Influence des activités anthropiques sur le profil entomologique du paludismeen zone équatoriale camerounaise: cas de Simbock et Nyabessan

Abstract

Environmental changes resulting from development projects such as dams, urbanization activities create breeding grounds favourable for mosquito development, vectors of tropical diseases such as malaria. Indeed, in many sub-Sahara African countries, it has been shown that hydraulic infrastructures and urbanization affect the interactions between humans and insect vectors of pathogens, with a significant implication on disease transmission. The present study aimed to assess the influence of development projects (construction of a hydroelectric dam and certain urbanizational projects) on the transmission profile of malaria in the equatorial zone of Cameroon. Specifically ; to identify possible changes in the composition of the anopheline fauna ensuring malaria transmission and to determine the susceptibility to insecticides of Anopheles gambiae s.l., an emerging vector of malaria in a disturbed ecological context. Human landing catches of adult mosquitoes were carried out by volunteers before and during the construction of the dam in Nyabessan and the process of urbanization in Simbock, between 2000 and 2006 (retrospective study), then between 2014 and 2016 (prospective study). The same study was conducted simultaneously in Olama village which was considered as the control site. Malaria vectors were identified using morphological identification keys and analyzed by the ELISA technique for the detection of the circumsporozoite protein of Plasmodium falciparum. Anopheles gambiae s.l. was identified down to species using molecular markers. Larval collections were carried out between 2014 and 2017 to perform susceptibility tests to insecticides (deltamethrin, permethrin, DDT and Bendiocarb). The use of the synergist PBO (Piperonyl Butoxide) made it possible to explore the mechanisms of metabolic resistance to pyrethroids. Mechanisms of resistance to pyrethroids and DDT by modification of the target (kdr mutation) were also screened using molecular markers. A total of 17,530 adult mosquitoes belonging to 13 species were collected, including 6,127 in Nyabessan, 5,341 in Simbock and 6,062 in Olama. In Nyabessan, A. moucheti and A. ovengensis which were the main species before the construction of the dam (27.4 - 37.5 bites/man/night- b/m/n, 0.48 - 0.22 infective bites/ man/night-ib/m/n), showed a decrease in their role in malaria transmission during the construction of the dam (14.5 -11.6 p/h/n, 0.12 - 0, 01 ib/m/n) (p<0.005). Conversely, A. gambiae s.l. and A. paludis showed an increased role in transmission during dam construction (6.3 - 9.3 b/m/n, 0.24 - 0.2 ib/m/n) (p<0.001). At Simbock, A. moucheti which was the major species before urbanization (8.1b/m/n, 0.12 ib/m/n) was not caught between 2014 and 2016 (during the urbanization process). In Olama, A. moucheti remained the main vector of malaria throughout the study between 2000 and 2016. Populations of A. gambiae s.l. from Nyabessan, Simbock and Olama were resistant to deltamethrin (46-95% mortality), permethrin (7 - 66%) and DDT (5-10%) but susceptible to bendiocarb (98-100%). Exposure of mosquitoes to PBO before susceptibility testing significantly restored vector susceptibility to deltamethrin (98% - 100%) but did not change the resistance status of anophelines to permethrin (22%). The L1014F allele responsible for kdr resistance was found at high frequencies in the three study sites, including 0.68 in Nyabessan, 0.64 in Simbock and 0.98 in Olama. Large-scale anthropogenic modifications of the environment (such as urbanization and dam construction) generate a profound modification of the entomological profile, consisting of a substitution of secondary vectors (A. moucheti) by major vectors (A. gambiae s.l.), thereby increasing the risk of malaria infection. This is even more serious as these major vectors are resistant to common public health insecticides, including combinations of kdr and metabolic resistance mechanisms. These results underline on one hand, the need to set up an effective surveillance system against vector-borne diseases around developmental projects which are likely to modify the environment, and on the other hand the urgency of adopting the use of combinations of insecticides as a resistance management method.