Don't feed fresh garlic, period. IF
a horse is going to develop anemia from garlic, it is the allicin in RAW garlic that's going to cause it.
I have been feeding powdered equine garlic, seasonally, for tick control since 2004.
It is 85% efficient for ticks, not-so-much for flies which, sweet itch is caused by the bites of Midge Flies.
Increased Vitamin E can help.
Change of diet can help because certain feeds will permeate thru the skin, in turn, attracting all sorts of flies.
I have read on other forums that folks have fed their horses really massive doses of Vitamin A. However, it stores in the liver (Vitamin E does not) and can become toxic, so I won't use it.
My Arab has always had a terrible time with sweet itch. Changing his diet helped.
Worming him twice (two weeks apart) with a full tube of 100% pure Ivermectin helped; he's 13.3H & weighs 840 lbs., so a full tube of wormer was 1-1/2 doses for him. Bigger horses have been double-dosed.
Slathering his belly line every day with diaper rash cream helps because the flies can't land on him thru the diaper rash cream. It stays on all day.
Hope this helps