Sheep Lice Control for Ewes and Lambs

If dipping is contemplated prior to lambing, it is best carried out at least 6 weeks before lambing for two reasons. Firstly, it lessens the stress and health consequences of shearing and dipping late pregnant ewes, and secondly, it reduces the risk of treatment failure from carry over of lice to early born lambs.

Some dip products specify that ewes heavy in lamb should not be dipped. Whereas fast-acting ‘knock-down’ dip products can eradicate an infestation within a day of treatment, insect growth regulator (IGR) dip products may require 18 weeks to control all susceptible lice (see Sheep lice treatments – chemical group characteristics or the Products Tool for a list of products in different chemical groups).

If ewes are not in good condition, or if the imminent start of lambing excludes dipping as an option, a backline treatment must be used after shearing and these require 6 weeks (longer for IGRs) to remove all lice.

Unless lice are eradicated from ewes prior to lambing, there is a high risk that lice will transfer from ewes to lambs. Product labels contain restrictions such as mandatory isolation periods (time when treated sheep must not be mixed with untreated sheep) that apply in this situation. Mandatory isolation periods vary between products, but are 6 weeks for many backline products.

Some fast-acting dips may be effective in a shorter period, but it is generally unwise to dip ewes during late pregnancy. IGR dipping products require a longer time to control lice and it is recommended that lambs born to IGR-treated ewes be treated in the first 3 months of life.

Fluralaner must be administered orally to pregnant ewes 4 weeks or more before lambing starts, to ensure complete louse kill by the time lambs arrive.

Where possible, use the same method of treatment for ewes and lambs. Use of dipping products for ewes and backline products for lambs, or vice versa, may not eradicate lice because the protective effect of dipping may lapse before all lice on the backline-treated sheep have died. Dipping should only be conducted with ewes or lambs within 6 weeks after shearing. Treatment of either ewes or lambs with a long wool product (greater than 6 weeks after shearing) will reduce, but not eradicate, a lice infestation. These sheep must be retreated after their next shearing.

Oral fluralaner can be used on pregnant ewes up to 4 weeks prior to lambing, or once lambing has completed and all lambs weigh over 6 kg. In both situations, all sheep must be treated. The wool length does not matter.

Products vary in whether or not they can be used on unshorn lambs and on what age/size lambs they can be used; always check the label claim before choosing a product.

Oral fluralaner can be used in lambs as small as 6 kg.

One product containing a neonicotinoid insecticide can be used on unshorn lambs up to 2 months of age, and some IGR backline products can be used on lambs up to 3 months of age.

Label instructions indicate that the macrocyclic lactone backliners should only be applied to shorn lambs and not be used on lambs less than 10 kg bodyweight.

For health reasons dipping young lambs is usually inadvisable. There is also the risk of smaller and weaker lambs getting down in the yards or being caught under other sheep in the dip.

Oral fluralaner can be used in all classes of ewes, including in long wool but is not suitable for ewes who will ever produce milk for human consumption. It is not advised to treat during mating, as this in general disturbs sheep behaviour during this critical time.

Most short wool backline treatments must be applied within 24 hours after shearing, although some products can be used up to one week after sheep are shorn.

Depending on the product chosen, plunge and shower dipping must be conducted between 10 or 14 and 42 days after shearing (see label directions). As long as shearing cuts have healed, the optimum time is from 14–28 days after shearing.

Resistance is known to occur to synthetic pyrethroids and IGR compounds and can compromise the effectiveness of chemical treatment. Unless you are certain of their effectiveness against the lice on your property, avoid using chemicals to which resistance is known.

Oral fluralaner has no claimed protection period, so all sheep must be treated and re-infestation prevented.

Some IGR product labels carry a claim that they will protect sheep from becoming infested with IGR-susceptible lice for 12 weeks after treatment. The neonicotinoid-based product Avenge™ claims to protect treated sheep from reinfestation for a period of 4 weeks, while Fennec™ (imidacloprid + abamectin) has a registered protection claim of 5 weeks. Some other products have registered protection period claims.

Extended protective periods may be useful when treated ewes contact untreated lambs, or when treated lambs contact untreated lousy ewes.

Despite these claims it is unwise to rely on chemical treatments to protect sheep from becoming infested. It is best practice to manage mobs so that treated and untreated sheep do not contact each other. This is best achieved with good fences.

Check that these withholding periods can be met before using a product. The meat WHP and ESI will restrict choice of chemicals when treated sheep or lambs may be sold for slaughter for the domestic or export markets. Consideration of WHP and ESI will be particularly important when choosing treatments for prime lamb flocks.

The meat WHP and ESI for recently registered products are shown on the label, but can also be found listed on the National Sheep Vendor Declaration, on the Australian Pesticides and Veterinary Medicines Authority website, the Meat & Livestock Australia website or by telephoning 1800 023 100.

Consideration of the WHI, also shown on the label of most products, will be important when choosing a topical product for long wool treatment.

Table 1 shows the chemical groups and methods of application available to treat sheep lice in breeding ewe flocks. For a listing of products containing chemicals from the different groups, please refer to the LiceBoss Note: Sheep lice treatments—chemical group characteristics or the Products Tool Remember, lice will not be eradicated on ewes treated with a topical product in long wool, so they and their lambs will need to be treated again after their next shearing.