I wormed my mare last weekend and I thought she was having an allergic reaction to it, but after talking to several vets and friends and surfing google for a little while I found out it can cause chemical burns in their mouths.... I was not a happy momma and I felt so bad for her. She had sores and her whole mouth was swollen Posted via Mobile Device
They say if it gets in ther mouths is when it causes burns. She spit hers up. I felt awful. She had sores on her mouth and gums, her lip was hanging it was so swollen. I had to go to the vet and get her a shot Posted via Mobile Device
I've seen that happen when the dewormer was not placed at the back center of the tongue or the horse managed to spit it instead of swallowing. That can happen with any dewormer that gets caught between the cheek & the teeth or gums. It sits there & can burn.
If it's seen flushing with water will clean it out.
The good news is that the mouth heals quickly.