I wonder how hot a fire has to be to kill fleas. The burning of London was sort of a blessing in disguise, though I am sure at the time people did not think so. It is interesting that Katherine's abilities have exceeded Morgana's, and that she is more in-tune with Mother nature. It's gonna be a cool battle of the wills when Katherine and Dominic do meet.
I think the fire drove away the rats population that harboured the fleas.