I do if the rabbit has come from an unknown background but I don't if I've adopted from a well known rescue. In my experience symptoms have been a result of a lowered immune system either through another illness or from stress. As a good rescue has probably had the rabbit at the vets for vaccs or neuter so essentially health checked and the bun has already dealt with the stress of a change of home I figure it unnecessary to put the new addition through the stress of 28 days of meds. I know this sounds ridiculous its just my weird logic. All my bunnies have probably been exposed at some point as I've had it in previous buns but none of them have ever displayed symptoms so my only use as a prevantative would be in the new bun itself as I doubt it would offer any protection to my exixting bunnies.