Anything I have read says Deet is a no-no for horses, I'm guessing because they nibble and scratch with their mouth, wipe their lips on legs etc. and so would ingest the stuff. I have seen some fly sprays which say don't use right on the horse, but you can spray it on certain external clothing of the horse, but I haven't used it myself. Thing is, if the flies are so thick they aggravate the horse just by sheer numbers, the sprays might only stop the bits and landing, but not the chasing and bugging.

While I'm not sure of the effect on deer flies (I barely know one fly from another outside of mosquitoes), I have been trying the thiamine supplement on Sharif. In previous years he got a nasty sore on his tummy from midges and diaper cream was the only defence, but I always seemed to be after the fact with it. This year I started thiamine well ahead and didn't really expect it to help. Either it's a coincidence or not, but he has no tummy sore this year (no diaper cream used so far) and I'm very happy about that. I don't think it made much difference to mosquitoes, but they are so bad this year, it might be cutting the bites down by half and you'd still have a bunch.