17 April 1998


Choosing the appropriate scab control product requires

careful consideration. Chris Lewis of Shrewsbury Vet

Investigation Centre outlines the pros and cons of each.

NO PRODUCT fulfils the total ideal for treatment and prevention of sheep scab. Choices have to be made.

But before choosing which product to use, some of the constraints imposed by the life cycle of the sheep scab mite must be remembered.

&#8226 During development from egg to adult, there are periods when feeding does not occur.

&#8226 Mites can survive off the sheep and remain infective for up to 16 days.

&#8226 The mite lives on the skin surface and does not burrow into the skin. The intense itchiness is caused by an allergic reaction between the mites faecal packets and the sheeps skin.

Products available

Three classes of products are available for controlling sheep scab.


&#8226 The organophosphates (OPs)

– diazinon

– propetamphos

&#8226 Synthetic pyrethroids (SPs)

– flumethrin

– high cis-cypermethrin


&#8226 Avermectin/Milbemycin

– ivermectin

– doramectin

No other products are licensed (have market authorisation) for use in the UK against sheep scab. No pour-ons are licensed or have market authorisation.

Efficacy of product

Before 1992 a product licensed as scab approved had to meet the criteria of being able, consistently, to produce a 100% kill of all mites on fully fleeced sheep, single plunge-dipped for one minute. In addition, protection had to be provided against re-infestation for 21 days, or longer in sheep with a fleece length of 1 cm (0.4in). Of the products listed above, only diazinon, propetamphos and flumethrin meet this criteria.

Since 1992, a licence has been issued to products achieving only 95% kill and offering little or no long term protection. It is essential that treatment with these products is administered twice at the prescribed intervals to achieve an effective kill.

Falling into this category are high cis-cypermethrin (two dips 14 days apart) and ivermectin (two subcutaneous injections seven days apart); neither provides long-term protection, so treated sheep must not be returned to pasture which has carried infected sheep for 16 days.

Lying between these two groups is doramectin which requires a single intramuscular dose but probably fails to protect for the full 16 days.

Advantages and disadvantages of the three product groups

OPs diazinon and propetamphos

&#8226 Advantages

OPs tend to translocate down the staple to bind with the lipids on the skins surface, so they are, therefore, in close contact with the mites.

They are also effective against all other commonly encountered ectoparasites of sheep at a single dose rate. No special pasture management is required after dipping due to the long term residual effect of the products varying from 28 days to over 40.

They are relatively inexpensive and both have comparatively short meat withdrawal periods but this is constantly under review. Virtually no cases of resistance have been reported.

&#8226 Disadvantages

They tend to strip out of the dip bath. This produces a saw tooth effect in concentration. In consequence, unless topping up (replenishment) is rigorously adhered to as per the manufacturers instructions, the equilibrium of the bath will be lost and the concentration of insecticide will be inadequate to eliminate all mites and provide reasonable long term protection.

In addition, it is necessary to hold a Certificate of Competence to purchase OP products. When using OPs, it is essential that they are used correctly and adequate protective clothing is used at all times.

Synthetic pyrethroids

&#8226 Advantages

Two synthetic pyrethroid dips are available. One is fully scab-approved and the other has been licensed since 1992. The compounds do not strip out of the dip bath so the bath concentration remains static despite volume loss.

There are considerable differences in the activity of flumethrin and high cis-cypermethrin. Flumethrin provides no protection against fly strike, whilst high cis-cypermethrin provides good protection against fly strike. These products have a short meat withdrawal period (nil-three days), making them useful for finishing lambs which are being drawn on a regular basis.

Currently no Certificate of Competence is required but this is to change in the near future.

&#8226 Disadvantages

The SPs are potentially up to 100 times more hazardous to the aquatic environment than OPs so the greatest care is required in their disposal.

They also tend to concentrate at the outer end of the wool fibre, away from the mite and this in turn leads to high wool residues. It also means that when dipping fully fleeced sheep, it is essential to ensure immersion for a full minute to achieve adequate concentration of insecticide at skin level. SPs are also generally considered more expensive than the OPs.

High cis-cypermethrin requires sheep to be plunge dipped twice at 14-day intervals and after each treatment to be moved to a pasture which has not carried sheep for the previous 16 days. It provides little or no residual actionso true prevention is not achieved. Having two SPs, only one of which has achieved the scab approved criteria, can and does lead to confusion in the market place.

Additionally, confusion arises from the requirement for different dip bath concentrations when treating different ectoparasites. The same brand name for both dips and pour-ons, which are ineffective against scab, can and has led to problems.


Again there is considerable difference between the two members of this group. Ivermectin is administered by the subcutaneous route (under the skin) and doramectin by the intramuscular route.

&#8226 Advantages

No specialist equipment is needed, except a weigh-crate to ensure that the sheep are dosed to the weight of the heaviest member of the flock. Most farmers and shepherds should be able to treat sheep by either the subcutaneous or intramuscular routes.

The products are particularly useful for heavily pregnant ewes or small groups of sheep away from the main flock. Both are useful in treating added animals since they are both effective wormers as well.

A Certificate of Competence is not necessary for the purchase of avermectin/milbemycin. As yet no resistance has been reported in the UK.

&#8226 Disadvantages

Ivermectin has no residual action so sheep must be treated twice at seven day intervals and then moved after each treatment to fresh pasture which has not carried sheep for the previous 16 days. In the case of doramectin protection is limited, so again after the single treatment sheep must be moved to a pasture which has not carried sheep for the previous 16 days. Lack of residual action means prevention is not achieved by either compound.

Because sheep are not washed and irritation is due to faecal packets, sheep will continue to scratch for two to three days after treatment. The requirement to treat twice with ivermectin makes treatment expensive in comparison to other compounds.

Both compounds have prolonged meat withdrawal periods, 42 days for ivermectin, 70 days for doramectin, which makes them unsuitable for use in finishing lambs.

Having weighed the advantages and disadvantages of the various compounds, the main factors involved in the choice of product depends upon a variety of factors:

&#8226 Whether treatment or prevention, or both, are required.

&#8226 Classes of sheep to be treated, as withdrawal periods vary widely.

&#8226 State of sheep to be treated – heavily pregnant or fully fleeced.

&#8226 Availability of scab clean grazing 16 days without sheep.

&#8226 Suitable facilities to dip sheep effectively.

&#8226 Provision or not of a Certificate of Competence.

&#8226 Environmental considerations regarding dip disposal.

&#8226 Degree of flock security from other sheep. &#42

Follow manufacturers instructions to the letter and ensure all sheep are dipped or injected to ensure effective scab control, says Chris Lewis.


l Protection (16 days+)

If protection only is required, both the OPs diazinon and propetamphos and the SP flumethrin are the only products available.

l Treatment only

All the six listed products are available so long as they are used exactly as manufacturers recommend.

l Treatment and protection (16 days+)

As protection (16+ days) above.


Product Sheep Scab Blowfly strike Lice, Ticks and Keds

Treat Prevent Treat Prevent Treat Prevent


Diazinon + + + + + +

Propetamphos + + + + + +


Flumethrin + + No No + +

High cis-cypermethrin +* No + + + +


Ivermectin +# No No No No No

Doramectin + No No No No No

* Requires two applications 14 days apart.

# Requires two doses, seven days apart.

Where products have no residual action, sheep must be moved after treatment to fresh pasture which has not carried sheep for the previous 16 days.

Choice of dipping product depends on

a variety of factors, but no product fulfils the total ideal says Chris Lewis.


Essential points for successful treatment and prevention:

&#8226 Ensure you have an accurate diagnosis that the skin condition really is sheep scab.

&#8226 Always follow manufacturers instructions absolutely.

&#8226 If using injections weigh and dose to the heaviest of the flock.

&#8226 Never miss one sheep, either dipped or injecting, for it will surely reinfect the remainder.

&#8226 When using two doses, ensure correct time interval and pasture management.

&#8226 Always observe withdrawal periods.

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