IT IS one of the most persistent and debilitating problems that sheep farmers face and one of the most difficult to get rid of which has serious welfare implications for affected animals.

In this article, the National Animal Disease Information Service (NADIS), gives practical advice on how to minimise risk from sheep scab, which is almost endemic throughout the country.

Economic importance - A failure to recognise and treat sheep scab promptly, or improper treatment, can incur serious economic loss due to increased feed costs to compensate for rapid loss of body condition. Also, it causes low birth weights and higher mortality occur in lambs born to ewes which suffered sheep scab during pregnancy.

Cause - Sheep scab is caused by the mite psoroptes ovis - cattle are not affected.

Mites are most commonly transmitted by direct contact with infested sheep, but can survive off sheep for 17 days and so can be transmitted on posts, vehicles, etc.

Sheep scab can be introduced into a flock by:

- Carrier sheep, including purchased animals;

- Sheep returning from grazing;

- Contact with neighbouring sheep/common fences.

Clinical signs - Animals are often seen at different stages of the disease within affected flocks.

On a flock basis, sheep scab is characterised by

- Intense itching;

- Repeated rubbing of the shoulders and flanks along the ground or against fences;

- Foot stamping, clawing at the flanks, and biting the shoulders;

- Tufts of wool are characteristically seen on fences and hedges;

- As the disease progresses, wool is lost and the skin becomes thickened and covered with thick scabs.

Handling of sheep with extensive scab lesions may precipitate seizure behaviour. New wool growth occurs as the disease regresses and lesions heal.

Diagnosis - Differential diagnoses - sheep scab must be differentiated from louse infestation and necessitates veterinary examination.

Diagnosing sheep scab is based on the clinical signs and veterinary confirmation of psoroptes ovis (PO) mites in superficial scrapings collected from the periphery of exudative lesions.

Confirmation of PO infestation is essential to ensure a correct diagnosis and treatment.

Treatment - How to treat should always be discussed with your veterinary surgeon, but the main methods are:

- Plunge dipping in organophosphorus sheep dips - these dips contain dimpylate (active substance) that will kill scab mites within 24 hours and afford residual protection for several weeks. Plunge dipping also treats any lice, blowflies, and ticks present on the host.

- Systemic endectocide injections, involving two subcutaneous injections, seven days apart, of 200 µg/kg ivermectin, or a single intramuscular injection of doramectin at a dose rate of 300 µg/kg provide effective control of sheep scab.

While a single injection of doramectin achieves some persistence, this may occasionally be insufficient to provide protection against re-infection for the whole 17-day period during which the scab mite can survive off the sheep.

A single subcutaneous injection of moxidectin at a dose rate of 200 µg/kg provides residual protection against sheep scab for at least 28 days, although the UK data sheet recommends two injections 10 days apart for the treatment of disease outbreaks.

Treated sheep must be kept isolated in a clean area for 7-14 days after treatment depending upon the product used.

Prevention, biosecurity and control - Scab control should form an important part of your veterinary flock health plan and important components include:

- Maintaining a closed flock wherever possible;

- Maintaining secure double-fenced perimeters where they contact other fields containing sheep;

- Treating all purchased sheep upon arrival on farm and quarantine in a clean area for at least two weeks;

- Investigating all cases of pruritus and fleece loss.