Join Date: Jul 2011
Location: Bethel, Ohio USA
• Horses: 0
Autoimmune response or toxicity was my first guess but these disorders typically effect all four feet; not just one.
Given the quick retreat of the pathology, a toxin seems more likely but I wouldn't rule out either. Evidence warrants a review of local pasture for known toxic weeds.
If autoimmune, pemphigus (foliaceus) would be worst case scenario but wouldn't retreat so quickly. Vasculitis appears similar to the photos but again, it's uncommon to see in a single foot. Verify by looking for similar lesions on the inside of the horses lips. Also check for edema of the distal limb.
Recent introduction of any pharmacologicals would be suspect. Diagnostics include histopathologic and immunohistopathologic examination of superficial and deep biopsies of the lesion margins.
While toxins are often suspect in autoimmune reactions most such occurrences are generally considered idiopathic in origin.
Are you sure there were not lesions on the other feet?