I've also only heard of mites in hay and straw - and this can be easily rectified by popping it in the freezer until you need it - or blasting it in the microwave for a few seconds just before use.
I'm fairly new to hamsters, I'm a 'rat girl' really
but I know that all rats carry mites and it's fine. However, when they are immuno-surpressed, it becomes a problem and they begin to show symptoms/become infested. I don't know if the same applies to hamsters.
I also know that injecting ivermectin into a rat can cause death - which was why I was interested to know what your vet injected. It's so easy to give a small animal an overdose, I would always ask before my vet injects anything into my hamster or my rats.