Septic arthritis, or joint infection, can be devastating to soundness, athletic careers, and even life for affected animals if treatment is delayed or inappropriate. In foals, bacteria from a distant site invade the bloodstream and cause an infection of the joint. The umbilicus is commonly blamed, but the gut and lungs can be equally responsible. Foals may also have diarrhea, pneumonia, or failure of passive transfer (when foals have not consumed a sufficient amount of high quality colostrum). In studies of foals with septic joints, 50% to 88% were identified with partial or complete failure of passive transfer.
In older horses, joint infections are usually caused by trauma. Wounds over joints (areas that bend) are the most common cause of infected joints. Joint infections can also occur during diagnosis or treatment of joint problems (such as joint injections or surgeries). Some medications have been associated with a higher risk of joint infection due to changes in the normal joint defenses. Tendon sheath and bursa infections are very similar to joint infections in causes, treatment and prognosis.
Lameness from a joint infection takes approximately 8 to 24 hours to develop. It appears after sufficient bacteria have started growing in the joint. The joint reacts to the bacteria with swelling due to increased fluid (effusion) and pain. In some joints, swelling may not be easy to detect due to overlying tissues. Fever may be noticed after the infection is established in adults. Foals are more sensitive, and usually run a fever even before any signs of lameness or joint swelling.
Diagnosis of joint infections can be challenging, especially in foals and horses with open lacerations. When joint fluid drains out through an open laceration, the horse is rarely lame and may not have swollen joints. Foals can have multiple and deep joints infected. Due to the difference in their immune response, the development of clinical signs may be delayed in foals.
Joint infection is usually confirmed by obtaining joint fluid for analysis. A sample of joint fluid is taken and then submitted for culture. If the joint is open, fluid may not be obtainable. These joints have to be checked with other methods including fluid injection, ultrasound and radiographs (x‐rays). Foals are also evaluated for signs of sepsis using blood work and a complete physical examination.
Radiographs are used to monitor for signs of deeper infection and arthritis. Radiographs may be taken initially to obtain a baseline view, and then repeated later to determine what changes are occurring due to the infection. It can take weeks to months for these changes to appear. Next month, treatment and prognosis of joint infections will be discussed.
By Rolf Modesto, DVM, Univ. of Pennsylvania
University of Minnesota Extension Horse Newsletter (Volume 5, Issue 5 - May 2009)
(Dr. Modesto was previously with the University of Minnesota).