Joint & Physis Infections in Foals

This is an image of a foal with a swollen knee, which is commonly seen in foals with an infected joint. (Photo courtesy of Cyberhorse Forum)

By Lauren Lamb, DVM

During foaling season, owners may notice that a foal is lame and suspect that the foal just got stepped on by the mare. While this could be the cause of the lameness, a more likely cause of this lameness is a septic physis or septic arthritis.

Septic arthritis means the foal has an infected joint. A septic physis means the foal has an infection in the growth plate. Both septic arthritis and a septic physis are life threatening conditions that need to have aggressive medical attention as soon as possible.

Foals have an increased risk of developing a septic joint or physis if they do not receive an adequate amount of colostrum from the mare. This colostrum needs to be in the foal’s gastrointestinal tract within 36 hours of being born. “Failure of passive transfer” is the phrase given to foals that do not receive enough colostrum.

Two common reasons a foal will not receive an adequate amount of colostrum — the foal not being able to stand and nurse the mare properly within the first 36 hours of being born is the most common cause of failure of passive transfer and due to a mare leaking a significant amount of colostrum prior to the foal being born. The colostrum contains anti-bodies that are vital for protecting the foal from infections caused by environmental bacteria.

Foals that are diagnosed with failure of passive transfer should receive one liter of plasma intra-venous. The plasma will have the anti-bodies that the foal will need to fight off any potential infections. Some veterinarians will also administer antibiotics to a foal with failure of passive transfer in additional to the liter of plasma. The antibiotics are used as another line of defense to prevent a foal from getting a septic joint or physis.

Foals have a unique blood flow at the physis and the joint, which is not seen in adult horses. The blood flow within the vessels in these locations will slow down. This slower moving blood will allow bacteria within the blood to attach to the vessel wall.

Once the bacteria attaches to the vessel wall, a bacterial infection can develop. The type of bacteria commonly isolated from septic joint and physis in foals are bacteria commonly seen in the foal’s environment. This is due the foal ingesting the bacteria as it explores the environment, or the bacteria can enter the body via the umbilicus.

Diagnosis of septic arthritis or physis is usually pretty straight forward. The foal will be lame (moderate to non-weight bearing) on the leg with the infection. Heat and swelling around the joint or physis that is infected frequently occurs. It will be very painful for the foal during palpation of the infected joint or physis. An infected joint will have an increased amount of fluid within it. The knee, fetlock, stifle and hock are the most commonly affected joints.

If a septic joint is suspected, a sample of fluid should be collected from the joint. This fluid should be cultured to identify the type of bacteria causing the infection. The fluid should also be analyzed to identify how many and what type of cells are present in the fluid.

Normal joint fluid will have less than 250 cells/mm3 and less than 10 percent of these cells should be neutrophils. A septic joint will have several 1000 cells/mm3 and 70 percent or more of these cells will be neutrophils. The neutrophils are the primary cells in the body that combat bacterial infections. If a bacterial infection is present, a larger number of these cells will be located in the joint fluid.

Radiographs of an infected joint should be taken to make sure that the infection in the joint has not spread to the bone. An ultrasound of the joint can be used to identify fibrin within the joint. The fibrin is produced secondary to the infection. The fibrin can act as a shelter for the bacteria to hide from the foal’s immune system and the antibiotics administered to fight the infection. If a large amount of fibrin is seen within the joint, the foal may need to have an arthroscopic surgery to remove the fibrin and improve the prognosis for a successful outcome.

Definitive diagnosis of a septic physis is confirmed with radiographs. Radiographs of a septic physis will have bone lysis (dark bone on radiographs) in the area where the infection is located. An ultrasound of the infected physis may show fluid in or around the physis.

Treatment of a septic joint involves a combination of local therapy and systemic therapy. Until the culture results are received, the foal will be started on broad spectrum systemic antibiotics.

Local therapy can be done either with injections of antibiotics into the joint or regional limb perfusions. Septic joints are usually flushed with several liters of sterile fluids to help remove bacteria and inflammatory mediators from the joint.

An arthroscopic surgery may need to be performed to flush the joint, remove any fibrin within the joint and debride any infected bone. Anti-inflammatory medication is used to help control the foal pain.

Pick up the May issue to learn more!