Few situations shout “Emergency!” louder than a horse refusing to bear weight on a grossly swollen leg. Suddenly one task supersedes the whole day’s plans: Call your veterinarian.

“A lameness with significant swelling should be seen by a veterinarian as soon as possible,” says Meg Hammond, DVM, of Woodside Equine Clinic in Ashland, Virginia. You’ll want to keep the horse still while you wait—if it’s an injury, making him walk even a few steps could make it worse.

The greatest fear, of course, is that the lameness is caused by a fracture or another severe orthopedic injury. But occasionally the source of the problem is something less dire yet still a challenge to treat: an infection called cellulitis.

“Cellulitis is somewhat common in the horse world, but it can be frightening for an owner,” says Hammond. “The leg can be normal one day and double or triple in size overnight. If the leg is infected, the swelling will not resolve with nursing care alone.”

Immediate, aggressive therapy with intravenous antibiotics and anti-inflammatory medications is needed to bring the swelling down and control the infection as quickly as possible. Most horses readily recover from a bout of cellulitis and return to their previous level of activity. But life-threatening complications are possible, especially if treatment is delayed. And for some horses, the initial bout of acute cellulitis will be just the first of many chronic flare-ups.

Making that emergency call to your veterinarian is the most important first step if your horse develops cellulitis. Here’s what you need to know about what comes next.

HOW THE CELLULITIS INFECTION DEVELOPS

Normal skin consists of three layers: the epidermis, the tough, external shield that forms one of the body’s first lines of defense; the underlying dermis, the flexible, sensitive layer that contains glands, hair follicles, capillaries, nerves and other structures; and the subcutis, which is made up largely of connective and fatty tissues.

Cellulitis develops when bacteria penetrate below the epidermis and multiply in the subcutis. The infection is diffuse, meaning that it spreads over a wide area without a specific point of origin. How the bacteria penetrate the dermis and subcutis is often unknown—a situation that is called primary cellulitis.

“In nearly half the cases we never really know what started the infection,” says Margaret Mudge, VMD, DACVS, DACVECC, of Ohio State University. “There is no known trauma or obvious wound. We don’t know if the bacteria were already in the horse’s body or if they were introduced through the skin. They may get dragged in through tiny punctures that we can’t see. They may get through the skin if there is enough damage to the underlying tissues that the barrier is compromised.”

On the other hand, secondary cellulitis develops when bacteria gain entry through a wound, surgical incision or another known route. Breaks in the surface caused by dermatitis, the technical name for inflammation of the outer layers of skin, can also allow bacteria in. “When the integrity of the skin is compromised, bacteria can gain entrance and replicate in the underlying tissue,” says Hammond. “It is impressive what can happen when that barrier is damaged.”

A variety of bacterial species have been implicated in cellulitis, but the infection is usually caused by Staphylococcus aureus and Streptococcus species. “These are common bacteria that are often found on skin,” Mudge says. Other bacterial species that may be involved include Enterococcus and Actinobacillus species. “Occasionally, cellulitis can be due to infection with Corynebacterium or Clostridium or a gram-negative bacterium such as Escherichia coli,” says Callie Fogle, DVM, of North Carolina State University.

Secondary cellulitis can develop anywhere on the body where a wound occurs. Primary cellulitis usually develops on a leg, and most often the hind legs. “The term ‘cellulitis’ is very general, referring to infection under the skin and sometimes involving the skin,” says Mudge. “But when we talk about cellulitis in horses we tend to think of the hind limb.”

Even when it initially seems mild, cellulitis is not an ailment to take lightly. The swelling can progress quickly, even within a few hours, to the point where fluid leaks from cracks in the overstretched skin.

Plus, if the infection is not controlled quickly, a number of serious complications can develop. For example, the bacteria may spread from the skin into the deeper tissues and structures of the leg. “A particularly aggressive or resistant bacteria may cause tissue necrosis or a more deep-seated infection, which in rare cases can affect the bone, tendon or synovial structures such as a joint or tendon sheath,” says Hammond.

Laminitis is also a possibility. “It’s usually a support-limb laminitis but it can also be laminitis in the affected leg,” says Fogle.

Systemic infections, such as sepsis, can also occur. “Horses can have further problems if the bacterial infection does not stay confined to that limb and goes throughout the body,” Fogle says. “The horse can become very sick from systemic infection. These are all risks with severe cellulitis, but are more likely in cases with a delay in the start of therapy.”

IDENTIFYING THE PROBLEM

Cellulitis is most common in a hind limb, but it can also occur in a front leg or on other parts of the body. “The classic form of cellulitis is unilateral, affecting just one limb, but it can affect multiple limbs,” says Fogle. Here are the common signs:

• Swelling that is dramatic and appears suddenly. “The leg is usually diffusely enlarged, sometimes all the way from the foot up to the stifle or beyond, and the typical case is generally swollen from at least the foot to the hock,” says Mudge. The leg may be two or three times larger than normal, and the swelling will be firm to the touch.

• Severe pain. “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,” Fogle says. “Generally, the horse will bear reasonable weight on the affected limb when not being asked to move, compared to a non-weight-bearing lameness that is commonly seen with a fracture or a joint infection.”

• Heat. “The leg is usually very warm and painful to the horse if touched,” says Mudge.

• Fever. The horse’s temperature is likely to be elevated, and his heart rate may be increased. His overall attitude may be dull, and his appetite low. [Click here to read more about fever in horses.] 

• Wetness on the surface of the skin may be noticeable, especially if the swelling is dramatic. “Depending on how severely swollen it is, the leg may be oozing serum, weeping through the skin,” says Mudge. These breaks in the skin may have been caused by the initial trauma, or the skin may be so overstretched that the yellowish serum seeps out.

Although some of these signs are distinctive, cellulitis is not necessarily easy to diagnose because many other problems—fracture, bruising, joint infection, to name a few—can cause sudden, painful swelling in a leg. Swelling may also hide evidence of a puncture wound, snakebite or another problem. “We might do radiographs or ultrasound 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 cellulitis,” says Fogle. “Imaging would also help us look for an abscess that might result from the bacterial infection.”

Bloodwork may show elevated white blood cells and other indicators of infection, while other laboratory tests may provide important clues. “Often the veterinarian will want to culture a sample of any drainage coming from the leg or from a wound,” says Mudge. “Sometimes he or she will ultrasound the leg and try to find a pocket of fluid to aspirate a sample to culture. This will show which bacteria are involved and enable us to know which antibiotic will be most effective.”

Your veterinarian may also suggest nuclear scintigraphy, especially if your horse has had cellulitis before. “The purpose of this imaging technique is to evaluate circulation to see how well the lymphatic and venous systems are working to return the blood and fluid from the limb,” Fogle says. “Often with cellulitis, particularly with chronic cellulitis, the function of the venous and lymphatic systems can be severely impaired.”

TREATMENT PLANS

Once cellulitis has been diagnosed, treatment will begin immediately. “The longer the leg stays swollen, the greater the risk for complications,” says Fogle. “It is crucial to treat acute cellulitis right away and be as aggressive as possible, within the owner’s financial ability.”

The primary treatment is an aggressive course of intravenous antibiotics as well as nonsteroidal anti-inflammatory medications such as phenylbutazone to address pain and inflammation. Initially a broad-spectrum antibiotic may be administered, but once the results of any cultures are in, the veterinarian may switch to another antibacterial agent that more specifically targets the source of the infection.

Cold hosing is an easy and fairly effective way to address the pain and swelling of cellulitis. “Cold water therapy is helpful, whether it’s one of the fancy boots that recirculates cold water, or just cold water from a hose,” says Fogle.

Bandaging may also help to control the swelling, if the horse will tolerate it. “Some horses will be so painful that they won’t tolerate having a tight bandage on the leg,” says Mudge. “They may protest the bandaging but tolerate cold hosing. Cold therapy can be a good way to reduce the heat and potentially some of the swelling.”

Finally, steroids might be an option, but they must be used judiciously. “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,” says Fogle. “Short-term steroid use with a bout of acute cellulitis is OK, but you wouldn’t want to use it very long.”

As a horse’s condition improves, walking can help improve his circulation and pull fluid out of the leg. “Exercise is not feasible when a horse is in an acute bout of cellulitis,” says Fogle, “but getting him moving once the severe symptoms are starting to wane is important.”

Milder cases of cellulitis may be treated on the farm, especially if the owner is comfortable with administering the pain medications and antibiotics as well as cold hosing and bandaging. However, says Mudge, “there are horses who are so painful that they need to be managed in a hospital with more continuous or higher-powered pain medications.”

Serious complications might also warrant hospitalization. “Some horses become systemically ill from this infection,” Mudge says. “If the horse has a fever and has gone off feed or is showing signs of laminitis, that horse might be referred to a hospital for more intensive treatment.”

THE ROAD TO RECOVERY

Most horses who develop cellulitis recover completely, especially if treatment begins promptly and they start to respond within the first 24 to 48 hours. “The majority of horses will respond to aggressive medical treatment,” says Fogle. “Often those horses won’t develop any of the chronic effects or complications of cellulitis.”

Horses with secondary, rather than primary, cellulitis tend to recover more quickly. “The prognosis is generally better in the cases where there’s a known injury such as a laceration or some kind of trauma, or cellulitis related to a surgical procedure,” Mudge says. “Those horses seem to do a little better than the horses that have recurrent bouts of primary cellulitis that are not due to a known skin injury. The primary cellulitis cases are often more challenging to resolve in the long term.”

But cellulitis can have long-term effects. Extreme inflammation can stretch and scar tissues enough to compromise the lymph system’s ability to draw out excess interstitial fluids (the fluid that fills the spaces between body cells) and return them to the bloodstream. “Damage to the lymph vessels will interfere with normal drainage of fluid from the limb,” Hammond explains. “With this decreased efficiency of the lymph system, these subcutaneous tissues may always hold a little extra fluid, making the leg appear slightly swollen even after it has healed. This is most likely to happen in the more severe cases or the ones that are not treated early.”

The change in the leg’s appearance may be permanent. “I warn owners that even when horses respond well, they may end up with a leg that is slightly bigger than the other one,” says Mudge. “Even if everything goes well and the horse makes it through and recovers without residual lameness, there may be some limb enlargement. It is yet to be determined whether prolonged bandaging or using things like compression cold therapy make a difference in the final outcome, though these strategies make a lot of sense in continued treatment for these horses. At this point in time, however, we don’t have strong evidence to say whether those will ultimately improve the cosmetic outcome.”

FROM ACUTE TO CHRONIC

Unfortunately, even a single episode of cellulitis can leave a horse susceptible to the chronic form of the condition—repeated episodes of severe, painful limb swelling. “A horse who has recovered from cellulitis is more likely to have a recurrence in that same limb,” says Hammond.

One reason may be that the infection and extreme swelling did some permanent damage to the skin, leaving it more porous. “Inflammation can cause damage to the skin and decrease its effectiveness as a protective barrier,” says Hammond.

Another possibility is that the permanent damage to the lymphatic and blood circulatory systems diminishes the ability to mount a new immune response. “We think that these repeat episodes may be due to scarring and permanent impairment of the venous and lymphatic systems in that limb,” says Fogle. “Once those systems are impaired, the horse is less able to fight infections in that limb. That limb is more vulnerable, and even a small amount of bacteria is capable of starting an infection.”

Chronic flare-ups of cellulitis behave a little differently than the initial acute disease. “It may come on more insidiously than suddenly, compared to acute cases,” Mudge says. And the physiological source of the swelling may differ as well: “The swelling with chronic cellulitis is commonly thought to be poor circulation and inflammation, with less contribution from bacterial infection of the deeper tissues,” says Fogle.

Horses prone to chronic cellulitis will need to be closely monitored; even the tiniest of wounds on the leg may spark a new bout of pain and swelling. “One of the things that owners can do to try to prevent recurrence in that limb is to be vigilant about feeling the skin in the fetlock and pastern area, checking daily for any scabs, scratches or abrasions,” says Fogle. “It’s wise to clip and clean any breaks in the skin on that limb with a gentle soap and allow it to dry.”

Regular turnout and exercise are also recommended to encourage circulation in the affected leg—with some cautions. For one, turnout in wet grass is not ideal. “When the skin is wet it becomes softer and more vulnerable to being nicked or scraped, and it’s easier for bacteria to gain access,” says Fogle. “Keeping the at-risk horse in until the grass is dry is best.”

Bandaging can help to limit swelling when the horse is in a stall. “Several different types of bandages can be useful,” says Fogle. “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.”

Shipping boots are a good idea while in the trailer, and polo wraps can protect the horse’s skin during exercise and competition. “Owners with horses that have chronic cellulitis are usually pretty careful about protecting the limb while they are riding or when trailering,” says Fogle. But the precautions are worth it: “The exercise is really helpful, to improve the blood flow to the limb and improve the circulatory system and lymphatic drainage,” she says, “so it’s often best to continue using the horse, as long as he is sound. As long as you protect the skin, exercise is great.”

Proactive antibiotic treatment is sometimes helpful, too. “I have several owners who have horses affected with chronic cellulitis,” says Fogle. “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 proactive approach to try to manage the limb and prevent further bouts of cellulitis.”

PREVENTIVE MEASURES

Why some horses develop cellulitis while others don’t isn’t well understood, so it’s impossible to identify definitive ways to prevent the infection. “Sometimes in a horse that had some dermatitis or we find a small scratch, we suspect that was what set it off,” says Margaret Mudge, VMD, of Ohio State University, “but there are many horses that have mild dermatitis or lots of cuts and scrapes that never develop cellulitis.”

Nevertheless, taking some basic precautions to protect the skin on your horse’s legs will not only keep him healthier and more comfortable, but might just help you ward off this terribly painful infection:

• Clean and disinfect even the smallest of wounds. “As soon as you notice anything abnormal, it should be treated promptly and appropriately,” says Meg Hammond, DVM, of Woodside Equine Clinic in Ashland, Virginia. Call your veterinarian for help with deeper, more serious wounds. 

• Maximize turnout and/or get the horse regular exercise. Moving around stimulates healthy circulation in any horse, but it’s especially critical for those prone to cellulitis.

• Keep the skin dry. Muddy, sloppy turnouts are unavoidable for farms in wet climates at certain times of the year. Do, however, make sure horses have access to dry shelters where they can find some respite. Laying gravel in low, muddy areas can also help keep their legs cleaner. Bringing turned-out horses inside once a day will give you the opportunity to clean and inspect their legs and let them dry out. 

• Go easy on the shampoo. Over-exuberant soaping up will dry out skin and may lead to cracking.

• Groom carefully. Removing long hair on the legs can help keep the skin drier, but be careful not to scratch the horse with the clippers. Also use only soft brushes and rags on the legs. 

• Sterilize grooming and bathing equipment periodically. Newer washing machines have a sterilization option that can heat rags and towels to a temperature high enough to kill most bacteria and other pathogens. Brushes and other tools can be sterilized by scrubbing them with soap and water before soaking them in a bleach solution and laying them in direct sunlight to dry. Avoid sharing tools among horses, especially if one is prone to chronic cellulitis.

Managing a horse prone to chronic cellulitis requires diligence. But with attentive care, there’s no reason he can’t live a long, healthy and productive life.

This article first appeared in EQUUS issue #461

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