic for neurocysticercosis (NCC). The objective of this thesis was to study the transmission patterns of T. solium taeniasis/cysticercosis in order to design appropriate and effective control strategies for this Neglected Zoonotic Disease in endemic areas, with special reference to Ecuador. Chapter 1 presents an overview of the current knowledge of T. solium including the life cycle of the parasite, the implications on public health and the economic impact of the disease, its diagnosis, epidemiology with an emphasis on Latin America, with particular attention to the situation in Ecuador. In addition, the current control measures available for T. solium taeniasis/cysticercosis are described. In Chapter 2, serological data on apparent prevalence of T. solium circulating antigens and/or seroprevalence of T. solium antibodies, apparent prevalence of human taeniasis and risk factors for HCC from endemic communities were gathered in order to understand the differences in patterns of exposure to the parasite and active infections with T. solium metacestodes in endemic areas around the world. A total of 39,271 participants from 19 countries, described in 37 articles were studied. A significant variation in the sero-epidemiological data was observed within each continent. Intrinsic factors in the human host such as age and immunity were the main determinants for the occurrence of infections while exposure was mostly related to environmental factors, which varied between communities. In Chapter 3, spatial and temporal variations in the incidence of hospitalized cases of epilepsy and NCC in Ecuadorian municipalities were analyzed in order to locate and characterize important clusters in space and time. Additionally, potential socio-economic and landscape indicators were evaluated to understand in part the macro-epidemiology of the T. solium taeniasis/cysticercosis complex. This study identified traditional endemic clusters in the highlands for both conditions as well as new clusters appearing in recent years in other zones considered non-endemic. Chapter 4 describes the dynamic nature of human T. solium larval infections in an Ecuadorian endemic community. In this study incidence rate and cumulative incidence figures of human T. solium larval infections are reported for the first time in Latin America. A sero-epidemiological cohort study was conducted in a south-Ecuadorian community to estimate the incidence rate of active cysticercosis and the incidence rate of exposure to T. solium. The proportion of infected individuals remained low and stable during the whole study year while more than 25% of the population showed at least one antibody seroconversion/seroreversion during the same time period. These estimates are of high importance to understand the epidemiology of T. solium in order to develop ad hoc cost-effective prevention and control programs. They are also essential to assess the burden of T. solium cysticercosis. The estimates generated here may now be incorporated in epidemiological models to simulate the temporal transmission of the parasite and the effects of control interventions on transmission. In Chapter 5, the findings of this research are discussed in the context of their contribution to the design of control strategies for T. solium, with special attention to the case of Ecuador. This research showed the implications of the variability of epidemiological patterns of T. solium in different endemic areas on the design of control strategies. Also, some tools were provided that could be used to help in the identification of priority areas of intervention within a country. The transmission dynamics of human cysticercosis were considered in order to give further recommendations when applying control measures in endemic rural and in urban areas of Ecuador. The approach presented in this research could be used to fine-tune the current control strategies of T. solium. However, further studies are needed to fill the gaps in the epidemiology of T. solium such as understanding the role of the environment in the transmission of taeniasis/cysticercosis.