Estimated reading time: 6 minutes
Sheep scab in small stock can lead to considerable damage if not treated early. Due to its prevalence, Dr Andries Lategan of Bult Street Animal Clinic in Upington is well-acquainted with the disease and often treats sheep that have contracted it. Stockfarm spoke to him about this problem.
According to Dr Lategan, sheep scab is a skin condition caused by the external parasite Psoroptes communis ovis. This sheep scab mite lives, feeds and produces on the skin, between the wool and hair of sheep. This causes severe irritation and the sheep experiences itching, which causes the animal to scratch and bite itself until lesions become visible on the skin.
Sheep scab causes physical discomfort, restlessness and general substandard performance in sheep.
The saying that one sheep-scab infested animal can infect the entire herd is indeed true. According to Dr Lategan, sheep scab is highly contagious and is transmitted primarily through direct and indirect contact.
“Sheep that flock together is one of the greatest concerns. This applies especially to animals that are kraaled, transported on trucks, fed in feedlots or lambing pens, or are bunched together at auctions and shows. When boundary fence wires are too far apart allowing sheep to crawl through, or when they constantly scratch against fences, the sheep scab mite is easily transferred from one farm to the next,” he explains.
“Sheep are curious and because the abnormal behaviour of the infected animal will pique another animal’s interest, the sheep will move closer to smell that animal, leading to disease transmission. The mites have no problem climbing from one animal to another as their legs end in suckers, while some legs have hair that provide better grip.”
Indirect transmission occurs when shearers, sheep dogs, other farm animals, sheep handlers, wool and the skins of infected animals come into contact with healthy animals.
Life cycle of the parasite
Dr Lategan says sheep scab mites complete their entire life cycle on the sheep’s skin. Immature females moult to the adult stage, after which mating takes place. Fertilised females lay approximately five eggs per day and can lay up to a total of 100 eggs. Eggs hatch within three days and the larval stage is reached after another three days. Three days later the nymph appears, turning into an adult mite after another three days.
Mites don’t burrow into the skin, but bite and pierce it. The punctured skin secretes moisture on which all stages of the mites then feed.
“These sites cause skin inflammation that is aggravated by the actions of scratching and biting. Inflammation produces even more secretions that form crusts and scales on the lesions,” he explains.
Signs of sheep scab
Signs of sheep scab are only pertinent in winter. Most parasites are dormant in summer, with no visible symptoms present.
Dr Lategan says the bites itch severely, leading to sheep scratching, rubbing and biting the affected areas. Bald spots will start appearing as the animals pull hair or wool from the affected areas. This wool or hair tends to get caught between their teeth and are also visible on objects such as wires, fences, shrubs and posts.
These bald spots are covered in crusts or scales (brought on by inflammation and the excretions it produces) and can quickly become bigger, as the mites are concentrated at the edges of the legions where they thrive. Fresh lesions appear purulent and wet, and crusts only develop after five or six days. Old lesions have a drier, flakier appearance.
“In wool sheep purulent lesions are more common, with lesions occurring on the shoulders and groin of the animal. In hairy sheep the lesions generally have a drier appearance and are situated more towards the animal’s tail and back.
“Sheep that are driven usually have a higher body temperature, which increases parasitic activity. As a result, the scratching and biting symptoms become more noticeable.”
Treating the condition is straightforward and effective, but the secret lies in the fact that the entire herd must be treated. Re-infestation should be avoided at all costs.
“Various injectable antiparasitic agents are available, most of which are registered for effectiveness with a single treatment. There are also several types of dips available on the market. Administered at the correct strength and employing the correct method, these treatments should have the desired effect,” says Dr Lategan.
Since the disease occurs mainly in winter, which coincides with the period during which lambs and ewes are together, he advises small-stock producers to use injectable treatments instead of the cold-water dipping method. “Applying the latter method in winter could result in secondary respiratory infections and ewes abandoning their lambs.”
A few general principles must apply in order to prevent the disease from spreading:
- Boundary fences between farms should be completely impenetrable.
- Avoid buying in animals of questionable origin.
- No new animal must be released on a farm without having been properly quarantined. Modern biosecurity is becoming increasingly important in this regard.
- Each farm’s general policy needs to include this quarantine period. All vaccinations, dosages and treatments must be administered during this period. Treating sheep scab, even if no symptoms are visible, should form part of this policy.
- A good rule of thumb is to administer an injectable external parasite control agent at least once during the winter months.
“Everyone has a duty and responsibility to report suspicious skin lesions or other symptoms of sheep scab to the state veterinarian. The state will then take over and apply disease control measures under specific conditions.
“Sheep-scab positive farms (as well as neighbouring farms) are placed under quarantine without delay and all animals are treated under state supervision. Animals are treated twice, seven to ten days apart. The quarantine status is lifted roughly six weeks after the last treatment and once a sheep-scab negative inspection has been carried out by an official of the state veterinarian’s office,” explains Dr Lategan. – Koos du Pisanie, Stockfarm
For more information, phone Dr Andries Lategan on 082 788 2448.