Septic arthritis hypothesis

Septic arthritis is a common cause of lameness in young foals.  Though this foal is nursing normally and presumably had successful passive transfer of immunity from the mother, we felt that this was a possibility to keep in mind until we have good evidence for another cause or sufficient evidence to rule it out.  Septic arthritis can affect a single joint or multiple joints.  In cases where failure of passive transfer is a predisposing factor, with hematogenous spread, you generally see multiple joints affected.  In this case, since there is only one joint affected, the etiology of the disease would be from direct inoculation of the joint (rather than hematogenous spread, such as from an umbilical infection). With only one joint affected, we would not necessarily expect to see fever in the foal, so lack of fever or systemic illness does not rule out septic arthritis.
	Even a small puncture wound, easily missed by the owner, could have introduced bacteria into the sterile diarthrodial joint of the fetlock.  If the invading organism were able to bind to tissues and establish growth, infection of the joint would ensue.  Even with successful passive transfer of immunity, it seems that a young foal might be more susceptible to joint infection because of the immature immune system.   With infection, neutrophils are chemotactically drawn to the joint, where they release inflammatory mediators and degenerate, leading to local tissue damage.  The inflammatory response, coupled with toxins released by the bacteria, leads to loss of proteoglycan and collagen from the articular cartilage.  You also get inflammation of the synovial membrane, with hyperplasia and fibrosis.  Because the synovial membrane and fibrous joint capsule are richly innervated, the animal experiences intense pain from the inflammation, leading to toe touching and eventually, nonweight-bearing lameness.  If not treated aggressively, the destruction and remodeling of joint tissues from septic arthritis can lead to osteoarthritis.
	If this foal's trouble is septic arthritis, we would expect to see turbid synovial fluid with greater than 300,000 WBC per ml and a total protein of at least 4 g/dl. The swelling and pitting edema seen in this foal are consistent with inflammation from septic arthritis.