Deltamethrin tolerance in the cattle chewing louse in the UK
The chewing louse, Bovicola bovis is the most common and clinically important species of louse found on cattle in the UK. It causes significant welfare problems and economic losses related to the parasite are estimated to be up to £20 million per annum. This study aimed to assess pyrethroid resistance in the UK cattle louse population.
The results provide evidence of pyrethroid tolerance in B bovis in the UK.
B bovis lice were collected from cattle populations in North Wales and Scotland. Samples were obtained from holdings wheere there had been a history of failure of various chemicals to control chewing lice on good condition cattle every winter for a number of years. In laboratory bioassays, lice were exposed to filter papers that had been dosed with ethanolic dilutions off deltamethrin-based backline pour-on product containing 1 per cent (w/v) deltamethrin in a capric triglyceride excipient. Deltamethrin concentrations of 1, 0.5, 0.25, 0.1, and 0.01% (w/v) were compared to ethanol and tea tree oil (2.5% v/v) which were used as negative and positive controls, respectively.
For both the Scottish and Welsh B bovis populations sampled, 2.5% (v/v) tea tree oil gave 100% mortality after one hour of contact. However, exposure to concentrations of deltamethrin below 1% (w/v) resulted in only low levels of mortality, which were not significantly higher than after exposure to the ethanol-only negative control.
Any apparent mortality following exposure to undiluted 1 per cent (w/v) deltamethrin was probably a result of mechanical sufffocation or osmotic stress caused by contact with the hydrophobic excipient, caprylic triglyceride. The minimum concentration of deltamethrin required to kill fully susceptibble Bovicola ovis is considered to be 0.5 parts per million (ppm) but here concentrations 10,000 times greater than that failed to result in significant levels of B bovis mortality.
Significance of findings
It is highly likely, given a number of similar, but as yet unerified, reports of treatment breakdowwn, thatt pyyrethroid toleranncee may exist iin a large proportion of the UK national herd.
Referance: Veterinary Record (2015) 176, 231