Treatment choice – key considerations
High risk of transfer of lice from ewe to lamb
Treated sheep must be kept separate from un-infested untreated sheep (isolation period). For this reason, both the ewe and her lambs must be treated at the same time. Pregnant ewes should be treated at least six weeks prior to lambing to prevent surviving lice on the ewe being transferred to the newborn lamb and subsequently back onto the treated ewe. This applies to both dips and suitable backline products as outlined below.
Product use restrictions
Some products cannot be used in late pregnancy or are unsuitable for use with ewes and lambs, for example Eureka Gold (a consequence of the time required for chemicals in the backline formulation to move around the sheep’s body and contact lice rather than a toxicity issue) and Avenge (label states ‘Treated sheep must not be mixed with lice free sheep until six weeks after treatment. Live lice on backline treated ewes within six weeks of lambing can infest the lambs of these ewes.’)
Dipping period before lambing
Dipping is best carried at least six weeks prior to lambing for two reasons. Firstly, it reduces risk of eradication failure as a result of early born lambs, and secondly, avoids stress and health consequences of dipping late pregnant ewes. This is in line with the Code of Practice for Sheep in Western Australia in which Section 9.4 states that dips or showers should be constructed, maintained and operated in a manner that minimises injury, disease and stress to sheep.
Consistent product application in the ewe-lamb unit
Shorn ewes and lambs with up to 12 weeks' wool must receive the same treatment chemical formulation. Use of dipping agents for ewes and backline applications for lambs in long wool will not achieve eradication. Dip products are not registered for use on lambs older than 12 weeks without shearing. Only Extinosad PO has registration for shorn ewes with lambs at foot without wool length specification. Remember that where treatments are applied in longer wool, eradication will be less certain.
Avoid using chemicals with known resistance such as synthetic pyrethroids or unmeasured resistance as in the Insect Growth Regulators (IGR), unless known to be effective. At present, the reliability of IGR chemicals for lice treatment may be doubtful where the lice to be treated may have originated from a strain that have had multiple previous exposures. As there is no way of testing whether sheep lice on a particular farm may be from a strain resistant to IGR chemicals, the IGR chemical group has not been included as treatment recommendations in this information for this reason.
Chemicals without extended protective periods
The mode of action of the IGRs occurs at the louse moult and when fully effective had persistency for 18-24 weeks. Such long protective periods make split shearing applications in ewe-lamb and other flocks possible. However resistance development now leaves this group with unreliable efficacy. Remaining chemicals (other than IGRs) are 'knock-down' chemicals that kill as they contact lice once the correct chemical concentration is achieved on sheep skin. The lack of an extended protective period and the knock-down nature of current chemicals means that eradication will only occur when chemicals are used strictly in accordance with label recommendations. Knock-down chemicals applied as split treatments to ewes and lambs (or any flock), will risk lice transfer between flocks.
Residues in meat and wool
Before using a product, check that the meat and wool withholding periods and Export Slaughter Interval (ESI) can be met. The ESI will restrict use of chemicals when expected turn-off time for the animals is within the ESI. Choice of chemical and the associated ESI need consideration when treatments are sought in prime lamb production. New product ESIs are listed on labels, or otherwise can be found on the Australian Pesticides and Veterinary Medicines Authority website.