Lameness resulting from sepsis
A basic understanding of the pathogenesis of septic osteoarthritis in foals will help
the veterinarian to recognise and diagnose the various different presentations. It is
believed that orthopaedic infections in foals arise from haematogenous spread from
a primary source, most commonly the lungs, gastrointestinal tract, urachus, or
neonatal septicaemia. Sluggish blood flow through the complex arrangement of
microvascular loops at the metaphysis, physis and epiphysis of growing long bones
creates an ideal environment for the colonisation of bacteria at these sites. Similarly
the slow flowing microvasculature of the synovial villi allows sequestration of bacteria
into synovial fluid, resulting in septic arthritis.
Five types of septic osteoarthritis have been classified:
1. S-type, where the septic arthritis is a result of innoculation from the synovium,
most common in young foals (14 days and younger).
2. E-type involves the epiphysis and infection of the subchondral bone and may
inoculate the joint. Radiographic changes like focal subchondral lysis of the
epiphysis are common.
3. P-type involves the physis and results in radiographic lysis of the physis and
metaphysis of foals from 7 days to 4 months old.
4. T-type involves the cuboidal bones, tarsal and carpal, and may result in
cuboidal bone collapse.
5. I-type, which is a periarticular soft tissue abscess that infects either a physis
or a joint, and is more common in older foals (2 months plus).
Typically foals with septic osteoarthritis are acutely lame, have obvious focal synovial
effusion or regional swelling, and are pyrexic or have a history of pyrexia. However,
in some cases the source of lameness may be very difficult to determine. Infection of
the proximal joints, hips, pelvis and shoulders, vertebral bodies and some tendon
sheaths are particularly hard to locate.
In acute cases the systemic haematology profile may even be normal due to the
localised nature of the infection but the white cell count n