Sheep scab, caused by the skin mite Psoroptesovis, remains an important cause of wool loss, skin disease and ill thrift in Irish sheep. Biting of the skin from these non-burrowing mites leads to an acute or chronic form of allergic dermatitis which can have severe welfare implications in an affected flock of sheep.

However, sheep with subclinical scab can look perfectly normal and can unknowingly be introduced to a flock. Sheep scab is a winter disease with most cases occurring between September and April. It is a notifiable condition in sheep, i.e. sheep scab outbreaks in flocks should be notified to the Department of Agriculture, Fisheries and Food.

Sheep with early infestations may show no signs or be restless, rubbing against fence posts, etc., exhibit soiled and stained areas of wool (picture 1) and head tossing.

The mites cause small vesicles on the skin that fill up with serum and burst, leading to the formation of small yellow scabs. Later stages of infestation are characterised by a rapid increase in mite numbers and lesion spread. The lesion gradually spreads outwards as the mite population increases. Rubbing and head tossing become more excessive and areas of wool loss may appear, together with open, bleeding wounds. Sheep rapidly lose condition and can even occasionally show signs of fitting. While lesions are initially confined to the top of the back, all areas of the body can become affected in severe untreated cases (Figure 2). The mite population will eventually decline with a natural recovery occurring in affected animals.

Treatment

Treatment of sheep scab involves the use of sheep-dipping or injectable avermectins. In terms of sheep dipping, both organophosphate and pyrethroid dips are on the market. The important points for effective sheep dipping include the immersion of the whole body of the sheep in the dip for at least 60 seconds and the use of the correct concentration of dip. Operator safety is also crucial, especially when using organophosphates which can be harmful to human health. Ivermectin, doramectin and moxidectin are all effective against Psoroptes ovis, but two injections of ivermectin (seven days apart) may be required. Pour-ons are not suitable for the treatment of sheep scab.

Control/prevention

All new animals, including ewes and rams, should be held in isolation for three weeks prior to mixing with the main flock. During this time, they should be inspected for signs of ectoparasites and treated if necessary. Routine treatment for sheep scab and other ectoparasites should be carried out on a whole-flock basis at least once a year, usually in autumn, or if evidence of disease is noted. It is important to prevent sheep breaking into the flock, and breaking out into other neighbouring flocks, by ensuring that fencing is well maintained.

Other external parasites

Other ectoparasitic mites can occasionally cause skin disease in sheep and should be managed similarly to sheep scab. Lice is not as big a problem in sheep flocks.