Vector competence and filariasis transmission in Mali

• Main Author: Coulibaly, Y. I.
• Published: University of Liverpool 2016
• Subjects: 616.9
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.724481

Lymphatic filariasis (LF) is a public health problem in 73 countries and is associated with marked morbidity and disability. It is unique because of its transmission by five main genera of mosquitoes, including Culex, Aedes, Anopheles, Mansonia and Ochlerotatus. In Mali, LF endemicity mapping in 2004 found all eight administrative regions to be endemic for LF. Prior to the National LF Elimination Programme (NPELF), six pilot sentinel villages were selected for baseline research studies to inform the most appropriate strategy for monitoring the impact of the proposed elimination programme based on treatment with ivermectin in combination with albendazole. The following three objectives form the basis of my PhD studies:(i) investigate LF vector population and associated transmission patterns before, during and after the initiation of mass drug administration (MDA) (ii) assess efficacy of new entomological trapping tools for LF post-MDA xenomonitoring and (iii) determine transmission potential in a urban environment in Mali. The overall design is a descriptive study including cross sectional entomological surveys along with longitudinal human surveys to assess the MDA impact. I used standard infection status assessment methods as well as recently developed methods; including the antibody test for Wb123. I conducted these studies in both rural (Sikasso and Kolondieba districts) and urban areas (Bamako, the capital city). My thesis is the first report of the outcome of up to five years post-MDA annual assessment of W. bancrofti transmission using both entomological and parasitological data in an Anopheles transmission area where albendazole plus ivermectin is the recommended drug regimen and Anopheles gambiae s.l the main vector for LF transmission. These features are found mainly in the Western part of Africa. In the pilot sentinel sites in Mali, made of six neighbouring villages, seven MDA rounds with the albendazole plus ivermectin were successful not only at stopping LF transmission (infection rates within 6-7 years old children < 2%) in the short term, but also at sustaining it for up to five years after the last MDA. In contrast, impact assessment in another hyper endemic area (two neighbouring villages treated by the NPELF in the district of Kolondieba) did not demonstrate interruption of transmission after the sixth and seventh MDA rounds. The reasons of these different outcomes of MDA implementation in the different areas are discussed. Of note, the failure in the latter villages was detected using only the ICT card, a method that has been found to overestimate the infection rate in children when compared to the circulating filarial antigen test (Og4C3 ELISA) and the Wb123 antibody test in the pilot sentinel area. In Anopheles transmission areas, it has been observed that focal low-level transmission can exist without being a real threat for re-emergence of transmission, due to lower capacity of the vector to transmit when parasite density is low. Nevertheless, in areas that fail the Transmission Assessment Survey (TAS), adult populations should be checked in addition to the recommended 6-7 year-old age group. Additionally, this thesis showed very promising results for using the Ifakara tent trap type C (ITTC), a human baited trap alternative to the human landing catch. Anopheles yields and infection rates using ITTC were strongly correlated with results using human landing catch (HLC) overall, as well as monthly, in two villages with significantly different Anopheles densities. Finally, it appears that the current version of the TAS needs more tools and additional directions for human infection status determination especially when the baseline endemicity level is high. Further evaluation the ITTC after reducing its bulkiness is required to confirm its usefulness for LF entomological studies in Anopheles transmission areas. From the 6,174 Culex spp and the 16 Anopheles gambiae s.l processed and 1,002 volunteers tested, there was no evidence of LF transmission in the urban environment of Bamako in Mali.