Green’s Dictionary of Slang

any form of itchy rash contracted via insects in a dirty place.

1833		Bell’s Life in London 13 Oct. 2/5: Scotchmen are seldom averse to the scratch.
1881		Dundee Courier (Scot.) 18 Mar. 7/4: Somehow or another we’d caught the ‘scratch’ and we lay up in the old workhouse [...] for a fortnight.