Lice Treatment for Sheep

Conditions and Environment

Finding lice

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.

Life cycle

Life cycle

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.

Cost of Lice

Sheep lice are a major cause of reduced productivity in all classes and ages of sheep and the cost of control is significant. It is stated that lice cause a 10% reduction in harvested wool weight due to rubbing and wool lost on fences and the remaining wool has 10% less value due to damage and discolouration. Lice can affect productivity too as sheep spend less time grazing.

Management Strategies

Biosecurity Management

Good biosecurity is essential for effective lice control. The main way lice spread is via direct contact between sheep so good boundary fences are crucial to keeping stray sheep out. A common source of lice infestations is stray sheep coming from a neighbouring infested flock via a faulty fence line. Don’t simply return stray sheep by tipping them back over the fence. It is useful to talk with your neighbours and adopt coordinated control strategies and work together to maintain good fences. A number of common farm practices can make lice control more challenging. Split shearing can result in treated clean sheep coming in contact with untreated, infested sheep unless strict separation can be maintained. Failure of clean shearing, for instance due to skin infections such as lumpy wool or “dermo” can reduce the efficacy of lice treatments and should be addressed. Also having lambs at foot at shearing poses another challenge. Either lambs should be weaned before shearing or if lambs are still on their dams at shearing, they need to be treated at the same time with a suitable registered product.

Quarantine Management

In general, all introduced sheep need to be quarantined until they are shown to be free of lice or treated appropriately. If introduced sheep are treated for lice, keep them apart from any clean sheep for 6 weeks to allow time for the treatment to kill all lice on them

Chemical Treatments

After shearing, there are two options in terms of treatment methods, either a pour-on backliner (e.g. Viper ) or a sheep dip (e.g.Piranha®) via either plunge or cage dip. Both methods are highly effective and both have their advantages.


Sheep can be dipped up to 6 weeks post shearing so this gives more flexibility if weather conditions are not ideal. Once the process is up and running the first few batches of sheep should be checked for thorough wetting. Wetting can be checked by using a water activated pencil. If sheep are not thoroughly wet they need to be run through again to ensure adequate time in the dip wash. The length of the dip should be sufficient to allow for adequate time to achieve wetting and at least 2 dunkings. 9 meters is considered a minimum length. When dunking sheep, a push backwards helps open up the wool and slow progress through the dip.

Dipping hot, thirsty sheep should be avoided. Yarding them the previous night will let them settle down and if kept off feed will reduce faecal contamination of the dip. Avoid dipping sheep with shearing cuts or grass seed damage. Waiting at least 10 days after shearing for cuts to heal is important to minimise infection transfer. Dip sheep in groups of similar size to avoid inhalation of dip which can lead to pneumonia.

Backline chemicals

Backliners work by delivering a concentrated dose of chemical to the sheep’s back which then moves via gravity and diffusion in the wool grease layer around the sheep’s body. Sheep can be treated off the board or soon after shearing so no re-muster is required, infections through shearing cuts such as abscesses, cheesy gland or arthritis are less likely and backliners are ready to use so there is no need for measuring volumes and mixing.

However, correct application is important. Good application involves means draughting sheep according to weight and dosing to the heaviest in the group, using serviced and calibrated applicators, treating all sheep and avoiding having poorly shorn sheep or sheep with “dermo”.

VIPER is a next generation sheep lousicide containing thiacloprid, a high potency neonicotinoid insecticide that knocks down lice quickly by affecting their nervous system, resulting in insect paralysis and death. Thiacloprid has been shown in laboratory studies to be 6 times more potent against lice than imidacloprid.