Onchocerciasis | equimed.com
Got midges or small biting flies? I have one that was loaded with what looked like sweet itch every year. Once I started treating her for NTW it is almost gone. She still gets itchy and hates flies but now at least she isn't bald.
Pretty much that ^^^ BUT when you say "filly" how young are we talking? She may be too young to put two tubes of Ivermectin in her
With great respect to the folks who use MTG in this sort of situation, I have read of others having to call the vet because the horse had a reaction to MTG. It is sulphur-based and if the horse has open seeping sores, MTG is not the stuff to use.
The filly's main issue is that she has a poor immune system. It doesn't matter how perfect you are feeding her, how consistent you worm and get her vaccinated, she has a poor immune system and is having severe allergic reaction to Midge Fly bites.
I could suggest mixing Vitamin A in her feed pan BUT again, how young is she?
The Rule of Thumb that folks swear works is 5 CC of injectable cattle Vitamin A, once a week for two weeks, three weeks maximum BUT AGAIN, that is for mature horses.
Topical things that might work, along with keeping a mesh sheet on her:
Order some MalAcetic. It is an all-natural shampoo, created by a vet. Dechra MalAcetic Shampoo for Dogs and Cats (ONE GALLON)
Yes, it's very expensive BUT a little bit of this shampoo goes a long way.
The other thing you might try to calm her skin down is to shampoo her with Dawn Dish soap.
I use Dawn Dish Soap on one horse on his back for rain rot and on all the horses' legs to get the Dallis grass sap off them. They all seem to find it soothing, plus if it's good enough to clean baby ducks after an oil spill, it's good enough to wash my horses skin issues.
Feeding some Magnesium oxide might also help her immune system but it will take some time for that to work thru her system. Again, her age would dictate the amount.
Not a whole lot of help, I'm afraid; mostly because I don't know which end of the age scale "filly" means in this case. If she's closer to maturity than not, I would go full bore with one of the oral supplements.
I'm still not sure I'd double dose her with wormer unless she is rock solid healthy except for this Sweet Itch