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Hi there. Thanks for the question. Let me see if I can help. Does your defecate in his water bucket? If so, then they could be coming from him. If not, there is no way a horse intestinal parasite could get into the water bucket except from fecal contamination. If he does defecate in his bucket, then they are likely one of two types of parasites. If they are large and white-ish, then they are ascarids (large round worms). If they are small, thin and reddish in color, then they are small strongyles or sometimes called bloodworms. I hope this helps. best of luck.