Addressing veterinary questions on sheep scab and lice

Q: I have noticed ewes scratching their neck and front shoulders in the last week or two, with wet underfoot conditions highlighting the issue much quicker. I am considering injecting the ewes, but am afraid it may be lice and the injection won’t work. Is there any way to tell?

A: It is difficult but possible to visually identify the parasite causing the problem. The mites causing sheep scab, called Psoroptes ovis, are brown in colour and round, but are tiny – barely visible to the naked eye. They feed on flesh and are generally found on the skin, as opposed to lice which generally persist and move through the wool.

In advanced stages, characteristic signs of sheep scab include scabby lesions developing and wool loss. Lice can be seen easier with the naked eye and are long, shaped similar to a wasp, with a yellow or pale brown colour. They are still very small, although not as small as mites.

If plunge dipping, there would generally be no need to check as all ecto-parasites are covered.

However, with injectable treatment, it is important to diagnose first, as incorrect treatment can essentially be a waste of money, as injectable products that treat sheep scab generally do not cover lice.

Your vet will be able to diagnose by examining using magnification or collecting a skin or wool sample for examining under a microscope.

Q: If it is lice and I have no access to a plunge dipper, what product should I select?

A: Be careful with product selection. Some pour-on products treat and provide a level of protection against lice. Others provide a level of treatment, but do not completely prevent lice.

If the infestation is bad, retreatment may be necessary. In this case, it is advised to explore potential avenues for plunge dipping. Plunge dipping provides cover for biting and sucking lice. It is also important to consider that both lice and sheep scab can be present at the same time.

Q: Will sheep showers or jet systems work in place of dipping for treating sheep scab?

A: Showers or jetting equipment are not recommended as providing 100% effective treatment for sheep scab. Immersing sheep in dip for 60 seconds is advised to kill all mites present. While these alternative methods may provide a level of treatment, they are not recognised by the veterinary profession as providing total protection.

Q: Injectable products differ in their cover of sheep scab. What do I need to take into account?

A: Some injectable products with the active ingredients ivermectin, moxidectin and doramectin treat sheep scab.

Doramectin-based products have a single treatment claim, while Moxidectin-based products have a residual activity meaning they also provide a protective component. Be careful when selecting to read the manufacturer’s guidelines closely, as some products require repeat treatment seven to 14 days later.

Also, withdrawal periods can be very long, ranging from 50 to 100 days and this should be taken into account if treating lambs intended for slaughter.