Culicoides, often referred to as midges, are tiny blood-sucking flies that play a huge role in the spread of an often deadly equine virus: African horse sickness (AHS). Although the disease is currently restricted to Africa, Culicoides are prevalent worldwide, meaning there’s a real potential for AHS to spread to countries currently void of it.

Concerns about potential AHS spread recently led the United Kingdom’s Department for Environment, Food, and Rural Affairs (DEFRA) to issue recommendations about steps to take in the event of an outbreak in Great Britain. One of those suggestions is to apply deltamethrin, a permethrin insecticide, directly on a horse to control the disease spread, said Matthew Baylis, chair of veterinary epidemiology and head of the Liverpool University Climate and Infectious Diseases of Animals group at the University of Liverpool.

However, there was no data available regarding the insecticide’s efficacy in horses. So Baylis and colleagues took a closer look. Unfortunately, their results didn’t support DEFRA’s recommendations.

“Our evidence is that this is unlikely to directly benefit the horses, as midges will still feed on them and so potentially spread AHS,” he said. “There may be an effect on the population of midges but, we do not know this for sure