Life cycle and transmission
The entire life cycle of the mite is spent in and on the skin of sheep. Over a period of five weeks newly laid eggs develop (through several nymphal stages) into an adult mite. 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). Transmission is also 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% of a flock to show signs of itch mite. Mites may be present in a flock for several years before they are noticed.
Mites should be suspected only after lice or other possible causes of fleece derangement are ruled out. 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 on the surface of the skin may also be seen with mite infestations (which may vary from sparse white powdery particles to thick white or yellow flakes). One practical difficulty in confirming the presence of itch mite involves selecting sheep to be examined as the level of fleece damage is not always proportionate to the amount of mites present. Therefore, 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 are best conducted in spring when itch mite numbers are greatest.
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, usually beginning in early spring. During the period that the sheep are shedding their fleeces, they commonly rub and scratch themselves.
- Exotic diseases — some exotic diseases, such as scrapie, sheep pox, lumpy skin disease and Aujeszky’s disease, may cause sheep to rub or chew their fleeces, among other signs.
Itch mites are harder to control than lice and complete eradication is probably not possible with the chemicals currently available.
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.
- ML-containing anthelmintic drench or injection, or ML-containing lice or fly treatment. The macrocyclic lactone group of chemicals (such as ivermectin, abamectin and moxidectin) will control itch mite. There is rarely any need to treat all flocks on the farm with an ML-containing product solely because of itch mite. The timing of use of any of these products should be based on the need for internal or external parasite control, and used at the correct label rate, with the secondary possible benefit of some control of itch mite.
- Cull. Some sheep may continue to be affected (rub and chew the fleece) when the flock is generally normal. Because this only involves a small number, an option is to cull these animals, regardless of the cause.
Experiments have shown that treatment is most effective in spring.
In rare cases where itch mite is suspected to be a significant problem, newly shorn infested sheep should 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.