Is she limping and how bad? Have you check the hoof for cracks or something in there? Is the hock area hot? We have had alot of hoof problems this year.
Our heifer looked a lot like that. The Vet thinks that she was bit by a copperhead or water snake. He gave her a steriod to reduce the swelling and pencillian (sp) just in case it was infected. So, maybe a snake bit is not too far fetched.
I took her to the vet and he felt like she was bitten by something and had a reaction. She never limped, looked listless, or went off of feed. He gave her a shot of Dex and some antibiotics and said give it time. The swelling is almost gone.
