Sheep scab is caused by the non burrowing, mite Psoroptes ovis, which just visible to the naked eye. The adult female is pearly white and approximately 1.0 mm in length. The life cycle of P ovis takes 14 days in ideal conditions from egg to adult, and consists of 4 motile stages, but the adult female can live for up to 40 to 50 days, depositing 1 or 2 eggs per day. The mite will remain infestive off the host for 15 to 16 days and an infestation can he initiated by only one egg laying female. Transmission of sheep scab can be either Direct or Indirect.

1. Direct: Through forced sheep to sheep contact at market, in livestock lorries, at feeding troughs or any time that sheep are gathered and kept tightly packed together.
2. Indirect. Through contact with residual mites in tags of wool or scab attached to brambles, fencing, farm machinery, animal housing etc. Shearing combs and cutters and contaminated clothing can also spread scab.

There are no other hosts for P. ovis in the United Kingdom, other than sheep. P ovis can infest restrained cattle, during which period the mite is still infestive to sheep but psoroptic mange in cattle is not endemic to the United Kingdom (although it is a major problem in mainland Europe).

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P. ovis grazes the moist periphery of the scab lesion, taking in nutrients with the serous exudate, skin secretions and lipid but the scab itself is not a direct result of the mite feeding but is in fact a form of allergic dermatitis to the mite. The mite exploiting this allergic reaction: the heat and humidity produced by the inflammation forms the micro-climate needed for mite survival and the leakage of serous exudate forms the basis of the mites nutrition.

Early lesions are undetectable, with the mite adjusting to the new host and the host responding to the mite. This "sub-clinical" phase can extend for 10 days or as long as several weeks or even months. If the infesting numbers of mites are low and the sheep is unable to mount an allergic response to the mite, the mites will die out. If on the other hand the sheep is immunologically responsive, the mite population and the lesion will enter a rapid growth phase.

In the early stages of disease excess serous fluid will dry to form a yellow scab the size of new penny, with a moist, faintly green periphery. The lesion gradually spreads outwards as the mite population increases. Eventually the whole sheep can be covered in scab, at which time the infestation enters a decline or regressive phase, and the mite population declines rapidly. After this stage sheep appear to recover completely but in fact may still harbour a small population of mites, waiting to reinfest the sheep once normal skin conditions are restored. The severity of scab depends on the individual animal. Animals with a strong response to the mite demonstrate more extensive lesions, compared to those that respond weekly.
Examining sheep

Part the fleece in several areas, suspect scab if scales or scabs are apparent. Consult your veterinary surgeon if you are not sure.

Early Disease

Generally speaking there are few definite symptoms of early, subclinical scab (while the disease is in the sub clinical phase). Sheep with sub-clinical scab can look perfectly normal and can easily be introduced to a flock via market purchases. Eventually a pattern of symptoms can be seen as the disease progresses. Initially these include restlessness, rubbing against fence posts etc, soiled and stained areas of wool (particularly on the shoulders), head tossing and deranged or tagged fleece. These could also be the symptoms of other ectoparasite infestations (e.g. chewing lice (Bovicola ovis), blowfly strike (Lucilia spp), fly bites, even scrapie).

Established Disease

In the later stages of infestations, the rubbing and head tossing become more excessive, areas of wool loss appear, together with open, bleeding wounds. Sheep rapidly lose condition and epileptiform fitting may be evident.

Numbers of scab infested sheep within the flock can vary from 1 or 2 in the early days of infestation, to the whole flock as the disease takes hold (depending on their immune status of each individual sheep). Throughout the flock there will be animals with non established lesions (that will eventually die out), young subclinical lesions together with animals with obvious extensive disease. All sheep should be considered to be infested and the whole flock should be treated for scab. One missed sheep could reinfect the whole flock.

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Apart from the obvious welfare issue, sheep scab can have significant economic effects within a flock including considerably reduced fleece and leather quality, reduced conception rates, poor lamb growth and in extreme cases, fatalities. The presence of sheep that are not healthy within a flock clearly has welfare implications for those animals and their companions. Sheep infested with sheep scab suffer immensely, so it is rightly an animal welfare issue.

P. ovis infestations cause intense irritation and animals can become exhausted and rapidly debilitated from continual scratching, rubbing etc. The entire animal can become covered in scab within 8 to 12 weeks. Fleece loss can occur due to destroyed wool follicles, rubbing off, or lifting away with the rising scab, leaving the animal totally naked. Scrotal mange can severly affect ram fertility through increased temperature of the testes.

Tups infested on the ventral surfaces may be unwilling to mount ewes or if they do they only remain for a short period of time and ewes infested on the back may not accept the tup.

Tups may also transfer mites to the backs of ewes and ewes may infest tups on the belly. Conception rates may therefore be low. Infestation during pregnancy could affect the normal foetal development. In-lamb ewes may be too pre-occupied with rubbing and scratching to ingest enough nutrient. Lambs may therefore be born weak or still born. Milk production may also be adversely affected. If the lamb survives it can be infested from the mother. Lambs less than 1 month old tend not to present clinical signs of sheep scab, usually only manifesting circumscribed areas of wool tugging, with intense irritation. Growing lambs can show a 30% loss in weight gain and lambs born to infested mothers have been shown to be 10% lighter at birth.

Sheep scab is a winter disease, with the majority of cases occuring between September and April, although a significant number of cases do occur in the summer months particularly on animals still full fleeced (lambs, hoggs etc) and on "ridges" of longer fleece on poorly shorn sheep. Shearing can halt the progress of disease by removing the micro-climate, leaving the mites exposed to dehydration etc.

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A. Oncoming Sheep

1. Isolate all new stock and shared tups for at least three weeks prior to mixing with the main flock.
2. Observe isolated stock regularly for signs of scab.
3. Seek veterinary advice if an ectoparasite is suspected. The parasite(s) should be identified professionally.
4. All isolated sheep should be treated with the CORRECT insecticide/acaricide for the diagnosed ectoparasite.
5. Isolated sheep should not be released into the main flock until treatment is completed and the infestation has been shown to be cured.
6. Infested sheep must be moved to clean housing directly after treatment to prevent reinfestation.
7. Building in which infested sheep had been isolated must be thoroughly cleaned and disinfected with a DEFRA approved disinfectant. All litter must be burnt or deposited out of sheep contact and all tags of wool must be collected and burnt.
8. Sheep should not be introduced into quarantine housing and paddocks for at least two weeks after disinfestation.

B. At Grazing

1. Fencing must be effective in preventing straying on or off the property and direct contact with neighbouring sheep.
2. On common or unfenced grazing co-operation must be sought with neighbouring properties to attain equal standards of health. All flocks should be treated simultaneously.
3. Examine sheep for signs of scab at gathering for foot trimming, drenching etc, and particularly before winter housing.

C.Human Contact

1. Scab can be introduced via contaminated shearing equipment and clothing. Stockowners should enquire if contract shearers had encountered infested sheep within the previous two weeks of shearing their flock.
2. Protective clothing, shearing hand pieces, combs and cutters can be disinfested using strong disinfectant, sheep dip or where suitable, boiling water.
3. Scab mites can survive on clothing and the human body (particularly under finger nails) for a period of time. Anybody having contact with an infested flock must disinfest their protective clothing and wash exposed areas of skin with water (as hot as bearable) before leaving the premises. It is unwise to visit another sheep flock after contact with infested sheep, without a hot shower and a change of clothing between visits.

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Outbreaks of scab require chemical treatment. Currently the only effective methods of control are plunge dipping in wash containing the organophosphorous (OP) acaricides, diazinon or propetamphos or the synthetic pyrethroid (SP) acaricides, flumethrin or high cis cypermethrin (HCC) or by injections of Endectocides (doramectin, ivermectin or moxidectin). Pyrethroid pour-ons are not effective against sheep scab.

A: General Considerations

1. FOLLOW THE INSTRUCTIONS CAREFULLY: always read the product label.
2. SHEEP MOVEMENTS: Avoid moving sheep on or off the property until treatment is completed.
3. NEIGHBOURS: If possible neighbours should be informed of scab; they may unknowingly be the source of infestation.

B: Choosing the Correct Treatment.

1. ACCURATE DIAGNOSIS of the ectoparasite. A number of other ectoparasites, associated with intense itching and wool loss (eg lice, ticks and blowfly maggots), can affect sheep in the UK. It is important that the cause of any wool loss or itching is identified in order to administer the correct treatment and prevent the development of insecticide resistance in scab mites and other ectoparasites.
2. THE SIZE OF THE FLOCK to be treated. Apart from the cost, the inconvenience and detail of some treatment methods may lead to inadequate application at the expense of speed. All contact sheep must be treated, not just those clinically affected.
3. PHYSIOLOGICAL CONDITION OF THE SHEEP. Avoid "dipping stress" to in-jamb ewes (particularly within the first and last months of pregnancy) and there may also be problems while ewes are lactating. Some treatments are not advisable to young lambs and tups and fat sheep are susceptible to "immersion shock" while plunge dipping. Read the manufacturers instructions thoroughly before use.
4. THE END PRODUCT (eg. meat, milk. breeding stock etc). The withdrawal period of some products may not be acceptable at certain times of the year.
5. AVAILABILITY OF LABOUR AND FACILITIES. Some treatment procedures are time consuming and need extrat labour and fixed equipment.
6. SIMULTANEOUS CONTROL OF OTHER PARASITES. Some treatments also offer anthelmintic activity. Timing could lead to a more cost effective flock treatment.
7. OPERATOR SAFETY: People can be adversely affected by certain active ingredients and should not contemplate their use if under medical advice. Stockowners must obtain a certificate of competence in order to purchase organophosphate based dip formulations.
8. THE ENVIRONMENT. Facilities should be available for the safe disposal of used dipwash either on farm or via an authorised contractor.
9. USE OF CONCURRENT MEDICINES. Some treatment methods are not compatible with other farm medicines. Eg. organophosphatc or imidazothiazole based anthelmintic drenches (eg levamisole) and organophosphate based plunge dips (eg diazinon and propetamphos) should not be used within fourteen days of each other.
10. THE WEATHER. Plunge dipping should be avoided in extremes of heat and cold, as well as thundery and wet weather. As far as possible do not carry out dipping in the rain or when the fleece is wet. Hypothermia can occur if sheep are plunge dipped late in the day if the weather is cold. Allow enough time for dipped sheep to dry out before nightfall.
11. SHEARING, BLOOM DIPPING OR WASHING should not be carried out for at least four weeks following dipping
12. HANDLING. Procedures requiring handling of sheep should be carried out before or as long as practically possible after the application of plunge dips.

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There are three types of dip bath; short swim through, long swim through and circular. Short swim baths are generally for small flocks, holding one or two sheep at a time. These baths are very labour intensive, as sheep need to be moved around in the bath for total fleece penetration. Long swim baths are less labour intensive, requiring less manipulation of the sheep. Long swim through baths require a gate at one end and sheep must not be able to turn around or overtake one another. Circular baths offer better control over the sheep, leading to more accurate immersion times.

In all baths sheep must be kept moving: swimming action displaces the air in the fleece and aids dipwash penetration.

Sheep dip formulations can be divided into stripping dips (eg the organophosphate formulations, diazinon and propetamphos) and non stripping dips (eg the synthetic pyrethroid, flumethrin).

1. Stripping Dips. Stripping is the removal of the active ingredient from the dipwash, resulting in greater amounts of active ingredient remaining in the wool. Water is also retained by the fleece, the volume dependent on the size of the sheep and the length of fleece. Stripping relies on the removal of fat soluble suspended or emulsified active ingredient particles by active absorption into wool grease.
2. Non Stripping Dips. Non stripping dip formulations contain the synthetic pyrethroid acaricide, flumethrin. In these types of dip the active ingredient does not strip, so that the concentration of the dipwash in the bath remains the same throughout dipping.

Depletion and Replenishment

All sheep remove active ingredient and water from the dipwash. If active ingredient is not replaced the concentration will eventually become too low, a process known as depletion. Adequate concentrations of active ingredient and volume of dipwash must be maintained at all times. Depleted dipwash must be replenished. Replenishment is the traditional method whereby concentrate and water are added either after a certain number of sheep (head count method) or after a specified drop in volume. For stripping dips replenishment is the regular addition of dip concentrate (at a level greater than the initial charge) and water to compensate for depletion.

For non stripping dips the wash is topped up with wash (concentrate and water) at the same concentration and therefore no need to wait for a specific drop in diwash volume or a specified number of sheep, topping up can be carried out when convenient (as long as there is enough dipwash to adequately immerse the sheep.

OP dips are effective after a single dipping. All synthetic pyrethroid dips (flumethrin or HCC) require a second dip after 14 days.

Dipping for 30 seconds is adequate for the control of lice, blowfly or ticks but sheep need to be immersed for 60 second (with the head under twice) in order to eradicate sheep scab. Double the amount of acaricide is absorbed by dipping for 60 seconds compared to 30 seconds.

Wet sheep can add water to the dipwash. Acaricides have a strong affinity to fleece lipid, wet sheep will therefore remove little water from the dipwash but will remove relatively the same amount of active ingredient as dry sheep. Similarly sheep with short fleeces will remove relatively the same amount of active ingredient as full fleeced sheep. Thus dipwash volume will be slow to fall, despite depletion of the active ingredient. Replenishment by a drop in volume does not take into account wet or shorn fleeced sheep. Replenishment by head count is advisable.

Heavy rain during or directly after dipping can wash active ingredient out of the fleece, as can susequent washing or bloom dipping. Continuous heavy rain falling on the draining pens and water contained in the fleece can also dilute the dipwash.

It is not unusual for dipwash to contain 3.0 to 5.0% organic matter (faeces, soil etc) at the end of dipping. Active ingredient has an affinity for organic matter leading to 60% less active ingredient being taken up by the fleece.

Acaricidal Resistance

Sheep dipping is an exact science and attention must be paid to detail. If dipping is not carried out correctly, the sheep scab mite can be exposed to sublethal concentrations of acaricide and could develop resistance. In South America the sheep scab mite has developed resistance to the organochlorine, lindane (ý BHC) and to the organophosphate, diazinon. In the UK the mite has recently developed resistance to the SP, flumethrin (possibly potentiated by the inappropriate use of SP pour-ons, which are ineffective against P ovis) and the OP, propetamphos.

Protection Against Reinfestation

Flockowners are warned to read the instructions on the product label. The label will state whether a product is effective against sheep scab and whether it protects against reinfestation as well as cures active scab. The sheep scab mite can live off the host and remain infestive for 15 to 17 days. "Approved dip" formulations (licensed prior to 1992) containing the 0P's diazinon or propetamphos or the SP, flumethrin, are guaranteed to protect against reinfestation for at least three weeks on sheep with 1.0 cm of fleece. In reality this would be considerably longer on full fleeced sheep. Consequently sheep dipped in one of the "approved dips" can be returned to an infested pasture, yard or barn directly after dipping, without risk of reinfestation. Dip formulations licensed after 1992 (ie. high cis cypermethrin) are no longer required to offer protection against scab and dipped sheep should not be returned to the infested pasture after dipping.

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A: Pre Dipping

1. Read the Health and Safety Executive (HSE)?Department of the Environment booklet "SheepDipping" (AS 29) and all the instructions enclosed with the product before commencing disposal.
2. Check all personal protective equipment (PPE) is in good order and there is a set for all involved in the dipping operation. DO NOT START DIPPING WITHOUT EFFECTIVE PROTECTIVE CLOTHING.
3. Know the capacity of your dipbath. Calibrate using a vessel of known capacity (eg an old milk churn) or use a water meter. Prepare a calibration stick marking off the initial level and all the necessary drops in replenishment.
4. Do not dip tired, heated or thirsty sheep. If possible do not feed the night before dipping and rest them quietly near the bath for a few hours before dipping.
5. For efficient dipping sheep must have at least 1.0 cm (1/2 inch) of fleece.
6. Check you have sufficient dip concentrate. Don't forget replenishment in your calculations. Only buy enough dip for immediate use.
7. Know the chemistry of your water. Hard or salty water can affect the stability of dipwash emulsions. Check label for instructions.
8. If using an OP dip check if sheep have been drenched with an OP based anthelmintic within the last 14 days.
9. In order to minimise sheep handling after dipping carry out all stock tasks before dipping. Dagging and foot trimming prior to dipping will also reduce fouling of the dipwash (see 8.0 above).
10. Clean dipbath thoroughly, hose down and sweep collecting and draining pens. Unblock and clean sump if present.

B: Effective Dipping

1. Read the label instructions thoroughly.
2. Scab mites can be found on the face and ears, the head must therefore be immersed at least twice.
3. Do not crowd sheep in the bath. Each animal must have it's lower neck and shoulders covered at all times.
4. The time of immersion is essential. Each sheep must be immersed for a full minute.
5. Only use the method of replenishment designated on the concentrate can.
6. Fouling absorbs active ingredient. At replenishment remove floating faeces, wool, kemp, etc. Occasionally brush down (DO NOT HOSE DOWN) the collecting and draining pens, brushing all contamination away from the dipbath. A solids filter in the sump will help reduce contamination.
7. Keep sheep moving in the bath to displace air and allow for maximum penetration of dipwash.
8. Add the correct amount of active ingredient initially and at replenishment. Mix in a bucket of water before adding to the bath. Mix wash thoroughly.
9. Ensure run off returns to the dipbath. This applies particularly to mobile dips.
10. Dip ewes and lambs seperately to prevent drowning.
11. Work at a steady pace to avoid accidents and excessive splashing. Allow enough breaks.
12. It is essential to clean out the dipbath completely and recharge before the wash becomes too fouled. Do not exceed 3 sheep per gallon of dipwash

C. Post Dipping

1. If dipwash is to be used the following day, add the appropriate disinfectant at the correct rate. This will be designated on the dip can. Do not continually re-use dipwash. Fouled wash can cause infections (eg. post dipping lameness) and active ingredient can become deactivated by contaminants.
2. Empty dipbath at the end of dipping, clean out thoroughly and cover. Clean sump, hose down and sweep collecting and draining pens.
3. Sheep dips are harmful to wildlife and the environment. Dispose of spent dipwash correctly. Careless disposal can pollute watercourses and groundwater reserves.
4. Under EC Groundwater Directive 80168/EEC a farmer who wishes to dispose of spent sheep dip onto land which might lead to a direct pollution incident will have to apply to the appropriate Licensing Agency for authorisation or pay a contractor to remove the spent dipwash.
5. Methods have been developed to degrade OP and SP dipwash. eg the addition of agricultural soaked lime to flumethrin (SP) wash or hypochlorite to propetamphos (OP) wash. Read the instructions on the container to see if this is possible.
6. Dispose of empty dip containers in a safe manner. Store full and partially used containers out of reach of children. Preferably in a lockable "Chemsafe".
7. Rinse empty containers at least three times and add washings to the spent dipwash.
8. Thoroughly hose down PPE and wash hands and exposed skin.
9. Record the COSHH all relevant assessments.
10. Records of treatment given must be kept to ensure food safety regulations are met.

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Systemic endectocides have both acaricidal and anthelmintic properties. That is they kill both scab mites and gut worms. At present all systemic endectocides are administered as injections. Correct doses (according to body weight) control ectoparasites anywhere on the body through the ingestion of the active ingredient.

Endectocides containing the acaricides doramectin, ivermectin or moxidectin are all licensed for the control of sheep scab in the UK. As endectocides are carried throughout the body it is important to observe the required withdrawal period for meat and milk.

Some endectocides are effective after a single injection (eg. doramectin), others require two injections (after 7 days for ivernectin and after 10 days for moxidectin). Second injections of ivermectin or moxidectin are essential in order to kill parasites emerging from eggs deposited before treatment. Single injections of ivermectin or moxidectin can be only 90% effective in the control of scab after 10 days and live mites can be present on sheep as long as 84 days following a single injection. The life cycle of Psoroptes ovis (egg to adult) is 14 days. A time interval between the two injections greater than 10 days could therefore allow for a build up of residual mites not completely controlled by the second injection.

There are slight differences in administration between the three endectocides. Doramectin is administered as a single intra-muscular injection at a dose rate of 300 pg per kg of body weight (not a subcutaneous injection at a dose of 200 gg per kg of body weight as for cattle mange). Ivermectin and moxidectin are both administered as double subcutaneous injections, at a rate of 200 pig per kg of body weight, 7 and 10 days apart, respectively.

The washing process and drying of the microclimate seen in dipped sheep is not generally observed after the use of an injectable endectocide. Consquently lesions may take longer to resolve, even after a second injection. Animals can still exhibit severe bouts of irritation and hypersensitivity as long as the lesion or part of the lesion, is still in contact with the skin (even though the mites themselves have been killed). Irritation should decrease as the fleece grows, lifting the scab away from the skin. Prolonged periods of irritation are seen in sheep with simultaneous, infestions of both scab and chewing lice at the time of treatment. None of the endectocides are effective against chewing lice, consequently, once the scab mites have been killed, the lice populations grow rapidly as they feed on the scab lesion, thus continuing the irritation.

Endectocides have varying periods of protection. Read the label for details of the products capacity to protect against reinfestation. Rubbing and scratching of heavily infested sheep will deposit live mites into the environment (eg. fencing, bushes, lorries, shearing combs etc). Mites can survive off the host for at least 16 days. Consequently if sheep are reintroduced to the original contaminated accomodation, they are likely to be reinfested. The prolonged period for complete resolution of clinical disease increases the chances of sheep rubbing and thus contracting residual mites from the environment.

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