Control of lice on haired and fleece-shedding sheep

Note: For brevity, the term ‘new breeds‘ is used in this page, though it is not strictly correct. The sheep breeds referred to are those that shed their fleeces and/or have fleeces that are wholly or partly kemp or hair. Some of these sheep have coloured fleeces. The Dohne Merino, the South African Meat Merino (SAMM) and the Afrino are relatively new to Australia but are not included because they are wool producing, non-shedding Merino sheep.

New breeds

Breeds such as those listed in Table 1 are often collectively called the ‘new’, ‘alternative’ or ‘exotic’ sheep breeds. Most of these breeds are no longer particularly new to Australia and they are no more alternative than any other breeds (they are exotic but so are all the sheep in Australia). Most of these breeds originate from Africa or the Middle East. The Wiltshire Horn and Wiltipoll breeds shed their fleeces annually but they are not usually described as new breeds because the Wiltshire was introduced into Australia from England in 1952 and the Wiltipoll was developed in Australia from 1996. There are some new breeds of fleece-shedding sheep, developed in Australia, but not mentioned in Table 1.

Claims have been made that external parasites do not establish on some new sheep breeds but this is not true. Anecdotal evidence in Western Australia suggests however that lice are less frequently detected on these breeds and their crosses than on woolled, non-shedding breeds. The fleeces and skins of some are quite different from those of a Merino, so perhaps the new breeds are poor hosts for external parasites.

Why treat new breeds for lice?

The direct cost of lice infestation of sheep may be small if their fleeces have little or no market value. However effective lice treatments will probably be required at times for other reasons, such as the following:

• Transfer of lice to or from wool producing sheep. If there are wool producing sheep on the same farm as louse infested, non-wool producing sheep, then treatment of the latter to protect the former will be essential. Conversely, if wool producing sheep have lice and lice eradication is desired, then all sheep on the farm, including any new breeds, will need to be treated.
• Good neighbourly relations. If neighbours with wool producing flocks have (understandable) concerns about the risk of spread of lice infestation to their own sheep.
• Skin and property damage from rubbing. The cost of damage to potentially valuable sheepskins and to fences, gates and other structures, caused by sheep rubbing, can be difficult to quantify but may be important.
• Cockle. Lice can cause a skin allergic response, resulting in a type of skin damage called cockle. This is only apparent when skins are processed. Cockle can reduce the value of high-grade skins, such as those that the new breeds are expected to produce.

Not all itchy sheep are lousy sheep

When any sheep rub or chew at their fleeces, lice infestation is just one of a number of possible causes. Some exotic diseases, such as scrapie, sheep pox, lumpy skin disease, screw worm fly and Aujeszky’s disease may cause sheep to rub or chew their fleeces, among other signs (see biosecurity reminder below).

Sheep that shed their fleeces annually tend to rub when their fleeces are being shed in spring and summer. Other parasites may be responsible for skin irritation, such as itch mites or fly maggots. Grass seeds, photosensitisation of the face and ears following some plant poisonings, or sunburn on the back following shearing may all cause rubbing or the appearance of fleece derangement.  Before assuming that itchy, rubbing sheep of any breed are lousy, inspect them carefully and seek expert help if you are uncertain, especially if they are showing other signs of illness.

Issues with lice treatment of new breeds

All the chemicals registered for the control of lice on sheep have been developed, tested and registered in the expectation that they will be used on wool producing sheep that are shorn annually.

The recommendations for treatment of fleece-shedding or haired sheep breeds are the same as those for wool sheep. Any product that is registered for lice treatment of sheep may be used, according to label instructions, on new breeds. None of the registered treatments, whether for long wool or off-shears use, has a label claim for eradication of lice, although eradication may be achieved, especially with off-shears treatment.

Some issues with lice treatment of new breeds include:

• Insecticide spread on individual sheep. The spread of chemical over the skin from a pour-on or spray-on treatment may occur differently on some new breeds compared with wool sheep, due to lower levels of lanolin in the former. Treatment efficacy may not necessarily be reduced though and it is even possible that these treatments may be more successful (on average) against lice on new breeds than on wool sheep.
• Insecticide persistence. The persistence of insecticides may be reduced compared with wool sheep because some new breeds have less lanolin in their fleeces. None of the insecticides used for sheep lice treatment kills louse eggs and so insecticides must remain at effective levels long enough to kill lice that hatch from eggs. If you decide to re-treat sheep in case chemical persistence is inadequate, then the second treatment should be given not less than 10 days or more than 30 days after the first treatment. This interval allows time for all eggs to hatch but not enough time for adult lice to start laying eggs.
• Itch mites. Itch mites (Psorergates (Psorobia) ovis) live under the skin surface and may cause sheep to chew and rub their fleeces. If uncontrolled, itch mite infestation may cause economic losses, particularly in Merino sheep. The prevalence and importance of itch mites in new breeds in Australia is unknown. Unless itch mite infestation is diagnosed, there is no point in treating new sheep breeds for itch mite. Macrocylic lactone drenches, used against worms, will also control itch mite.
• Chemical residues. The choice of which lice treatment to use for wool sheep can be influenced by concerns about chemical residues in wool. If sheep are not shorn and the fleeces not sold or processed, then there need be little concern about wool withholding periods when choosing chemicals. However, meat withholding periods and occupational health and safety recommendations will need to be observed. If the sheep are dairy animals, any milk withholding period must also be observed.

Options for lice treatment of new breeds

Off-shears lice treatment

We recommend this option.

Although the sheep may not require shearing and their fleeces may have little or no value, treatment off-shears will give the best chance of lice eradication. Shearing itself will markedly reduce lice numbers. Off-shears treatments should be applied according to label directions. That is, backline treatments should be applied within 24 hours or seven days (depending on the product used) and dips within six weeks after shearing but preferably within two to three weeks.

There are numerous products registered for lice treatment of sheep off-shears and many factors to take into account when deciding which product to use. As well as cost per dose, also consider the possibility of lice resistance to the chemical, operator risk, meat (and possibly fleece and milk) withholding periods, ease of application, stress to sheep and operator during treatment and the risk of other problems or diseases associated with particular treatment methods.

Long wool lice treatment (without first shearing the sheep)

We do not recommend this option.

It is better to wait and treat off-shears. If sheep are treated in long wool and not subsequently shorn and re-treated, the best that can be hoped for is that lice numbers will be suppressed at a low, tolerable level. None of the chemicals registered for use on sheep with long wool (more than six months since shearing) will reliably eradicate lice. If eradication is your ultimate goal, this option will cost more because a second treatment will still be needed off-shears. It also carries the risk of selecting lice for chemical resistance, leaving chemical residues in meat or fleece and exposing workers to the hazards of chemical use.

Biosecurity reminder

Always ask your veterinarian to investigate whenever you have sheep with neurological signs and rubbing or any other unusual combination of clinical signs occur in your livestock. The cause might not be what you think. Call a private or Department of Primary Industries and Regional Development (DPIRD) veterinarian, or the Emergency Animal Disease hotline 1800 675 888 after hours.

The Subsidised Disease Investigation Program, delivered through the Department of Primary Industries and Regional Development, provides subsidised veterinary investigations (subject to approval by the local DPIRD Veterinary Officer) for livestock diseases with high stock losses or which have similar disease signs to an exotic or reportable disease.

When purchasing any sheep, ask the vendor for a national sheep health declaration which covers lice, ovine Johne’s disease, footrot, ovine brucellosis and drenching and vaccination history.

All livestock disease investigations provide evidence of Australia’s internationally high health status, supporting the export of Australian livestock and livestock products around the world.