Species Composition Distribution and Ecology of Anopheles Mosquitoes in Relation to Malaria Transmission and Control in Dembiya District Northwestern Ethiopia
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Date
2022-05-04
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Addis Ababa University
Abstract
Malaria is an important vector borne disease transmitted by the infective bite of female Anopheles mosquitoes in malaria endemic areas in Ethiopia. Malaria vector control requires field and clinical data on malaria transmission and ecology of local vectors. A six-year retrospective malaria data set from health facilities was analyzed to determine trends in malaria prevalence in the two malaria-prone areas of Dembiya District, Northwestern Ethiopia. A cross-sectional parasitological study was conducted to determine the prevalence of malaria during the peak transmission season in the two Kebeles of Dembiya district.
A longitudinal entomological study on the species composition and ecology of adult and immature Anopheles mosquitoes was conducted from June 2018 to May 2019. Larvae and pupae of Anopheles mosquitoeswere collected from different mosquito breeding habitats using a 350 ml standard dipper, and physicochemical characteristics of the larval breeding habitats were measured in conjunction with larval sampling. CDC light traps, pyrethrum spray catches (PSCs) and artificial pit shelters were used to collect host seeking and resting Anopheles mosquitoes from indoors and outdoors. Using morphological keys, collected Anopheles mosquitoes were identified to the species level and An. gambiae s.l (sensu lato) were further identified to sibling species using polymerase chain reaction (PCR). Enzyme-linked immunosorbent assay (ELISA) was used to examine the blood meal source of blood fed Anopheles mosquitoes, and to detect
Plasmodium species using circum-sporozoite proteins (CSP). A WHO test tube bioassay was used to assess the susceptibility status of Anopheles arabiensis to four insecticides such as pyrethroids, deltamethrin, bendiocarb and fenitrothion.
Malaria is endemic in the area according to retrospective malaria data from health facilities. Over the past six-years, the overall prevalence of malaria cases was 22.4% (484/2157). Plasmodium falciparum was responsible for 75.1 % (367/484) of the malaria cases in the study area, while P. vivax was responsible for 18.2% (88/484) the malaria cases. The remaining 5.9% (29/484) were mixed infections. Malaria parasites were found in 3.5% (26) of 735 blood smears stained with 3% geimsa and microscopically examined slides. Plasmodium falciparum and P. vivax were responsible for 65% (17) and 19% (5/26) of the malaria infection, respectively, with 15% (4/26) being mixed infections. Males (18/382; 4.7%) were 2.6 times more likely to be infected with malaria than females (8/353; 2.3%) (AOR = 2.6; 95% CI: 1.041- 6.412; p= 0.041).
A total of 1,629 Anopheles larvae and 185 pupae were collected from different breeding habitats. Eight species of Anopheles mosquitoes were identified from female mosquitoes that emerged from field collected larvae and pupae, including Anopheles arabiensis, An. pharoensis, An. coustani, An. christyi, An. squamosus, An. demeilloni, An. danicalicus and An. cinereus. Anopheles arabiensis (59.2%) was the most common followed by An. pharoensis (35.3%).
Anopheles mosquitoes belonging to 11 species were identified from 2,055 field collected adult specimens during this study: An. pharoensis, An. arabiensis, An. coustani, An. demeilloni, An. cinereus, An. funestus, An. ardensis, and An. squamosus were identified from both Guramba Bata and Arebiya study sites, whereas An. garnhami, An. christyi and
An. nili were identified only from Guramba Bata. Anopheles pharoensis was the dominant species identified in both Arebiya and Guramba Bata study sites, accounting for 46.4% (953/2,055), while An. Arabiensis was also relatively dominant in both study sites (38.3%; 776/2055).
Anopheles larvae were more abundant in drainage canals (14.7 ± 3.5 larvae/dip) than in other types of breeding habitats such as river side pools (2.0 ± 0.9), hoof prints (3.0 ± 1.2), swamps (3.8 ± 1.2), and puddles (2.7 ± 2.7) (F8,99 = 9.85; p<0.001). The presence or absence of Anopheles larvae was associated with physical characteristics of larval breeding habitats such as turbidity (mid turbid) (AOR = 66.03; 95% CI: 2.01-2168.24, p= 0.019) and presence of grass (AOR= 12.62; 95% CI: 1.29-122.78, p= 0.029).
The mean outdoor density of Anopheles mosquitoes collected with a CDC light trap (4.8 ± 1.8 mosquitoes/trap/night) was slightly higher than mean indoor density of Anopheles mosquitoes (4.3 ± 1.7 mosquitoes/trap/night) in Arebiya study site. Similarly, in Guramba Bata, the mean density of outdoor Anopheles mosquito collected with CDC light trap (8.1 ± 2.6 mosquitoes/trap/night) was higher than indoor Anopheles mosquito density (5.5 ± 1.7 mosquitoes/trap/night). The human blood indices (HBI) of indoor and outdoor hosts seeking An. arabiensis were 17.4% and 15.3%, respectively. The overall sporozoite rate of An. arabiensis, An. pharoensis and An. coustani was 0.3%, 0.9% and 5.9%, respectively. Whereas, the annual Entomological inoculation rate (EIR) of outdoor hosts seeking An. arabiensiswas 4.7 infective bites/person/year. Anopheles arabiensis was resistant to deltamethrin and permethrin.
In conclusion, the dominant Anopheles vector species in the study area were An. arabiensis and An. pharoensis.The season, and type and physicochemical characteristics
of the breeding habitats, influenced the distribution of Anopheles mosquitoes. Anopheles arabiensis, An. pharoensis and An. coustani showed relatively strong exophilic, exophagic and zoophilic tendencies in the study area, which was likely influenced by decades of indoor malaria interventions with IRS and ITNs. Anopheles arabiensis has developed resistance to pyrethroids and deltamethrin that have been used over the years. As a result, malaria control and elimination programmes should target outdoor biting and resting Anopheles mosquitoes with appropriate resistance management measures.
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Keywords
Malaria, Larvae, Behaviour, Susceptibility, Plasmodium Vivax, Plasmodium Falciparum, Anopheles Pharoensis, Dembiya, Blood Meal Index, Entomological Inoculation Rate, Anopheles Arabiensis