I don't understand why but, believe me double-dosing with pure Ivermectin kills them. It took me three years of shaking my head in the wrong direction before I finally broke down in 2010 and did the double-dosing.
For the first time in many years, my Arab finally had hair on his belly line instead open sores from Midge Flies that I had to keep covered with diaper rash cream every day.
Many folks suggest double-dosing again in two weeks to get any newly incubated NTW's that weren't killed off the first time.
I didn't do that because my horses seemed to heal quickly without any recurring sores. I did double-dose them again in early Fall of 2010 and that was the last I had to do it until last month.
I am pretty sure that double-dosing is what brought on the colic in my 24+ with long-time metabolic issues. The vet diagnosed him with hind gut ulcers and he's going to be on Succeed for the rest of the summer.
That's I why I suggest consulting with the vet, especially if the horse tends toward colic and/or ulcers. Thevet should recommend something really high dose to give the horse during worming. Which is something that should be done even when doing a standard worming for the worms we are all familiar with.
You may want to print the NTW info off to show your vet. You'd be surprised the vets that aren't up to speed on them either.
I have also read of folks having the vet do a skin scraping and it comes back negative, yet they do the double-dose of Ivermectin, the sores really erupt as the microfiliae die off, then all is quiet and the horse quits itching.
Most folks don't know about them until they have to deal with them, so don't beat yourself up.
It's a shaky moment and tough to wrap ones head around the words "double dose" when wormers are being discussed.
I have read of folks using Equimax in a double dose but I stick to the original concept of pure Ivermectin. I've got enough problems and my Heart Horse of 22 years colicked anyway because I didn't realize he is dealing with hind gut ulcers