Diagnosis
Septic Arthritis due to Local Traumatic Injury

The radiographs of the fetlock joint indicate possible inflammation of the distal 
physis of the cannon bone. No obvious fractures or soft tissue injuries are apparent.  
Clinical signs of lameness and edema and the CBC results lead us to a diagnosis of 
localized septic arthritis.  Leukocytosis, which would indicate a systemic infection, was 
not apparent.  This is further indicated by the fact that the lameness is localized to one 
joint only.  Hematogenous spread of bacteria to the joint is not likely in this case because 
the foal appeared to receive adequate colostrum. 
In this case, the foal was possibly stepped on by a mare and received a blow to the 
metatarsophalangeal (fetlock) joint.  This type of traumatic injury can lead to a localized 
inflammation due to possible physeal injury.  Salter-Harris fractures type I and II) are 
common in horses, but not evident radiographically in this case.  More importantly, 
inoculation of environmental pathogens is possible if integument is ruptured.  Inoculation 
of the synovial membrane or synovial fluid incites an inflammatory reaction.
The inoculation of the synovium leads to an inflammatory reaction which 
provides an environment for entrapment and colonization of bacteria.  In this case, 
Klebsiella (Enterobacteriaceae, environmental pathogen) was most likely inoculated into 
the joint at the time of injury.  The resulting inflammatory cascade results in the loss of 
proteoglycan and collagen from the articular cartilage.  The loss of cartilage with 
mechanical trauma leads to rapid, irreversible and progressive cartilage destruction.  Over 
time, periarticular fibrosis often results that may limit range of motion and compromise 
synovial membrane function.   Our foal is showing clinical signs of the initial stages of 
septic arthritis.  In the absence of treatment, the foal may develop cartilage destruction 
and lytic bone lesions.  The possibility of premature closure of the physis is great as the 
physis heals.