Farmers need to assume all bought-in sheep have scab and treat accordingly as anecdotal case numbers reach an all-time high, warns Lesley Stubbings of SCOPS.
Sheep scab is caused by an infection with the mite Psoroptes ovis, but it can take weeks or even months before clinical signs are obvious, so farmers cannot tell just by looking at a sheep whether it is infected or not.
Ms Stubbings says: “If you are buying in sheep, or they have come into contact with sheep of an unknown scab status, then you must assume they have it.”
Sheep scab is endemic within the UK with figures suggesting between seven and ten thousand clinical cases a year, at an estimated cost of £12-18 a ewe.
She says: “This is the worst year I have known. Our local mobile sheep dipper is run off his feet. Part of this could be that endemically infected flocks may not have clinical signs and they do not realise scab is still active in their sheep, so they are not controlling it.
"It could also be associated with resistance to some injectables.”
Dr Peter Bates from Veterinary Medical Entomology Consultancy (VMEC), says most scab is controlled at farm level, mostly on a reactive basis, once the parasite has taken hold in the flock.
However, he warns this firefighting approach can be expensive and can result in overuse and misuse of the few treatments available.
He says: “The use of diazinon plunge dip formulations via non-validated methods of application such as showers has the potential to generate resistance in scab mites.
"The overuse of macrocyclic lactone (ML) injections for scab control can generate resistant gut worms, increasing the problem of anthelmintic resistance. There are also the negative economic effects of unplanned scab treatments, particularly the excessive meat withdrawal periods (MWPs) on the marketing of finished lamb.
“For effective in-flock eradication, all sheep on/off the main holding should be treated at a convenient/cost-effective time. Winter is ideal, with the lowest number of sheep, more tolerable MWPs and a full fleece which is ideal for plunge dipping.
"Once eradicated, a plan must be formulated to keep scab out and this plan must be included in a flock health and welfare plan."