However - no one can predict what a horse (or human) may have a reaction to.
Vaccines, wormer, feed, fly spray, shampoo . . . .
That's right, and her other horses were fine... it was just the one. It's enough of a red flag with the reaction being so traumatic, that why push it.
There's plenty of other wormers that are effective, and much safer than Quest. I'm sticking with those.
I have a mare that one year had a reaction to the flu/rhino vaccine. Never an issue up to that point. She no longer can have it. My other horses never had a problem with flu/rhino. 3 veterinarians said it's that particular type of rhino vaccine (don't remember the details it's been to long ago) but you can't get a flu shot without it