It happened to a fellow rider when we camped at Anza Borrego Desert
State Park. Pulled out some cholla spikes, thought no big deal since
the horse continued sound throughout that day and the next.
Perhaps a week later, swelling, heat, lameness. It did last for over a
month, the horse went totally sound. The vet was unable to pinpoint the
cause as cholla; the "tincture of time" remedy did seem to do the trick.