This study presented the current geographical distribution, the level of infestation, and the factors governing the presence of
Ae. albopictus and
Ae. aegypti in Yaoundé as well as the entomological risk for large arbovirus outbreaks based on
Stegomyia indices. Our analysis confirmed the co-occurrence of both
Ae. aegypti and
Ae. albopictus across Yaoundé with a predominance of
Ae. albopictus in downtown and suburban areas. This observation is in accordance with previous data collected in Yaoundé in 2007 highlighting the predominance of
Ae. albopictus in this city [
18]. A spot of
Ae. aegypti persisting downtown matches the previous observations made in 2017 in the same city [
13]. In fact,
Ae. albopictus was first recorded in Cameroon in the early 2000s [
11] and has rapidly colonized all human-domesticated environments in the southern part of the country [
33,
34]. The predominance of
Ae. albopictus across Yaoundé confirms the competitive advantage of this species on the native species
Ae. aegypti as was suspected previously [
18]. These observations are consistent with other studies reported in several countries in the world in areas invaded by
Ae. albopictus such as in Brazil [
22,
45], Florida [
14,
24], and Australia [
26]. However, the mechanisms by which competition takes place are not well known, but some authors believe that it could happen at the pre-imaginal phase and that several factors such as temperature, precipitation, response to symbionts, predators, and chemical interferences that retard growth are the main drivers [
25,
46]. Also, other work has shown that the two species are able to mate in nature and that
Ae. albopictus males effectively sterilize
Ae. aegypti females [
47,
48,
49]. The authors suggest that this form of mating interference, called satyrization, could explain the competitive displacement of resident
Ae. aegypti by the invasive
Ae. albopictus where they co-occur. Surprisingly, the coexistence of
Ae. aegypti and
Ae. albopictus was reported in Peninsular Florida (USA) two decades after competitive displacement, suggesting a resistance to mating interference [
17]. The abundance of
Ae. aegypti found in rural areas located in two different boroughs in Yaoundé could suggest a resistance to mating interference with
Ae. albopictus among this population, which allows them to co-occur in this area, and further investigations are needed to elucidate. It is important to underline that this study was carried out during the raining season only, although it was demonstrated that seasonality can affect the pattern of abundance of
Ae. aegypti and
Ae. albopictus [
19,
50]. The variation in abundance between the two species would probably be due to the difference in the tolerance of desiccation of the eggs of both species [
51]. However, previous data collected in Central Africa suggest that the variation in abundance between
Ae. aegypti and
Ae. albopictus depends on the difference of time between the rainy season and the dry season among locations [
13]. Surprisingly, no immature stages were found in the Briquetrie (Muslim) neighbourhood in the houses surveyed; however, residents notified us of the use of larvicide to treat the potential breeding sites. Further studies including socio-anthropological aspects are needed to elucidate further.
Overall, the immature stages of both
Ae. aegypti and
Ae. albopictus preferentially colonized peri-domestic containers, particularly discarded tanks and used tyres. These observations are consistent with previous results reported in Central Africa [
13,
19]. However, the opposite situation was found in other parts of the world, particularly in Asia, where domestic containers such as water storage tanks represent the bulk of infested containers for
Ae. aegypti [
52]. Interestingly, it has been clearly established that in major unplanned urban cities, improved waste management through the physical removal of used containers reduces the quantity of mosquito breeding sites and thus decreases
Aedes densities [
1,
53,
54].
Both species breed in the same type of container, notably flowerpots, discarded tanks, used tyres, and car wrecks filled with turbid water, and are associated with plant debris inside. These outcomes highlight the impact of micro-environmental factors on the presence of
Aedes spp. inside breeding sites. In fact, the presence of organic matters inside the larval habitats could serve as food resources [
19,
55] or a micro-habitat to hide and avoid predators [
55,
56]. The sympatric situations found in certain containers suggest possible competition for resources and other ecological interactions in the larval stage which may influence physiological characteristics like body size and wing length, and thus affect adult vector competence for arboviruses [
54,
57].
Overall, the infestation indices were higher compared to the thresholds established by WHO for dengue virus (DENV) [
43] and yellow fever virus (YFV) [
44] transmission. In fact, estimated risk values suggest that Yaoundé is at high potential risk of dengue outbreaks for both species and a high potential risk of yellow fever outbreaks for
Ae. albopictus. These results highlight a higher potential of human exposure to the bites of
Ae. aegypti and/or
Ae. albopictus females in Yaoundé depending to the borough or neighbourhood. Indeed, previous studies based on
Stegomyia indices have shown that high indices coincided with dengue outbreaks in some African countries such as Kenya [
58], Ethiopia [
59], and Tanzania [
60]. The higher indices for
Ae. albopictus compared to those of
Ae. aegypti previously recorded in 2007 in Yaoundé [
18] show a stability of the potential risk in this city. Such potential risk is also similar to that observed in other central African cities such as Bangui in the Central African Republic [
19]. Interestingly, it was recently demonstrated that
Ae. aegypti and
Ae. albopictus populations from Yaoundé are able to transmit DENV [
41] and Zika virus (ZIKV) [
42], further increasing such potential risk. The same was observed of the
Ae. aegypti population for YFV [
40]. Additional studies including a dynamic of abundance of each species depending on the season and trophic behaviour of each species are required to establish the epidemiological importance of each species.