Harry's facial markings indicated he was predisposed to Megacolon. When he entered his first episode it was confirmed that he did in fact have Megacolon.
Megacolon hasn't been well research as of yet and currently there is no way to formally diagnose. Currently the only way to diagnose is through a necropsy.
When the pattern across the nose is broken at any point during the marking.
Check out some examples of a broken butterfly!
Signs of Megacolon
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