Control of Sheep Scab
Sheep scab is caused by the host-specific mite Psoroptes ovis. Sheep scab can be transmitted by direct contact when sheep are gathered and held tightly together such as during sales or contact with infested scab material on fenceposts or vehicles used for animal transport etc.
Sheep scab was rarely seen until withdrawal of compulsory dipping in 1989; now such infestations are rampant causing serious welfare concerns. It is a grave concern that the UK sheep industry with highly efficacious dips and ectoparasiticides at their disposal cannot achieve sheep scab eradication in the 21st century which New Zealand farmers so effectively achieved themselves over a century ago with nicotine dips derived from tobacco dust.
In the UK, disease is typically encountered during the autumn/winter months from October to March. During the early stages of infestation some sheep in the group have disturbed grazing patterns and are observed kicking at their flanks with their hindfeet and/or rubbing themselves against fence posts etc. which leads to loss of wool and a dirty ragged appearance to the fleece. There is serum exudation which gives the fleece overlying the skin lesions a moist yellow appearance. Some infested sheep have low mite numbers and show few clinical signs.
Fig 1: Sheep scab was rarely seen until withdrawal of compulsory dipping in 1989; now such infestations are rampant causing serious welfare concerns
Fig 2: It is a grave concern that the UK sheep industry with highly efficacious dips and ectoparasiticides at their disposal cannot achieve sheep scab eradication in the 21st century.
Fig 3: Sheep scab continues to cause serious welfare problems throughout the UK concerns
The fleece is wet, sticky, yellow, and frequently contaminated with dirt from the hind feet. It may prove difficult to part the wool fibres overlying the lesions due to serum exudation which has formed a thick layer within the fleece. Typically after eight weeks' infestation or so the hair loss on the flanks may extend to 20 cm diameter surrounded by an area of reddening and serum exudation at the periphery. The skin at the denuded centre of the lesions becomes thickened. The skin is thrown into thickened corrugations in many advanced cases. By this stage, infested sheep have lost considerable body condition and are frequently emaciated. Death may result in some sheep. In particularly severe cases, cutaneous stimulation during handling procedures may precipitate seizure activity which lasts for two to five minutes before animals recover fully.
Confirmation is essential by demonstration of live mites by a veterinary surgeon. Skin scrapings taken using a scalpel drawn at right angles over the skin surface at the periphery of active lesions demonstrate large numbers of mites under X100 magnification. Lice and keds can be visualised on careful examination of the fleece/skin. The best treatment option may vary between farms and it is essential that any proposed treatment is discussed with the farmer's veterinary practice who may wish to help coordinate local action.
Prevention & Treatment
Prevention depends on co-ordinated, careful plunge dipping or endectocide injection involving all neighbouring flocks and taking into account the following principles -
1. A flock infestation of sheep scab can be instigated by only one egg-laying female mite.
2. It is essential that all sheep are gathered and correctly treated at the same time using an appropriate dip or systemic endectocide injection.
3. Sheep scab mites can survive off the sheep for up to 17 days, survival being longest when the weather is cold and damp.
4. The only treatments that guarantee persistence for longer than 17 days are diazinon (organophosphate) and moxidectin injection.
5. Doramectin injection persists for marginally less than 17 days, but this is usually sufficient for sheep scab control.
Fig 4: It is essential that all sheep are gathered and treated.
Fig 5: All sheep introduced must be treated for sheep scab upon arrival or strictly isolated until treated.
6. One ivermectin injection repeated after 7 days, is effective for the treatment of sheep scab, but does not achieve significant persistence.
7. Systemic endectocide injections (ivermectin, doramectin and moxidectin) may take several days to kill all the sheep scab mites, while effective plunge dips, when used correctly, kill mites immediately.
8. Pour-ons and plunge dip solutions applied in shower dippers or jetting races are ineffective for sheep scab control.
Fig 6: In the case of plunge dipping, animals must be immersed for at least 1 minute, with their heads dunked twice during this period.
The practical relevance of these principles is -
1. Whenever it is necessary to return sheep to fields, handling pens or buildings used by untreated animals during the previous 17 days, only plunge dips or endectocide injections that persist for more than 17 days should be used for sheep scab control.
2. Ivermectin injection should only be used for sheep scab control when it is possible to avoid fields, handling pens or buildings used by untreated animals for the subsequent 17 days.
3. All sheep introduced, or returning from grazing away from home after whole flock control measures have been taken must be treated on arrival, following the same principles outlined above. If these animals are treated with a systemic endectocide, they must not be mixed with the main flock, or placed in areas used by the main flock for at least 7 days after treatment.
Fig 7: All dips must be correctly disposed of.
4. When purchased animals are certified as dipped before sale it is important to determine what they were plunge dipped in, and when they were dipped. It is also important to be confident that they were correctly dipped or injected.
5. Avoiding contact with strays, neighbouring sheep, fomites or shared handling facilities such as shearing trailers of scanning races may be impossible. It is therefore beneficial to ensure that all sheep flocks within a defined geographical area are treated with a preparation with residual action during the same 3 weeks period.
6. Shared handling equipment should not be allowed near to the sheep flock, unless it has been scrupulously cleaned beforehand.
Fig 8: Sheep scab, lice or simply poor nutrition?
Fig 9: All suspicious cases of wool loss and itching seen after October should be investigated by a veterinary surgeon.
Practical considerations -
1. In the case of plunge dipping, animals must be immersed for at least 1 minute, with their heads dunked twice during this period.
2. It is essential that the sump volume of the dipper is known, the correct initial dip concentration used, the correct replenishment rate used, and care taken to limit faecal contamination of the dip solution
3. Sheep should be yarded overnight before dipping and never dipped when hot, tired or thirsty.
4. Dippers should be emptied and cleaned at the end of each day's dipping, or after more than one sheep per 2 litres of sump volume has been dipped.
5. After dipping, sheep should be stood in a drainage pen until the dip ceases to run from their fleeces, before being turning onto shaded pasture, away from watercourses.
6. There can be problems associated with disposal of plunge dip solutions, or environmental contamination by recently dipped sheep. Unused dip solution can be disposed of by spreading on pasture in accordance with the data sheet recommendations, although ground needs to be licensed before it can be used for dip disposal. Alternatively, some dipping contractors are able to take used dip solution away for disposal on a licensed area elsewhere.
7. In the case of systemic endectocides, a sample of sheep should be weighed, syringes calibrated and great care taken to ensure that all sheep actually receive the correct drug dose.
Fig 10: Skin infestations in sheep are a major welfare concern not least because they could be eradicated from the UK within days.
1. The best timing for a concerted sheep scab treatment would be during the first three weeks of October, after most replacements have been brought home and before tupping for most flocks.
2. For most flocks, the most appropriate treatment would be either plunge dipping in diazinon or injection with doramectin or moxidectin.
3. There is a need to treat or cull all stray and feral sheep in the area.
4. Individual flock difficulties associated with dip disposal, meat withdrawal periods in store lambs and organic production are acknowledged. Where these difficulties cannot easily be addressed, other methods of scab control should be employed, based on the principles outlined above; in particular the need for strict separation of treated and untreated sheep. In these cases, individual veterinary consultation should be sought.
5. The importance of involving all flocks within a defined area is emphasised.
6. All suspicious cases of wool loss and itching seen after October should be investigated.
Fig 11: Is this sheep scab, lice or both infestations?
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