NADIS disease bulletins are written specifically for farmers, to increase awareness of prevalent conditions and promote disease prevention and control, in order to benefit animal health and welfare.

Farmers are advised to discuss their individual farm circumstances with their veterinary surgeon.

Prevention of Sheep Scab

During 2003/4 sheep scab was effectively controlled in several regions in the south-east of Scotland. Unfortunately, sheep scab continued to cause problems in other areas.

SHEEP SCAB IS AN IMPORTANT PRODUCTION LIMITING DISEASE AND WELFARE CONCERN

IT OUGHT TO BE POSSIBLE TO PREVENT SHEEP SCAB THROUGHOUT THE UK. SUCH PREVENTION DEPENDS ON CO-ORDINATED, CAREFUL PLUNGE DIPPING OR ENDECTOCIDE INJECTION, INVOLVING ALL FLOCKS AND TAKING INTO ACCOUNT THE FOLLOWING PRINCIPLES -

PSOROPTES OVIS - THE CAUSE OF SHEEP SCAB

1. A flock infestation of sheep scab can be instigated by only one egg-laying female mite.

2. It is essential that all sheep are gathered and correctly treated at the same time using an appropriate dip or systemic endectocide injection.

3. Sheep scab mites can survive off the sheep for up to 17 days, survival being longest when the weather is cold and damp.

4. The only treatments which guarantee persistence for longer than 17 days are diazinon (organophosphate) and some high-cis cypermethrin plunge dips (refer to table) and moxidectin injections.

5. Doramectin injection persists for marginally less than 17 days, but this is usually sufficient for sheep scab control.

6. There are concerns about scab mite resistance to high-cis cypermethrin in UK flocks.

7. One high-cis cypermethrin plunge dip (refer to table) or ivermectin injection repeated after 7 days, are effective for the treatment of sheep scab, but do not achieve significant persistence.

8. Systemic endectocide injections (ivermectin, doramectin and moxidectin) may take several days to kill all of the sheep scab mites, while effective plunge dips, when used correctly, kill mites immediately.

9. Pour-ons and plunge dip solutions applied in shower dippers or jetting races are ineffective for sheep scab control.

PYRETHROID OR INSECT GROWTH REGULATOR POUR-ONS AND SOLUTIONS APPLIED IN SHOWER DIPPERS ARE INEFFECTIVE FOR THE TREATMENT AND CONTROL OF SHEEP SCAB.

THE PRACTICAL RELEVANCE OF THESE PRINCIPLES IS -

1. Whenever it is necessary to return sheep to fields, handling pens or buildings used by untreated animals during the previous 17 days, only plunge dips or endectocide injections which persist for more than 17 days should be used for sheep scab control.

2. Non-persistent high-cis cypermethrin plunge dips and ivermectin injections should only be used for sheep scab control when it is possible to avoid fields, handling pens or buildings used by untreated animals for the subsequent 17 days.

3. All animals introduced, or returning from grazing away from home after whole flock control measures have been taken should be treated on arrival, following the same principles outlined above. If these animals are treated with a systemic endectocide, they must not be mixed with the main flock, or placed in areas used by the main flock for at least 7 days after treatment.

4. When purchased animals are certified as dipped before sale it is important to determine what they were plunge dipped in and when they were dipped. Non-persistent dips or systemic endectocide injections may not protect against infection acquired during the sale. It is also important to be confident that they were correctly dipped or injected.

5. Avoiding contact with strays, neighbouring sheep, fomites or shared handling facilities such as shearing trailers of scanning races may be impossible. It is therefore beneficial to ensure that all sheep flocks within a defined geographical area are treated with a residual acaricide during the same 3 week period.

6. Shared handling equipment should not be allowed near to the sheep flock, unless it has been scrupulously cleaned beforehand.

THE EWE ON THE RIGHT IS SHOWING OBVIOUS SIGNS OF SHEEP SCAB, WHILE THE OTHERS IN THE PEN APPEAR TO BE UNAFFECTED. HOWEVER, ALL OF THESE ANIMALS WOULD SPREAD THE DISEASE IF MIXED WITH UNAFFECTED SHEEP.

PRACTICAL CONSIDERATIONS -

1. In the case of plunge dipping, animals must be immersed for at least 1 minute, with their heads dunked twice during this period.

2. It is essential that the sump volume of the dipper is known, the correct initial dip concentration used, the correct replenishment rate used, and care taken to limit faecal contamination of the dip solution.

3. Sheep should be yarded overnight before dipping and never dipped when hot, tired or thirsty.

4. Dippers should be emptied and cleaned at the end of each day’s dipping, or after more than one sheep per 2 litres of sump volume has been dipped.

5. After dipping, sheep should be stood in a drainage pen until the dip ceases to run from their fleeces, before being turning onto shaded pasture, away from watercourses.

6. There can be problems associated with disposal of plunge dip solutions, or environmental contamination by recently dipped sheep. Unused dip solution can be disposed of by spreading on pasture in accordance with the data sheet recommendations, although ground needs to be licensed before it can be used for dip disposal. Alternatively, some dipping contractors are able to take used dip solution away for disposal on a licensed area elsewhere.

7. Pyrethroid dips are extremely toxic to aquatic life and can also cause skin problems in humans. Farm staff should be specially trained for dipping sheep and always wear the recommended appropriate protective clothing.

8. In the case of systemic endectocides, a sample of sheep should be weighed, syringes calibrated and great care taken to ensure that all sheep actually receive the correct drug dose.

HOW MANY SHEEP ARE CORRECTLY DIPPED IN A SETUP LIKE THIS?

SHEEP NEED TO REMAIN IN THE DIP BATH FOR AT LEAST ONE MINUTE, WITH THEIR HEADS SUBMERGED TWICE DURING THIS PERIOD

FAECAL CONTAMINATION SHOULD BE LIMITED BY YARDING OVERNIGHT AND WALKING OVER SLATS OR COARSE STONES BEFORE DIPPING

SHEEP SOMETIMES CONTINUE TO SHOW SIGNS OF PRURITUS FOR UP TO 30 DAYS AFTER SYSTEMIC ENDECTOCIDE TREATMENT. THIS IS BECAUSE IT MAY TAKE UP TO SEVEN DAYS BEFORE ALL MITES ARE KILLED. FURTHERMORE, DEAD MITES AND MITE EXCRETORY PRODUCTS REMAIN ON THE SKIN LONG AFTER MITES HAVE DIED, RETAINING THE ABILITY TO EVOKE AN ALLERGIC DERMATITIS

ACTION -

1. The best timing for a concerted sheep scab treatment would be during the first 3 weeks of October, after most replacements have been brought home and before tupping for most flocks.

2. For most flocks, the most appropriate treatment would be either plunge dipping in diazinon or injection with doramectin or moxidectin.

3. There is a need to treat or cull all stray and feral sheep in the area.

4. Individual flock difficulties associated with dip disposal, meat withdrawal periods in store lambs and organic production are acknowledged. Where these difficulties cannot easily be addressed, other methods of scab control should be employed, based on the principles outlined above; in particular the need for strict separation of treated and untreated sheep. In these cases, individual veterinary consultation should be sought.

5. The importance of involving all flocks within a defined area is emphasised.

6. Any suspicious case of wool loss and itching seen after October should be investigated.

Neil Sargison BA VetMB DSHP FRCVS