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
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
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
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
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
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
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
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.
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
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
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
6. All suspicious cases of
wool loss and itching seen after October should be
Fig 11: Is this sheep
scab, lice or both infestations?