Most farmers only realise their sheep are infested with biting lice during late autumn and winter, when sheep having more than 6 months’ wool start biting and scratching themselves.
A number of conditions may cause sheep to itch. The farmer will see sheep rubbing themselves against fences and gateposts, leaving tufts of wool stuck to the fence.
The sheep will also bite and tug at their wool. Both actions damage the wool. If this damage is caused by a low-grade infestation of lice, it can result in a 0.2 kg loss of wool at shearing, but it can escalate to a 1 kg (30% of annual wool production) loss in cases of heavy infestation.
Also read: Is there an ideal time to shear sheep?
The cause of itching must be investigated immediately. A sheep suffering from sheep scab will also tug at its wool, but there will be an area where the wool is wet and matted.
The wool will also fall out, leaving a wet circular wound which rapidly becomes bigger. If sheep scab is suspected, summon a State veterinarian without delay.
LICE AND GRASS SEEDS
- Other causes are lice infestation and grass seeds.
- Both can be observed by parting the wool and looking at the skin at various places on the body.
- During winter, grass seeds get into the wool and move down the wool fibres to the skin.
- The sharp ends of the seed can cause irritation and even small wounds, which will appear as red spots.
- Biting lice infestation can also be seen with the naked eye.
- The parasite is about 2 mm long, has a reddish head and lighter body and moves around.
- If only one louse is observed during each parting of the wool, the total infestation on the sheep is estimated at 3 000 to 4 000 lice.
- A heavily infested sheep may carry as many as 100 000 lice.
- During shearing, more than 60% of the lice are removed.
- The remainder are exposed to the sun and other elements and most of them die.
- The survivors will start to reproduce as the wool gets longer and provides protection.
- Adult female lice produce 1 to 2 eggs every 3 days.
- After shearing the initial increase may be slow, but the rate will soon increase because of the louse’s short life cycle of about 30 days.
- 6 months after shearing the population will have increased more than tenfold on a monthly basis.
- This is when signs of the disease become apparent.
The treatment options in long-wool sheep are limited, because the condition tends to coincide with winter. One option is to shear earlier, but that can only be determined by the season.
An even bigger problem is that the use of pesticides in sheep with wool more than 6 months old must be limited to avoid high pesticide residues in wool at shearing. For this reason, dipping long-wool sheep is not common practice.
At this stage sheep can be treated by spraying them with a registered insecticide. This is termed “jetting”, because an extended nozzle has to be used to get to the skin. You will need an average of 3 litres of dipwash per sheep.
Although injection-type ivermectins work against sucking (blue) lice sometimes seen on the face and legs, they aren’t effective against biting lice.
A “long-wool” treatment cannot eliminate a louse infestation. It can only reduce the number of lice and wool loss up to the time of shearing, when the infestation can be eradicated by using an effective treatment.
After shearing, a registered pour-on ‒ but preferably dipping ‒ product can be used for eradication, but the procedure must be managed very well to prevent transfer of infestation between ewes and young lambs or between treated and untreated sheep. Ask a veterinarian to provide an eradication plan.
The most important point to remember is that these lice only live on the sheep. They will otherwise die within a month. Buying infested sheep is the most common cause of flock infestation, but stray sheep mixing with your flock may also be a cause.
Biosecurity measures must be in place to ensure that your sheep don’t mix with other sheep. In a communal system all sheep and goats must be treated after the sheep have been sheared to control lice in a grazing system where sheep from different owners mix every day.
The threefold approach to managing lice in sheep:
- Prevent new infestations
- Detect a new infestation early
- Eradicate infestation after shearing
- This article was written by dr. Danie Odendaal and first appeared in Farming SA.