Assessment of the Prevalence, Breeding Sites and Insecticide Resistance Among Mosquitoes of Medical Importance in Two Selected Cities in Sierra Leone Open Access

Abstract

The prevalence of arboviral diseases transmitted by Aedes aegypti mosquitoes in West Africa has increased in recent years, with outbreaks of Dengue, Yellow Fever, and Chikungunya reported throughout most of the region. However, little basic entomological information is known about the vector in the region, which limits the ability to prevent and control its spread and hence transmission of the associated viruses. Therefore, we performed an entomological survey of Ae. aegypti in Sierra Leone, where Ae. aegypti has not been studied in recent decades. Between June and August 2017, we conducted egg, immature, and adult sampling of Ae. aegypti in two locations in Sierra Leone (Bombali and Bo Districts). Eggs were collected using ovitraps to estimate densities and then used to rear adults in the insectary. Lab-reared females were tested for resistance to six different insecticides as well as for the prevalence of knock-down resistance (kdr) mutations. Household surveys for immatures were conducted to identify common breeding sites and calculate standard Aedes infestation indices. Wild-caught adult females were collected from houses using traps and aspirators and were later tested for arboviruses. The overall mean eggs per trap was 23.7 while 58 adult females were captured from 194 houses in total. Immature indices were high in general across both sites, ranging from 13-17 for the container index, 18-61 for the house index, and 65-153 for the Breteau index. Tires and bottles were the most common breeding sites, accounting for 44% and 20% (respectively) of all positive container types. Adults reared from eggs in both locations were 100% susceptible to deltamethrin and resistant to permethrin (53% mortality). Evidence of higher kdr allele frequencies among permethrin-resistant mosquitoes was found in Bombali. Testing of adult mosquitoes for flaviviruses and chikungunya was negative, however the sample size was small. Recommendations arising from these findings are to focus on tires and bottles for clean-up campaigns, continue arboviral surveillance in mosquitoes, and use deltamethrin rather than permethrin if chemical control measures begin for Ae. aegypti in either location.

Table of Contents

Distribution Agreement…………………………………………………………… i

Approval Sheet……………………………………………………………………. ii

Abstract Cover Page………………………………………………………………. iii

Abstract……………………………………………………………………………. iv

Acknowledgement………………………………………………………………… vii

Dedication…………………………………………………………………………  x

List of Figures…………………………………………………………………….   xi

List of Tables……………………………………………………………………… xiii

Chapter One: Introduction

1.1 Background and Importance…………………………………………………  1

1.2 Vector Surveillance………………………………………………………….   2

1.3 Issues of Insecticides Resistance…………………………………………….   2

1.4 Gaps in Surveillance and Monitoring of Resistance………………………...   4

1.5 Purpose ……………………………………………………………………...   4

1.6 Research Questions …………………………………………………………    5

1.6.1     Null Hypothesis…………………………………………………………  5

1.6.2     General Objective………………………………………………………   5

1.6.3     Specific Objectives……………………………………………………… 6

1.7 Definition of Terms……………………………………………………………  6

Chapter Two: LITERATURE RELEVANT TO THE STUDY

2.1 Introduction………………………………………………………………………. 8

2.2 Mosquitoes of Medical Importance………………………………………….      9

2.2.1     Life Cycle………………………………………………………………     9

2.2.2     Habitat and it’s Impact on Disease Transmission………………………     9

2.2.3     Aedes as a vector of Human Diseases………………………………….      10

2.2.4     Aedes species and Human Impact..……………………………………..     10

2.3 Global distribution of Aedes Aegypti and other Species of interest…………       11

2.4 Breeding Sites and Behavior of Aedes Aegypti and other species of interest…   13

2.5 Core Vector Control Methods.…………………………………………………   14

2.6 Insecticide Resistance in Aedes Aegypti and other species of interest…………  15

2.7 Managing Insecticide Resistance………………………………………………   16

2.8 Gaps in the Literature…………………………………………………………..   17

Chapter Three: Research Methodology

3.1 Preamble ………………………………………………………………………     18

3.2 Research Design………………………………………………………………      19

3.3 Sampling ………………………………………………………………………     19 

3.3.1     Aedes Aegypti Egg Collection…………………………………………..      20

3.3.2     Adult Mosquito collection using Prokopack (Back Pack Aspirators) and Biogent (BG) Sentinel Traps ... 21

3.3.3     Larval and Pupal Surveys……………………………………………….        22

3.3.4     Data Management and Analysis………………………………………...        24

3.4 Laboratory…………………………………………………………………….         24

3.4.1     Rearing …………………………………………………………………         24

3.4.2     Resistance Testing………………………………………………………         25

3.4.3     Knock Down Resistance (KDR) Testing……………………………….          26

3.4.4     Virus Testing……………………………………………………………          29

3.5 Ethical Considerations…………………………………………………………         32

3.6 Limitations and delimitations…………………………………………………          33

Chapter Four: Results

4.1 Introduction……………………………………………………………………         34

4.2 Aedes Aegypti Egg collection………………………………………………...           34

4.3 Adult Mosquito collection using Prokopack and BG sentinel traps…………            34

4.4 Larval and Pupal surveys………………………………………………………         35

4.5 Insecticides Resistance testing…………………………………………………         39

4.6 Knock Down Resistance testing……………………………………………….          40

4.7 Viral Testing……………………………………………………………………         48

Chapter Five: Discussion, Conclusion, and Public Health Implications

5.1 Discussion ………………………………………………………………………        50

5.1.1     Vector Prevalence…………………………………………………………        50

5.1.2     Breeding sites……………………………………………………………...        51

5.1.3     Insecticides Resistance…………………………………………………….        52

5.1.4     Virus Testing……………………………………………………………….       52

5.2 Limitations ……………………………………………………………………….      53

5.3 Public Health Implications ……………………………………………………….     54

5.4 Conclusions……………………………………………………………………….     55

References……………………………………………………………………………….     57

About this Master's Thesis

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School	Rollins School of Public Health
Department	Hubert Department of Global Health
Degree	M.P.H.
Submission	Master's Thesis
Language	English
Research Field	Biology, Entomology
Keyword	Resistance
Committee Chair / Thesis Advisor	McFarland, Deborah A., Emory University
Committee Members	Levine, Rebecca A., Evans, Dabney P., Emory University
Partnering Agencies	CDC

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