Hypothesis 3:  Arthritis

Infectious arthritis
When the foal sustained a trauma to the fetlock (whether it was from the mare 
stepping on it or otherwise), a laceration was produced that allowed for bacteria to enter 
the joint spaces surrounding the fetlock.  The bacteria release toxins and initiate the 
release of cytokines and other inflammatory mediators that lead to loss of proteoglycans 
from the articular cartilage.  Once the cartilaginous support is lost, mechanical trauma 
leads to additional cartilage destruction.  The synovial membrane is inflamed and fibrotic, 
which limits the range of motion.  Fibrin deposition into the joint cavity, as a result of the 
inflammation, limits the nutrition and function of the synovia and cartilage.
	Clinical signs of infectious arthritis include: progressive lameness, heat and 
swelling of joint, and fever.  The only sign this foal is showing is swelling around the 
joint.  Also, trauma is usually the cause of infectious arthritis in older horses while 
infectious arthritis in foals usually results from extension of osteomyelitis or a 
hematogenous spread of a systemic infection.  Therefore, we consider infectious arthritis 
to be low on the rule-out list.

Traumatic arthritis
	Any traumatic episode has the possibility to progress to osteoarthritis; however, 
this usually is progressive and takes longer than what was presented here.  Osteoarthritic 
changes include: swelling of the proteoglycan matrix, thickening of the subchondral bone 
and hypertrophy and hyperplasia of the synovia.