title: Insecticidal effects of deltamethrin in laboratory and field populations of Culicoides species: how effective are host-contact reduction methods in India?
abstract: Bluetongue virus (BTV) is transmitted by Culicoides biting midges and causes bluetongue (BT), a clinical disease observed primarily in sheep. BT has a detrimental effect on subsistence farmers in India, where hyperendemic outbreaks impact on smallholdings in the southern states of the country. In this study, we establish a reliable method for testing the toxic effects of deltamethrin on Culicoides and then compare deltamethrin with traditional control methods used by farmers in India.
  
  
  
  Effects of deltamethrin were initially tested using a colonised strain of Culicoides nubeculosus Meigen and a modified World Health Organisation exposure assay. This method was then applied to field populations of Culicoides spp. in India. The field population of C. oxystoma in India had a greater LC50(0.012 ± 0.009%) for deltamethrin than laboratory-reared C.nubeculosus (0.0013 ± 0.0002%). Exposure of C. nubeculosus to deltamethrin at higher ambient temperatures resulted in greater rates of knockdown but a lower mortality rate at 24 h post-exposure. Behavioural assays with C. nubeculosus in WHO tubes provided evidence for contact irritancy and spatial repellence caused by deltamethrin. The field experiments in India, however, provided no evidence for repellent or toxic effects of deltamethrin. Traditional methods such as the application of neem oil and burning of neem leaves also provided no protection.
  
  
  
  Our study demonstrates that field-collected Culicoides in India are less susceptible to deltamethrin exposure than laboratory-bred C. nubeculosus and traditional methods of insect control do not provide protection to sheep. These low levels of susceptibility to deltamethrin have not been recorded before in field populations of Culicoides and suggest resistance to synthetic pyrethrioids. Alternative insect control methods, in addition to vaccination, may be needed to protect Indian livestock from BTV transmission.
authors.affiliation: Department of Disease Control, Faculty of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, Keppel Street, London, WC1E 7HT, UK.
authors.forename: Rien
authors.initials: R
authors.lastname: De Keyser
authors.affiliation: Department of Disease Control, Faculty of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, Keppel Street, London, WC1E 7HT, UK.
authors.forename: Clare
authors.initials: C
authors.lastname: Cassidy
authors.affiliation: Vaccine Research Centre-Viral Vaccines, Centre for Animal Health Studies, Tamil Nadu Veterinary and Animal Sciences University, Madhavaram Milk Colony, Chennai, 600 051, India.
authors.forename: Swathi
authors.initials: S
authors.lastname: Laban
authors.affiliation: Vaccine Research Centre-Viral Vaccines, Centre for Animal Health Studies, Tamil Nadu Veterinary and Animal Sciences University, Madhavaram Milk Colony, Chennai, 600 051, India.
authors.forename: Prakash
authors.initials: P
authors.lastname: Gopal
authors.affiliation: Rothamsted Research, West Common, Harpenden, Hertfordshire, AL5 2JQ, UK.
authors.forename: John A
authors.initials: JA
authors.lastname: Pickett
authors.affiliation: Vaccine Research Centre-Viral Vaccines, Centre for Animal Health Studies, Tamil Nadu Veterinary and Animal Sciences University, Madhavaram Milk Colony, Chennai, 600 051, India.
authors.forename: Yarabolu K
authors.initials: YK
authors.lastname: Reddy
authors.affiliation: Department of Animal Biotechnology, College of Veterinary Science, Lala Lajpat Rai University of Veterinary and Animal Sciences, Hisar, 125004, Haryana, India.
authors.forename: Minakshi
authors.initials: M
authors.lastname: Prasad
authors.affiliation: Indian Council of Agricultural Research, New Delhi, 110 001, India.
authors.forename: Gaya
authors.initials: G
authors.lastname: Prasad
authors.affiliation: Postgraduate Research Institute in Animal Sciences, Kattupakkam, 603 203, India.
authors.forename: Sreekumar
authors.initials: S
authors.lastname: Chirukandoth
authors.affiliation: Veterinary University Training and Research Centre, Karur, 639 006, India.
authors.forename: Kandasamy
authors.initials: K
authors.lastname: Senthilven
authors.affiliation: Vector-borne viral diseases Programme, The Pirbright Institute, Ash Road, Woking, GU24 0NF, UK.
authors.forename: Simon
authors.initials: S
authors.lastname: Carpenter
authors.affiliation: Department of Disease Control, Faculty of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, Keppel Street, London, WC1E 7HT, UK. james.logan@lshtm.ac.uk.
authors.forename: James G
authors.initials: JG
authors.lastname: Logan
mesh_terms: D000383:Agriculture; D000818:Animals; D001819:Bluetongue / Q000517:prevention & control* / Q000635:transmission / Q000821:virology; D001820:Bluetongue virus / Q000302:isolation & purification; D002519:Ceratopogonidae; D004196:Disease Outbreaks / Q000662:veterinary; D007194:India; D007300:Insect Control / Q000379:methods; D007302:Insect Repellents; D007303:Insect Vectors; D007306:Insecticides; D009570:Nitriles / Q000633:toxicity; D011722:Pyrethrins / Q000633:toxicity; D012756:Sheep; D014944:World Health Organization
chemical_list: D007302:Insect Repellents; D007306:Insecticides; D009570:Nitriles; D011722:Pyrethrins; C017180:decamethrin
keywords: Arbovirus; BTV; Bluetongue virus; Ceratopogonidae; Ectoparasite; Insecticide; Vector-borne disease