September 2021 Polo Players' Edition- Cellulitis In Horses

Cellulitis In Horses

Swelling of a Limb is Often Associated with Bacterial Infection

By Heather Smith Thomas

Cellulitis is an inflammatory process of the skin and deeper tissues, and is commonly associated with bacterial infection. In horses, cellulitis is not very common, but it usually appears as a severely swollen limb. It typically involves only one limb, most often a hind limb. This and the fact that the leg is usually painful, makes it different from typical “stocking up” of the hind limbs.

Callie Fogle, DVM, North Carolina State University, says that cellulitis or a horse with a severely swollen limb can be a diagnostic and therapeutic dilemma for veterinarians since it can be challenging to figure out the cause.

“A puncture, a tendon or ligament trauma, or an infected joint can be masked by a very swollen, lame leg,” she says. “It can also be difficult to isolate the type of bacteria causing the cellulitis, and even with appropriate, aggressive therapy, this infection sometimes does not respond quickly, or the swelling may not resolve.”

Signs of cellulitis and importance of diagnosis

“Often these horses have acute, severe swelling, usually affecting an entire limb. However, there are some other things that could present this way, or problems that could be masked by the severe swelling. When a horse owner notices acute, severe swelling, it is important to have a veterinarian examine the horse and look for other possible problems,” she says.

“Radiographs and ultrasound are commonly performed to rule out a fracture and to look for sepsis or infection in a joint. It would be important to rule those things out before beginning treatment for a cellulitis,” she explains.

“Imaging would also help us look for an abscess that might result from the bacterial infection. Another useful diagnostic test might be a culture of the serum that is oozing from the skin, or to culture a subcutaneous biopsy of deeper tissues,” she says.

“The infection in deeper tissues of the limb can result in major tissue damage, so cellulitis is not necessarily a less severe disease, but the treatment would be different from some of the other possibilities, like fractures or soft tissue trauma. It is important that the cause of swelling is recognized early, and treated properly,” she says.

Cellulitis is most common in a hind limb, but can also occur in a front leg. “The classic form of cellulitis is unilateral, affecting just one limb, but it can affect multiple limbs. The horse may have a fever and increased heart rate. Often the swelling is firm and hard, and the leg is hot. It is usually very painful to touch. There may be breaks in the skin that ooze serum. These breaks can be caused by a precipitating trauma or from just the stretching of the skin due to the severe swelling,” explains Fogle.

“These horses are generally very lame, but often the pain occurs when advancing the limb rather than from standing on that leg; it’s difficult or painful to move the limb. Generally the horse will bear reasonable weight on the affected limb, when not being asked to move, compared to a non-weightbearing lameness that is commonly seen with a fracture or a joint infection,” she says.

Occasionally (more often with chronic rather than acute cellulitis) the veterinarian might use scintigraphy (bone scan), in which a radioactive compound is injected into the subcutaneous tissues.

“The purpose of this imaging technique is to evaluate circulation of the distal limb— and see how well the lymphatic and venous system are working to return the blood and fluid from the distal limb. Often with cellulitis, particularly with chronic cellulitis, the function of the venous and lymphatic systems can be severely impaired,” she says.

Causes

Acute cellulitis in horses is most commonly due to bacterial infection, usually with something like Staphylococcus aureus or Staphylococcus intermedius, or Streptococcus. These are common bacteria that are often found on the skin, and they’ve simply found a break in the skin to infect deeper tissues.

“Occasionally, cellulitis can be due to infection with Corynebacterium or Clostridium or a gram negative bacteria such as E. coli,” says Fogle.

Acute cellulitis can be a bit different from chronic cellulitis in which a horse suffers from repeated episodes of severe, painful limb swelling. “The swelling with chronic cellulitis may also differ from that with acute cellulitis, as it is commonly thought to be more of a primary inflammatory process, with less contribution from bacterial infection of the deeper tissues,” she explains.

“A horse that has never had a swollen limb before, that suddenly develops an acute, severe swelling of a limb and doesn’t respond very quickly to aggressive treatment can develop changes in the limb that predispose to future bouts of cellulitis. This may take different forms. Some horses may have a chronically thick leg that feels hard and fibrous like scar tissue. That limb may always carry a bit of swelling and look just a little thicker,” says Fogle.

Plenty of turnout is recommended for horses with chronic cellulitis, as increased circulation will help with swelling. However, wet grass is not ideal because wet skin is softer and thus easier to nick and scrape.

“Other horses resolve the swelling but seem more likely to develop subsequent episodes of acute, severe swelling of that leg. We think that these repeat episodes may be due to scarring and permanent impairment of the venous and lymphatic systems in that limb. The acute bacterial infection can start that process, and once those systems are impaired, the horse is less able to fight bacterial infections in that limb. The skin barrier to bacterial invasion in that limb is not as good as in the other limbs. That limb is more vulnerable to infection,” she says.

Owners of a horse with chronic cellulitis might notice that even a tiny abrasion, small scratch or scab on the skin can precipitate another bout of cellulitis, with the leg once again becoming swollen and painful.

“Even a small amount of bacteria is capable of starting an infection; the horse is unable to mount an appropriate immune response in that limb because of the impairment of venous and lymphatic drainage,” she explains.

Treatment

“It is crucial to treat acute cellulitis right away and be as aggressive as possible, within the owner’s financial ability. Sometimes there are severe complications, especially if treatment is not begun immediately with aggressive broad-spectrum antibiotic coverage. Most of the time this will entail intravenous antibiotics, and use of a non-steroidal anti-inflammatory medication like phenylbutazone to try to address pain and inflammation in that limb,” Fogle says.

“Steroids are commonly used by veterinarians to address minor distal limb swellings, but we can run into trouble with steroids to treat severe cellulitis because they can effectively mask whether or not we’re successfully resolving the bacterial infection in the limb. Short-term steroid use with a bout of acute cellulitis is okay, but you wouldn’t want to use it very long. Steroids are not routinely part of my treatment regime for severe cellulitis,” says Fogle. Long-term steroid use tends to hinder the immune system.

Lymphangitis is different than cellulitis as it is more likely to affect two legs, or even four, and is generally in the lower portion of the limbs.

“Along with antibiotics and NSAIDs, we also use bandaging and supportive treatment to help reduce the swelling in that leg. Cold water therapy is helpful, whether it’s one of the fancy boots that recirculates cold water, or just cold water from a hose. Cold saltwater therapy in the form of an equine saltwater spa can be extremely beneficial in quickly reducing the limb swelling and improving venous and lymphatic fluid return from the limb. These are all important points of treatment for cellulitis,” she explains.

Aggressive, prompt treatment is important to prevent laminitis, reduce the risk of deep tissue necrosis, and reduce the risk of developing permanent damage to the circulatory system in that leg. “The longer the leg stays swollen, the more risk for complications. Horses can develop laminitis as a secondary complication of cellulitis. It’s usually a support limb laminitis (from too much weight on the normal leg) but it can also be laminitis in the affected leg,” she says. Getting the horse comfortable quickly on the limb with cellulitis is crucial, through medication and supportive therapy to reduce the pain and swelling.

“Horses can have further problems if the bacterial infection does not stay confined to that limb and goes throughout the body. The horse can become very sick from systemic infection (sepsis, or bacteremia). Horses can also develop superficial and/or deep necrosis of the tissues, anywhere within the affected limb. Often this is in the fetlock or pastern area where massive amounts of tissue slough away. This problem may also accompany bone infection or tendon/ligament infection or joint infection. These are all risks with severe cellulitis, but are more likely in cases with a delay in the start of therapy,” she says.

Treatment of acute cellulitis can be frustrating for both the owners and veterinarians, particularly when the symptoms are not resolving as quickly as everyone might like. “It can be hard to choose an antibiotic before we have a culture, but we don’t wait for the culture results to come back because we need to start treating quickly. Often the antibiotic we choose is something we think will work, but with cellulitis we sometimes find that our first choice may not be the right choice. We might have to change after we get the culture results back. Or, if we don’t get a positive culture for bacteria and the horse is not responding as hoped, we may end up making another educated guess, using another antibiotic in an effort try to get the swelling down as fast as possible,” she says.

Lymphangitis vs. Cellulitis

“Classically we think of lymphangitis as something that has a breed predisposition, commonly affecting draft horses or cold-blood breeds. It can affect horses of other breeds. Lymphangitis and cellulitis can both affect lymphatic drainage to the distal limb, particularly in chronic or recurrent cases. Lymphangitis is more likely to be bilateral, however, affecting both front legs, or both hind legs, or even all four limbs. Usually the hind limbs are worse, but we do also see cases in which the forelimbs are worse,” says Fogle.

The swelling is generally in the lower portion of the limbs. “It can look a little like stocking up, because the swelling is most dramatic in the lowest part of the limb, and is not an acute process like we find in cellulitis. With cellulitis you suddenly find the horse with a huge leg, but with lymphangitis it’s more insidious and gradual and is slowly progressive. The legs may have some scabs or scratches (pastern dermatitis) on the back of the fetlocks and pasterns, as well as firm, fibrous nodules. Those crusts, ulcerations and nodules may be a sign that the lymphatic drainage is not effective. Like a horse with chronic cellulitis, a horse with lymphangitis is also unable to fight infection or mount a good response against invading bacteria in the distal limbs,” she explains.

“These horses tend to have low-grade infections in the skin in areas where the lymphangitis is affecting the distal limbs. The swelling doesn’t go completely away with exercise, like it would in a horse that is simply stocked up (distal limb edema). Horses may stock up, especially in the hind limbs, if they are accustomed to being out moving around and are then confined overnight in a stall or small pen. In the morning they may have a little edema, but as soon as you turn them out or ride them, the swelling goes away.” The increased circulation dissipates the edema.

“Lymphangitis can be responsive to exercise and improve a little when the horse is turned out, but the limbs don’t get completely back to normal. Besides the edema and skin lesions there are often some small nodules of scar tissue in the pastern and fetlock areas that you can feel with your fingers. The nodules are thought to be due to inflammation of the lymphatic system. It starts as inflammation and then ends up as scarring and impairment of the lymphatic system in the distal limbs, with secondary bacterial infection,” explains Fogle.

Management for Chronic Cellulitis

“Horses that have had a bout of acute cellulitis and now have a chronically thick limb, or a limb that the owner knows is more likely to swell if anything happens to it, need to be monitored closely. One of the things that owners can do to try to prevent recurrence in that limb is to be vigilant about feeling (palpating) the skin in the fetlock and pastern area, checking daily for any scabs, scratches or abrasions. If they notice any breaks in the skin on that limb, it’s wise to clip and clean them with a gentle soap and allow it to dry before placing a lower leg wrap. This can sometimes prevent or reduce the risk for swelling (giant limb) and lameness that can occur with minor abrasions. This is something owners can do if they have a horse that tends to develop cellulitis whenever they get a break in the skin,” she says.

“Other things I recommend for owners of horses affected by chronic cellulitis is plenty of turnout—but being aware that turnout in wet grass is not ideal. When the skin is wet it becomes softer and more vulnerable to being nicked or scraped. It’s easier for bacteria to gain access through wet skin. Keeping the at-risk horse in until the grass is dry is best. If the horse has to be stalled, bandaging the legs while in the stall is helpful. There are several different types of bandages that can be used. A quilt and polo wrap or special bandages called short stretch bandages can be used to try to minimize the amount of edema that develops when a horse is stalled. Once these horses are turned out, they don’t need the bandaging,” says Fogle.

Horses with chronic cellulitis need to be regularly monitored and put on a round of antibiotics whenever a break in the skin is found to prevent exacerbation of cellulitis.

“I have several owners who have horses affected with chronic cellulitis. When they identify a break in the skin, they contact their veterinarian and get the horse on a round of antibiotics to try to prevent another exacerbation of cellulitis. They have developed a very successful pro-active approach to try to manage the limb and prevent further bouts of cellulitis,” she says.

When the horse is in between bouts of cellulitis, it is usually safe to go ahead and ride/compete with that horse. “Owners with horses that have a chronic cellulitis are usually pretty careful about protecting the limb while they are riding or when trailering. The exercise is really helpful, to improve the blood flow to the limb and improve the circulatory system and lymphatic drainage so it’s often best to continue using the horse, as long as he is sound,” she says.

“When a horse is in an acute bout of cellulitis, however, exercise is not feasible. Getting the horse moving once the severe symptoms are starting to wane is important, and in between bouts. A regular exercise schedule is very helpful, so as long as you can protect the skin, exercise is great.”

Prognosis

“For the most part, prognosis for horses with cellulitis is good, particularly if the horse responds quickly to aggressive medical therapy (within the first 24 to 48 hours). Often those horses won’t develop any of the chronic effects or complications of cellulitis. The ones we generally struggle with are the ones that don’t respond quickly to the treatment. The things we worry about are abscesses, necrosis and sloughing of deep tissue, laminitis, and secondary involvement of joint, bone or soft tissue support structures of the limb, which can be severe complications of cellulitis,” she says.

“The majority of horses will respond to aggressive medical treatment, however, and early aggressive treatment is key to avoiding severe complications and development of chronic cellulitis.” It pays to get a diagnosis and get right on it.