Insecticidal Decay Effects of Long-Lasting Insecticide Nets and Indoor Residual Spraying on Anopheles Gambiae and Anopheles Arabiensis in Western Kenya
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Date
2015
Authors
Wanjala, Christine L.
Zhou, Guofa
Mbugi, Jernard
Simbauni, Jemimah A.
Afrane, Yaw A.
Ototo, Ednah
Gesuge, Maxwell
Atieli, Harrysone
Githeko, Andrew K.
Yan, Guiyun
Journal Title
Journal ISSN
Volume Title
Publisher
BioMed Central
Abstract
Background: Indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs) are the first-line tools for
malaria prevention and control in Africa. Vector resistance to insecticides has been extensively studied, however
the insecticidal effects of the nets and sprayed walls on pyrethroid resistant mosquitoes has not been studied
thoroughly. We evaluated the bioefficacy of LLINs of different ages and lambda-cyhalothrin (ICON 10cs) on the
sprayed mud walls for a period of time on malaria vector survivorship.
Methods: WHO tube bioassay was performed using diagnostic doses of lambda-cyhalothrin (0.05 %), permethrin
(0.75 %) and deltamethrin (0.05 %). Cone bioassays were conducted on netting materials from 0 to 3 years
old long-lasting insecticide-impregnated nets. Wall bioassays were performed monthly on mud slabs sprayed
with lambdacyhalothrin over a period of seven months. All bioassays used An. gambiae mosquitoes collected
from the field and the laboratory susceptible reference Kisumu strain. Concentration of the insecticides on the
netting materials was examined using the gas chromatography method. Mosquitoes were identified to species level
using PCR and genotyped for the kdr gene mutation frequencies.
Results: WHO bioassays results showed that populations from five sites were highly resistant to the pyrethroids
(mortalities ranged from 52.5 to 75.3 %), and two sites were moderately resistant to these insecticides (80.4 – 87.2 %).
Homozygote kdr mutations of L1014S ranged from 73 to 88 % in An. gambiae s.s. dominant populations whereas
L1014S mutation frequencies were relatively low (7–31 %) in An. arabiensis dominant populations. There was a
significant decrease (P < 0.05) in mosquito mortality with time after the spray with both lambda-cyhalothrin
(75 % mortality after six months) and with the age of LLINs (60 % mortality after 24 month). Field collected
mosquitoes were able to survive exposure to both IRS and LLINs even with newly sprayed walls (86.6–93.5 %
mortality) and new LLINs (77.5–85.0 % mortality), Wild mosquitoes collected from the field had significantly
lower mortality rates to LLINs (59.6–85.0 %) than laboratory reared susceptible strain (100 %). Insecticide
concentration decreased significantly from 0.14 μg/ml in the new nets to 0.077 μg/ml in nets older than
18 months (P < 0.05).
Conclusion: This study confirms that insecticide decay and developing levels of resistance have a negative
contribution to reduced efficacy of ITN and IRS in western Kenya. These factors contribute to decreased efficacy of
pyrethroid insectides in ongoing malaria control programs. In order to mitigate against the impact of insecticide
resistance and decay it is important to follow the WHO policy to provide the residents with new LLINs every three
years of use while maintaining a high level of LLINs coverage and usage. There is also need for urgent development
and deployment of non-pyrethroid based vector control tools.
Description
An Article Published in Parasites & Vectors
Keywords
Malaria, Insecticidal decay, Insecticide resistance, Indoor residual spraying, Long-lasting insecticide nets