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With breeding sheep and store lambs changing hands across NI at present, it means there is a higher risk of scab spreading between flocks.
The highly contagious disease is caused by a mite and can lead to itching, wool loss, reduced animal performance and welfare issues.
Reports from local farmers and vets indicate that the prevalence of scab in NI has increased in recent years. A common starting point for an outbreak is when newly bought-in stock join a flock.
Prevention is more difficult for farmers who buy-in sheep from multiple sources over a prolonged period, such as store lamb finishers.
Injectable avermectin products can kill mites on bought-in sheep, but the length of time that treated animals are safe from being re-infected varies between products.
“If you are injecting lambs on arrival with a product that has a residual effect which lasts for several weeks, the last sheep in should not infect the first sheep,” explains Larne-based vet Paul Crawford.
“If you use a less persistent product, you will kill the mites on the first bunch, but there is a chance that mites from later arrivals will transfer to the first bunch before they die off,” he said.
Another option is to keep all bought-in stock together and hold off treating them until buying has been completed for the year. All stock can then be treated together by injection or plunge dipping.
However, care will be needed that untreated stock do not come into contact with existing sheep in the flock. It should also be noted that mites can live without a host, including on livestock handling facilities, for up to 17 days.
Survival period
Crawford points out that this long survival period means newly bought-in sheep could be carrying mites, even if the host flock did not have a scab problem.
Indirect sources of infection can include mart pens or haulage lorries, so all bought-in stock should be subject to an effective treatment programme, or at least quarantined for an appropriate period.
“If you don’t want to routinely treat everything, you could keep them separate for a few weeks and then get your vet to take blood samples to test for the presence of scab,” Crawford said.
He maintains that just visually observing bought-in sheep for signs of scratching is not an effective method of identifying scab.
This is because bought-in sheep can be infected with a small number of mites that can take a long time to develop into clinical scab.
“You will have to keep sheep separate for three or four months if you want to just check them yourself for signs of scratching. You should either treat or test,” Crawford advised.
Other sources
As well as bought-in stock, other sources of scab include neighbouring flocks or shared handling facilities, such as sheep scanning equipment.
Crawford urges farmers to speak to their vet if they suspect they have a scab problem. He points out that not all scratching sheep have scab, as the likes of lice infestation cause similar symptoms.
If scab is confirmed, treatment options include injectable avermectins or plunge dipping, but veterinary advice should be sought beforehand to make sure an effective control plan is in place.
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