In both surveys and in all communes, the larval indices (HI, CI, and BI) were higher than the arbovirus transmission threshold values (BI of 5 set out by the World Health Organization) (29)(30). The Breteau index was on average 45 per 100 houses in the rainy season, and in comparison to a house index of on average 27%, it is clear that one house can have different Aedes spp. positive breeding sites. These high larval and pupal Aedes infestations suggests that if an arbovirus starts circulating, transmission can rapidly occur in Kinshasa. This has been confirmed during the recent chikungunya outbreak in 2019 (11).
The strength of this study lies in the use of standardized procedures in four different communes of Kinshasa during the rainy and dry seasons. The entomological team was trained beforehand and was largely the same for both surveys. A weakness is that the study took place over only one year time and only once per season. As inspection of breeding sites depends on the rigor and professionality of the team doing the fieldwork, quality control was established, namely a fixed supervisor was available in the field site during the survey and there was regular extra control from the international integrant of the study-team. Due to operational issues we were not in a position to identify all larvae to species level, which was another weakness, hence we could not calculate the relative importance of Ae. aegypti and Ae. albopictus, neither which species has predilection for which container type.
The larval indices are remarkable high for a place without important endemicity of arboviruses and only sporadical reporting of cases of dengue (14)(12)(15)(13), outbreaks of chikungunya (11)(9) and yellow fever (31). The level of infestation is on the higher end of what has been published in other African settings: in south-eastern Tanzania with HI of 4.9—6.6, CI of 14.6–18.9 (32); in Burkina Faso with HI of 70, CI of 35 and BI of 10 (33); in north-west Ethiopia with HI of 25.5, CI of 32.9 and BI of 48.4 (34); in Mozambique with CI of 22 (35); and in Angola with HI of 4.3—27.9, CI of 2.1–9.3 and BI of 5.8–42.2 (36). But the Breteau Index in Kinshasa was much lower than the one observed in Kenya during a dengue outbreak in 2013–14, where BI reached a value of 270/100 houses (37).
In contrast to findings in Latin-America (38), in Kinshasa Aedes breeding sites were mainly found outdoors, a characteristic also seen in other African countries (39). The prevalence of Aedes breeding sites outdoors suggests a close human-mosquito contact. The inhabitants stay most of their day-time outside the house, in the backyard or in the open place in front of their house, where also the water storage is for domestic activities. This human behavior favors the development of the mosquito cycle since the mosquito finds there breeding sites and at the same time sources of blood, as Aedes spp. bites during day-time. The low presence of Aedes immature stages inside the homes can also be due to rapid use and cleaning of the few containers found there. Outdoors, emptying and cleaning the many potential breeding sites is a daunting job that needs to be realized all weeks of the year if one wants to stop the mosquito development cycle. These results indicate that for controlling Aedes in Kinshasa, management strategies need to target outdoor spaces for breeding sites destruction or reduction.
Used car tires, water storage containers and artificial breeding sites (type trash) were the main containers chosen by Aedes mosquitoes for the oviposition coinciding with other studies conducted in several countries of the African continent (33–37,39)(40). The water storage containers were also found to be the most productive for Aedes pupae, which is a stage in the mosquito cycle which does not need nutrients and which is just before the adult stage of the mosquito. These containers are all always filled (partially or fully) with water, also when there is no rain to fill the small trash deposits outdoors. This makes them a preferred breeding site, even despite being constantly subject to anthropogenic action. In the rainy season, in Lingwala and N’Djili, artificials, favored by rain, are most productive for Aedes pupae, while in Mont Ngafula and Kalamu, small water containers, where rain water is captured, are most productive. In the dry season, it is clear that water storage containers (whether big or small) are the most productive, pointing out that water storage is important for the households due to a deficient water supply system, especially in the communes of N’Djili and Mont Ngafula. In the rainy season, about 35% of all pupae are found in tires, while tires are only representing 11% of the potential breeding sites. In the dry season this is less pronounced as expected since they contain water dependent on the occurrence of rainfall. Discarded tires might also be stored for longer durations and harbor mosquito larvae undisturbed, making them prolific breeding containers (41). Moreover, the weather conditions inside tires, such as temperature, humidity, and reduced light, create a suitable environment for Aedes mosquito breeding (42,43). Eggs attached to the tires may also play a role in the preservation of the Aedes mosquito population throughout the dry season (44). These containers preserve water, organic and mineral substances for a long time and allow vectors to reach adulthood. The importance of used tires in the maintenance of Aedes mosquitoes populations is well documented by several previous works in different parts of the world, and it is well described how to handle them, such as recycling (39) or storing them under a roof to avoid filling with rainwater.
Small water deposits were found in large number in all the study sites and were also the main pupae producing containers. A slightly lower productivity by this group of container type was seen in the rainy season, in comparison to the dry season, and can be due to their short-term use for water storage and being subject to frequent emptying and cleaning, which effectively interrupt the breeding cycles of Aedes mosquitoes.
In this study, Aedes species were dominant in the inspected potential breeding sites, in and around the houses. Also other mosquito genera were found, such as Culex and Anopheles. Culex species are common species in urban settings using similar breeding sites as the urban Aedes species. It is of note that Anopheles species were found together with Aedes in the same breeding sites. Anopheles usually prefers other types of breeding sites, such as ponds with clear water and are not particularly attracted to small containers. The presence of Anopheles in urban settings is primarily associated with urban agriculture, as in Mont Ngafula (45), though we found Anopheles in all four communes in the rainy season, also in the center of Kinshasa.