It might be pinworms, which most wormers don't kill as they don't reach the far end of the intestinal tract. There was a big article on it in Horse and Hound (British weekly horse magazine), but here's the condensed version.

http://www.horseandhound....rsecare/5125/295975.html

Alternatively, he might just have an itchy arse.

Last Edited By: IndigoMoose Jul 23 10 5:15 PM. Edited 1 times.