We occasionally experience Sweet Itch in our cow ponies here in the western U.S. The malady, at least here, is seasonal since it's primary cause are certain varities of flies often lumped together under the name of "midge". Our Vet says there are basically two types of flies that are causal. One, the most common, bites the horse on the upper parts of the horses body, especially around the head, wethers and base of the tail. The other, less common, bites lower parts of the horses body. This causes itching, which the horse tries to alleviate by rubbing, often with disatrous, infectious results if left untreated
Our flies tend to breed and stay fairly near a source of water... especially pasture ponds. Keeping the horse as far away from the sources helps. Keep in mind that the flies are seasonal... spring through late summer. We do spray the water areas for flies since they can also cause problems with the cattle.
I realize many like to use "natural" or homeopathic cures, but, in all seriousness, we've found that cortisonal salves supplied by the Vet to be most effective.
Barring that, if you can, stable your pony in the venening until late morning since the flies tend to be most active during that period. You could also cover the pony with a light weight rug and hood during the daylight hours. There are preventive sprays available that are very effective but are chemically compounded...
Best of luck!