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.