First off if the horse got hurt falling into that septic tank and they did not get medical help for him...that was just so wrong. The reason he bites at his legs is he is in pain. I would about bet you he has some sort of infection from his injury being neglected . Where are you located ?Maybe I can help you find some close to you that can help !
Thanks,
Betty Sue
Ps..I just moved the Helpahorse site to a new server. Check it out when you can http://www.helpahorse.net
