Course: Diseases and pests in sheep | Last Updates: 12th October 2015
If sheep scab is an uncomfortable issue for the industry, this does not compare to the discomfort suffered by the afflicted animals. Scab is a nasty, painful and highly contagious condition that not only dents production, but can quickly become a welfare issue.
Until 1992, regulated dipping kept scab under reasonable control. Since then, the incidence of scab has escalated year on year. It is widespread in all sheep-populated areas, but greater awareness and a willingness among sheep farmers to work together to keep it under control has proven highly successful in some parts of the country.
What causes scab?
Scab is caused by a mite, Psoroptes ovis (pictured), that is specific to sheep. Contrary to common perception it does not bite, but feeds on dead skin and debris close to the skin. This gives the host sheep allergic dermatitis, an irritating skin inflammation.
Mites are most active, and reproduce quicker, during winter months – preferring cool, damp conditions. A single female mite can establish an infestation, and all stages of the organism – larvae, nymph and adult – feed off the host and can be found on an infested animal. The lifecycle is approximately 11-19 days from egg to egg under optimal conditions, and adult mites can live for 40 days.
How does it become a problem?
Three Golden Rules
- Get a correct vet diagnosis and make sure you spot a problem early on
- Quarantine incoming sheep for two weeks and treat on entry
- Selecting the best treatment option and product for your flock is essential – don’t go it alone and always seek advice from your vet
Scab is highly contagious. Mites pass easily between animals. Adult mites can also survive away from the host for up to 17 days. Wool tags rubbed off on a fence are a classic way for transfer to take place between holdings. Scab mites can be easily brought in on replacement sheep and will quickly spread to the rest of the flock.
Scab will affect all breeds but tends to be less severe on hill sheep. This could be because the breeds are kept more extensively and have a looser fleece. Mites thrive in the tightly packed fleece of down-type breeds such as Suffolks and Dorsets.
What effect does it have on sheep?
Scab can often go unnoticed, particularly in summer months, when sheep can carry a latent infestation. However, sheep are still infective during this latency. Early sheep scab shows first as a moist eczema, evident as dampness in the wool around the shoulders.
Look closely and you will see red macules, or sores, with yellow staining of the wool near the skin. These develop into large yellowish crusted lesions that spread down the back and sides and the wool comes out.
The condition is incredibly itchy and afflicted animals will constantly scratch and rub to the point of self trauma, resulting in large angry red patches developing on the back and shoulders.
This preoccupation causes productivity to drop, resulting in smaller lambs being born to infected ewes and lower milk production. A large amount of protein is also lost through the weeping skin and the resulting hypo-proteinaemia can also seriously affect the growth rate of fattening lambs.
How do you diagnose a problem?
Mites are invisible to the naked eye, mainly because they are very small, whitish and cannot be seen against the fleece. Vets can diagnose a problem using sticky tape on the skin to pick up mites. And inspection under a microscope will confirm the presence of sheep scab.
Lice can be easily misdiagnosed as scab mites, and vice-versa. Lice are darker in colour and larger and the effect is usually less severe. A correct diagnosis is important as treatments for lice and mite can differ.
What treatment is needed?
Full-plunge dipping in OP solutions and injectables are the only effective treatments for scab. Pour-ons and shower treatments do not offer scab control and should not be used. The crucial element to successful treatment is to ensure every animal is treated at the same time – just one miss and the exercise will have been worthless.
Organophosphate (OP) dips offer an instant kill of all mites present at all life stages and the product has a persistant activity for 28 days. Dipping with OP solutions is highly regulated, which dissuades many producers. There are a range of injectables available that offer between 17 and 60 days protection. Scab mites can take up to seven days to die after injectable treatment so this must be taken into consideration. The best option and specific product will depend on circumstance, facilities, available staff and a host of other factors, so it is important to discuss these with your vet.
Whatever option you choose, it must be administered correctly to be effective (see panel right).
How do you prevent scab affecting your flock?
The first step is to ensure you have an effective control programme on your own holding. When buying in, always assume stock may be infected with sheep scab – apply a treatment and quarantine them for at least two weeks until you can be sure they are clear.
Neighbouring and stray sheep can be a problem. Taking part in a regional control programme has proven to be an effective way to control scab. It relies on farmers in a region working together and coordinating control. Typically there will be a period in autumn when all sheep farmers will treat their flock. It is worth at least consulting your neighbours to find out their control strategies.
Regional control programmes are only as robust as their weakest link and it is difficult to enforce control programmes on individual farmers. But the Scottish government is bringing in legislation to give more power to local authorities to deal with any problem areas. If it proves effective, it may be a model that is rolled out elsewhere.
Scab symptoms begin as dampness in the wool around the shoulders, which eventually develops into large, itchy red patches on the back and shoulders.
Treatment essentials: Top tips for effective control
Dipping in OP solution
- The solution must be made up and replenished to the correct concentration. This is crucial to remember when topping up.
- Keep the solution clean – mud and faeces bind up the OP product, stripping out the dip. Dag animals beforehand and pass them through a water footbath in front of the dip.
- Each animal must be immersed for at least one minute, with the head dipped under the surface twice.
- Sheep should never be dipped when hot, tired or thirsty and should be stood in a drainage pen after dipping until the dip ceases to run from their fleeces.
- Abide by the regulations – you need a license to purchase and dispose of the product and correct protective clothing must be worn. Seek advice and follow label recommendations.
- Calibrate the gun correctly – dose for the heaviest animal you will treat as under-dosing is ineffective.
- Pass your flock through a race or equivalent to ensure you have treated every animal. Omitting one or two sheep can be easily done when injecting.
- Make sure you inject properly – there is a high risk you may fail to penetrate the skin.
- Assume your flock is still carrying live mites for seven days after treatment and handle them accordingly. The same goes for bought-in stock.
- Good training and skills will help ensure treatments are effective.
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