Anopheles stephensi: The emerging vector of malaria in the Republic of Djibouti, Horn of Africa | IJB 2023

Larval collections from A. stephensi breeding sites.

In this study we identified A. stephensi and A. gambiae SL. as the two major malaria vectors in the Republic of Djibouti. A. stephensi mosquitoes was present in all the 6 regions investigated suggesting that this species has become established in the country after its first detection in only Djibouti-ville in 2013. A. nili somalicus, A. dthali and A. azaniae were the secondary malaria vectors collected across the South-North transect. Insecticide resistance testing of the A. stephensi populations from Djibouti showed phenotypic resistance to organochlorine, pyrethroids, organophosphates and carbamates revealing the urgency to develop and implement a programme for monitoring and managing insecticide resistance in local vector populations with efficient control strategies at country level.

Author Name

Renaud Govoetchan*, Mohamed Mousse Ibrahim, Arthur Sovi, Houssein Mouhamed Omar, Abdillahi Omar Boulhan, and Houssein Youssouf Darar

Journal Name

International Journal of Biosciences | IJB |

Publisher Name

International Network For Natural Sciences | INNSpub

Abstract

The present study investigated mosquito species composition and phenotypic insecticide resistance profile to support decision-making in vector control in the Republic of Djibouti at the Horn of Africa. Adult mosquitoes were collected between December 2016 and December 2017 across 20 sentinel sites established in the 6 regions of the country using both Centers for Disease Control (CDC) miniature light traps and pyrethrum spray catches (PSC). Female mosquitoes were kept aside, for morphological identification to species by an expert entomologist using appropriate taxonomic keys by Gillies & Coetzee and Glick. Bioassays were also conducted in An. stephensi from Djibouti-ville against nine insecticides used in public health. A total number of 12,538 host-seeking mosquitoes belonging to four genera (Anopheles, Culex, Aedes, Uranotaenia) comprising 12 species were collected. Among these, A. gambiae S.L. and A. stephensi were the two major malaria vectors identified while secondary malaria vectors such as A. nili somalicus, A. dthali and A. azaniae were also collected. Culex quinquefasciatus was the most abundant mosquito species in the 6 regions. WHO susceptibility tests performed on A. stephensi population from Djibouti-ville showed resistance to pyrethroids, organophosphates, carbamates and DDT. The resistance intensity bioassays indicated low to moderate intensity of resistance with pyrethroid insecticides and the organophosphate pirimiphos methyl. Meanwhile pre-exposure to PBO suggested involvement of P450 detoxification enzymes in pyrethroid resistance. These findings revealed the urgent need to develop and implement a programme for monitoring and managing insecticide resistance in local vector populations with efficient control strategies in Djibouti.

Larval collections from sites

Introduction

Located in the Horn of Africa, the Republic of Djibouti shares borders with Eritrea, Ethiopia and Somalia and has just over 300 km of coastline along the Red Sea and Gulf of Aden (Hatem, 1996). With an area of 23,200km² consisting mainly of plateau, plains and highlands, the Republic of Djibouti has a hot and humid climate, with a temperature that varies between 30°C in January and 43°C in July with an average humidity of 69% (Schulman, 2019). Rainfall is rare (<130mm annually), however unusual rains may occur across the country causing heavy rainfall and flooding in residential areas of the capital Djibouti-ville (Schulman, 2019).

In the Republic of Djibouti, mosquito-borne diseases are primarily transmitted by malaria vectors and secondarily dengue fever vectors, followed by other diseases with suspected cases such as West Nile virus and Leishmaniasis (Faulde et al., 2012; Rodier, 1995). While the Horn of Africa is known to be highly susceptible to mosquito-borne diseases, the Republic of Djibouti was historically thought to be a malaria meso to hypo-endemic country with intermittent epidemics (Rodier, 1995). However local populations have experienced only low and unstable malaria transmission and intermittent epidemics (Fox et al., 1991; Fox et al., 1989) with most of cases detected in people returning from neighbouring countries (Khaireh et al., 2013; Crowell et al., 2012).

The first case of malaria was reported at the very beginning of the 20th century, but it was not common until 1984 that malaria became a public health problem after a significant number of imported malaria cases were recorded. Since then, malaria cases have continued to increase, most importantly in Djibouti-ville. From 1988, there was an increase in malaria cases throughout the country, even in the regions of Tadjourah and Obock that were not affected in the past. In order to address the progression of the disease, the country implemented its first vector control programs based initially on larval control (Louis, 1988; Carteron et al., 1979) strengthened later with the introduction of insecticide-treated bed nets.

Unfortunately, investments and control efforts did not commensurate with the risks of disease, and major malaria outbreaks were reported. In 1991, a record of 7,338 microscopically confirmed malaria cases declining to 4,770 cases in 1993 (Rodier, 1995). 

Contrastingly, an unusual increase of malaria cases was reported in 1999 in and around Djibouti-ville due to the emergence of chloroquine resistance (Rogier et al., 2011). The cases detected were mainly caused by Plasmodium falciparum but also P. vivax which was responsible of 3% of malaria burden (UNDP, 2013). The last uncommon urban outbreak of malaria was observed in 2013 with 1228 reported cases of which 83% were from Djibouti-ville alone (UNDP, 2013). 

The entomological surveillance in Republic of Djibouti was not a routine exercise and as such the monitoring programme at country level is very poor. Most of the existing data were generated as part of the international European Union Naval Force Somalia mission “Atalanta” (Faulde et al., 2012; Faulde & Ahmed, 2010) by the country's military partners that have troops based at various locations throughout the territory. The local malaria transmission was attributed to Anopheles arabiensis in all the 6 regions of the country (WHO, 2012). 

However, a larval survey has reported the presence of A. nili in Ali Sabieh, southern region of Djibouti. More recently, A. stephensi, the Asian malaria mosquito vector of Plasmodium falciparum and P. vivax was incriminated in the outbreak which occurred in 2013 (Faulde et al., 2014). 

The current study updates malaria entomological data mandatory for decision making on control strategies implementation. The data was generated over a recent entomological surveillance conducted between December 2016 and December 2017, in 20 sentinel sites located along the south-north transect of Djibouti Republic. Mosquito species compositions, as well as insecticide resistance status of the main malaria vector identified in the Djibouti-ville were assessed to support decision-making for the implementation of an efficient control program. Check out more Anopheles stephensi: The emerging vector of malaria in the Republic of Djibouti, Horn of Africa

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