Although synthetic pyrethroids are 80-90% effective in their control of ticks there was some anecdotal evidence of resistance developing in some flocks, says Peter Bates of the Veterinary Laboratories Agency, Weybridge.

“Tick problems are now far more widespread in UK sheep flocks.

The tick selects a site where capillaries are close to the surface of the sheep’s skin and although there’s no pain associated with the bite it does open up a lesion allowing infection to enter through the skin.”

But he added that studies have shown an SP pour-on used for three years in the same flock may not reduce the incidence of tick-borne fever or tick pyaemia.