I was told just last week at our feed store that Zimecterin use to be the only brand on the market. I was asking what one is the best brand. They said that Merial that makes Zimecterin, lost the patten for about 5-8 months and that's when all the other brands came out so fast on the market. Now there are several to chose from. They all have the same ingredence so they told me, with the exception of Zimecterin Gold. I have used Zimecterin, IverCare and also Ivermectin and have never had problems with any of them. I also used them with preg, mini mares.
I did however have the Vet come out last month for one of my mines that was acting like she was going to die. She wouldn't eat, drink and didn't have any energy at all. She had her head laying in my lap and just wouldn't get up. The Vet dewormed her, I just had 8 weeks earlier, and that was her problem. You might want to check for worms. That might have been some of your problem.
I'm so very sorry to hear about your lose.