Use of World Health Organization susceptibility test for malaria vector control in South-Eastern Nigeria: A discourse

Joel D Akilah; Ibanga Eyo Ekong

doi:10.4103/ed.ed_4_19

PERSPECTIVE

Year : 2019 | Volume : 4 | Issue : 2 | Page : 29-32

Use of World Health Organization susceptibility test for malaria vector control in South-Eastern Nigeria: A discourse

Joel D Akilah¹, Ibanga Eyo Ekong²
¹ Department of Public Health, Federal Ministry of Health, Abuja, Nigeria
² Department of Community Health, University of Uyo, Uyo, Akwa Ibom, Nigeria

Date of Submission	14-Jan-2019
Date of Acceptance	25-Mar-2019
Date of Web Publication	17-Jun-2019

Correspondence Address:
Dr Ibanga Eyo Ekong
Department of Community Health, University of Uyo, P.M.B. 1017, Uyo, Akwa Ibom
Nigeria

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/ed.ed_4_19

Abstract

Background: With growing evidence of mosquito vector resistance to pyrethroids and other classes of insecticides globally, we employed one of the two important tools for the monitoring of insecticide resistance in selected states in southeastern part of Nigeria via the World Health Organization (WHO) susceptibility test.
Aim: This study tested the effectiveness of WHO susceptibility test for the use in identifying and monitoring the resistance of Anopheles and Aedes mosquitoes which transmit malaria and arbovirus-related diseases in Nigeria.
Methods: Larval sampling of mosquitoes was embarked upon with ovitraps set in various study sites; eggs were hatched and reared and subsequently subjected to susceptibility tests according to the WHO test procedures.
Summary and Conclusion: Resistance to dichloro diphenyl trichloroethane and the pyrethroids has been established to a large extent. The loss of efficacy of pyrethroids may result in operational failure of disease control programs.

Keywords: Mosquitoes, pyrethroids, susceptibility test

How to cite this article:
Akilah JD, Ekong IE. Use of World Health Organization susceptibility test for malaria vector control in South-Eastern Nigeria: A discourse. Environ Dis 2019;4:29-32

How to cite this URL:
Akilah JD, Ekong IE. Use of World Health Organization susceptibility test for malaria vector control in South-Eastern Nigeria: A discourse. Environ Dis [serial online] 2019 [cited 2023 Mar 30];4:29-32. Available from: http://www.environmentmed.org/text.asp?2019/4/2/29/260516

Introduction

Malaria vector control is a major pillar in the elimination of this disease globally. The disease is prevalent in Nigeria which accounts for 25% of the 219 million cases global morbidity and 19% of the 435,000 global mortality.^[1] In almost all parts of the country, favorable environmental conditions and efficient malaria vectors are present for a sustained transmission of the parasite. Comparing achievements over the years indicates that integrated vector control contributes substantially to the reduction of the global burden of malaria.^[2]

Vector control is usually implemented using insecticides of various classes. These are done through the use of long-lasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS). By doing so, the vectors of malaria, Anopheles spp., have over the years developed varying degrees of resistance to some classes of insecticides, especially the pyrethroids, which is the dominant insecticide used in LLINs and IRS in Nigeria.^[3] Resistance to other classes such as the dichloro diphenyl trichloroethane (DDT) has long been established; interestingly, it has been reported that mosquitoes resistant to DDT are also resistant to pyrethroids, hence the discontinuation of its use for malaria control purposes.^[4] Addressing the burning issue of resistance would result in efficiencies in terms of quality and program costs.

Two important tools for the monitoring of insecticide resistance are the World Health Organization (WHO) susceptibility test and United States' Centers for Disease Control and Prevention bottle bioassay. This review is concentrating on the WHO susceptibility test to see its effectiveness for use in identifying and monitoring the resistance of Anopheles and Aedes mosquitoes which transmit malaria and arbovirus-related diseases in Nigeria.

Context

The National Arbo-Virus Research Center (NAVRC) was host to this resistance monitoring study, which involved various parts of the South-East geopolitical zone in 2016. Eleven WHO test papers were received from the National Malaria Elimination Programme, and states in the region were sampled for malaria and/or arbovirus vectors. Immature stages of the vectors were collected and reared to adults at the Center's insectary.

Larval sampling for Anopheles species

To ensure that populations of all possible vectors breeding in the study area were collected, extensive larval sampling was embarked upon. Puddles, rivers, streams, rice fields and other farms, containers, excavations, tire tracks, hoof prints and crab holes, among others, were sampled. Adequate use of dippers, ladles, siphons and pipettes was made to ensure that each site in the study area was combed in the course of prospecting for larvae and pupae.

All the larvae and pupae were put in well-labeled and adequate containers and transported to the insectary of NAVRC. They were then reared to adults, for the susceptibility tests.

Ovitrap setting for Aedes species

Ovitraps were set in various study sites where the study was conducted. The traps targeted mosquitoes of the Aedes species. They were set in shaded areas and left for 48 h before they were retrieved. On retrieval, the eggs were dried under room temperature and preserved in air-tight containers.

Soaking/hatching of Aedes eggs

At the insectary, the eggs were introduced into bowls containing nonchlorinated water. The bowls and their contents were then covered with an untreated net to avoid “intruders” introducing foreign eggs. Hatching commenced in a few hours.

Rearing of larvae and handling of pupae

The eggs were hatched in a designated compartment of the insectary and reared in separate bowls. The larvae were fed with Quaker oats and animal feed. This was done in the normal 12 h light and dark periods of the day. Rearing water was changed when necessary and feed was replaced, until larvae pupated.

The pupae were introduced into small “pupal cups” by means of disposable pipettes immediately when they were seen. The cups and their contents were then transferred into rearing cages in readiness for adult emergence.

Rearing of adults

As soon as the adults emerged, cotton wool soaked in 10% sugar solution, placed in a Petri dish ^{More Details}, was introduced into each cage. The mosquitoes were maintained with the sugar solution which served as their food. This was changed every other day until the mosquitoes were old enough and sufficient to be utilized for the test.

World Health Organization susceptibility test

To determine the resistance status of the mosquitoes, larvae were collected from the study area and reared to adults at the NAVRC insectary. Nonblood-fed females that were 3–5 days postemergence were subjected to susceptibility tests according to the WHO test procedures.^[5] Adults from all the sites in the study area were tested against with all the four classes of WHOPES-approved insecticides as shown in [Table 1].{Table 1}

Summary of Findings and Discussion

Species identification

Only Anopheles gambiae complex, which transmits the malaria parasite, was collected from larval sampling done in the 5 southeastern states. This could be attributable to the fact that most of the breeding sites where the larvae were collected from were puddles, ditches, and guttars – known breeding sites of the complex. Some studies agree with this assertion.^[6],[7],[8] However, a few other studies claim otherwise that breeding sites also include swamps, vegetations, canals, and other farm sites.^[9],[10]

On the other part, Aedes aegypti, Aedes albopictus, and Aedes simpsoni complex were the three species harvested from ovitrapping in three (Anambra, Enugu, and Abia States) of the five states. Studies have shown these species to be present in the southeastern zone of Nigeria.^[11],[12] They are responsible for transmitting parasites that cause malaria and lymphatic filariasis.

A. gambiaes. l populations from the five southeast states were susceptible to only insecticide in the carbamate group, bendiocarb. They were all resistant to the pyrethroids (deltamethrin), organophosphates (pirimiphos-methyl) and organochlorines (DDT). Resistance studies on A. gambiae s. l conducted in this zone and contiguous zones reveal a similar pattern.^[13],[14]

On the part of Aedes mosquitoes, there was clear resistance to the organochlorine (DDT) family of insecticides. Both A. aegypti and A. albopictus were susceptible to the three other classes of insecticides in Anambra and Enugu states. A. aegypti showed suspected resistance to the pyrethroid, deltamethrin, in Abia state. This has been demonstrated by Self and Pant in a Nigerian study.^[15] However, resistance studies conducted in some African countries (Senegal and Nigeria) for Aedes spp. reveal susceptibility to permethrin.^[16],[17] Worthy of mention is that only few studies have studied the susceptibility of the Aedes spp. to permethrin and DDT.^[18],[19]

Emerging issues: New members of Anopheles Gambiae spp.

Dominance of Anopheles coluzzii, a relatively new member of the Anopheles Gambiae spp., has been observed in Ebonyi State, and is worth further investigation. A. coluzzii has been reported to share the same resources such as vertebrate hosts or freshwater habitats with A. Gambiae s. s.^{[20],[21],[22]} Resistance has also been reported in A. coluzzii.^[23]

Perceiving the fact that lymphatic filariasis and malaria are transmitted by the same vectors in Nigeria, there needs to be a concerted effort to jointly control the two diseases by aligning resources, goals, and strategies. This has been agreed to by some studies.^[24],[25]

Conclusion

This study has given a broad view on the susceptibility of A. gambiae s. l and Aedes species in the southeastern zone of Nigeria. Generally, resistance to DDT and the pyrethroids has been established, to a large extent. The loss of efficacy of pyrethroids may result in the operational failure of disease control programs, hence a need for synergy.

Study limitation

Paucity of funds affected the number of study sites covered; an increased number of sites would have provided a broader picture on the mosquito vector resistance to insecticides.

Acknowledgments

We acknowledge the NAVRC, Enugu, and the National Malaria Elimination Programme for the support provided.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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