Leptospirosis is another disease that may be transmitted by a rat.
I've also heard comments elsewhere complaining about Frontline, but I haven't seen any official responses yet, or comparisons as to which products use which chemicals, or if it's not the chemical but something ELSE that prevents it from persisting the full four weeks.  I think it is quite possible that other medications/feed supplements etc could prevent frontline from working.  Don't have time to look into it in detail right now though.
But which tick/flea preventative a dog can be given depends sonmewhat on breed (especially sight hounds, collie type breeds - ask your vet if the product is appropriate for your dog BEFORE PURCHASING ANY flea/tick preventative.