Like keeping the pistons of an engine lubricated for smooth, easy movement, the tendon sheaths of a horse function similarly by providing a friction-free environment in which tendons can move. But if the lubricant becomes fouled, engine malfunction follows. It’s the same with a horse’s tendons. That can mean big-time problems: Neither the four-wheeled or four-legged vehicle will run without serious repair!

Tendons run down the front and back of a horse’s lower limb. Clinical signs of septic tenosynovitis include swelling of the limb and tendon sheath due to excess fluid, heat, pain on palpation, and lameness. | Dr. Robin Peterson Illustration

The tendon sheath is a fluid-filled sleeve that envelops and lubricates the tendon and provides an environment in which the tendon can smoothly move and change direction where there is joint movement. But if sheath-penetrating injury occurs, infection can follow. Even a simple wound can become septic, putting the horse at risk for permanent lameness and even death.

Septic Insights

Septic tenosynovitis, or a septic tendon sheath, describes an infection that produces inflammation in a tendon sheath. Usually caused by a puncture, laceration, or trauma, septic tenosynovitis most often occurs in the digital tendon sheath (behind the fetlock) and the tarsal sheath (behind the hock), as these are the structures most likely involved with punctures or lacerations. Additionally, infection can occur when an aseptic tenosynovitis (an uninfected tendon sheath wound) becomes infected following a therapeutic injection into the sheath, although this is very rare.

Nathaniel A. White II, DVM, MS, Dipl. ACVS, Director of the Marion duPont Scott Equine Medical Center, Virginia-Maryland Regional College of Veterinary Medicine, and Theodora Ayer Randolph Professor of Surgery, further explains septic tenosynovitis: “Bacteria or other contaminants can invade the sheath as a consequence of an injury. Once there, these contaminants inoculate the synovial fluid and lining, allowing bacterial growth and subsequent infection. Because the tendon sheath has synovial fluid within the sheath, the infection can fulminate (explode) and become trapped within the sheath. As there is always movement of the tendon within the sheath with weight bearing and walking, the wound or traumatized area is irritated.”

Swelling, pain, and marked lameness result. The condition can be acute or chronic.

“Chronic tenosynovitis starts out as an acute problem due to some initiating problem,” White says. “Tenosynovitis can fester for a long, long time, for two reasons. First, the sheath responds to inflammation by producing fibrin, a protein normally found in the blood and the primary component of blood clots; this is a normal inflammatory response. In the tendon sheath, however, this can lead to permanent adhesions, which prevent normal movement of the tendon through the sheath. The sheath can be fully healed and not painful, but because the tendon is tied up due to the adhesions and unable to move properly, the horse can end up with stiffness or lameness.”

The second reason acute tenosynovitis can become chronic is when the bacteria or contaminants become encapsulated within the sheath. “Essentially, you have bacteria hidden from the rest of the body,” White explains. “It looks like you’ve cured the problem, but down the line the horse suffers a relapse and more infection.”

Clinical signs of septic tenosynovitis include tendon sheath and limb swelling due to excess fluid, heat, pain on palpation, and lameness. “Normally a wound is present and synovial fluid from the sheath may be seen draining from the wound,” White adds.

Diagnosis is usually straightforward and is based on clinical signs (sheath swelling) and sampling fluid from the sheath. “The fluid from the infected sheath will be cloudy and often red-tinged (compared to normal fluid, which is yellow and clear),” says White. “There will be more white blood cells and protein than normal in the fluid. The increased number of white blood cells is sufficient to suggest an infection, but culture of the bacteria with an antibiotic sensitivity test is desired to provide the appropriate treatment.” Ultrasound can often help visualize and confirm extra fluid in the sheath.

Two conditions can obscure the diagnosis. “If the horse has an infection not only in the sheath but also subcutaneously (just beneath the skin), where the whole limb swells up with infection, then the diagnosis can be much more difficult because you’ve got an infection of the tissues underneath the skin (cellulitis) that can cover up the tendon sheath infection,” White says. “The veterinarian might think it’s just cellulitis, not a septic tendon sheath.”

And with infection in the subcutaneous tissues, passing a needle into the tendon sheath to aspirate fluid risks carrying the infection into an uninfected tendon sheath.

Continues White, “The other time when it’s sometimes difficult to get an accurate diagnosis is when the wound enters the tendon sheath and all the fluid drains out; the sheath itself can look totally normal, the leg looks fine, and yet you’re still at the stage when you can start to have infection. The veterinarian dresses the wound and administers antibiotics, then three days later, the wound is sealed because of the fibrin and the tendon sheath is hot and swollen with infection. To see if a wound penetrated the tendon sheath, we recommend injecting sterile balanced electrolyte solution into the sheath to see if it drains out of the wound. Any drainage indicates sheath penetration and the chance for injury or infection. If it does, you need to treat it aggressively, early, and either prevent the infection or treat the infection before it becomes chronic.”

Dealing With It

Treatment begins with local and systemic antibiotic therapy and flushing the wound with a sterile balanced electrolyte solution. “We often recommend using the arthroscope to examine the sheath to remove fibrin and to examine the tendons for damage,” White advises. “Injection of hyaluronic acid after the flush, and regional perfusion with antibiotics or a constant infusion of antibiotics, are also recommended. Open drainage of the sheath has sometimes been recommended, but will often cause chronic tenosynovitis. This is used when the acute therapy is not able to cure the infection.”

Treatment is often handled at a referral center because of the need for flushing and the possibility of surgery required to look inside the sheath.

Resting the horse until the inflammation is gone is essential. “That could be a couple of weeks or it could be a couple of months, depending on the severity, whether the tendon has been injured, and how quickly the treatment is effective,” White says.

There is debate about whether the area should be immobilized with a cast or bandage. White notes that some texts recommend keeping the area mobile to avoid adhesions or scars. “This is a delicate issue because if you mobilize an inflamed sheath too soon and too often, you potentially will keep it inflamed and cause more adhesions,” he says. “On the other hand, mobilizing the sheath at the right time helps bring normal physiology back to the sheath. You don’t want to mobilize the sheath before it can manage the inflammation. I’m not sure I have a formula, except I’m very conservative and will rest the horse, keep a bandage in place for support, start walking the horse for very short periods, and over time increase the amount of walking.”

Prognosis varies from complete resolution to permanent lameness, chronic relapses, or death. Of the horses treated over the years at the Equine Medical Center, most achieved total resolution.

“But septic tenosynovitis is life threatening and needs to be treated aggressively,” White warns. “Early treatment is best to ensure total resolution without adhesions and scarring of the tendons within the sheath. If the sheath has severe trauma with or without tendon injury, even with immediate treatment scarring and adhesions may occur.”

Risk Reduction

Although septic tendon sheaths are not especially common, they aren’t rare either. Therefore, the horse owner should seek immediate veterinary attention for any wound/injury near a joint or tendon sheath. “Immediate treatment could prevent occurrence of infection and the severe inflammatory reaction that leads to the permanent damage,” White says. “This is a serious disease that can cause permanent, debilitating lameness, so early treatment is essential. Although it costs more to fully evaluate the problem, waiting until complications arise will increase the cost and decrease the prognosis.

“Septic tenosynovitis is one of the toughest local infections we deal with,” White continues. “Particularly, the tendon sheaths around the hocks are difficult to cure; they’re very discouraging and frustrating, as you have an isolated pocket that is hard to drain and to prevent inflammation. Frankly, I think septic tenosynovitis is a huge emergency that needs to be treated very aggressively, very early. Our results show that in the early phase–meaning immediately upon having wounding or within the next few days–is really a critical period. Once the wound becomes chronic, you may save the horse, but you often end up with chronic (tendon) scarring.

“We’ve had what appeared to be very simple wounds that turn out to be horrible septic tenosynovitis,” he says. “If there is any chance a tendon sheath may be involved, you should step up a notch in your diagnostics and care.”