Lice infestations have increased significantly since the de-regulation of scab control in 19921. Clinical signs can be similar to scab, and it is important to make an accurate diagnosis. Please consult your veterinary surgeon.
Lice are permanent parasites, completing all life stages on the sheep host, feeding on wool and skin debris. The nymphal stages last for one to three weeks. Adults live for up to one month. An important fact is that lice can survive off the host for up to 17 days.
Most lice infestations occur in the winter.
Sheep can carry quite significant numbers without obvious clinical signs. The signs are irritation and wool loss. Sheep biting their fleece or rubbing against fence posts are displaying classic signs of the irritation these lice cause. This can be confused with sheep scab, although it should be remembered that it is possible for both infestations to occur simultaneously.
A good biosecurity plan should be devised to prevent contact with infested sheep - quarantine all incoming stock, ensure good fencing, strict cleansing and disinfection of transport vehicles, try to maintain closed flocks etc. The onset of summer causes the natural decrease of lice infestation. Shearing will reduce residual populations by up to two thirds.
If you find one infested animal, the whole flock should be treated with an appropriate ectoparasiticide, ideally just after shearing. Treatments include spot-ons containing deltamethrin or cypermethrin and alphacypermethrin pour-on products. Diazinon dipping is also effective against lice.
1 Bisdorff B, Wall R, Milnes A - Prevalence and regional distribution of scab, lice and blowfly strike in Great Britain Veterinary Record 2006 Jun 3;158(22):749-52