Lice DO NOT fly, jump or circle the sheep’s body every 24 hours
Symptoms of the Disease/Condition.
Lice are external parasites that can live on most animals. They are very host specific so can only breed on their own hosts.
Sheep lice breed only on sheep and complete their entire life cycle on the animal. They usually spread by direct contact between sheep.
Lice-infested sheep will bite at themselves and rub against anything that is available. This damages the fleece by matting the fibres and tearing the tip of the staple.
Up to 90 per cent of lice are removed with the fleece at shearing. Shearing is by far the most effective time to achieve control through chemical application.
The economic effects of lice will depend on the severity of infestation, the class of sheep affected and the proportion of each mob affected.
There are two species of lice that affect NZ sheep:
Lice generally do not live for more than four or five days, in ideal conditions, once separated from sheep. If lice are unprotected and exposed to sunlight they are unlikely to survive for more than a couple of hours (e.g .on wool in trucks, yards, catching pens, fences or in the paddock). Lice can be transmitted on shearers' moccasins (surviving for up to 10 days), but microwaving boots for five minutes in a plastic bag kills these lice.
Sheep lice can transfer to goats and survive the remainder of their normal life span on the goat. Sheep and goats need to be running very closely together (e.g. yarded or shedded) for this to occur. Sheep lice will not breed on goats and are very unlikely to be the cause of re-infestation. Sheep lice will not transfer to other animals.
Lice do not move far from the site of hatching. Most movement is up and down the wool fibre and is governed by temperature and light. In hot, bright conditions, lice stay close to the skin surface. In moderate (cool to warm) and shaded conditions, they move towards the tip of the fibre.
This is important, as lice are most likely to spread when near the tip of the fibre. Conditions that favour transfer will therefore occur between late afternoon and early morning (ie at sheep camps, while watering), on cloudy days and during the cooler months of the year.
Cost/Impact on Herd/Farm Revenue.
The main cost is lower value of wool because of fleece damage as result of rubbing and scratching by louse infested sheep.
A ewe shearing 6kg greasy wool with 75% yield (4.5kg clean) at wool worth $4.00 per kg clean income is $18.00.
A ewe with a medium lice burden shears 5.7kg greasy wool with 73% yield (4.16kg clean) at wool worth $3.80 income is $15.80.
Management and Control.
Effective lice control is dependant on an appropriate mix of animal management and chemical usage. The degree to which chemicals are needed will vary with the level of control that can be achieved.
Every effort must be made to reduce the over-reliance on chemical control methods and reduce chemical residues on wool. This is particularly important in relation to chemicals applied later than six weeks after shearing.