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Monday, 11 May 2015

Control of Sheep Scab
·         Sheep scab is caused by the host-specific mite Psoroptes ovis. It can be transmitted by direct contact when sheep are housed tightly; prime examples are at sales or contact with objects or transport used for sheep.
·         Sheep scab was rarely seen until withdrawal of compulsory dipping in 1989; now, such infestations are rampant causing serious welfare concerns. In theory, given the availability of efficacious dips and ectoparasiticides, eradication of a disease of this nature should be achievable in the 21st century in one year of complete treatment of the disease leaving no place for survival of the mite in areas that accommodate sheep.
·         In the UK, disease is typically encountered during the autumn/winter months from Oct – Mar.
·         During the early stages of the infestation some sheep in the group have disturbed grazing patterns and are observed kicking at the flanks with their hindfeet and/or rubbing themselves excessively against fence posts etc, which leads to loss of wool and a dirty ragged appearance to the fleece. There is serum exudation which gives the fleece overlying the skin lesions a moist yellow appearance. Some infested sheep have low mite numbers and show few clinical signs; these sheep may be of great importance as they can remain dormant, acting as a reservoir for the mite and suddenly cause outbreak.
·         Infested fleece is typically wet, sticky, yellow and frequently contaminated with dirt from the hind feet. It may prove difficult to part the wool fives overlying the lesions due to serum exudation which has formed a thick layer within the fleece. At around the 8th week of infestation, the hair loss on the flanks may extend to 20cm diameter surrounded by an area of reddening serum exudation at the periphery. The skin at the denuded centre of the lesions becomes thickened; it is thrown into thickening corrugations in many advanced cases. By this stage, the sheep will have lost considerable body condition and frequently emaciated. Death may result in some sheep.
Diagnosis
·         Confirmation is essential by demonstration of live mites by a vet. Skin scrapings taken using a scalpel at right angles over the skin surface at the periphery of active lesions demonstrate large number of mites under X100 magnification. Lice and keds can be visualised on careful examination of the fleece/skin.

Prevention & Treatment
·         Prevention depends on co-ordinated, careful plunge dipping or endectocide injection involving all neighbouring flocks and taking into account the following principles…
1) Flock infestation or sheep scab can be instigated by only one egg-laying female.
2) It is essential that all sheep are gathered and treated correctly at the same time using an appropriate dip or systemic endectocide injection.
3) Sheep scab mites can survive off the sheep for up to 17 days, survival being longest when the weather is cold and damp.
4) The only treatments that guarantee persistence for longer than 17 days are diazinon (organophosphate) and moxidectin injection.
5) Doramectin injection persists for marginally less than 17 days, but this is usually sufficient for sheep scab control.
6) One Ivermectin injection repeated after 7 days is effective for the treatment of sheep scab, but does not achieve significant persistence.
7) Systemic endectocide injections (Ivermectin, Doramectin and Moxidectin) may take several days to kill all the sheep mites, while effective plunge dips, when used correctly, kill mites immediately.
8) Pour-ons and plunge dip solutions applied in shower dippers or jetting races are ineffective for sheep scab control.
·        
The practical relevance of these principles is…
1) Whenever it is necessary to return sheep to fields, handling pens or buildings used by untreated animals during the previous 17 days, only plunge dips or endectocide injections that persist for more than 17 days should be used on those sheep returning.
2) Ivermectin injections should only be used when it is possible to avoid fields or contact with untreated animals for the subsequent 17 days after application.
3) All sheep new to the system or returning from grazing elsewhere should be subject to the control measures outlined above. If injection is used they should be kept away from the main flock for at least 7 days.
4) When purchased animals are certified as dipped before sale, it is important to determine what they were plunged in and when they were dipped. If there are any doubts then seek vet consult as re-treatment may be necessary.
5) Avoiding contact with strays, neighbouring sheep, fomites or shared handling facilities such as shearing or trailers or scanning races may be impossible. It is therefore beneficial to ensure that all sheep flocks within a defined geographical area are treated with a drug with residual action during the same 3 weeks period.
6) Shared handling equipment
should only be used when it is thoroughly cleaned before and after use.
·         
Practical Considerations…
1) In the case of plunge dipping, animals must be immersed for at least 1 minute, with their heads dunked twice during this period.
2) It is essential that the sump volume of the dipper is known, the correct initial dip concentration used, the correct replenishment rate used and care taken to limit faecal contamination.
3) Sheep should be yarded overnight before dipping and never dipped when hot, tired or thirsty.
4) Dippers should be cleaned and emptied at the end of each day or after more than one sheep per 2 litres.
5) After dipping, sheep should be stood in a drainage area until the dip ceases to run from their fleeces, before turning onto pasture away from any watercourses.
6) Dips should be disposed of properly to ensure they do not contaminate water sources. Unused dip can be spread on some pastures in accordance with the data sheet recommendations; alternatively some dippers will take a dispose of the dip themselves.
7) In case of systemic endectocides, a sample of sheep should be weighed, syringes calibrated and great care taken to ensure that all sheep actually receive the correct drug dose.

·         
Action…
1) The best timing for a concerted sheep scab treatment would be during the first 3 weeks of October, after most replacements have been bought home and before tupping.
2) For most flocks, the most appropriate treatment would be either plunge dipping in diazinon or injection with Doramectin or Moxidectin.
3) There is a need to treat or cull all stray and feral sheep in the area.
4) Individual flock difficulties associated with dip disposal, meat withdrawal periods in store lambs and organic production are acknowledged. Where these difficulties cannot be easily addressed, other methods of scab control should be employed based on the principles above; in particular the need for strict separation of treated and untreated sheep.
5) The importance of involving all flocks in a defined area is emphasised.
6)
All suspicious cases of wool loss and itching seen after October should be investigated.



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