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Itch mite in sheep
Text only version of Farmnote 449 - PDF Available
Itch mites in a nut shell
- Tiny mites that live in surface skin of sheep. Impossible to see without magnification
- Present in some WA sheep but the prevalence is low
- Cause some sensitised sheep to rub and chew at their fleeces, but the economic effect is small
- Spread ewe to lamb and between shorn sheep
- Very slow to spread on individuals and through a flock
- Fine-wool Merinos and poorly fed sheep are more susceptible
- No eradication method. Cull affected sheep or give ML drench
The itch mite, Psorobia ovis (formerly Psorergates ovis), is a microscopic parasite that lives on the skin of sheep. It causes intense irritation resulting in rubbing and scratching which may lead to severe fleece damage. This mite is found in sheep throughout Australia but because it is difficult to detect, its prevalence is not known.
During the 1980s, reports of itch mite were fairly common but since the introduction and use of macrocyclic lactone worm drenches (ivermectin, abamectin and moxidectin), reports of this pest have been rare. As well, confirmation of suspect cases has not been common because farmers usually opt to treat infestations rather than seek a diagnosis.
Signs of itch mite infestation relate to the irritation it causes. Affected sheep rub, scratch and bite at their fleece resulting in the fleece having a ragged, tufted appearance.
Sheep with itch mite bite more at their wool than do sheep with lice, resulting in tassels that hang down along the flanks. The skin may also have excess dry scurf and scale, and may be mottled with greyish patches.
Infested flocks usually show a range of signs. Most sheep show no fleece damage at all, or may have some tufting of wool along the flanks. Very few sheep (usually one per cent) have severely damaged fleeces. Itch mite mainly affects older sheep and is rarely seen in young sheep.
Other causes of rubbing
Before a suspect case of itch mite is fully investigated, the following causes of rubbing and biting should be eliminated:
- Lice—Lice are difficult to detect in sheep with less than six months wool. Rubbing sheep should be inspected by parting the wool in five sites on each side including the neck, shoulder and flank.
- Break in the wool—A break in the wool, whether induced by disease or nutritional stress, can give the appearance of pulled wool. Generally the break affects the entire fleece and checking a few staples of wool will quickly confirm the diagnosis.
- Grass seeds—The main seeds which cause irritation are barley grass, wild oats, brome grass and storksbill (Erodium) during late spring/early summer. The seed works its way down the wool fibre and starts to penetrate the skin, creating severe discomfort and causing the sheep to bite and pull at its fleece.
- Dermo (lumpy wool or dermatophilosis)—Serum from skin inflamed by dermo infection binds with growing wool, resulting in hard, blocky lumps in the fleece. The fleece of sheep affected by dermo may have a matted appearance which can be made worse by flystrike. Affected sheep may rub.
- Fleece rot—Fleece rot, especially when associated with flystrike or dermo, will give the appearance of pulled wool. Look for tell-tale discolouration of the wool.
- Flystrike—Whether associated with dermo or fleece rot or on its own, flystrike with its associated tissue damage and fluid leakage, will cause wool to mat and strand.
- Fleece shedding sheep—Breeds such as Dorper, Damara, Wiltshire Horn and Wiltipoll naturally shed their fleeces each year, beginning in early spring. During the period that the sheep are shedding their fleeces, they commonly rub and scratch themselves.
The economic importance of itch mite infestation to the sheep industry has not been clearly established.
Severely affected fleeces may be discounted at sale, but this usually involves very few sheep. Where there is only tufting of the surface wool, fleeces are usually not downgraded. Wool cuts on badly affected individual sheep may be reduced but a reduction in value of the total fleece usually occurs in less than one per cent of the mob.
The major impact of mite infestation may be to cause concern about the flock’s appearance, especially on stud properties. Because the signs of itch mite infestation are similar to those caused by lice, itch mite infestations often lead to a false suspicion of louse infestation.
As with other parasitic diseases, poorly fed sheep are likely to become more severely affected.
The entire life cycle of the mite is spent in and on the skin of sheep. Newly laid eggs develop through several nymphal stages to the adult mite over five weeks. Only the adult is freely mobile, so it is during this stage that infestation is most likely to spread from sheep to sheep.
Itch mites die quickly when not in contact with sheep.
Generally, mite populations increase in winter and spring and decline over summer. Transmission does not occur readily, but when it does it is usually between shorn sheep and between ewes and lambs. The greatest opportunity for transmission occurs when sheep are shorn in late winter or spring when mite numbers are greatest.
Spread is greatest in yarded sheep immediately after shearing.
Mite populations are slow to build up, and usually spread slowly through a flock. It is rare for more than 5–10 per cent of a flock to show signs of itch mite. Mites may be present in a flock for several years before they are noticed.
When flock owners see ragged, pulled fleeces they usually suspect a louse infestation.
Mites are suspected only after no lice, or other possible cause of the fleece derangement is found. Confirmation of a mite infestation is made by examining a skin scraping under a microscope as mites can not be seen with the naked eye.
Scurf (which may vary from sparse white powdery particles to thick white or yellow flakes) on the surface of the skin may also be seen with mite infestations. One practical difficulty in confirming the presence of itch mite involves selecting sheep to be examined. Sheep showing the greatest signs of fleece damage may have only low or moderate mite numbers, whereas sheep with many mites may show comparatively little fleece damage. The results from skin scrapings are hence often disappointing, and it may be necessary to examine several sheep before mites are found, though a negative result is still not conclusive. If fleece rubbing continues, and no other cause can be found, another attempt at scraping may be necessary. Scrapings should be done in spring when itch mite numbers are greatest.
Itch mites are harder to control than lice and complete eradication is probably not possible with the chemicals currently available. Experiments have shown that treatment is best done in spring. Treatment should significantly reduce the signs of fleece damage and, because of the slow build up in numbers, no further signs should be seen for several years.
- Drench—the macrocyclic lactone group of chemicals (such as ivermectin, abamectin and moxidectin) will control itch mite. It is not usually economical to drench all sheep on the farm solely because of itch mite, so it is recommended to drench at a time that fits in with the farm worm control program.
- Cull—some sheep may continue to be affected when the flock is generally normal; because this only involves a small number, an option is to cull these animals, regardless of the cause.
It is important that newly shorn infested sheep do not mix with 'clean' sheep, especially in spring. If itch mite is present in a ewe flock, shearing should not be done before lambing, but delayed until after the lambs are weaned to reduce the chance of spread to lambs.
This is the Text only version of Farmnote 449 - full PDF Available
Page amended: November 2010