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Sheep Scab - A Major Pest to Sheep Production

Dipping ceased to be compulsory in 1992 leading to a greater incidence of sheep scab with an estimated 60-fold increase in disease on UK farms1.

In spite of having 4 medicinal active ingredients available with 16 different brands/preparations licenced and available in the UK, we are still struggling to get this disease under control.

Scab is still a major concern for UK sheep producers as it has a significant impact on the health, welfare and productivity of affected animals.

 

What is Sheep Scab?

life cycle of psoroptes ovis

Sheep scab is a disease caused by the mite Psoroptes ovis which lives on the skin surface where it feeds. The faeces produced by the sheep scab mite cause a severe allergic dermatitis resulting in the ‘scabby’ lesions which we associate with the later stages of the disease.

The mites are transferred from animal to animal by direct contact or on ‘fomites’; pieces of wool containing sheep scab mites. These mites are able to survive for 17 days without a host to feed from making it challenging to control and the potential for re-infection high.

Infestations can be debilitating, have a detrimental impact on welfare and can lead to severe economic losses. The Sheep Health and Welfare Group report 2016 states that the annual estimated cost of sheep scab in the UK is £8.3 million!2

Clinical signs include:

  • Restlessness,
  • Rubbing against fence posts,
  • Soiled/stained areas of wool,
  • Head tossing/biting,
  • Pulled wool appearance leading to eventual wool loss,
  • Open bleeding wounds and ‘scabby’ lesions,
  • Loss of condition,
  • Death


Diagnosis

First and foremost get a diagnosis from your vet. The clinical signs of lice and sheep scab infestation can be identical- particularly in the early stages.

The other complicating factor is that both diseases are contracted in the same way; through poor biosecurity. Therefore this means there is nothing to prevent animals from being dual infected with sheep scab and lice. Just because you can visualise lice it does not confirm that the animals are not also infected with sheep scab!

Your vet can perform skin scrapes on clinically affected animals or can perform blood samples to detect antibodies to infection before clinical signs become apparent.

Treatment

In the UK we have four options for controlling sheep scab (table 1). A key consideration for parasite control strategies is ensuring that we use the correct active ingredient at the correct time.

When we use dual endo- and ecto-parasiticides for sheep scab control (the macrocyclic lactones) we are targeting both internal and external parasites.

When we dip (using Diazinon) we only target external parasites.

The highest incidence of sheep scab occurs during the winter months. At this time of year gastrointestinal roundworms are generally present in lower burdens and less likely to cause disease requiring treatment than during the grazing season. To continuously expose these small burdens to anthelmintics (wormers) increases the rate of development of resistance.

SCOPS mirror this sentiment: “For the macrocyclic-lactone (clear 3-ML) wormers there has been a marked increase in recent years, probably linked to their widespread use as endectocides for the treatment of sheep scab. Action to try to preserve this group is now imperative.”3

Remember that the sheep scab mite can survive for 17 days without a host to feed from so re-infection must be controlled by using a product/protocol which provides protection for longer than this or by moving them to clean grazing/housing.

Table 1. Active Ingredients Licenced for the control of sheep scab

Group

Active Ingredient

Preparation

Notes

Organophosphate dip

Diazinon

Dip

Dipping treats and protects for up to 4 weeks

Macrocyclic Lactones

Ivermectin

Injectable

2 injections 7 days apart

Doramectin

Injectable

1 injection and move to clean area

Moxidectin 1%

Injectable

2 injections 10 days apart

Moxidectin 2%

Injectable

1 injection provides 60 days protection

Prevention

Good biosecurity is key to controlling this disease. It is impossible to tell simply by looking at animals if they have been recently exposed to Psoroptes ovis. In the early stages the disease can be asymptomatic with no visible evidence of itchy sheep. Just because the animals do not appear to be itchy and do not have pulled wool/bald patches/lesions, does not mean that they are not carrying sheep scab mites.

When purchasing new animals they should either be presumed infected and treated or an ELISA test should be utilised to assess the risk. The flock should be kept separate from other sheep at the periphery of the farm (e.g. double fencing).


About the Author
Rachel Mallet is a Veterinary Surgeon, who now works as a Professional Services Vet providing technical support to vets, SQPs and farmers in the UK. Rachel is passionate about animal health and about promoting best practice and preventative medicine amongst farmers.


Use medicines responsibly. Noah.co.uk/responsible.

Date Editorial Prepared: August 2017
Goldfleece (60.8% Diazinon) is a POM-VPS medicine.
Consult your vet or SQP before using any medicines for ectoparasite control


References:

  1. Rose H, Wall R.(2011)“Endemic sheep scab: risk factors and the behaviour of upland sheep flocks” Prev Vet Med 104 (1-2)
  2. Sheep Health & Welfare Report for Great Britain 
  3. Sustainable Control of Parasites in Sheep