Sheep Scab

This parasite was discussed slightly in one of my earlier blogs on mites. Sheep scab is a disease which is endemic in many parts of the UK, and is therefore very important. This is why it is discussed here in further detail. 

Species

• Psoroptes ovis- obligate parasite of sheep. May also be found on cattle (bovis) and horses (equi). 
• Psoroptes cuniculi- parasite of domestic rabbits ears. May also affect sheep and horses. 
• Non-burrowing mite.
• Has jointed pedicels and a 'trumpet' shaped sucker for identification purposes.
• Feeds on the skin surface- lipids, superficial fluid, bacteria, cells & lymph fluid.
• Sensitive to dessication (drying out) hence a long sheep fleece is ideal. 
• Reservoir in the axilla, skin folds and ears. 

Transmission

• Direct contact. 
• Contaminated environment (can survive off the host up to 16 days).
• On fomites e.g. humans/pets.
• Introducing sheep which are not treated on arrival, mixing flocks, flocks on common land or rented land, poor fencing, and non-disinfected lorries.

Life Cycle

• Winter disease- due to fleece length, climate, mixing and stocking density increases as well as an increase in off host survival times. 
• 13 day life cycle.
• Live for 40-60 days.
• Produce 60-70 eggs. 

Pathogenesis

• Chronic exudative superficial dermatitis.
• Type 1 hypersensitivity to excretory-secretory antigens.
• Hyperkeratosis.
• Aural Haematoma.
• Irritation, weight loss and epileptiform seizures.
• Death.
• Inflammation with serous exudate which forms scales and crusting skin.
• Mites appear at the moist edge of the lesion. 
• Lesion can cover 3/4 of the body in 6-8 weeks. 

Clinical Signs 

• Severe pruritis and irritation.
• Scratch posts and show wool loss.
• Sheep isolated and depressed.
• Clean areas of fleece where biting.
• Dirty fleece where rubbing.
• 'Nibble reflex.'
• Lesions on neck, back, shoulders and flanks.
• Weight loss.
• Secondary bacterial infections.

Diagnosis

• Clinical signs.
• ID tick from skin scrapings taken from the edge of lesions and from several places and animals.
• Boil material in 10% KOH for 5-10 minutes before examination under the microscope. 

Control

• Dipping.
• Injectable Ivermection.
• Regular flock inspection.
• Quarantine sheep.
• Biosecurity- no common land grazing.