How to stop the itching?

[Victoria A Thompson <toriandsteve@juno.com>]

OK, folks, I'm at my wits end.  I no longer know what to do to relieve my
poor horse of her itchy skin.  I suppose I should start from the
beginning.

It's bugs.  We assume mosquitoes, but we (we meaning my vet, me and
everybody else along the trail where I keep my horse) really don't know
for sure.  Taffy (the blood donor) is literally the only horse on the
trail being eaten.  There are 29 other horses in the barns around us and
Taffy is the only one with the problem.  It is not a food allergy,
definitely bugs.  It is NOT sweet itch (yes, I know, sweet itch is bugs -
gnats - but we have a different problem).  At first it was just a whole
bunch of bites - normal size bites - but instead of one or two it would
be 15 to 20.  Then the bites became bigger welts and now we're into full
blown hives.  BIG, HONKING HIVES.  It is NOT fungus.

I have tried everything to keep her from being bitten.  Fly spray at tick
strength is a daily dosing.  A day sheet is kept on her but they just
dine from her neck, face and belly.  Now they're even going under the
sheet (or through it) because she has some bites on her sides again.  I
removed all old bedding and replaced it with cedar shavings (specifically
CedaRest).  I had her hay checked and there are no mites in it.  She
doesn't have fleas.

My normally stoic horse goes into quivers of joy when I scratch her -
anywhere, not just on the bites.  She doesn't want me to stop either. 
She wraps herself around me and begs me to scratch her.  It's heart
breaking to have her so miserable.  Someone mentioned Benadryl to help
with the hives, but I don't know how much to give her.  My vet is off
with her horse at a clinic for a week and I don't want to bother her with
a none emergency call.  Have any of you used Benadryl Allergy Relief on
your horses?  Also, when I was a kid my mother used to make us soak in
oatmeal baths to give us relief from the itch of poison oak.  Do you
think sponging some of that on Taffy would give her any relief?  I can't
find any kind of shampoos for itchy skin that isn't for fungal infections
too.  Should I try one of those or something else?  My husband suggested
using flea shampoo for dogs.  

I'm lost.  I've run out of ideas.  This has been going on since
mid-February when she first started losing her winter coat.  We're in
Southern California over on the coast, not out in the desert.  The only
thing different that Taffy gets that the other horses around her don't is
a daily turnout on the green grass along the trail.  I took her off the
grass 5 days ago thinking that it was something in the grass, but  the
bites haven't stopped.  Just between yesterday and today she has three
new hives, one new welt and seven new bites.  

Any helpful hints or ideas would be greatly appreciated.

Tori (at my wits end) and
Taffy (who loves moms fingernails)