Diagnosis:

Our final diagnosis would be a combination of our hypothesis one and two, infectious 
and trauma induced arthritis.  The joint tap was positive for Klebsiela which is indicative 
of infectious arthritis.  Infectious arthritis in foals under 45 days of age is usually caused 
by hematogenous spread via the umbilicus (navel ill) or trauma induced.  In this case, we 
feel the history of the mare possibly stepping on the foal, Clinical Pathology results and 
radiographs support trauma induced arthritis.  The endotoxins of Klebsiela cause 
chemotaxis of inflammaotry cells.  This explains the pain response found on presentation.  
The CBC indicated that there was not a systemic infection due to the lack of leukocytosis.  
The joint tap taken from the right hind fetlock showed evidence of effusion of 
inflammatory cells.  Again, the localization of inflammation would be supportive of 
trauma induced rather than hemtogenous spread.  Synovial effusion and bone lysis were 
shown on the radiographs of the right hind fetlock.

Our theory would be that the mare stepped on the foal causing a penetrating wound that 
would allow the introduction of Klebsiela.  Although we cannot completely rule out navel 
ill, the fact that the foal did receive adequate colostrum and that multiple joints are not 
affected suggests hematogneous spread is unlikely.