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Plunge dipping of sheep in water both to wash fleeces and as a way to control ectoparasites has been carried out for centuries. Sheep are plagued with many external parasites who find their dense moist fleece an ideal habitat in which to carry part of all of their life cycle.

In the UK the main problems are lice, ticks, sheep scab mites and most importantly blowflies laying eggs onto soiled areas of fleece. The resultant maggots eventually eat their way into the flesh of the unfortunate animal leading to death. Wingless flies (keds) have also been a problem in the past.

These are problems found worldwide both in intensive and extensive husbandry systems. Some such as ticks and fly strike are impossible to eradicate as the parasites occur widely and can live on a wide range of other animals whilst those specific to sheep and which spend their entire life cycle on the sheep (sheep scab, keds, lice) are amenable to local or national control programs. Sheep scab was apparently eradicated from mainland UK for many years following a compulsory sheep dipping campaign but reappeared in the mid 1960s and has proved impossible to eradicate since then.

The sheep fleece is a very dense coat with great variation between breeds in respect of the fineness of the wool varying from the ultra fine Merino to the essentially hairy Herdwick breed found in the Lake District. All fleeces however contain an oily waterproof layer close to the skin consisting of waxy lanolin. Almost all insecticides are highly oil soluble and partition into the lanolin layer after external application providing a long term depot which provides sustained protection against reinfection. This in itself can also lead to problems with both meat residues and residual activity in the fleece, from lanolin which is removed during wool processing and may cause environmental pollution.

Control of parasites was a major problem until the advent of the chemical industry in the Victorian era. Toxic compounds such as salts of arsenic, antimony and mercury could be suspended in a dip bath and then absorbed into the fleece by sheep swimming through. These chemicals poisoned the parasites but also could affect the sheep and the environment. These were replaced with phenolic compounds which went into the lanolin and then with a range of insecticides/acaricides such as the organochlorines (DDT, Dieldrin, ? BHC), the organophosphorus compounds (OPs -Dursban, Coumaphos, Diazinon) and from the 1970s synthetic pyrethroids (permethrin, cypermethrin, flumethrin). All of these compounds are highly fat soluble and are concentrated in the lanolin. The organochlorines were banned for reasons of environmental persistence and toxicity as were all the OPs other than Diazinon. Synthetic pyrethroids were formulated both into dips and pour on formulations. Insect growth regulators such as dycyclanil and cyromazine are extensively used as pour on formulations to control fly strike and injections of macro cyclic lactones are effective against sheep scab mites and some other external parasites.

The great advantage of dipping is that the animal rece...