11 April 1997


FIRST case of sheep scab mite resistance to organophosphate (OP) dips in the UK and Europe was confirmed last year.

In 1994, mites in Somerset and Caithness were found to be resistant to dip containing the synthetic pyrethroid (SP), flum-ethrin. Another two cases were reported a year later in Cumbria.

To date there are no reported cases of avermectin resistance in the UK or elsewhere in the world.

Widespread resistance by mites in Argentina to organophosphate dips containing diazinon has been evident since the 1970s. Earlier, in 1962, there were also reports of mite resistance to lindane.

The rapid spread was mainly due to poor sheep husbandry – one of seven factors which aid the development of insecticide resistance listed in the table.

"During the years of compulsory dipping there was no suspicion of organo chloride, OP or SP resistance in the UK," says Peter Bates, head of entomology at the Central Veterinary Laboratory. "In fact, there was little chance of resistance developing due to the overkill nature of compulsory dipping. This practice also included the supervised double dipping of confirmed scab-infested flocks, as well as strict government control of insecticides on the market. Organophosphate and SP resistant mites emerged after deregulation which removed the requirement for supervised dipping resulting in the potential for ineffective treatment.

The routine use of SP pour-ons for the control of blowfly and ticks could have induced resistance to SP dips or even augment existing SP tolerance. Mr Bates, therefore, warns that it is unwise to treat a flock with a SP dip if SP pour-ons are used routinely. "By the same token," says Mr Bates, "oral anthelmintic drenches containing ivermectin may also select for populations of scab mites less susceptible to ivermectin administered via subcutaneous injection."

The development of OP resistance is purely the result of poor dipping practice, says Mr Bates.

"If all dippings are carried out according to the manufacturers instructions, under-dosing would not occur. Sheep dipping is an exact science, not a haphazard operation," he warns.

"Insecticide-resistant strains of ectoparasite are not widespread in the UK. However, we can prevent their spread by reducing the use of insecticides and quarantining all incoming stock. If ectoparasites are suspected, have them professionally identified. Ensure that only a product licensed for the control of that parasite is administered and administered correctly."n

Seven factors aid development insecticide resistance

&#8226 Widespread use and reliance on insecticide. Only use if absolutely necessary. Avoid annual blanket treatments. All incoming stock should be quarantined and only treated if an ectoparasite has been diagnosed.

&#8226 Prolonged exposure to a single group of insecticides. Rotate groups of insecticides used on the flock. Do not use SP dip if SP pour-on is routinely used for tick or headfly control.

&#8226 High selection pressure. Where no refuge for the population exposed to the insecticide. Important in the case of permanent parasites such as sheep scab mite or lice.

&#8226 Licensing of veterinary insecticides. Prior to 1992 all dip requiring scab approval had to be 100% effective against sheep scab mite as well as residual in the fleece or skin for 21 days after dipping to protect against reinfestation. Also conferred 100% efficacy against blowfly and chewing lice. After deregulation of scab in 1992, EU stipulated only 95-98% efficacy required for licensing of ectoparasites and residual protection no longer needed. Leaves 2-5% of parasites not susceptible to treatment.

&#8226 Method of insecticide application. Not all insecticides or their methods of application effective against all ectoparasites. Professionally identify infesting parasite and administer correct, licensed treatment.

&#8226 Ease of use in field. Easier the method, more effective the treatment.

&#8226 Biology of the parasite. Ectoparasite have short generation times, producing large numbers of offspring a generation. Product label instructions must be carried out so if two treatments stated, two must be administered. First treatment will only kill active stages of parasite present on sheep at time of treatment. Second treatment kills eggs hatched since first treatment.