Sherlock Holmes S06 E02 The Last Vampyre

"Jeremy Brett is Sherlock Holmes."

Holmes investigates a killing which has all the hallmarks of the worst kind of Ruritanian vampire. He is sceptical (to say the least) about the possibility that the undead are up to their mischief just a short train journey from his rooms in Baker Street. If a local resident really found his South American wife sucking blood from their young son, what might it signify?