Hypothesis 2: Infection induced by trauma to leg

When the mare stepped on the foal, or from another puncture wound, there may have 
been a break in the skin of the fetlock where bacteria was introduced.  The bacteria would 
trigger an inflammatory response to attract neutrophils and macrophages.  This would 
cause swelling of the joint and surrounding areas, pain due to the inflammatory 
mediators, and a reluctance to use the leg.

Septic arthritis