Did you know that Sheep Scab can infect up to 60% of your flock before any clinical signs are seen? So it’s a pretty important condition to control. Dan Stevenson gets us up to speed…

Sheep Scab, also called Psoroptic Mange, is caused by the mite Psoroptes Ovis. Infected sheep have an allergic-type reaction to the presence of the live mites and as a result, are intensely itchy. Eventually, this causes wool loss and severe weight loss and can lead to death. The full life cycle of the mite takes 2 weeks and they can only survive off the sheep for up to 17 days.

In the late 1980s sheep Scab was nearly eradicated through a programme of compulsory preventative treatment. Now, the diagnosis of Sheep Scab is on the up with 167 confirmed diagnoses in 2018. This is compared to 52 in 2015, made by surveillance laboratories throughout Great Britain.

Treatment and control have typically relied upon the use of clear wormers in injectable preparations, such as Dectomax, Cydectin and Ivermectin products. These have proved popular as they are widely available, are relatively easy to administer, do not require special handling training and are safe for the operator when used correctly. However, they are also used widely for the control of worm infestations.

The use to control Scab is not specific and so whenever they are used in this way all parasites in the sheep are exposed to the product. While this might sound like a good thing it is rarely necessary. Therefore this has contributed to the development of worm resistance to clear wormers. Depending on the exact preparation used, some persistent action can be expected (theoretically up to 60 days in the case of Cydectin 2%). Although two injections may be required for treatment (in the case of Ivermectin products).

Sheep with Scab

Over recent years we have been increasingly concerned within the practice about the possibility of Scab mites developing resistance to clear wormers. This may have manifested as persistent signs following treatment or recurrent disease. Unfortunately, we are often involved late on in these cases when several treatments have been tried with no success.

A study published last year, in the Vet Record has confirmed that this is indeed the case. Mites were collected from four farms where resistance was suspected when infected sheep had failed to respond to injectable clear wormers. These mites were then exposed to a clear wormer within the laboratory along with control samples of mites that had never been exposed to any treatment products. Worryingly, the mites collected from the four farms showed similar low levels of death when exposed to the clear wormer product as the control mites did when exposed to no product at all.

This suggests that the product had little effect on the mortality of the mites from the farm outbreaks. The fact that the control mites (that had never been exposed to treatment products before) nearly all died on exposure to the product shows that it is still effective but that resistance had developed on the four farms as a result of over-exposure to these products in the past. This was the first demonstration of resistance to clear products in the UK and it is likely that this represents some degree of resistance across all the clear products.

Whilst this is not great news for the control of a disease that has serious economic and welfare consequences, there are several things we can do to ensure effective Scab management:
  • If you don’t have Scab then talk to us about effective quarantine treatments and biosecurity to keep it this way.
  • If you suspect Scab then involve us early to make sure the diagnosis is correct. This way you will not waste time and money with ineffective treatments.
  • Dipping provides very effective treatment and protection for up to four weeks. There are increasing numbers of contractors offering fully licensed, mobile dipping services making this a realistic option. All of our clients having used these services, including those where clear wormer resistance has been suspected, have been very pleased with the process and results.

Whilst Sheep Scab is on the rise and resistance to clear wormer products is here to stay there is much we can do to keep the disease under control on your farms. Please give us a ring to arrange a time to discuss your individual Scab control plan.