Sheep Scab
Introduction
Sheep scab, caused by the mite Psoroptes ovis, is a major welfare concern and source of economic losses to the UK sheep flock. Compulsory treatment was enforced in 1928 in an attempt to eradicate the disease from the UK, but abandoned in 1992 with the disease deregulated due to failure. More recently, it was re-categorised as notifiable in Scotland in 2010, whilst in England and Wales treatment of flocks visibly affected by sheep scab has been a legal requirement since 1997.
Figure 1: Affected sheep can have extensive wool loss
Cause
Psoroptes ovis mites live and feed exclusively on the skin surface. Their faeces causes an allergic reaction leading to intense itching and rubbing. Onset of clinical disease is usually slow, meaning infestation may be widespread within a flock potentially before an outbreak is detected. Females lay eggs which hatch and develop into adults over a period of 10-14 days. The mites prefer warm, moist conditions, with outbreaks most commonly found on fully fleeced animals housed over winter, although outbreaks can occur in the summer in fully fleeced or poorly shorn animals. Shorn sheep can carry live mites without showing any signs.
Figure 2: Scab mites can be spread by direct and indirect contact, and can survive in the environment for up to 17 days
Sheep scab is spread through contact with live mites. This is usually through direct sheep-to-sheep contact, although adult mites can survive for up to 17 days in the environment enabling indirect spread via interaction with rubbing posts, pens, handling facilities, shearing equipment, clothing etc. contaminated with wool tags or scabs. It is also thought cattle may be able to carry mites for a limited time.
Welfare and Economics
Sheep scab currently endemic in the UK, with cases reported in most regions and Wales appearing to have the highest prevalence.
Sheep scab can be widespread and severe, causing weight-loss in ewes, poor growth rates in lambs, significant wool and skin damage and, in severe cases, death. Sheep scab has previously been estimated to in the region of £8-14 million per annum cost the UK sheep industry as a result of stock losses and poor weight gain.
Lower birth weights and higher mortality rates have been observed in lambs born to ewes infested with sheep scab during pregnancy, whilst slaughterhouse condemnation of carcasses due to furthers the economic losses. The costs associated with preventing and treating infestations are also substantial for individual farmers.
Clinical signs and Diagnosis
During the early stages of sheep scab, infestations are not obvious. Mite numbers are initially low and lesions are small and virtually undetectable. It can take 40-50 days after infection before signs are seen by which time they can be widespread within a flock, with up to 60% of animals affected. Early signs can include restlessness, rubbing against fence posts, soiled and stained areas of wool, head tossing and loose tags of fleece.
The allergic reaction and resulting from skin damage caused by initial infestation sets up ideal conditions for further mite survival and multiplication. In these later stages very high numbers of mites may be present at the edges of characteristic spreading lesions. These are scabbed and crusted areas of thickened, corrugated skin. Active scab lesions are associated with intense rubbing of the shoulders and flanks along the ground or against fences, foot stamping, clawing at the flanks, and biting, nibbling and lip smacking behaviour when animals are handled. Damaged skin and open, bleeding wounds may also become infected with secondary bacterial pathogens. Head tossing and deranged behaviour becomes more excessive as a result of the intense itching and large areas of fleece can be lost. This is often wet, sticky and discoloured due to serum discharge from lesions.
Figure 3: A severe case of sheep scab with extensive wool loss and thickening of the skin
The distraction caused by intense itching also reduces feeding leading to rapid weight loss and significant loss in body condition. In serious cases, animals may start fitting, particularly if moved or handled. Loss of fleece and poor nutrition may lead to death through pneumonia or hypothermia.
Figure 4: Rubbing or nibbling at the flanks may indicate early scab infestation, but can also be a sign of lice
Where sheep scab is suspected further investigation is necessary to differentiate it from louse infestations which can present with similar signs, particularly in the early stages. This is important, as treatment regimens for scab and lice differ markedly. Falsely attributing clinical signs to lice will allow continued spread of scab within the flock and administration of ineffective treatments. Similarly, louse infestations assumed to be sheep scab may lead to unnecessary use of inappropriate treatments.
Farmers must therefore seek veterinary advice to get a correct diagnosis. Lice can be easily identified in wool samples taken from affected regions of fleece. To confirm sheep scab, skin scrapes need to be taken from the edge of an active crusting lesion and examined under the microscope. Observation of psoroptic mites in skin scrapes definitively confirms current infestation and indicates treatment in all sheep in the affected group, although mites can be difficult to detect in the early stages of disease and will not usually be present in skin in the middle of an active lesion. It is also important to note that sheep scab and lice can occur together, meaning both wool and skin scrapes should be taken to get an accurate diagnosis.
In addition to microscopy, a blood test has been developed which detects antibodies against sheep scab. This can detect infestations from two weeks onwards. This test is of particular value when buying in stock as part of routine quarantine measures, as it allows identification and treatment of sub-clinical infestations. Positive results confirm recent exposure to scab mites, but it is important to note antibody levels may remain high even following successful treatment for several months. For use in evaluating treatment efficacy, blood samples need to be taken before and 2 weeks after treatment in order to observe falling antibody levels.
Figure 5: Scab mites are almost invisible to the naked eye, so diagnosis must be confirmed by microscopic examination of the scabs from the skin. Photo credit: Dr Joseph Angell, Wern Veterinary Surgeons
With unusual presentations, other diseases causing itching in sheep should also be considered. These include bacterial and fly-bite dermatitis, ked and forage mite infestations, rain scald, lumpy wool and scrapie. Wool loss can occur naturally in some breeds, and may be seen in ewes after lambing. Wool break may also occur after serious illness such as mastitis, twin lamb disease and after eating poisonous plants such as St. John’s Wort or rape.
Figure 6: Antibody testing is possible, and will identify infected sheep two weeks following infection. Photo credit: Valentina Bustin, University of Glasgow
Prevention
Transmission of sheep scab is predominantly direct from sheep to sheep. It is, however important to remember adult mites can survive in the environment for up to 17 days, meaning pens, pastures and handling systems previously occupied by infested animals can pose a risk to currently unaffected animals, as can people and inadequately cleaned clothing and machinery and equipment such as scanning and shearing equipment.
Figure 7: Unsecured farm boundaries provide an easy way for sheep scan to be introduced onto the farm.
Where scab is not present on farm, strict biosecurity measures should be put in place to prevent its introduction. Farmers must have robust biosecurity and treatment plans established with their vets as part of their flock health plan. Maintaining a closed flock where possible will reduce risk considerably, although neighbouring flocks may still allow its introduction so it is important to ensure all boundaries are stock proof.
Quarantine measures should be applied to all animals coming onto the farm, including bought-in replacements, animals returning from away grazing and rams may be carrying scab mites even if they have no clinical signs of disease. Bought-in animals should be considered as potential carriers. Even sheep from scab free flocks can be infected at markets, in transport or from contaminated clothing and equipment. New quarantine guidelines from the SCOPS group advise assessing the risk of scab in all brought-in stock and treating and housing all high risk animals on arrival, or in lower risk animals housing on arrival and testing by ELISA after 2 weeks to determine infection status and treat and isolate as required.
Figure 8: All incoming animals must be quarantined and either tested or presumed infected and treated appropriately before mixing with the rest of the flock
Studies show sheep on common grazing are twice as likely to be infected as those on private land. In order to gain control, all graziers with animals on the same common should cooperate and, on established “off-days”, round up all sheep from the common. This is difficult in practice and some studies estimate 10% of upland sheep can be missed at gathering. Where outbreaks occur, selected treatments will require both residual activity and preventative action if animals are returning to the common afterwards to reduce risk of re-infestation from other flocks. As part of the Sheep Scab Orders for England and Wales (1997) and Scotland (2010), local authorities have certain legal powers to clear animals from common land and restrict movements on premises, common or otherwise, until effective scab treatments have been administered. It is also important to note sheep scab is a notifiable disease in Scotland.
Treatment
There are only two products available to treat or prevent sheep scab in the UK: plunge dipping with diazinon, an organophosphate, or certain injectable group 3-MLs. It is essential where treatments are administered to check the datasheets for a licence of efficacy, and to follow the manufacturer’s directions on dosage, route of administration, timing of repeated treatments and withdrawal periods.
Diazinon dips are only effective against sheep scab when applied by plunge dipping and should under no circumstances be applied by other means. Dip operators must have a formal certificate of competence obtained through an approved training course. They must take all the necessary precautions to avoid human and animal health risks and have permissions and licenses for safe disposal. Dipping kills scab mites within 24 hours and offers residual protection from reinfection several weeks and also aids in the control of blowfly, lice, keds and ticks.
Figure 9: Plunge dipping must be carried out according to the manufacturer's instructions by an approved dip operator
Certain injectable group 3-ML products can be effective against scab when administered correctly and carry a licence for this purpose. However, requirement for, and timing of repeat treatments as well as length of residual activity against sheep scab varies considerably between products. Moxidectin 2% is injected in the base of the ear and should be performed by an operator competent in this technique. Sheep treated with a group 3-ML may remain infective for up to 14 days so must be kept separately over this period. Treatments with no persistent activity can also leave animals susceptible to re-infestation, particularly if mixed with other sheep and/ or are re-introduced to their previous environment within 17 days. Furthermore, resistance of mites to treatment with certain group 3-ML products has been reported recently in a number of parts of the UK. Consequently, veterinary advice should be sought where treatment failure is suspected. Follow-up diagnostic testing may be advisable to determine whether treatments have been effective, whether further treatments are indicated and what these should be.
It should be noted that injectable group 3-MLs are also very valuable wormers, and over-reliance on them for sheep scab control will increase the risk of worm resistance developing on farm. It should also be noted that injectable group 3-MLs do not carry a licence to control blowflies, lice or ticks.