Dengue 1 outbreak in Rosso, northern Senegal, October 2021: entomologic investigations.

Senegal has experienced periodic epidemics of dengue in urban areas with increased incidence in recent years. However, few data are available on the local ecology of the epidemic vectors. In October 2021, a dengue outbreak was reported in northern Senegal to the Institute Pasteur de Dakar. Entomologic investigations then were undertaken to identify the areas at risk of transmission and to identify the vector(s). Adult mosquitoes were collected indoors and outdoors at selected households, while containers with water were inspected for mosquito larvae. All the Aedes aegypti (L.) collected were tested for dengue virus NS1 protein using a rapid diagnostic test (RDT), and positive samples were confirmed by real-time RT-PCR. The qRT-PCR positive samples were subjected to whole genome sequencing using Nanopore technology. The majority of the larvae-positive containers (83.1%) were used for water storage. The Breteau and Container indices exceeded the WHO-recommended thresholds for the risk of dengue virus transmission except at 2 localities. Ae. aegypti, the only reputed dengue vector, was collected resting indoors as well as outdoors and biting during the day and night. The NS1 protein was detected in 22 mosquito pools, including one pool of females emerging from field-collected larvae. All NS1-positive results were confirmed by RT-PCR. Virus serotyping showed that the outbreak was caused by DENV-1. This study demonstrates the need for continuous control of adult and aquatic stages of Ae. aegypti to prevent future dengue epidemics in Senegal. RDTs appear to be a promising tool for dengue diagnostics and surveillance.

Differential insecticide resistance in Anopheles arabiensis populations in the seaside area of Mbour and its suburbs in Senegal.

Regular monitoring of insecticide resistance status is an important step in implementing appropriate and adapted insecticide-based strategies for vector control. In Senegal, Indoor Residual Spraying (IRS) and a national distribution campaign for long-lasting insecticide-treated net (LLIN) have been implemented since 2007 and 2009, respectively to prevent malaria transmission. To expand and ensure the sustainability of these strategies, we conducted a study on the status of insecticide resistance in malaria vectors in the seaside area of Mbour and its suburbs where no data were previously available. Anopheles larvae were sampled from four study sites (two in both coastal and inland areas) and reared to adulthood in the insectarium. Non-blood-fed females aged 3-5 days were then tested for susceptibility to permethrin, deltamethrin, lambdacyhalothrin, bendiocarb and pirimiphos-methyl. PCR amplification was used to identify sibling species of the An. gambiae complex and genotyping for the presence of resistance knockdown (kdr) L1014S, L1014F and Ace-1 G119S. Anopheles arabiensis was the only species present in the area. At all four sites, mosquitoes were resistant to deltamethrin, permethrin, and lambdacyhalothrin, and exhibited varying degrees of resistance to bendiocarb and pirimiphos-methyl. Overall, high levels of leucine-serine/phenylalanine substitutions at position 1014 (L1014S/L1014F) were observed, with frequencies ranging from 76.4 to 85.2 % for L1014F, and from 43.2 to 66.7 % for L1014S, compared to 8.1 to 28.3 for the Ace-1 G119S mutation. These results indicate a high level of phenotypic and genotypic resistance to insecticides, which is alarming, as it could have a significant impact on the operational effectiveness of current vector control tools that rely on pyrethroids. However, in the case of bendiocarb and pirimiphos-methyl, while some level of tolerance was observed, their potential use requires regular monitoring to prevent operational failure, as their deployment could potentially lead to an increase in resistance to them.

Host feeding preferences of malaria vectors in an area of low malaria transmission.

Studying the behaviour and trophic preferences of mosquitoes is an important step in understanding the exposure of vertebrate hosts to vector-borne diseases. In the case of human malaria, transmission increases when mosquitoes feed more on humans than on other animals. Therefore, understanding the spatio-temporal dynamics of vectors and their feeding preferences is essential for improving vector control measures. In this study, we investigated the feeding behaviour of Anopheles mosquitoes at two sites in the Sudanian areas of Senegal where transmission is low following the implementation of vector control measures. Blood-fed mosquitoes were collected monthly from July to November 2022 by pyrethrum spray catches in sleeping rooms of almost all houses in Dielmo and Ndiop villages, and blood meals were identified as from human, bovine, ovine, equine and chicken by ELISA. Species from the An. gambiae complex were identified by PCR. The types and numbers of potential domestic animal hosts were recorded in each village. The Human Blood Index (HBI) and the Manly Selection Ratio (MSR) were calculated to determine whether hosts were selected in proportion to their abundance. Spatio-temporal variation in HBI was examined using the Moran's index. A total of 1251 endophilic Anopheles females were collected in 115 bedrooms, including 864 blood fed females of 6 species. An. arabiensis and An. funestus were predominant in Dielmo and Ndiop, respectively. Of the 864 blood meals tested, 853 gave a single host positive result mainly on bovine, equine, human, ovine and chicken in decreasing order in both villages. Overall, these hosts were not selected in proportion to their abundance. The human host was under-selected, highlighting a marked zoophily for the vectors. Over time and space, the HBI were low with no obvious trend, with higher and lower values observed in each of the five months at different points in each village. These results highlight the zoophilic and exophagic behaviour of malaria vectors. This behaviour is likely to be a consequence of the distribution and use of LLINs in both villages and may increase risk of residual outdoor transmission. This underlines the need to study the feeding host profile of outdoor resting populations and how domestic animals may influence malaria epidemiology in order to tailor effective malaria vector control strategies in the two villages.

Blood Feeding Patterns of Aedes aegypti Populations in Senegal.

Aedes aegypti plays an important role in the transmission of several arboviruses of medical importance. The availability of information on the blood-feeding preferences of mosquito vectors is a critical step in the understanding of the transmission of human pathogens and implementation of control strategies. In Senegal, no data currently exist on the feeding pattern of Ae. aegypti in urban areas. To fill this gap, Ae. aegypti blood-fed females were collected in five localities by aspiration and using BG Sentinel 2 traps. Collections were carried out monthly between July and November 2019 inside and outside human dwellings. The origin of the blood meal of Ae. aegypti females were identified by an ELISA technique. A total of 1,710 blood-engorged females were examined and showed that Ae. aegypti preferentially fed on human with 78.6% of the identified blood meals. The other blood meals were from animals including dog, cat, horse, cattle, sheep, and rat. This is the first report on the feeding behavior of Ae. aegypti in urban settings in West Africa. It demonstrated that this species is highly anthropophilic.

Resting Behavior of Blood-Fed Females and Host Feeding Preferences of Aedes aegypti (Diptera: Culicidae) Morphological Forms in Senegal.

Aedes aegypti (Linnaeus) is the main vector of most arboviruses in tropical and subtropical urban areas. In West Africa, particularly in Senegal, domestic and wild populations have been described. Both Ae. aegypti aegypti (Aaa) and Ae. aegypti formosus (Aaf) were found in progenies of Ae. aegypti families from several localities of Senegal. However, nothing is known about their resting and trophic behavior, which are key data for vector control. To fill this gap, blood-fed mosquitoes were collected monthly indoors and outdoors with BackPack aspirators and BG-Sentinel 2 traps between July and November 2019 from four urban sites. The enzyme-linked immunosorbent assay technique was used to analyze blood-fed Aaa and Aaf specimens. Both forms were found resting in all investigated places with the highest proportions found in scrap metals (51.7% for Aaa and 44.1% for Aaf) and used tires (19.2% for Aaa and 26.1% for Aaf). Blood-fed Aaf females showed lower occupation of the indoors environment compared to Aaa. Overall, the percentages of single bloodmeals from human were 80.5% (916/1138) for Aaa and 71.1% (263/370) for Aaf. A low frequency of other domestic hosts, including bovine, ovine, and cat were detected for both forms. This study provides the first data on resting and trophic behavior of Aaa and Aaf in Senegal. Both forms showed differences in their resting behavior but fed primarily on human and highlight the risk of arboviruses transmission in urban areas.