Sheep scab is caused by a tiny, pearly-white mite
(Psoroptes ovis) just visible to the naked eye. Mites live on the skin surface causing a skin allergy to their droppings. Initially the lesions are very small (less than the diameter of 1p coin) as the mites multiply the scab spreads, eventually covering the whole sheep. Mites can be found around the edge of the growing lesion.
- Early disease is difficult to spot – animals look and behave normally.
- As the scab spreads sheep become increasingly irritated, excessively rubbing, scratching and digging with their back legs.
- After 10 to 15 weeks the scab covers the entire animal. Wool can fall out, revealing a dry, scabby skin beneath.
- By now scab can have spread throughout the flock making control expensive.
- Infested animals can demonstrate a violent biting reflex on handling (similar to scrapie) sometimes leading to fitting.
- Scab can be found throughout the year, but the majority of cases occur between October and March.
- Scab mites live their entire lives on the sheep, introduced into a flock through contact or purchase of infested stock.
- Mites can however live off the sheep for 16 to 17 days in tags of scabby wool, clothing, barns or livestock transport.
- Quarantine incoming stock for at least 3 weeks
- Ensure fencing is sound to prevent straying on or off
- Disinfect livestock trailers after use
- Remove all debris (wool etc) from contaminated housing
- Do not re-stock for at least 3 weeks after decontamination
- Don’t wait for wool to fall out - act as soon as you see the sheep rubbing and scratching.
- Ask your vet to examine the sheep promptly to identify any parasite and advise on treatment. Not involving a vet will be far more costly than prompt veterinary inspection.
- The signs of scab can also easily be confused with those of lice and resistance may occur in both ectoparasites if they are not professionally identified and treated. Sheep can also have mixed infestations of sheep scab and chewing lice.
- Sheep scab can be controlled by using a diazinon based dip or by doramectin, ivermectin or moxidectin based injections.
- Remember that injections are also effective wormers and misuse / overuse could lead to drug resistant worms – consult your vet.
This guide is a practical introduction to the external parasites most likely to jeopardise the welfare and productivity of sheep: blowfly strike, lice, ticks and scab.