This is the time of year when sheep sales are happening. With this comes the risk of buying in sheep that appear clinically normal yet may be infested with sheep scab. It is not safe to assume that sheep will have been treated correctly pre-sale and they may pick up sheep scab at the mart. If buying in individual sheep and when sheep cannot be kept separated from the home flock for at least 2 weeks, then treatment will be necessary. However when purchasing stores or replacements it may be possible to check whether treatment is indicated by testing a proportion of the sheep to check for evidence of exposure to the mites. If buying from multiple sources and mixing then wait at least 2 weeks before testing.
Also in the autumn many flocks undertake a precautionary treatment for sheep scab, whether that is with an injectable macrocyclic lactone or using OP plunge dipping. For many low risk flocks (those with good biosecurity, no contact with neighbouring flocks and appropriate quarantine measures for purchased and returning sheep) the ‘just in case’ treatment for sheep scab may not be necessary.
One way of deciding if treatment is necessary is to screen for exposure to sheep scab mites using the ELISA blood antibody test. Testing 12 sheep from the flock / group, including any itchy sheep in those to be tested, can be used as a screen to detect sheep that have met sheep scab mites. The number of positives and the antibody levels, together with history of use of any products active against sheep scab in recent months, will allow vets and farmers to make decisions as to whether this represents active sheep scab. Flocks testing positive can therefore treat to eliminate disease, whereas if all sheep test negative the flock is considered seronegative and treatment can be avoided. Testing 12 sheep provides a 95% confidence of detecting high levels of antibodies in at least one of the animals sampled where prevalence of infestation is 20% or more. In most flocks with sheep scab the prevalence is much higher than this.
It is crucial, given confirmed reports of resistance in scab mites to the 3-ML injectables and with these products also being important anthelmintics, that we avoid unnecessary treatments where this is no evidence of sheep scab. This also makes sense financially, as flocks stand to save over £800 on labour and drug costs if they test clear and can justify leaving the sheep untreated (based on a 500 ewe flock, ADAS 2013).
The sheep scab ELISA blood test is offered by Biobest and there is a dedicated submission form and Frequently Asked Questions (FAQ) guide on the website. The test costs £6 per sample if 12 or more sheep are tested.