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This article was published in 1982
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Some Technical Aspects of Lice Control in Sheep

A.N. Sinclair, Senior Lecturer, School of Wool and Pastoral Sciences, Faculty of Applied Science, University of N.S.W.

The grazier must consider lice control in the whole management situation. Similarly we need to include lice control in the context of all sheep external parasites to see the matter in true perspective. In addition we should consider the sheep itself, the host-parasite interface, and the operators carrying out treatments we advise or dictate.

It is convenient to begin by classifying external parasites of sheep on the basis of life history, which includes the type of lifecycle, feeding mechanism and bionomics, i.e. changes in numbers under natural influences. Also it is useful to review recent information on the host-parasite interface, scene of the action for treatment. With such basic information we are then in a position to assess treatments either traditional or novel.

PARASITE CLASSIFICATION

There is practical value in dividing ectoparasites into three groups: field, nest and host-resident - definitions of a Canadian group led by Nelson.

(a) Field Parasites: e.g. blowfly.

These have free-living forms in the field not parasitic on sheep. When sheep are treated for field parasites there is direct effect only on parasitic forms, e.g. larvae, but not field forms, e.g. pupae or adults. The extent of this unaffected proportion of parasite population is unknown. Estimates for internal parasites are that at best we reach 5% with drug treatment missing 95% or more. It seems reasonable to guess that most of the blowfly population is similarly inaccessible to direct influence by treating sheep. This leads [us] to believe that future approach will be best directed to reducing host susceptibility rather than by using insecticides, as is happening with other field parasites such as the cattle tick.

(b) Nest Parasites: None are known on Australian sheep.

These arthropods can complete their life-cycle on the same host. Eradication is feasible with synchronous effective treatment of all sheep on the same property causing destruction of all life-cycle forms before another cycle can be completed.

FEEDING MECHANISMS

Much needs to be learned of this aspect in the life history of ectoparasite species, particularly the mites, on Australian sheep. It seems reasonable to suppose a treatment involves either contact poisoning or food poisoning of parasites so the more we understand feeding mechanisms the better chance of devising efficient treatment. There are two broad classifications of species: superficial feeders and epidermal piercers.

(a) Superficial feeding arthropods

Within available knowledge limits these are Damalinia (chewing louse) Psoroergates (itch mite) Chorioptes (Mange mite) Demodex (follicle mites). So far as we know, these parasites live on the superficial stratum corneum which will be described in detail later.

(b) Epidermal piercing arthropods

In Australian sheep these are usually insects - Melophagus (ked) Linognathus (blood-sucking lice). These parasites pierce epidermis and feed off blood in the dermis. It is not certain whether these insects are strictly solenophage (vessel feeders) or Telmophage (pool feeders. The size of the mouthparts suggests the latter is likely.

BIONOMICS

With the exception of Demodex, about which little is known, most parasites follow a general trend of maximum numbers in spring, a rapid decline in summer, low numbers throughout autumn, a winter increase to the next spring peak. The mechanism controlling populations is not known. It is speculated that a successful parasite would have evolved with peak numbers coinciding with birth of the next host generation. The practical importance of bionomics should be remembered. Eradication treatments have better results when the parasite population is lowest. Unfortunately traditional treatments are dictated by shearing date rather than parasite bionomics so we often attempt eradication at the most difficult season, i.e. spring, when parasite numbers are at their peak.

THE HOST-PARASITE INTERFACE

For a long time knowledge of this area was limited and misleading. The handicap to realistic study lay in the means available for study. The usual approach was to take skin biopsies and preserve them in fluids containing formalin. It was shown by David Lloyd that in this technique outer skin layers, in which we are interested, can be found in the bottom of the bottle, detached from the specimen we wish to study, so there is little chance of seeing important structures in the subsequent microscope sections. The usual technique was to embed the skin specimen (minus the valuable outer parts thrown down the sink!) in paraffin wax, for cutting thin sections. These were then dehydrated and treated to remove any wax, including skin lipids, for staining and microscope study. Thus traditional skin sections, though providing a useful model for building a picture of important structures of the skin organ, miss some vital components. Recently, Jenkinson and Lloyd showed how much more can be learned by placing the biopsy in liquid nitrogen then cutting frozen sections to reveal lipid or placing the preparation in a Scanning Electron Microscope to give three-dimensional representations of the skin surface.

It is useful here to give a brief summary of superficial skin anatomy which helps to describe parasite feeding and behaviour of insecticide formulations. Skin is the largest body organ. It may seem inert to the casual observer but parts of it are extremely 'busy'. Despite study there remain unknowns in our descriptions of skin anatomy and physiology which seems anomalous because the tissue appears accessible to study. Sheep skin presents three distinct lateral aspects to parasites and treatment techniques (a) glabrous or hairless skin confined to areas surrounding natural orifices and between the digits (b) hair covered skin on face, axilla, inguinal regions, legs and near glabrous skin (c) fleece covered skin which, in our sheep, is the major proportion of total surface. There is practical value in appreciating differences between hair and fleece zone skin. The important factor is the presence of a considerable amount of lipid, wax or 'yolk' in fleece zone. Vertically skin is divided into two major regions, the deeper dermis, which supplies the superficial epidermis. Important structures of dermis are vessels, nerves, cells of the immune response and wound repair, structural components such as collagen and elastic fibres. It is assured that dermis controls change in epidermis, particularly in response to damage. One control mechanism is called 'epidermal growth factor' but its nature still requires accurate description. Dermis also contains fibre follicles and their supply glands. Sebaceous glands supply all follicles; sweat glands are specific to primary follicles. In Merino sheep the ratio of primary to secondary follicles about 1:20 or more) is different from other breeds of sheep (about 1:5 or less than 1:10). Epidermis is of major interest in this discussion. It contains two distinct cell series. The keratinocytes form distinct layers. Starting at the dermo-epidermal junction is the basal layer, covered by the prickle cell layer. Cells in both layers can divide and are misleadingly called 'living' epidermis. All cell layers are living. Though the more superficial cells do not divide, their function is vital to life. An analogy may be made with red blood cells which also have become highly specialised and have lost the potential to divide. The superficial layers are the granular layer, incomplete in fleece zone skin, though usually present around the follicle mouth, prominent in wound repair, and the outermost stratum corneum. Lloyd and his colleagues have shown that stratum corneum is much thicker than was appreciated from paraffin sections. It is composed of an inner dense region, and an outer loose zone. The latter is impregnated with complex lipid emulsion which forms a superficial layer over the outer loose stratum corneum.

Outer loose stratum corneum and lipid emulsion are vital to understanding the concepts of insecticide application and effect. The lipid emulsion is produced by combined sebaceous and sweat gland secretions. There is an additional, probably small, contribution from the keratinisation of upper keratinocytes. The lipid emulsion contains fats / salty acids, proteins, liquid from sweat gland and antibodies. It may be contaminated with minerals and other material from the environment. The lipid emulsion is a major barrier against infection and has been studied in important skin disease such as mycotic dermatitis and Pseudomonas infections of fleece rot. This complex material is an important diet item of the superficial feeding external parasites.

The other cell series in skin is not so well known. It has a variety of names which may be confusing. Lyne and Hollis called the 'dendritic' cells because of the dendritic processes similar to nerve cells, and proposed that these cellular processes control keratinocyte functions, probably keratinisation. This has yet to be proved but an adequate explanation has yet to be given so dendritic cells could well be a factor in keratinocyte control mechanisms. Some dendritic cells contain melanin granules and are called melanocytes. Others, in the basal layer, called Merkel cells, have nerve connections to dermis and provide sensors of pressure changes on epidermis. In 1868 other cells in this series were described by Langerhans better known for his description of islets in the pancreas). It is remarkable that the function of Langerhans cells has only just been described, more than a century later. They are now considered part of the immune system, being formed elsewhere in the body and migrating to the skin. While We have no description of the role dendritic cells play in the pathogenesis of skin lesions resulting from external parasites it seems reasonable to believe that they are involved in responses to infestation by the host. We are concerned here with eradication of host resident ectoparasites by insecticide treatments so this aspect of skin physiology is not of immediate interest.

THE PARASITE HABITAT

With the exception of Demodex all other ectoparasites live either on the fibres and/or in the lipid emulsion and loose outer stratum corneum. Immature Psorergates are found in inner dense stratum corneum. If other parasites also penetrate this area evidence has yet to be seen. It is surprising we know so little about essential details of the habitat for parasitic mites even though we presume to control them. Also it seems anomalous that evidence has not been seen to establish the nature of the critical contact between parasites and insecticide. It is presumed to be either trans-cuticular contact poison) or by ingestion of poisoned foot [sic]. Without this information our chances of improving target precision for treatments is handicapped. Here we will assume the important phase of insecticide contact with parasites in their food supply, particularly in fleece zone skin.

THE LIPID EMULSION

When discussing behaviour of insecticide in fleece it is an oversimplification, but practically convenient, to imagine the fleece as a 'solution' of lipid emulsion pierced by fibres. The volume of this solution varies directly with fleece fibre length. Insecticide mixed with this lipid solution follows a simple relationship between weight of applied drug and volume of the solution in which it is 'dissolved'. Being a solution it follows the law of mass action i.e. within normal temperature range the concentration of insecticide in lipid will tend to equalise throughout the entire volume of the lipid emulsion. Thus insecticide applied either at the fleece surface, or systemically through the epidermis, will eventually disperse throughout the entire lipid emulsion but its concentration will decrease with increasing wool length unless there are compensatory increases in drug dose.

DILUTION WITH WOOL LENGTH

In most traditional treatments with insecticide the amount of insecticide applied to each sheep remains relatively constant, unless special adjustments are made, e.g. by increased wash concentration. It is therefore generally true, though often overlooked, that the longer the wool at time of treatment the lower the final concentration of insecticide in lipid emulsion. Hence it is likely that topical treatments in longer wool are less effective unless compensatory dose increases are made. This may not be feasible due to increased toxicity and/or cost. This handicap to topical treatment in long wool is distinct from another important hazard, that of longer wool sheltering host-resident arthropods at skin level. The combined handicap leads to a slogan which should be emphasised in all topical aspects of host-resident parasite control - THE SHORTER THE WOOL THE BEITEH THE TREATMENT.

THE IMPORTANCE OF INSECTICIDE PERSISTENCE

Sheep fleece is unique in the proportion of lipid emulsion present, particularly the Merino breed. This has practical implications. Insecticide combines with lipid emulsion and persists for prolonged periods. A single treatment such as showering or dipping will be effective against lice for several months. Stronger applications, e.g. dusts and surface sprays may be detected by chemical analysis a year later, though it is not certain how effective these prolonged deposits may be against lice. In practical terms a single treatment of sheep with insecticide persists longer than the life-cycle, particularly in the 'in egg' phase of the parasite. Thus it is feasible to anticipate eradication of fleece zone parasites by a single treatment. In hair zone insecticide can only persist a day or two. The practical implication is that in control of hair zone parasites repeated treatments, at about two week intervals, are needed to eradicate host-resident parasites which otherwise survive 'in egg' and emerge after the insecticide has deteriorated to ineffective levels.

The practical importance of insecticide persistence must be considered in two ways. Firstly, although insecticide persistence in fleece zone aids eradication with a single treatment it should not be used as an excuse to avoid synchronous treatment of all sheep on a property. This should be discouraged since breakdowns of eradication campaigns can be traced to unnecessary postponements of treatment. On the other hand it is important to realise that hair zone parasites on any animal species need repeated treatments at 9 to 14 days intervals for eradication.

SPEED OF THE INSECTICIDE-LIPID COMBINATION

This process seems rapid. Wash samples taken at the nozzles and drain outlet of a sheep shower show a different insecticide concentration, the decrease being known as 'stripping'. In the laboratory a glass slide coated with wool lipid, passed through a dip wash solution will immediately take up insecticide. The bond between insecticide and lipid seems robust. Attempts to wash insecticide from the fleece of dipped sheep are usually unsuccessful. Similarly sheep dusted or treated by surface spray, then subjected to heavy rain showers, are still freed of lice and ked.

STRIPPING OF DIP WASH

This may be subject to misconceptions. It need not be complicated provided several relatively simple factors are clearly understood. The wash ~~from~~ draining from sheep either in a shower or a plunge bath contains lower insecticide concentration than that applied. The inference is that the fleece has selectively absorbed insecticide into the lipid emulsion. It may also be absorbed into mineral contaminants such as dust or organic contaminants e.g. faeces. It is possible that a small fraction may be absorbed onto surfaces of the equipment e.g. concrete. Fortunately the major proportion of stripped insecticide may be assumed to be absorbed by the fleece lipid emulsion. Thus stripping is a helpful process in transferring insecticide to the site of action against the parasites. Stripping is only harmful if not corrected. One technique is to add dip concentrate to the wash at regular intervals during treatment. This is rarely successful. The operator may forget to do so, or the grazier may resent adding expensive dip concentrate without obvious result. A successful technique, now widely adopted, is that of continuous replenishment. In its simplest terms this consists of mixing up fresh dip wash in a tank beside the shower or plunge bath. The wash is trickled in during showering or dipping at a rate sufficient to compensate for the volume of wash removed by treated sheep. Operators can see the necessity for compensating the volume and this is usually done. Many people get confused by the habit of trying to explain the system in relation to depletion rates. It is much easier to understand that the replenishing wash contains insecticide which is going in at a rate sufficient to maintain the wash concentration at a slightly lower level than that running in and this is adequate for louse control. Contrary to official uncertainties about the system, judging by confusing regulations, it is sufficient to mix replenishment wash at normal dipping concentrations.

INFECTION HAZARDS IN SHOWERS AND PLUNGE BATHS

If the slogan THE SHORTER THE WOOL THE BETTER THE TREATMENT is taken to its logical extreme an ideal occasion for topical treatments is off-shears. One could postulate the 'perfect' system of a shearing hand piece which also delivered the insecticide, though such an invention seems unacceptable on an industrial basis. Off-shears showering and occasionally plunge dipping is practiced by many graziers. There remains the myth that insecticide can be absorbed through skin cuts, hence the hazard of this practice. In fact insecticides are readily absorbed through intact skin. The hazard lies in infections of skin cuts. If one thinks about fresh cuts the outflow of blood or serum seems likely to hinder, rather than aid, insecticide absorption.

HYGIENE AT SHOWERING OR DIPPING

Hygiene can be practised in several ways. The most useful is also cheap. At the end of showering or dipping each day pump all foul wash from the sump, rinse with clean water, leave dry overnight or until next dipping session. A small amount of antiseptic may be added. It is unlikely that sufficient antiseptic could be combined in a dip formulation to control build-up of infection in foul dip wash left overnight in warm weather. The bacteriostat included in dip formulations is usually specific for Erysipelothrix, a cause of post-dipping lameness. A major hazard from infection is the practice of dipping or showering grass-seed infested sheep. The result is usually multiple skin abscesses, which of themselves may not be fatal but affected sheep are reluctant to walk to water, and are hyperthermic, so may die of thirst. In such cases it is a reasonable 'therapy' to hold the sheep on water and if necessary provide feed.

INSECTICIDES ARE POISONS

Insecticides are poisonous to sheep, operators and parasites. Proportional dose rates are in our favour. A 50kg sheep exposed to 1 gram of insecticide is dosed at the rate of 1:50,000 which is usually safe. It takes about 20,000 lice to weigh a gram so the parasite dose rate is 1:1 or thereabouts which is usually fatal. Operators may find to their dismay that insecticide readily penetrates intact skin when they suffer various forms of toxicity and skin disease. Sensible precautions are to avoid prolonged exposure in clothing wet with dip wash, and to avoid handling dip concentrate without gloves. It is also a useful precaution to provide numerous drinking vessels of clean water around the dipping site so that heated dogs lap at water and not dip wash.

INSECTICIDE APPLICATION TECHNIQUES

Having discussed essential preliminaries it is now possible to come to the major topic. With the basic knowledge it is possible for informed persons to make their own appreciation of any application technique either traditional or novel. Insecticide treatments may be broadly classified as either topical or systemic. 'Pour on' treatments are a combination of both though essentially systemic.

(a) Topical treatments.

These are usually traditional methods. Perhaps the earliest recorded are seen in the Old Testament where it was common to anoint the head with oil, either sheep or kings. This results in clogging the tracheae of parasitic arthropods leading to their eradication. In primitive husbandry to this day oil applications are still employed. About the beginning of the 1800's there is evidence of a 'new era' in sheep treatment. One interesting account has been found in the journal of Charles Rowe, brought to my attention by Norman Graham, that doyen of sheep ectoparasite research. It is quoted here for its historical interest as well as a pertinent comment on hair zone applications.

'My father took up Naringal Hill in 1841 after walking from Melbourne. Scab got amongst the sheep and as no-one knew how to cure it there was a great deal of unnecessary work. We stretched a bullock hide upon a frame and put the dip stuff in, and two men held every sheep and soaked it. It was then placed on some pieces of board at one end and the surplus stuff scraped out with a piece of hoop iron. At that time we used arsenic and killed a good many sheep besides spoiling the wool by making it dry and harsh. Spotting, or pouring stuff on where sheep had plucked the wool was also done but it was no good. Afterwards when tobacco and sulphur were discovered to be a cure it was a very simple matter to get rid of it. The way was to dip one day and again nine days later and again nine days later, which if done properly and the sheep kept from mixing with others will cure any scab.'

The use of repeated treatments for hair-zone parasites has been mentioned previously. The scab mite inhabits both fleece and hair zone skin.

The plunge bath in various forms has had about two centuries of use. It can be effective but has handicaps, e.g. arduous labour involved forcing reluctant sheep into the dip, the hazard of infection, and some claim adverse effect on sheep body weight. Misconceptions about plunge baths are of practical importance, perhaps the worst being the notion, widely held yet without proof, that sheep stay in the bath for thirty seconds or more. This legend does not stand close examination. A simple head count and a stopwatch will reveal an average time in the bath around ten to fifteen seconds. Head ducking tends to be fortuitous and parasites may survive on the poll. Because cleaning is a major task it tends to be postponed.

The sheep shower was devised to overcome plunge bath deficiencies. Early designs before 1920 usually failed with clogged nozzles. In 1929 Wass patented a design which, as a student, I saw in operation at 'Bundemar'. It consisted of a shed holding two pens of sheep. The roof was pierced with holes which were kept free by a lad on the roof armed with a broom. The wash was pumped from a sump, which collected drainings, a typical re-cycling shower. Early observers, including officials, condemned the design because it lacked floor mounted nozzles to spray the undersides of sheep. This matter of the floor mounted nozzles has become a major legend in shower design. It is tenaciously held that without floor mounted nozzles showers are ineffective, yet objective experiments do not support the proposition. Sheep can be held in a cage over a floor mounted nozzle for prolonged periods of showering without demonstrating fleece penetration. One can also demonstrate effective treatment of sheep infested with lice, keds or itch mite, by showers using only top nozzles. There must be some psychological factor involved since graziers pay hundreds of dollars for the redundant equipment. It is recommended that top showers be left on for the entire showering period. If operators must fiddle with the floor mounted nozzles then only use them for the final minute. Showers have advantages, e.g. control of the time sheep stay in them. Contrary to legend one cannot drown standing sheep in a shower. Consideration of respiratory tract anatomy will show why. Investigations of drowning in showers usually reveal that sheep have fallen to the floor and have inhaled or ingested dip wash while lying down and being trodden on. Showers have the advantage of easily achieved hygiene. As with all topical treatments the shorter the wool the better the treatment. Long wool showering is tedious, taking as long as ten or fifteen minutes per pen, and the wool is often stained and thus downgraded. Reports of showering without recycling wash for blowfly control should specify that this is for only 20 seconds. Prolonged showering without cycling wash would be prohibitively expensive.

Dusting and surface sprays had their vogue but have fallen into decline. This is a pity because they present considerable advantages off-shears. A major factor in their decline was use at times other than shearing without compensatory increases in dose of insecticide. That is unlikely to be popular because of cost. Another factor was the use of insecticides which do not simulate the 'pour-on' effect. McCosker and Osborne showed that aldrin concentrate applied to the head of sheep would cause death of lice all over the body within a week or so. Dieldrin works similarly. Current topical insecticides do not have this effect.

Topical insecticide treatments may have an immediate trans-cuticular effect by direct contact in very short wool, as well as a delayed effect when parasites ingest insecticide-impregnated lipid emulsion.

A novel topical insecticide treatment has recently been introduced. It has advantages, particularly operator convenience off-shears. It involves off-shears application of insecticide in a solvent, the result being rapid spread over and perhaps in the lipid emulsion so that within several days most parts of the body are covered with insecticide-impregnated lipid emulsion. There could be a systemic component but evidence for its existence has not been seen in published accounts. The technique has encountered unforeseen complications. The solvent in the formulation removes superficial lipid emulsion at the application site, and possibly removes some loose outer stratum corneum. This leaves the skin organ susceptible to infection because the protective barrier has been removed. Mycotic dermatitis is an obvious candidate. It is also reported that unspecified deterioration occurs in sheepskins but this requires careful evaluation and long term observation. No doubt both problems are receiving careful attention and we await the next formulation. The technique, though convenient, is still tied to shearing time, which may not be the optimum season for eradication treatments, as mentioned before in parasite bionomics.

(b) Systemic Insecticides.

In 1957 McCosker and Osborne demonstrated the systemic effect of concentrated aldrin. Since then less toxic insecticide formulations may be administered as 'pour-ons' to hair zone skin of cattle. The mechanism is transcutaneous absorption followed by secretion through sebaceous and possibly sweat glands so that the surface lipid emulsion is universally impregnated with insecticide, thus compensating deficiencies of topical application. Similar formulations have not been seen in commercial use for sheep. Perhaps significant amounts are bound in the lipid emulsion at the application site, thus being unavailable for absorption.

As a treatment technique a strong case can be made for systemic treatments. Oral dosing and/or parenteral injection may prove nearly as convenient as 'pour-on' yet less expensive. These could be used irrespective of shearing time, thus at seasons such as midsummer when populations are low and more easily eradicated. Systemic insecticides will probably have an immediate effect on epidermal piercing parasites which suck blood and tissue fluids. A delayed effect seems probable in the case of superficial feeders as it takes about a week for radio-labelled injections to reach the skin surface in sebaceous gland secretions. It is possible that systemic insecticides will have differential efficacies against epidermal piercers compared to superficial feeders and this should be remembered when evaluating systemic compounds.

The vexed 'political' problem of tissue residues is outside the scope of this discussion.

OPERATOR EFFECT

Whatever treatment technique is used it is prudent to consider operator effect. Some graziers can achieve success with means which fail for others. This seems inevitable for any technology involving people not specifically trained. In addition graziers have to manage multiple problems. While it is reasonable to anticipate eradication success by dedicated personnel such as experimental workers, without distractions, able to concentrate on their tests, usually with discrete flocks, it is a different proposition when one works on a large flock while trying to cope with other demands, particularly around shearing time. It is my belief that if nationwide eradication is to be a political objective we must consider the managerial and personal factors as well as the scientific technology of drug use.