Human activity without adequate planning and infrastructure in forest areas of the Brazilian Amazon can promote greater contact between man and vector and, consequently, intensify malaria transmission in different locations [12, 35]. Furthermore, alterations in the land use could completely change the richness and composition of anopheline species, and thus change the dynamics of local malaria infections [36, 37]. The malaria control and prevention strategies currently developed by the Ministry of Health are not effective in areas in the Brazilian Amazon, such as rural settlements, that suffer increasing urbanization since entomological and epidemiological knowledge regarding these locations is limited.
In the current study, Ny. darlingi was the dominant species in settlement areas in the municipality of Candeias do Jamari, Rondonia, Brazil. This observation is accordance with the findings of other studies carried out in rural settlements in the Brazilian Amazon [12, 15, 38]. This mosquito species is abundant and contact rate with humans is high in studies of the first stage of frontier malaria [20]. Furthermore, Ny. darlingi is a species that is well-adapted to anthropogenic changes in the forest environment when compared to other anopheline species [17, 20].
The more recent settlement in the present study (AF) showed the highest density of Ny. darlingi and this was where other anopheline species were collected. Studies in rural settlements in Acre have reported a higher density of Ny. darlingi in sample collections near a recent settlement, which suggests that a higher level of colonization decreases the vector presence [12]. If we consider the “frontier” malaria concept, our data regarding anopheline density confirm what occurs in the early stages of frontier settlement. The first phase of “frontier” malaria involves changes in the natural forest landscape, which alter the abiotic characteristics and ecology of larval habitats, and this leads to an increase in the abundance of the local vector [9].
The two other species of mosquitoes collected at the AF (Ny. triannulatus and An. peryassui) have already been described as vectors that inhabit the forest edge and stay away from domestic environments, but may be associated with human activity such as deforestation in the Amazon regions [39]. The houses in the AF settlement are closer to the forest, and deforestation is common because of the human colonization process in the area.
Nyssorhynchus darlingi was the only species of mosquitoes that was collected at the PAFJ and presented low density. This settlement has existed for more than ten years, the houses are further from the forest and the exploration of land is no longer as intense. Some studies have demonstrated that environmental alterations may affect mosquito populations in term of abundance and species composition [40–42].
Few field studies focus on the structure of anophelines population in areas with different degrees of human activity in rural settlements in the Amazon [17, 43]. However, it is a fact that Ny. darlingi is the main vector of malaria in both settlements, regardless of the length of time the land has been in use. The main vector in different areas of Rondonia, as well as in most of the Brazilian Amazon is Ny. darlingi [26, 40, 44]. This species is found in great abundance in the region and it is recognized as a highly anthropophilic vector [15, 18, 19, 45, 46]. Biting activity of Ny. darlingi, in general is bimodal, occurring at the beginning of dusk and at dawn [47–49]. However, patterns of biting activity can be influenced by location, vector density, seasonality, presence of hosts, types of housing and the distance between dwellings and the forest [21, 26, 40, 50, 51]. Biting activity in our areas of study was constant during the whole night, though more activity was registered at dusk. Similar results were reported in other Ny. darlingi studies in other areas of Amazon Basin such as endemic areas in the state of Rondonia, Brazil [52] and Iquitos, Peru [53]. The habits of settlers, housing conditions and distance from forest fringe observed at both locations may explain this pattern of anopheline activity. The settlement which showed the highest concentration of Ny. darlingi at the first part of the night was the more recently settled area, where settlers were more exposed to the vector due poor local housing conditions and the type of human activity found there.
Our data regarding HBR also confirm the effect of “frontier” malaria concept in areas which are undergoing transformation. The human-biting rate (HBR) was higher in the AF (5.70 bites/person/hour) than in the PAFJ (0.71 bites/person/hour). Therefore, an individual who lives or visits the AF during the period of activity of Ny. darlingi is almost five times more exposed to mosquito bites than an individual in the PAFJ. However, for the number of infective bites represented by the EIR, our data showed that the settlers and visitors of the PAFJ are more exposed to infective bites (0.05 infective bites/person/hour) than settlers and visitors of the PAFJ (0.02 infection bites/person/hour). The low EIR recorded at the AF was possibly determined by the low SR (0.36%) and high vector density of the location. The PAFJ showed the highest SR (7.35%) of the two locations, although it did present a lower HBR. However, it is known that Ny. darlingi can maintain transmission even at very low density [54–56].
The API at the PAFJ was higher (> 50) in 2018 and 2019, although, since 2012, the API in PAFJ has decreased with 295.08 in 2017. At the last stage of “frontier” malaria concept, malaria declines after 10 years of colonization and development in the settlement, and reaches low and stable levels of transmission because of the reduction in environmental changes. However, the infection risk in these older settlements could also be determined by behavioral factors of the population [9]. The risk of contracting malaria in PAFJ was greater in the first part of night, since the majority of infected mosquitoes were captured before midnight and only one was captured after midnight. In addition, individuals are unprotected outside due the existence of bars and the frequency of truckers that spend the night at the location. Another important point is that the lack of local healthcare facilities and the precarious access roads, which make the rapid diagnosis and effective treatment of malaria difficult, mainly during rainy season.
The unexpectedly low EIR at the AF probably do not truly represent the risk of malaria infection at this recent settlement. The API in this settlement has also been higher (> 50) in 2018 and 2019, and was higher than PAFJ, which is as expected for a recent settlement; in addition, the density of Ny. darlingi and HBR reported here was higher than the older settlement.
A parameter that was not assessed here was the parity of female mosquitoes collected and this is a factor that could have interfered with the EIR. If the majority of female mosquitoes from AF were nulliparous females, these females were not infected because they were getting their first blood meal [40], which may explain the low EIR in recent settlement. Regarding the risk of contracting malaria at this location, this is most likely in the middle of the night, when the majority of infected mosquitoes were captured and when people are inside their dwellings, though the conditions of the dwellings do not offer much protection since they are built with holes between the wooden slats and often have unscreened windows.
In general, although our study has some limitations, these entomological data may be used for planning and implementing vector control measures that are aligned with the malaria transmission dynamics of each of the settlements. Considering that the few strategies of control were implemented without any information about the local mosquito vector, the information presented here could aid in implementing a more effective control strategy.