Marty you honestly need to put a book together... That one is on my barn door now.
Just because one horse is with others does Not mean that they will all have it. Often they will only get them when they are stressed out or run down a bit. (At least down here in FL that's the case, It may be differnt in a northern climate.)
Recently I bought a weanling colt, a stallion and a gelding from the same farm, the stallion was rather thin and run down, but the weany and gelding looked great. When we got home I realised much to my dismay, that the stallion was Covered in lice... ewwwwww. I have used 7 dust and ivermectin past, and clipped him and all the little suckers went right that away!
Asides that though, some horses can get sensitive to food allergies when the seasons change, or the extra hair lets the sweat collect, and therefore buggies, ergo itchies.
Just be careful with all the bathes that you do not dry out his coat.
I have used feed through fly control systems, (garlic, natural earth, grape seed extract, and apple cider viniger,) with a great deal of suscess in this type of horse. A
lack of sweet feeds in their diest helps also.
Mine are all on the COB diet with added flax seed and minerals. I have noticed that it has cut down on the buggies alot as well....