Hi Caroline,
I don't know if these UTI's are particularly common in bassets as a rule, but you can add my Scully (8-y-o) to the list of houndies that have had issues with them! She had her first one when she was about 5 or so, then another one maybe a year and a bit later, and then the whopper of an infection just this year. The last stubborn one was an E.coli infection in her bladder and it took about 5 months (maybe 6, I lost count) and quite a few courses of antibiotics to finally wipe it out (first Clavamox, then Chloramphenicol, and then Zeniquin). In the end, it was a 6-week course of Zeniquin that worked! The vet also did make sure to do a urine culture about 10 days after each course of antibiotics to determine whether or not the infection was actually gone...the last few times they did a clean sample (cysto). During that last long round of antibiotics, the vet also did a culture while Scully was actually on the antibiotic to make sure it was working.....no sense in being on it for so long if it wasn't doing the trick I guess. As well, they did x-rays to rule out stones, and a blood panel to rule out kidney disease. Now we just have to keep an eye out for symptoms, and have her urine tested every 6 months to make sure all is ok. In Scully's case, they thought that her infections may be due to some slight sphincter incontinence that is not severe enough to ever cause her to leak urine, but allows bacteria to get in.
I hope that you have success in wiping out Cassie's infection!! Some people swear by cranberry supplements as a preventative measure, although we haven't tried that.