What are itch mites?
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. Itch 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. Since the introduction and use of macrocyclic lactone (ML) worm drenches (ivermectin, abamectin and moxidectin), and more recently ML-containing anthelmintic injections and spray-on, jet or pour-on insecticide formulations, reports have been rare. Confirmation of suspect cases has not been common because farmers usually opt to treat infestations rather than seek a diagnosis.
Itch mites in a nutshell
- Tiny mites that live on the skin surface of sheep, they are impossible to see without magnification.
- Present in some WA sheep but the prevalence of visible infestations is low.
- Cause some sensitised sheep to rub and chew at their fleeces but the economic effect is small.
- Can be spread from 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 available but ML drenches are highly effective.
Signs
Signs of itch mite infestation relate to the irritation it causes however most sheep show limited or no signs at all. Some affected sheep rub, scratch and bite at their fleece resulting in the fleece having a ragged, tufted appearance. Biting at the wool appears to be more common than in lice infestations and results 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. Very few sheep (usually 1%) have severely damaged fleeces.
Itch mite mainly affects older sheep and is rarely seen in young sheep.
Economic impact
The economic impact of itch mite infestation to the sheep industry has not been clearly established. 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 1% of the mob.
The major impact of mite infestation usually relates to concern about the flock’s appearance, especially on stud properties. Because the signs of itch mite infestation are similar to those caused by lice, 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.