Insecticide treated nets (ITNs) can offer valuable protection to horses from Culicoides biting midges, which can transmit the devastating African horse sickness virus (AHSV), a new study by scientists at The Pirbright Institute has shown.
The research, recently published in Parasites & Vectors, is the first of its kind to investigate the mortality rate of Culicoides when exposed to insecticide treated nets (ITNs) using standardised World Health Organization (WHO) cone bioassay trials, and the first to assess how well ITNs can protect horses from Culicoides, using field experiments in the UK.
Researchers set out to understand the effectiveness of ITNs, following concerns about the threat to the UK from AHSV, which can cause up to 90% mortality in susceptible horses, and the limited information on the effectiveness of commercially available insecticides against Culicoides.
Although AHSV is currently restricted to sub-Saharan Africa, it has a history of emergence into southern Europe, and the cost of a major European outbreak to the UK equine industry, has been estimated at up to £3.5 billion. Hypersensitivity reactions to the bites of Culicoides are also the primary cause of equine summer seasonal recurrent allergic dermatitis in the UK, commonly known as ‘sweet-itch’, which affects up to 12% of horses.
Seven commercially available pyrethroid insecticides and three repellent compounds commonly used by horse owners to reduce biting rates (all licensed for amateur use), were tested in standardised WHO cone bioassay trials. This revealed a significant variation in their effectiveness at both causing mortality in exposed Culicoides and reducing bloodfeeding post-exposure in surviving Culicoides.
The most effective insecticide identified during the WHO cone bioassay trials1 was selected for use in field trials. The treated and untreated mesh nets were found to significantly reduce the number of Culicoides able to access the test frames, with the treated nets providing 100% protection, as well as significantly reducing the number of Culicoides able to access stables whose entrance had been covered with mesh.
The study also examined the impact of the repellent compounds on Culicoides survival when it was applied to mesh netting. Although the three products tested had little impact on Culicoides mortality as expected, one product2 did reduce the bloodfeeding response of exposed Culicoides for up to 14 days after application.
Dr Lara Harrup, a member of the research team based at The Pirbright Institute, said: ‘This study means that for the first time we have evidence that screening horses during transportation and in stables with ITNs would offer significant protection in the event of an outbreak of AHSV in the UK; particularly if combined with other control measures such as topical repellents and insecticide treatment in stables.
‘Vaccines for AHSV are available; however none are currently licensed for use within the UK. In their absence, this study provides strong evidence that the use of ITNs, can have a significant impact on reducing contact between Culicoides and horses, and provide a substantial degree of protection against the transmission of AHSV’, she explained.
Dr Simon Carpenter, Head of Entomology at the Pirbright Institute, and a member of the research team, said, ‘Another important outcome of this study is that it has highlighted significant variations in the effectiveness of many of the insecticides licensed for amateur use, and further research would help improve these products. The ability of the repellent compounds to reduce the bloodfeeding response of exposed Culicoides also warrants further investigation’.
ENDS
For more information please contact:
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Notes
Baker T, Carpenter S, Gubbins S, Newton R, Lo Iacono G, Wood J, and Harrup LE. 2015. Can insecticide-treated netting provide protection for Equids from Culicoides biting midges in the United Kingdom? Parasite & Vectors 8, 604 DOI 610.1186/s13071-13015-11182-x.
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