When lice are found, economic and animal welfare considerations dictate the need for sheep to be treated. However, the timing and method of such treatment will be determined by a range of factors (see Sheep lice: selective chemical treatments). The best time to treat sheep for lice is after shearing. Post shearing treatments should be able to reduce lice to non-detectable levels if applied properly. Products from a number of different chemical groups are registered for use on sheep. Resistance has occurred to some of these groups and management to prevent resistance should be a key consideration when determining a treatment strategy.

Methods for application of post shearing treatments include backliners and plunge, shower, and cage dipping.

Long wool treatments (products applied to sheep more than 6 weeks after shearing) will not eradicate lice. Where a long wool treatment has been applied the sheep will need to be treated again after their next shearing.

Two situations that can compromise the effects of chemical treatments and which require special consideration are where mobs on one property are shorn at different times (split shearings) or where pregnant ewes or ewes with lambs at foot need treatment.

Chemical Groups

Chemicals registered for lice control include organophosphates, synthetic pyrethroids, insect growth regulators, spinosad, ivermectin and a combination of magnesium fluorosilicate, rotenone and sulphur. Registration of diazinon
(an organophosphate) for use as a short wool dip and for long wool jetting was recently suspended. Products manufactured prior to May 2007 that display the registered label claim can still be used according to label directions but new product will no longer include this claim and must not be used for dipping or jetting. For details of the suspension of diazinon and allowable uses follow the link.

It is important to be aware of the characteristics of the different chemical groups, in particular their safety profile, residue consequences, withholding periods, mode of action and whether resistance has been detected.

All of the products currently registered for lice control in Australia, the chemical groups to which they belong, their method of application and the wool and meat withholding periods for each, together with their approximate prices are listed in the LiceBoss products module.

Backline application 

Approximately 80% of treatments applied for lice control in Australia are backline treatments. These are not absorbed but spread over the surface of the body, assisted by contact between animals. Chemicals available for application by this method include insect growth regulators, synthetic pyrethroids, an organophosphate and spinosad.

This method is very popular because of its ease of application. However, it is critical for good effect that chemical is applied to the whole length of the backline of the sheep from head to rump, that the dose rate used is calculated exactly according to label directions and that equipment is working properly and delivering the correct dose. For information about ensuring effective use of backliners, see Offshears backline treatments.

Plunge dipping 

This is the traditional method of treatment in which sheep are completely immersed in dipping fluid. It is very difficult to completely wet sheep to the skin because of the waterproofing effect of wool. The dip should be deep enough for sheep to be completely submerged and long enough to allow time for sheep to become completely wet so that all lice are treated. Lice can occur on the head so it is important that each sheep is dunked twice.

Some compounds used for dipping are subject to ‘stripping’. This is when the dipping active is removed or ‘stripped’ from the dip at a faster rate than dip wash, leaving a lower concentration of active compound. With products which strip it is important that reinforcement and topping up instructions on the label are followed to maintain adequate concentrations of lousicide in the dip wash.

Detailed information is available in the LiceBoss note Plunge dipping.

Shower dipping 

Showering relies on applying a high volume of dip wash from the top nozzles, and to a lesser extent lower sprays, to completely wet the sheep. Shower dips need to be correctly set up and working efficiently. Sufficiently long treatment times, correct boom rotation rates and adequate pressure will be needed to achieve lice eradication. It is extremely common to find that shower dips are not working properly. Directions for optimal setup and operation of shower dips should be consulted. These are given in Shower dipping for lice eradication.

Cage dipping 

Cage dips are a relatively new innovation in which sheep are enclosed in a cage and dipped as a group. Currently cage dips are used mainly by contractors.

Split shearings, pregnant ewes and ewes with lambs at foot 

Split shearings (where there is more than one shearing time on a property), and ewes with lambs at foot or due to lamb soon after treatment can cause difficulties in achieving eradication, even when treatments are applied properly. This is because backline treatments and insect growth regulator lousicides may take some time to bring about the death of all lice. During this time if there is contact with other untreated sheep there is the potential for lice to spread.

Where split shearings occur it is critical that treated mobs are kept separate from untreated sheep for the period specified on the label if a property is to remain lice free.

Managing ewes with lambs at foot or ewes due to lamb soon after treatment is more complex and the ewe-lamb module within LiceBoss or factsheet on Sheep lice control for ewes and lambs should be consulted to determine the best course of action for this particular situation.