Regular worming for "regular worms" does not affect Neck Threadworms
I know it's a contradiction to double dose a horse with pure Ivermectin and I don't get it but, double-dosing with pure Ivermectin does work and worming a horse for stomach worms on a regular basis does not
It is my understanding, a horse can have these microfiliae under their skin "forver", if they aren't wormed specifically to get rid of them.
These nasty little creatures can also migrate into the eyes. They can be the cause of uveitis and eventual blindness in some instances. That can all be Googled and substantiated.
While the sulphur sounds like a great idea, I would be careful applying it to open sores as it does burn. MTG is NOT something to use on open sores.
It's doubtful anyone would want to paint your horse, twice a day, with the ointment mixture I mentioned above; do you have some sort of soothing/medicated spray you could leave at the barn? It would at least be something until you get back.
Keep in mind, the topicals will help relieve the itch/pain but they will not get rid of the microfiliae that are under the skin. Only double-dosing with pure Ivermectin will do that and, again, I don't get how all that works, I just know it does.
I hestiated for a couple years on double-dosing two of my horses but their faces got so bad with sores that would never completely heal, that I held my breath and dosed them anyway, even though they both have ulcers
The double-dosing worked and, thankfully, I only needed to do that one time.
To reiterate, you know if you're dealing with Neck Threadworms if the sores violently erupt into massive itching after the worming, within 24 - 72 hours. That is the microfiliae dieing off and shortly thereafter the hair starts growing back and the horse is comfortable again.