NADIS disease bulletins are written specifically for farmers, to increase awareness of prevalent conditions and promote disease prevention and control, in order to benefit animal health and welfare.

Farmers are advised to discuss their individual farm circumstances with their veterinary surgeon.

Chewing Lice

There were numerous reports during February from sheep flocks throughout the country of foot stamping, scratching, rubbing against fences or partitions and biting at the shoulders, flanks and backs, displacing small tags of wool. In most cases, the problem was associated with chewing louse infestation. One case was reported where the flock of about 700 Greyface and ewes had been injected with doramectin two weeks previously, at a cost of about £0.60/head, after the owner was reportedly mis-informed that such treatment would remove lice as well as sheep scab mites. Unfortunately, systemic endectocides (ivermectin, doramectin and moxidectin injections) are ineffective in removing chewing louse burdens from sheep. This case highlights the importance of achieving an accurate diagnosis and gaining reliable advice about the management of the problem.

RUBBING AGAINST FENCES ASSOCIATED WITH CHEWING LOUSE INFESTATION

The incidence of chewing louse infestation is increasing in British sheep flocks, associated with the withdrawal of compulsory dipping and increasing usage of systemic endectocides for the control of sheep scab. Louse problems generally only become apparent at this time of year (January to March) associated with a slow build up in louse numbers from shearing and favourable conditions for louse survival in long-fleeced sheep.

THE SHEEP CHEWING LOUSE - BOVICOLA OVIS

Economic importance

There have been few controlled studies to investigate the economic effects of chewing lice in British sheep, but based on overseas studies, it seems unlikely that moderate burdens would have any effect on bodyweight or growth rates. Louse burdens vary between individuals, but the heaviest burdens are generally associated with young or old animals in poor health and low body condition. There is, therefore, a general perception that chewing louse infestations cause ill thrift and constitute a welfare problem, although it is not certain if louse infestations lead to a reduction in body condition or if they exploit sheep that are ill-thrifty for other reasons.

Overseas studies suggest that severe louse infestations have little effect on growth rates or wool yields, but do reduce wool quality. However, the reduction in wool quality is only likely to be economically significant in fine wooled (less than 24 microns) breeds. The most important economic effect of chewing louse infestation results from a nodular skin defect known as cockle, which is seen in pelts after they have been tanned. However, the presence of cockle is seldom reported to individual producers, and the substantial economic losses which it incurs are borne by all. Furthermore, potential hide damage will usually have already occurred by the time that the problem is diagnosed. However, should hides become traceable to the farm of origin, then chewing louse infestation would become economically important in British flocks. Louse infestation is an important differential diagnosis for sheep scab and few producers are prepared to accept the presence patchy wool loss and obvious discomfort in their flocks.

Diagnosis

Chewing lice are found in fleece-covered areas of the back and flanks of sheep of all ages.

SHEEP BITING LICE ARE 1 - 2MM LONG AND CAN JUST BE SEEN WITH THE NAKED EYE

The severity of infestation can be determined by counting lice at several 8 cm wool partings over these areas. Average counts of 1, 3 and 5 lice per parting indicate low, moderate and severe burdens respectively.

LOUSE BURDENS CAN BE ASSESSED BY PARTING THE FLEECE AT SEVERAL SITES

Treatment

Unfortunately, the management of lice in heavily pregnant, long-fleeced ewes is problematic. Shearing significantly reduces the louse burden by removing a large proportion of the lice and exposing those remaining to the lethal effects of desiccation, but is usually impractical during winter months.

Organophosphate and pyrethroid plunge dips can provide effective louse control, but winter dipping and disposal of dipwash solution is difficult on many farms. Shower dips are only effective in short-wooled sheep and thorough saturation is essential.

Most farmers rely on the use of long-wool pyrethroid pour-ons for the treatment of chewing louse infestations. Pyrethroid pour-ons translocate over the body in wool grease. The area of wool grease, which increases greatly after shearing, determines the effective concentration of pyrethroid, so the effective concentration of insecticide on the skin is determined by the wool length. High doses are required for long-wool treatments, which are expensive and seldom remove all of the lice present. Some animals continue to show signs of rubbing, nibbling and scratching for up to 6 weeks after treatment. This may be explained by the observations that pyrethroid pour-ons take up to 6 weeks to translocate over the body and when used in full-fleeced sheep. Treatments within 6 weeks of lambing fail to prevent infection of newborn lambs.

Pyrethroid pour ons may fail to reach lethal concentrations for chewing lice when used in long wooled sheep, which may select for resistance. Chewing louse resistance to pyrethroids is now widespread in Australia and is suspected, but not proven, in Britain.

Pyrethroid residues in wool are potentially dangerous to wool handlers and damaging to the environment when wastewater from scouring plants enters a watercourse. To avoid wool residue problems, dips or pour-ons should be avoided within 60 days of shearing or slaughter.

PYRETHROID POUR-ONS MAY ACHIEVE LITTLE MORE THAN A REDUCTION IN LOUSE NUMBERS WHEN USED IN LONG-FLEECED ANIMALS. FURTHERMORE, SUCH USE MAY SELECT FOR RESISTANCE AND CAUSE WOOL RESIDUE PROBLEMS

The decision to treat louse-infested sheep is not straightforward, because it is not certain that chewing lice limit animal production or constitute a welfare problem in British flocks. In some cases, the option of leaving animals untreated should be considered, although the rationale for and risks associated with this strategy should be weighed up.

The whole life-cycle of chewing lice takes place on sheep, but bionomics are greatly influenced by climate. Louse numbers fall dramatically during periods of hot or wet weather, which can influence populations for the subsequent 6 months or more. In some circumstances, the prospect of such weather conditions may, therefore, remove the need to treat long-fleeced sheep.

Prevention

Pyrethroid pour-ons are generally effective for the prevention of louse infestations when applied off-shears (immediately after shearing). Chewing louse populations on sheep only increase slowly and transfer of lice between animals requires close contact, so spread of infection is slow. Therefore, provided that sheep do not become debilitated for other reasons, satisfactory louse control, with less selection pressure for resistance and fewer concerns about wool residues would be achieved by a single off-shears application of pyrethroid pour-on during the summer.

Neil Sargison 9^th March 2003