Analyse du risque entomologique d’émergence des épidémies massives d’arboviroses (dengue, chikungunya et Zika) au Cameroun

Abstract

Dengue, chikungunya and Zika represent major public health problems in many parts of the world including Africa. The viruses responsible for these diseases are transmitted by an infected bite of female from the genus Aedes female, from which the species Aedes albopictus and Aedes aegypti are the major vectors. In Cameroon, Ae. aegypti is the indigenous species while Ae. albopictus is of recent introduction whose consequences are still underestimated. In the absence of an effective vaccine and treatment against these diseases, control is essentially based on vector control which requires a good knowledge of the bio-ecology of the vectors. Therefore, our study aims to update the geographical distribution in Cameroon, and assess the larval ecology and the level of colonisation in Yaoundé, the epidemiological role, as well as the genetic diversity of the main vectors, Ae. aegypti and Ae. albopictus. For this purpose, immature stages of Ae. aegypti and Ae. albopictus were collected in 28 localities to assess their distribution throughout the country. The level of colonisation and larval ecology of both species were assessed in 30 neighborhoods of the city of Yaoundé using a cluster random sampling design. Entomological indices were calculated to assess entomological risk. Determination of natural infection of the two species collected in Yaoundé by DENV, CHIKV and ZIKV was performed by real-time quantitative PCR. The origin of blood meals was determined by sequencing the cytochrome oxidase gene of vertebrates. Genetic diversity of Ae. albopictus samples from 17 localities and Ae. aegypti from 9 localities was assessed by analysis of the mitochondrial cytochrome oxidase subunit 1 (mtCOI) gene and the immune genes DOME and PIAS. Geographical distribution assessment resulted in the collection of 30381 adults of Aedes spp. with 69.28% for Ae. albopictus and 30.72% for Ae. aegypti. Aedes aegypti is present in all the surveyed localities while Ae. albopictus has a distribution limited to the southern part of the country around 6.4°N. This result suggests a climatic boundary to the establishment of the invasive species in the northern part of the country. Aedes albopictus was the predominant species over Ae. aegypti in almost all the localities prospected as well as in downtown and suburban environments. The study of larval ecology in Yaoundé showed high levels of colonisation for both species, with a predominance of Ae. albopictus in the downtown and suburban neighborhoods, which confirms the competitive advantage of this species over the indigenous species. Nevertheless, Ae. aegypti was persistent in high building and in rural areas. Both species developed primarily in used tires and discarded tanks associated with the presence of plant debris. The Stegomyian indices were above the established thresholds and highlight the existence of a potential risk for the emergence of arboviruses outbreaks. No specimens of Ae. aegypti and Ae. albopictus from Yaoundé were infected with DENV, CHIKV and ZIKV, and females of both vectors were feeding essentially on humans, confirming the anthropophagous nature of both vectors. In addition, other specimens of Ae. albopictus were found fed on monkey (Papio anubis) and bats, confirming the opportunistic nature of the species. Moreover, these observations highlight the risk of emergence of new human infections by new viruses or new variants of viruses already infecting humans. Analysis of the mitochondrial COI gene in Ae. albopictus populations revealed a low polymorphism of five haplotypes detected of which haplotype 1 was predominant. This confirms a recent introduction of the species into the country from an ancestral population. Analyses of the DOME and PIAS genes of Ae. aegypti populations revealed high polymorphism with 28 and 21 haplotypes, respectively, indicating high variability in these populations. All Tajima, Fu, and Fu and Li indices were negative for both genes and suggest the existence of an excess of rare alleles in the population. This work provides relevant data for the establishment of a vector surveillance program for arbovirus vectors responsible for potential epidemics in Cameroon. Key words: Aedes albopictus, Aedes aegypti, ,, genetic diversity,