Regular checking of your sheep for the signs of lice is important if infestations are to be identified early. Sheep can be infested with lice without showing overt signs so it is important during regular flock checks to catch some sheep for a closer inspection. To check sheep, choose somewhere with good light and look for lice along the sides of the neck, body and rump. Select sheep that are showings signs of rubbing. Part the fleece 20 times per side each parting being 10 cms. Adult lice are readily visible with the naked eye
Effects of lice
Sheep lice are obligate parasites which means they spend their entire lifecycle on sheep. Lice feed on flakes of dead skin, secretions and bacteria normally found at the surface of the skin. This causes irritation and therefore sheep bite, rub and pull at their wool. Signs of a sheep lice infestation includes cotted, discoloured fleece, fleece derangement, reduction in fleece weight and scouring yield.
Adult females lay 2-3 eggs every three days or so. The eggs are attached to wool fibres and hatch after about 10 days. The ideal temperature for egg laying and hatching to occur is 36-39°C and humidity 70-75%. Temperatures outside this range inhibit egg laying and humidity >90% prevents egg hatching. The newly hatched immature lice go through 3 nymph stages with a moult between each to become adult lice. The whole cycle can take 31-34 days.
The main way lice spread is via direct contact between sheep so good boundary fences are crucial to keeping stray sheep out. However, not all introduced sheep get in unintentionally. It is important to have biosecurity measures in place for all introduced sheep such as purchases, rams, sheep coming back from agistment or shows for example. Where possible, know the status of purchased sheep via the health history provided by the vendor. Other sources of infestation include shearing sheds where lice can survive for up to 2 weeks and on clothing and shearers’ footwear. Despite common belief, tags of wool on fences are not a major source of infestation because lice do not survive long under these conditions.
Some strains of lice have become resistant to certain chemical groups. Notably, there is widespread resistance in sheep lice populations to the synthetic pyrethroid (SP) and insect growth regulator (IGR) groups. In order to minimise the emergence and development of resistance, chemicals should be used strictly according to label instructions.