How did he get mange?

[Jamie714]

So a vet office opened about a month ago. The vet has an actual degree(hard to find here) and he seems to know enough about rabbits I'm willing to use him in emergencies and stuff.
So a couple weeks back I noticed a bald spot on his toe after we got back from a weekend trip.Since the skin looked healthy and didn't appear to be bothering him, I assumed it was from stress so I left it alone.
So the day before yesterday he wasn't eating much and he was limping on it and it was looking kind of crusty so I took him to the vet to see what was going on. The vet diagnosed it as mange but said there didn't seem to be many when he looked at a skin scraping. He gave him some Stronghold and said to come back in a couple weeks for another dose.
He said for the eating we should force feed him until he's doing good so he gave us critical care packets and showed us what to do.
He has been improving since then and is eating his normal food like a rude little piggy again. He's not quite as playful, but it seems like his foot is still a little sore, but he is getting more active than he was.
So the mystery is how did he get mange? He's not around other animals and I keep the room and his area really clean. His litter box is cleaned daily or sometimes every other day(I am 7 months pregnant, so I admit I have days I can't balance over the box very easily or I get lazy).
There are stray cats outside(no animal shelters here) and flies sometimes manage to sneak in from the balcony door. He's an inside only bunny, but he does occasionally sneak out onto the balcony if my husband is out there smoking.It's second floor, so it's not really possible for cats or other animals to get there.
I scrubbed everything down yesterday, I just want to make sure I do everything I can to avoid him getting it again.
And another question. Since he's eating again, does anyone know how long the critical care will stay good for after it's been opened?

 

[majorv]

I believe there's a date stamped on the bag somewhere. It stays good for a good while when kept in a cool dry place. Mites can be picked up in hay or accidentally brought in. Sometimes you don't know, but it's pretty easy to clear up as long as you clean/wash your bunny' s environment.

 

[JBun]

I agree with majorv. Could be the hay or could have hitched a ride some other way like the bedding. To be safe, it might be a good idea to ditch your current batch of hay and bedding/litter, and buy new stuff.

Critical care is usually good for quite some time as long as it is the dry mix and hasn't yet been mixed with water(which is really only good for about a day). I keep my bag of dry mix in the fridge or freezer to keep it fresh longer.

 

[Jamie714]

I'm thinking it could be from one of those stray cats. A couple have some pretty nasty looking bald spots and they will occasionally try to rub on people in hopes of food.
I replaced his hay and bedding to be sure and have cloroxed and vacuumed everything.
His toe is already starting to look better. It's looking less crusty and he doesn't seem to be as itchy on it. He's also starting to run around and play and just acting like himself again.