The efficacy of indoor residual spraying and insecticide susceptibility status of the Anopheles mosquito population from selected malaria endemic regions of Northern Namibia
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Date
2021
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University of Namibia
Abstract
Indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs) are the
primary malaria vector control interventions recommended by the World Health
Organisation (WHO). These interventions have been used in northern Namibia as the
main intervention for malaria vector control since the 1965 (IRS) and 2005 (LLINs).
However, insecticide resistance (IR) in Anopheles mosquitoes to pyrethroids, a major
insecticide class approved for use in both LLINs and IRS of modem structures, may
threaten continued use of these interventions. This study aimed to evaluate the
susceptibility status and composition of Anopheles mosquitoes and the efficacy of
indoor residual spraying insecticides in northern regions. Data was collected at one
point in time and not throughout the year. WHO bioassay tubes were used to
investigate the susceptibility status of Anopheles mosquitoes from Otjozondjupa,
Omusati, Oshana, Ohangwena, Kavango West, Kunene and Oshikoto region to DDT
(
4 %) and Deltamethrin (0.05 %) insecticide. The Anopheles mosquitoes were then
morphologically identified; as well as molecularly identified to a species level using
the Polymerase Chain Reaction (PCR) method. Residual efficacy of insecticides DDT,
Deltamethrin, pirimiphosmethyl (Actellic 300CS), and clothianidin (SumiShield
50
WG) sprayed on zinc, modem and traditional structures in Oshana region was also
tested with susceptible Anopheles arabiensis, using WHO cone bioassays. Findings
confirmed IR against Deltamethrin in Oshikoto and Kavango West region (78.75 %
and 90 % respectively) according to WHO protocols, with other malaria regions
showing reduced susceptibility. There is reduced susceptibility to DDT in Kavango
West region (93.75 %), while all other regions showed full susceptibility (100 %).
PCR results showed an overall predominance of Anopheles gambiae sensu stricto
(982) over Anopheles arabiensis (600) in northern Namibia; Otjozondjupa (214, 13 7),
Omusati (183, 123), Oshana (196, 48), Ohangwena (163, 167), Kavango West (41 ,
54), Kunene (113, 0), Oshikoto (72, 71), respectively. Five months after initial
application, SumiShield 50WG induced 100% mortality of mosquitoes exposed on
both modern and zinc structures, Deltamethrin killed more than 80 % of mosquitoes
on these structures, although Actellic 300CS and DDT were below the threshold
recommended. There is a prevalence of pyrethroid resistance in Namibian malaria
vectors in Kavango West and Oshikoto regions, which might hinder vector
management. However, SumiShield SOWG an environmentally acceptable insecticide
with a it ren t 111 oJc of action to pyrethroids may be a suitable alternative insecticide
for IRS in Nami bia, where resistance to Deltamethrin and DDT insecticide has started
to develop. SumiShield 50WG is a novel neonicotinoid insecticide that acts as an
agonist of the nicotinic acetylcholine receptor (nAChR). This receptor differs from
those of the existing recommended insecticide families (pyrethroids,
organophosphates, carbamates and organochlorine). Although, continued IR
surveillance is recommended in more than one sentinel site of the malaria endemic
regions of Namibia, monitoring should start earlier to avoid incomplete tests due to
insufficient sample size. Additionally, it is recommended that the residual efficacy
study should be repeated with a sufficient number of structures and sufficient mosquito
sample size
Description
A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science (Medical Entomology)
Keywords
Anopheles, Indoor residual spraying, Pyrethroids, University of Namibia, Namibia, Malaria endemic regions