Septic arthritis is a very serious condition in horses, caused by bacterial infection of the joint and surrounding tissues. The condition affects horses of all ages, however in foals it is referred to as 'joint ill', where it is discussed separately. In adult horses, most cases of septic arthritis are caused by bacteria entering the joint through a wound (traumatic or surgical) or through any procedure involving inserting a needle into the joint. Common veterinary procedures such as arthrocentesis (joint tap), used to collect a sample of joint fluid for analysis, or for injecting medication in the joint, such as when a horse receives corticosteroid or Adequan injections. Veterinarians will take certain precautions and care to minimize the possibility of a joint infection, however there will always be some degree of risk.

The most common bacterial organisms found in horses with septic arthritis are Staphylococcus aureus, Streptococcus pneumoniae, Escherichia coli, Proteus sp., Salmonella sp., Serratia marcescens, and Neisseria sp.. When bacteria enters the joint and into synovial structures, they cause severe inflammation of the synovial structures. The body's response to the infection also degrade the joint fluid and cause damage to the cartilage. Septic arthritis can result in severe damage to the joint within a few days of infection, leading to serious performance-limiting osteoarthritis.

Prompt, aggressive treatment is needed for horses with suspected septic arthritis. Treatment is aimed at controlling and eliminating the infection. Although most infections are caused by gram-negative bacterial organisms, ideally selection of an antibiotic should be based on results from culture and sensitivity testing as mixed infections with different types of bacteria commonly occur.