Livestock Pests |
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Sheep lice |
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Damalinia ovis
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Adult sheep louse (Damalina ovis)
© Agriculture Western Australia
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Description
The sheep body louse is a small insect up to 1.5 mm long, with special mouth parts adapted to feeding on the sheep's skin. It has a pale brown body with several dark transverse bands, and a red head.
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Life cycle
Lice require specific temperature conditions to breed and, even when these conditions are ideal, their rate of reproduction is not great compared with other parasites. The adult female louse lays only two or three eggs every five days and the minimum period from an egg being laid for a female louse to hatch, grow and begin laying its own eggs is 34 days. Even under the most favourable conditions, it takes 20 weeks for the louse population on a sheep to increase from 5 000 to half a million or more. This is based on the adult female louse living for 60 days. These ideal conditions for living, growing and reproducing occur during the late autumn, winter and early spring for all sheep and continue during summer in long woolled sheep. In sheep with short wool in summer, sunlight and high temperatures cause adults to die or lay less eggs and lice populations fall or do not increase. Similarly, heavy rainfall which saturates the fleece can kill adult lice and reduce breeding by surviving lice. Lice do not voluntarily drop off sheep and they are not easily brushed off. If they do become detached from their host they usually die within a few hours. They cannot breed off the host and, even under the most favourable conditions of darkness and moisture, their survival time is about five days. Sheep lice have been found experimentally to breed on angora goats. Lice spread by contact between infested and clean sheep. Spread is most likely in cool, cloudy weather when adult lice move out along the wool fibre and may be brushed from one sheep to another. The spread of lice is likely to be most rapid when sheep are frequently crowded together, especially in covered areas such as shearing sheds and sheep trucks. |
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Damaged wool from scratching sheep
© Agriculture Western Australia
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Damage
Lice infestations on sheep can lead to reductions in the value of wool by reducing fleece weight and affecting the fleece characteristics. During feeding, the lice bite the sheep and cause intense irritation. To relieve this irritation, the sheep rub against fences, posts and trees and, if heavily infested, they may scratch and bite at the wool. The wool becomes matted and discoloured, presenting a ragged, torn appearance. |
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Control
Sheep lice can be controlled by the application of insecticides. If all sheep are shorn and treated with an appropriate insecticide, lice can be eradicated. Methods of application for eradication at shearing are:
Backline: The insecticide is placed along the back of the sheep within 24 hours of shearing. It takes up to six weeks for sheep to become free of lice.
Dipping: The insecticidal solution is applied so that the solution contacts all lice. This can be done by immersion or spraying and should be undertaken within 30 days of shearing.
Treatments for eradication in longer wool are:
Handjetting: The insecticide is jetted along the back and sides of each sheep. It takes up to eight weeks for sheep to become free of lice.
Backline: The insecticide is placed along the back of the sheep and takes up to eight weeks to kill all lice.
For control, when shearing is not feasible, jetting will reduce the number of lice to a low level. An automatic jetter, which places insecticide at the tip of the wool all over the sheep, or a handjetter to saturate the wool and skin along the backfire, can be used. |
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