Abstract
BackgroundBluetongue 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.ResultsEffects 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.ConclusionsOur 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.
Highlights
Bluetongue virus (BTV) is transmitted by Culicoides biting midges and causes bluetongue (BT), a clinical disease observed primarily in sheep
A high rate of knockdown was found after 1 h exposure, even when using low concentrations of deltamethrin, producing a LC50 of 1.20*10-4% [8.94*10-5, 2.00*10-4] (0.043 mg/m2) and an LC90 of 3.85*10-4% [2.56*10-4, 5.00*10-4] (0.139 mg/m2)
The 24-h mortality rate at 0.001% deltamethrin was lower after 10 or 20 min exposure compared to 60 min (z = -6.976 and z = -5.785, df = 17, P < 0.0001), but at 0.005% deltamethrin, mortality rates between 20 and 60 min exposure did not differ, with 24-h mortality rates were both higher than that recorded in the 10 min exposure (z = -1.975, df = 17, P < 0.05 and z = -3.238, df = 17, P < 0.005 respectively; Fig. 3a)
Summary
Bluetongue virus (BTV) is transmitted by Culicoides biting midges and causes bluetongue (BT), a clinical disease observed primarily in sheep. While safe and efficacious single-serotype inactivated vaccines for BTV are available, limited crossprotection between serotypes and their cost inhibit use on a large scale, the recent development of an inactivated pentavalent vaccine may eventually increase uptake in poorer communities [5]. Where these vaccines are either unavailable or unaffordable, Culicoides control and animal movement restrictions are the only available responses to BT outbreaks [6, 7]. No quantitative data currently exist to assess the probable impact of such measures despite their widespread use and selection of appropriate measures is largely based on anecdotal experience and cost
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