yes they can still get it, rearguard helps PREVENT,
It never amazes me how quick the lil blighters can grow and spread, I've never had a case myself yet, but mums long haired cat caught it last year, now being old the hot weather turned her stools runny if it wasnt for the fact mum was checking every evening before the cats dinner lords knows, she rang me in a tiz as i told her about buns getting she wasnt sure on other animal lucky they were very few and tiny, so didnt need vet treatment
I found while people will say use fly traps etc they I find attract more flies because well thats the aim to catch them on the tape, so be carefull if you buy such things were you place them,
I use to put lavender mint and sage in pots all round the run or hutch to deter them
obviously regular cleaning too