Good biosecurity can keep sheep scab off farms
Sheep scab is a problem that is getting worse in the UK, costing as much as £19 per ewe in lost production and treatment in lowland flocks according to parisitologist Dr Peter Bates.
He says: “Sheep owners have a legal responsibility to prevent and cure infestations but control can be expensive, labour intensive and time consuming.
“Keeping ectoparasites off the farm at all times by having good biosecurity measures in place, is the best way of tackling the problem.”
Sheep scab is a progressive, debilitating mange caused by an allergic reaction to the faeces of scab mites living at the edge of expanding, pus-covered skin lesions. The mites do not feed on the sheep’s blood but on the pus. The irritation felt by the sheep is an allergic reaction to the mite’s saliva and faeces.
Mainly a winter problem, scab causes suffering, production loss and death. Early symptoms are mild. Animals may rub against fence posts and there may be tags of loose wool, although not in all cases. It is easy to mistake these early signs for other ectoparasites such as lice, which are also most prevalent during the winter (see table).
As the condition progresses, the lesion expands and in a third of cases, the mites migrate into the ear canal causing the sheep to shake its head violently. There is also excessive rubbing and wool loss, leading to open bleeding sores and loss of condition.
Not all sheep are affected to the same extent. Younger sheep are more susceptible, as are lowland breeds such as Suffolks and Texels with their tight, dense fleeces.
Mites also differ in their level of virulence. Studies have identified strains on Dartmoor that take six months to a year to develop into a full-blown infestation while a different strain in Shropshire takes only two weeks to take hold after arriving on a farm.
Scab mites are permanent parasites, completing their whole lifecycle on the sheep. However, the mites can live off the host animal for up to 17 days on bits of loose wool that have been scratched off, and can still infest sheep that come into contact them. It is therefore vital that all buildings, fences, gates and vehicles are disinfected after sheep have been in or near them.
More than a third of outbreaks are caused by lateral spread, from neighbouring flocks or strays. Stock-proof fencing backed up by an electronic fence will limit flock-to-flock contact.
A further 38 per cent of cases are initiated via imported sheep bought through markets and other sheep movements.
“Whenever new animals are being brought onto the farm, it is best to assume that all of them have scab, lice, footrot and resistant intestinal worms – regardless of where they have come from,” Dr Bates says.
“While they may all look pest free, lice and scab mites can take a couple of months to show themselves and sub-clinical signs are near impossible to detect.
“All incoming animals should be quarantined for at least 21 days, and watched to see if any symptoms develop. It is always easier and cheaper to treat a group of 50 newcomers than the whole flock a few weeks later.
“If there are any doubts call in the vet to take skin scrapes to identify the ectoparasite causing the problem, be it scab, lice or both, as the treatments for each are different.
“Blood tests for sub-clinical scab are being developed, but are not yet ready for on-farm use.”
Once identified, scab mites must be eradicated with an appropriate chemical treatment, although there are increasingly fewer options to choose from.
Mites have developed full resistance to plunge dips containing synthetic pyrethroids, and organophosphorous dips could be withdrawn on human health or environmental grounds in the future.
Dr Bates says: “Injectable treatments such as ivermectin deliver good control against scab, and avoid the need for dipping and all the problems associated with it.
“But incorrect use could jeopardise its efficacy against internal parasites.
“There is a real risk that we could be left with only one group of compounds to fight external parasites – and that is a very dangerous place to be.
“There is little prospect of new drugs being developed specifically for scab in the near future.
“This is why it is so important for farmers to use the chemical treatments they have now wisely, as well as putting strict biosecurity and quarantine procedures in place.”
MORE HELP: English sheep farmers can request a free copy of Eblex’s ‘Controlling external parasites for better returns’ by calling 08702 418829 or emailing firstname.lastname@example.org.