Board Thread:The Reference Desk/@comment-188432-20130505035914/@comment-188432-20130506174530

SmallerOnTheOutside wrote: Could you perhaps list the instances in which you think the script's referring to the leech or the venom as the Crimson Horror?
 * VASTRA: … and our greatest plague, the most virulent enemy, was the repulsive red leech
 * DOCTOR: Oooooh.  "The repulsive red leech"!  Nah, on balance, I think I prefer "the Crimson Horror". What was it, exactly?
 * VASTRA: A tiny parasite. It infected our drinking water.  And once in our systems, it secreted a fatal poison.
 * DOCTOR: So it's been hanging around, lurking in the shadows. Maybe it's evolved.  Maybe it's had help.

To me, this pretty clearly shows that they're unambiguously talking about the creature being the Crimson Horror.