Who uses sheep dip these days? A drench gun is used these days which is run down there back and across there hind quarters and not usually administered around shearing time .
Nowhere near as bad as the dip for sheep which can cause cancer and nerve problems in us
Edit: I think sheep dip is band in this country.
Sheep dip
Sheep farmer here. The gun down the back is for cypermethrin fly killer. It can definitely be used from late spring onwards through late autumn to combat fly strike, so before shearing July/August. Withdrawal times have to be observed for shearing because you're not allowed to present treated sheep to a shearer in case of skin reaction. We all try to avoid the use of fly strike meds completely before shearing, but with climate change I think this will become more and more difficult.
Cypermethrin kills flying insects that come in contact with it. The various permutations act in different ways on various stages of the insect and have short-to-very long efficacy times. Fly strike is gruesome and often deadly, the fly lays eggs in the wool and within 24 hours the maggots are burrowing into the sheep's skin. Research in Australia and New Zealand has shown that there is a strong genetic component to susceptibility, and there are big efforts now to breed it out of the flock. We certainly don't retain anything that gets fly strike and if a ewe gets struck, her lambs are not kept for breeding either.
Sheep are normally dipped in autumn after shearing. Many people are dipping just now, in spring, last year's lambs mainly. The dip is diazanon. It is OP and not banned. In fact it is the only truly effective means of a 100% kill of sheep scab (a bloodsucking mange mite specific to sheep), a formerly reportable pest which is making a comeback thanks to people not dipping. Dipping used to be mandatory but I believe it was stopped during the foot and mouth at epidemic.
@jenkinsbrynmair dip doesn't clean the fleece, and dipping a sheep in full wool wouldn't be done because the sheep isn't in the bath long enough for the dip to get to the skin. Dye is often put in the dip and that's why you will see a field of sheep that look quite yellow or weirdly grey or brownish. It's's an indicator to a buyer that they have been dipped. Most pedigree breed sales require that the sheep are dipped before sale.
Dipping is a complex affair now, as you say the health risks are well known. Only licensed operators can do it, full hazmat kit etc. It doesn't affect the sheep as the effects have to accumulate over many years.
So yes you may get fleece that is treated. If using wool, check your source! Wool sheared in late summer should be perfectly safe because if the sheep were dipped, it would have been almost 10 months previous. Unless they have used flystrike stuff before shearing!