Culicoides biting midges

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CHAPTER 1: GENERAL INTRODUCTION

Culicoides biting midges (Diptera: Ceratopogonidae) are of veterinary significance worldwide, primarily as vectors of arboviruses affecting domestic livestock (Meiswinkel et al., 2004; Mellor et al., 2000; Purse et al., 2015). Based on the numbers collected near livestock Culicoides imicola is considered the principal vector of AHSV in South Africa (Meiswinkel et al., 2004; Nevill et al., 1992). Recent outbreaks of a novel orthobunyavirus, Schmallenberg virus (Hoffmann et al., 2012), and the spread of bluetongue virus (BTV) in northern Europe has raised concern of the introduction and spread of other midge-borne viruses, particularly AHSV, and the need for optimised preventative strategies (Backer and Nodelijk, 2011; Carpenter et al., 2008a; Carpenter et al., 2009; MacLachlan and Guthrie, 2010; MacLachlan and Mayo, 2013; Papadopoulos et al., 2010; Robin et al., 2014).
Clear recommendations for pre-export quarantine and testing of horses for AHSV have been in place for many years, however recent amendments to the OIE – Terrestrial Animal Health Code chapter on “Infection with African horse sickness virus” have included recommendations that mesh of appropriate gauge, impregnated with an approved insecticide be placed over containers during transport of horses through regions not free of AHSV (Anon, 2013). During international air transit from South Africa horses may be at short-term risk of exposure to Culicoides midges during loading of the jet stalls into an aircraft. Furthermore, there is the possibility that horses in jet stalls may be exposed to AHSV-infected midges during transit through an AHSV-infected country (DEFRA, 2008). Whilst limited information is available on the effects of microclimate on horses during air transport in insect-proof, climate-controlled enclosed containers (Thornton, 2000), the effects of enclosing solid top HMA-type commercial jet stalls (as used for export of horses from South Africa) with treated mesh on microclimate, clinical variables and stress of horses are unknown.
Monitoring of temperature and relative humidity (RH) inside jet stalls provides an indication of ventilation (Thornton, 2000), while monitoring of clinical variables and faecal glucocorticoid metabolites (FGM) provides a non-invasive means of assessing prolonged stress associated with transport in horses (Schmidt et al., 2010a,b,c). Investigation of effective and safe physical and chemical methods of protection to mitigate the risk of exposure of equidae to AHSV vectors during transhipment, such as jet stalls enclosed with HDPE mesh treated with an insecticide with proven efficacy against Culicoides midges, is required. These measures could ideally also be applied to protect horses from exposure to AHSV during high risk periods in endemic areas in South Africa.

Culicoides biting midges

Culicoides midges (Diptera: Ceratopogonidae) (Kettle, 1995) are small insects, measuring 1-3 mm in size (Meiswinkel et al., 2004). Over 1400 species of Culicoides midges have been identified worldwide, with approximately 30 species implicated in transmission of over 50 arboviruses (Mellor et al., 2000; Purse et al., 2015; Wilson et al., 2009). Over 120 species of Culicoides midges are known to occur in South Africa (Meiswinkel, 1998). Midges are most abundant during the warm summer months and exhibit predominantly crepuscular or nocturnal behaviour (Kettle, 1995). Dependent on the species, females lay up to 450 eggs (Kettle, 1995) on a moist substrate in which the four larval stages feed and pupate (Meiswinkel et al., 2004; Mellor et al., 2000).
The complete life cycle takes three to four weeks under favourable climatic conditions, facilitating development of several generations in a single season. Only female Culicoides midges blood-feed, which facilitates egg maturation (Kettle, 1995; Meiswinkel et al., 2004). Nulliparous females have never developed eggs and do not have follicular relics, parous females have laid eggs and have follicular relics, gravid females contain maturing eggs. Age-grading differentiation of midges is of importance as no transovarial transmission of virus has been demonstrated (Osborne et al., 2015), and evaluation of parous or gravid female midges is considered sufficient when aiming to detect Orbiviruses. Nulliparous and parous female midges can be differentiated based on the presence of burgundy-red pigmentation inside the abdominal wall of parous females (Dyce, 1969).

TABLE OF CONTENTS :

• DECLARATION OF ORIGINALITY
• DEDICATION
• ACKNOWLEDGEMENTS
• ABBREVIATIONS
• LIST OF FIGURES
• LIST OF TABLES
• LIST OF APPENDICES
• SUMMARY
• CHAPTER 1: GENERAL INTRODUCTION
• CHAPTER 2: LITERATURE REVIEW
  • 2.1. Culicoides biting midges
  • 2.2. Culicoides control measures
    • 2.2.1. Pyrethrins and synthetic pyrethroid insecticides
    • 2.2.2. Insect repellents
    • 2.2.3. Netting materials or mesh
  • 2.3. African horse sickness
  • 2.4. Equine encephalosis
  • 2.5. African horse sickness and international export of horses from South Africa
  • 2.6. Air transportation of horses
  • 2.7. Faecal glucocorticoid metabolite concentration as a stress indicator in horses
• CHAPTER 3: FIELD AND IN VITRO INSECTICIDAL EFFICACY OF ALPHACYPERMETHRIN-TREATED HIGH DENSITY POLYETHYLENE MESH AGAINST CULICOIDES BITING MIDGES IN SOUTH AFRICA
  • 3.1. Summary
  • 3.2. Introduction
  • 3.3. Materials and Methods
    • 3.3.1. Light trap assay
    • 3.3.2. Contact bioassay
    • 3.3.3. Statistical analyses
  • 3.4. Results
    • 3.4.1. Light trap assay
    • 3.4.2. Contact bioassay
  • 3.5. Discussion
• CHAPTER 4: EFFICACY OF ALPHACYPERMETHRIN-TREATED HIGH DENSITY POLYETHYLENE MESH APPLIED TO JET STALLS HOUSING HORSES AGAINST CULICOIDES BITING MIDGES IN SOUTH AFRICA
  • 4.1. Summary
  • 4.2. Introduction
  • 4.3. Materials and Methods
    • 4.3.1. Study site and design
    • 4.3.2. Jet stall treatment
    • 4.3.3. Mechanical aspiration of midges
    • 4.3.4. Light trap collection of midges
    • 4.3.5. Culicoides midge identification
    • 4.3.6. Statistical analyses
  • 4.4. Results
    • 4.4.1. Mechanical aspiration of midges
    • 4.4.2. Light trap collection of midges
  • 4.5. Discussion
• CHAPTER 5: THE EFFECT OF ALPHACYPERMETHRIN-TREATED HIGH DENSITY POLYETHYLENE MESH PROTECTION ON JET STALL MICROCLIMATE, CLINICAL VARIABLES AND FAECAL GLUCOCORTICOID METABOLITES OF HORSES
  • 5.1. Summary
  • 5.2. Introduction
  • 5.3. Materials and Methods
    • 5.3.1. Animals
    • 5.3.2. Study design
    • 5.3.3. Jet stalls
    • 5.3.4. Jet stall treatment
    • 5.3.5. Clinical variables
    • 5.3.6. Climatic variables
    • 5.3.7. Faecal sample collection and analysis
    • 5.3.8. Statistical analyses
  • 5.4. Results
    • 5.4.1. Climatic variables
    • 5.4.2. Clinical variables
    • 5.4.3. FGM concentrations
  • 5.5. Discussion
• CHAPTER 6: GENERAL CONCLUSIONS
  • REFERENCES
  • APPENDICES

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