Sheep scab infection
Sheep Scab (Psoroptic mange) mainly affects the woolly parts of the sheep where the wool offers warmth and shelter to the mites; the head and leg areas are rarely affected. However, mites are also known to inhabit areas around the ears and horns for long periods.
This is known as a latent infection and often occurs during the summer months and may be missed by the farmer. In the autumn and winter months when the fleece is longer, these mites move into the more woolly body area where the population increases and the infection become a serious problem.
The mites are very small (approximately 0.5mm in diameter) and live on the skin of the animal; they do not burrow into the skin. They puncture the skin with their mouthparts and suck up the lymph from which they obtain nutrients. The sheep’s immune system responds with an allergic reaction.
The skin becomes inflamed, the wool falls out and large moist scabs form on the skin surface. The mites are able to live off the exudates that the lesions produce and so a ‘snowball’ effect is created; more mites = more scabs = more food for mites = more mites. Within a very short space of time, the sheep is heavily infested and very ill.
The condition is extremely irritating to the animal and sheep are seen to scratch against anything they can. They will also scratch with their hind legs, horns or chew at their own skin. The sheep can be literally driven mad by the intense irritation; in advanced stages of the infection, the animal may even fall over, suffer fits and eventually die.
Sheep scab mites live their entire life on the sheep and the infestation is spread by direct contact. It may also be spread by mites that are left on fences etc after the sheep have been scratching themselves. However, the mites cannot live for more than two to three weeks away from their host.
The female mites lay about five eggs per day on the skin of the sheep where they hatch within ten days. Each adult female will lay around 90 eggs in her lifetime. The larvae feed on the skin for two to three days before moulting into nymphs. After around four days the nymphs turn into adult mites and the cycle is complete.
Damage to leather
Sheep scab causes the grain surface of the leather to become deeply pitted and disfigured and it is virtually impossible to disguise even with the heaviest of finishes.
Even after the infection has cleared, extensive scarring of the grain surface remains.
Wool-on skins are rendered useless due to the bald patches arising from the infection and damage to other areas of the fleece caused by the animal rubbing and chewing itself.
Because the mites are so small, they are not easy to identify until the infection has really taken hold. It is usually the animal’s behaviour that gives the game away in the early stages of infection as the sheep become restless and irritated.
Such is the importance of this disease, in 1997 in the UK the Sheep Scab Order was introduced to control outbreaks of the disease. The order, which is enforceable by law, states that it is an offence for any person to move a sheep visibly affected with sheep scab on or off any premise or move any sheep from a flock containing one or more sheep visibly affected with sheep scab except:
a) For treatment
b) For immediate slaughter
c) In accordance with a notice issued by a local authority (LA) requiring clearance of sheep from common land
d) Under the authority of a licence granted by an inspector.
Any movement must be carried out in a way that other sheep cannot become infected by contact with the infected sheep.
The Order also insists that affected sheep are treated with an approved insecticide within a specified period of time and this treatment is recorded by a veterinary surgeon. Failure to adhere with these regulations can result in fines of up to £5,000.
But, like everything, prevention is better than cure. A UK-wide initiative has been introduced led by the National Sheep Association which suggests an organised dipping programme. Dipping in organophosphate dip for at least one minute has been found to be most effective at treating and preventing an outbreak.
However, incidents of mites that are resistant to the effects of organophosphates have been reported. Sheep scab can also be treated by injectable products such as Avermectin, but this does not prevent re-infection.
As a precaution, it is recommended that when new sheep are purchased they should not be introduced into the rest of the flock until they have had at least three weeks quarantine without signs of scab.
Scientists have been successful in growing the sheep skin cells (keratinocytes) in the laboratory to form ‘skin-like’ structures2. This means that scientists can test mite products, without the use of live animals. Research such as this will enable veterinarians to understand the disease better and develop new and better products with which to treat and prevent it, eg a vaccine.