Do you know what they used as an injection?
As for the coccidia I havent had that experience in my snakes. I know most reptiles can carry low numbers of coccidia natrually. I assume snakes would as well. although I havent ran across a concerning count in my collection upon fecals. Mice could carry it. as with any animal. Do you get your feeders from a regular supplier?
Typically signs of high levels of coccidiosis is runny, stinky poop. and lack of appitite. It can be deadly at high levels. You could suggest to the vet showing you how to inject and doing it yourself. I know with a snake we rescued and treated an absess, the vet gave us 2 weeks of injections and we did it ourselves.