The survey conducted throughout two years in 15 localities among eight regions of Madagascar revealed that the distribution of Ae. albopictus and Ae. aegypti has changed considerably, in comparison to previous recorded distribution. Initially, the two species have been reported to be mainly allopatric with contrasted distribution [1, 2]. Indeed, Ae. aegypti has been found to occupy mainly dry and semi-arid western and southern regions of Madagascar, whereas Ae. albopictus was dominant in eastern coast and highland areas [1, 2]. Our data show that Ae. aegypti has become scarce within their previously delineated areas, and presently their population density is low. A decrease in Ae. aegypti distribution was also detected in the neighbour Reunion Island . In addition, the remaining Ae. aegypti populations of Madagascar may have displaced to occupy several sylvatic areas: two in allopatry (Ankarana and Kirindy) and two in sympatry with Ae. albopictus (Montagne d'Ambre and Ranomafana). Ae. aegypti has been reported to occupy other forest areas on Indian Ocean Islands [2, 14]. We also found two populations consisted of a few adults under bushes and fruit trees in Mahajanga city (western coast) and Diego Suarez city (north coast); which we suggest may be due to sporadic introduction by humans as this species was reported previously near the Ivato airport and in the south . In contrast to its behaviour in South Asia , the species Ae. aegypti is less anthropophilic in Madagascar [1, 14], consequently it is thus conceivable to hypothesize that the smaller size of such populations will provoke the extinction of the species in domestic environments.
The population density of Ae. albopictus was higher compared to that of Ae. aegypti, and also its distribution was wider, covering the eight regions surveyed from west to east and in the north. This study confirms the extension of the distribution area of Ae. albopictus which was already reported [2, 14]), albeit to a lesser extent. More evidence hereby was provided on the adaptation capabilities of Ae. albopictus to occupy various eco-climatic regions of Madagascar, from the high altitude with temperate conditions prevalent in the highland region up to typical tropical conditions of the low altitude of the coastal regions. These data undoubtedly confirm the environmental plasticity of which Ae. albopictus is known to be capable of i.e. adaptation in contrasting environments. For instance, it was shown that Ae. albopictus has the ability to adapt to cold temperatures, and these adaptative capacities are likely due to its ability to synthesize a high amount of lipids which consequently can provide eggs with substantial yolk resources to maintain them through diapause [28–30].
Ae. albopictus was predominantly found in artificial environments which formed its major oviposition sites. These included dumped containers, used and abandoned tires and buckets, coconuts and bamboo cut trees, that allow for the persistence of small numbers of mosquitoes or eggs. The population density significantly increases at the rainy season, and differences were found between regions with a high abundance found in the east coast of Antsinanana. Though Ae. albopictus is known to be highly anthropophilic, specimens were also found in sympatry with Ae. aegypti in some wild areas. Surprisingly, Ae. albopictus also predominated in such conditions as well, suggesting a better competitiveness [31, 32]. Although Ae. albopictus females have feeding preferences towards mammals, recent investigations highlighted the opportunistic and broad spectrum trophic behaviours to cold and other warm-blood vertebrates such as birds and reptiles [33, 34]. Overall, these features suggest that the wide distribution and invasiveness of Ae. albopictus in Madagascar can be attributed to their adaptive behaviour to both abiotic and biotic environmental factors, including resistance to agrochemicals [35, 36].
To examine whether the environment inhabited by mosquitoes generated genetic structuring amongst these populations, a phylogenetic analysis was performed using COI and ND5 mitochondrion markers . The haplotype sequences of Ae. albopictus from Madagascar were clearly separated from those from South America (Brazil) and South Asia (Cambodia, Thailand, Vietnam). However, no significant differences were found between sylvan and domestic populations of Madagascar. Malagasy populations of Ae. albopictus were intermixed with individuals from the Indian Ocean (La Reunion Island), Europe (Normandie, France), and North America (Jacksonville, USA). These results may suggest a recent invasion process or a trade-off between these regions.
Experimental infections of six Malagasy Ae. albopictus populations with CHIKV 06.21, a strain that circulated in the Indian Ocean , demonstrated their high susceptibility to infection. This high susceptibility has already been reported for this CHIKV strain 06.21 in most Ae. albopictus populations from the Indian Ocean , and was attributed to an A- > V amino acid substitution at position 226 in the viral E1 glycoprotein (8), leading to efficient dissemination and transmission by the vector [9, 37].