28 July 1995


Dr Dermot OBrien is acknowledged as one of the worlds leading lights on sheep scab. Here he discusses the disease, revealing it in its full horror, and lays out an agenda to ensure the mite is eradicated once and for all

SHEEP scab, psoroptic mange or scabies, caused by the mite Psoroptes ovis, has been a major impediment to sheep husbandry since biblical times.

The mite, just seen with the naked eye as a grey speck, has proven itself to be resilient. The disease it causes is both contagious and serious. Infected sheep suffer immensely, so it is rightly an animal welfare issue.

The incubation period, the time taken from contracting the disease to when the symptoms appear, varies from about three weeks to many months. During this time infected animals can spread the condition to other sheep.

The disease commences with uneasiness in the flock, stamping, tail twitching and head turning. This is followed by periodic scratching, with derangement of the fleece which shows as flecks of displaced wool. The scratching becomes more intense, then lesions show, commencing with reddening of the skin as wool is torn out. Lesions, which often start on the shoulders and sides, expand as large areas of abraded skin become exposed and plasma pours out. This congeals on the surface to give the condition its name scab.

Sheep so affected lose their appetites and spend long periods scratching – sometimes so frantically they go into convulsions from which they occasionally never recover. If left untreated, secondary bacterial infection ensues and one third of the affected sheep can die.

Those sheep which recover without treatment frequently harbour the mite for periods of up to two years. During this time they show no signs, but can spread the disease to other sheep.

The overt signs of sheep scab are obvious and recognised by most experienced sheep farmers. Paradoxically, this is probably the main reason why the disease remains a problem in the British Isles as only obvious cases are recognised. The disease may be in flocks in a suppressed state, and farmers frequently dont realise their flock is infected until they have already sold animals.

Sheep which are incubating the disease do not show symptoms, nor do recovered carrier sheep or those which have been incorrectly treated for the disease. Often shearing suppresses the symptoms until the fleece regrows, for the micro climate at the skin of shorn sheep does not favour the mites life cycle.

Various laws have been passed over the years in an attempt to eliminate the disease, commencing with King Hywell DDa (Howell the Good) of Wales in 949 AD. None have succeeded here. How-ever, Australia, USA, Canada and New Zealand all have eradicated the mite, some in the last century, with such archaic chemical methods as nicotine and arsenic dippings.

The control legislation in Britain, which was in place until recently, kept the disease at low levels. With reporting systems in place it was at least possible to map its existence.

If no treatment were to take place at all, up to 50% of flocks could be affected within five years. As compulsory yearly prophylactic treatment has gone and pressure for reporting and controlling has eased, we are sitting on a dam with the water rising relentlessly.

Although no firm figures can be given, all over the British Isles the incidence is increasing, with the attendant welfare and economic consequences.

Treatment of the disease is easy, yet not simple. Effective measures are available and easy to apply. But if they are not applied correctly they fail to work or temporarily suppress the disease and so become the cause of spread – rather than cure.

The use of OP dips requires attention to personal safety by means of protective clothing, and environmental safety by means of proper disposal. Correct dilution and reinforcing in fresh clean water ensures full efficacy.

Each sheep must be treated. One missed and it becomes a futile, expensive and exhausting exercise. Too little time in the bath and some mites may survive, leading to a suppressed disease, with later recurrence and further spread.

When using pyrethroid dips it is most important to follow instructions carefully to ensure a full kill of mites, as resistance by P ovis to pyrethroid can occur. Indeed, there have been recent reports of possible resistance.

At the laboratory many topical and pour-on systems have been examined, but none have effected complete elimination of mites.

The use of subcutaneous injections of avermectins and milbemycin (endectocides) offers a less labour-intensive and weather-dependent system of control. Although they do not fully control biting lice nor fly strike, they obviate the need to dispose of large quantities of environmentally-hazardous waste liquid. No special equipment or sites are required and they have the considerable benefit of also eliminating nematode worms.

But, it must be stressed that each sheep must be treated and that a generous sufficiency of dosage is vital. Farmers often under gauge sheep weight and dose rates. If this is the case, treatment will be unsuccessful and the suppressed state of the disease will ensue.

When injecting sheep with endectocides, wool should be parted in a vertical line down the shoulder so that the skin is exposed for injection. Each sheep should be clearly colour marked after injection.

Two subcutaneous injections of ivermectin at 200mcg/kg have proven successful in treatment of psoroptic mange of sheep. Likewise, when moxidectin, which is expected to be licenced shortly, is injected subcutaneously twice at 200mcg/kg it is effective. The second injection should be given 10 days after the first.

Extensive laboratory and field work has shown moxidectin has a prophylactic (preventative) action. Sheep injected with a single injection will withstand disease challenge for a month.

Although OP usage is now restricted there is, with the availability of the injectables, an excellent opportunity to eradicate the mite if a co-ordinated and agreed plan can be formulated.

The identification of self-contained, scab-free farms where agreed practices of sheep introductions (or reintroductions after sales failure) might be a start, followed by scab-free areas or counties all following agreed practices.

But any scheme devised must command farmer acceptance and shepherds willing to report cases and act as self monitors. Certainly, if there were a willingness to act, sheep psoroptic mange could be eradicated for all time.

Irelands Dr Dermot OBrien.

Sheep scab – Ewes show excessive irritation and rubbing.