Cellulitis and Lymphangitis in Horses

Cellulitis and Lymphangitis in Horses:

B­y Heather Smith Thomas

Cellulitis is a diffuse (widespread) inflammation/infection associated with bacterial infection of the skin and the soft tissue directly underneath. In horses, it generally involves only one limb. This is usually a way to differentiate it from “stocking up”, which is limb swelling associated with confinement or lack of exercise, affecting all four limbs or both hind limbs—with no evidence of pain. The horse may be a little stiff just from the swelling, but there is no lameness with stocked up legs.

According to Phoebe A. Smith, DVM, DACVIM (Riviera Equine, Santa Ynez, California), cellulitis causes rapid swelling of a limb, often accompanied by heat, pain when touched, and lameness. “In a severe case, the horse will not bear any weight on the affected limb, and may have yellowish-clear serum oozing from the most swollen areas.

“The swelling may include the entire limb or just the bottom half. Fever often accompanies acute (rapid onset) cases of cellulitis, with the horse’s temperature ranging from 102 to 105 degrees,” says Smith.

Kim A. Sprayberry, DVM, DACVIM (Hagyard Medical Institute, Lexington, Kentucky) says that the typical signs are heat and swelling, sometimes centered around a given area such as a hock or pastern, and sometimes a more diffuse swelling extending up and down much of the limb. “Even though this type of infection is thought to arise from a break in the skin, the break could be as small as the tiny puncture wound created by an insect bite, and not discernible even with careful inspection of the area. In most of the cases I see, the break isn’t obvious on haired skin, and to the horse’s owner or caretaker it appears as though the limb simply swelled up for no reason,” she says.

Lais Costa, MV, MS, PhD, Dipl ACVIM, ABVP (Tufts Cummings School of Veterinary Medicine, North Grafton, MA) says that in many cases we don’t know what caused the problem. “It is thought to be due to trauma, either blunt trauma like bruising, or a break in the skin. It may also be a sequel to some kind of surgery. Veterinarians often differentiate between the cases that have a known cause, such as in injection or some kind of surgical procedure, and the horses where there is no known cause. We often refer to it as secondary cellulitis if it develops following a known break on the skin such as a penetrating wound, a joint infection or arthroscopic surgery. We use the term primary cellulitis when there is no known cause,” she explains.

“Any kind of trauma to the limb carries some risk for development of cellulitis. The signs of cellulitis can take up to three weeks to appear after the trauma or break in the skin. A few weeks later the leg swells up. There are reported cases of cellulitis developing almost two months after a joint injection, for instance,” says Costa.

“In most cases of primary cellulitis, however, you never really find or figure out the inciting cause,” she says. The owner or trainer doesn’t recall any episode involving trauma, though it might have been a blunt trauma like a broad bruising that is not evident on observation.

“Painful, warm swelling of more than one limb may be a sign of systemic disease such as vasculitis. Whenever there are signs of pain associated with swelling of one or more limbs, a veterinarian should be called to examine the horse,” says Costa.

The severity of cellulitis varies, so having an accurate assessment of the case, ruling out other conditions and obtaining the proper diagnosis as early as possible is important. Your veterinarian can perform a careful examination and gather additional information by imaging the leg with radiographs and/or ultrasound. “If the horse is very lame, you need to rule out a bone problem or fracture, to make sure the horse won’t be causing more damage by putting weight on the leg,” explains Costa.

In some situations there might be a bone fragment from the trauma, or deeper infection in the bone or joint associated with the cellulitis. “These cases are a lot more serious and need to be dealt with immediately,” she cautions.

Some people may not want to call their veterinarian because of the expense, but getting a proper diagnosis will usually save money in the long run (by being able to treat this condition properly in the early stages and get it cleared up more quickly), and may also save the horse’s life. “It is important for horse owners to recognize the problem early and call the veterinarian—and use good judgment in the diagnostic tests that are done,” she says.

Some studies have suggested a seasonality in certain cases of recurring cellulitis. “Recurrence of cellulitis is very common. Some owners report the recurrences to occur at the same time each year. It could be that certain times of year the horse is more prone to suffer blunt trauma (during athletic competitions, for example) or more likely to suffer breaks in the skin—possibly associated with trail riding through brambles or brush, or insect bites, and there may not be a true seasonality,” says Costa.

“In most cases it is assumed that cellulitis infection is caused by staphylococcal organisms, since staph is the chief inhabitor of equine skin surfaces,” says Sprayberry. “In many instances, however, we can’t confirm the actual pathogen involved. The most common complaint is that the horse’s leg is suddenly swollen, the horse may be lame, and the limb is painful to pressure on the swollen area. Yet it is rare to find an associated wound,” says Sprayberry.

“The staphylococci can be nasty characters, and they produce several types of cytotoxins and enzymes that lead to severe local tissue damage, edema and spread of infection, all of which allow the bacteria to advance along tissue and fascial planes. Sometimes the infection is organized and corralled off by the body into an abscess, but often the inflammation remains diffuse and not localized enough to sample,” she explains.

Smith says the most common predisposing factors for cellulitis include wounds (large or tiny) on the limbs, frequent clipping and/or bathing, and shared bathing tools. “Less commonly, cellulitis may develop following joint injections, arthroscopy or other surgery on the limbs,” she says.

“Complications may include sloughing of the skin overlying the infected tissue, and laminitis in the supporting limbs, and these complications can be fatal if severe,” explains Smith. “Rapid recognition of cellulitis by the horse owner or trainer, and prompt treatment by the veterinarian is critical to the outcome. Delaying treatment for as little as 48 hours may significantly impact the outcome.”

Treatment for cellulitis varies with the severity of the case, and thus should be directed by your veterinarian. Antibiotics will be needed, and penicillin is most commonly used, according to Sprayberry.

“Cases with sudden onset and fever often benefit from intravenous antibiotics,” says Smith. “Giving antibiotics by IV provides high blood concentrations and thus excellent delivery of the drug to the affected tissues. Milder cases can be managed by oral antibiotics. Two to eight weeks of treatment with antibiotics are often required to completely resolve the infection. Premature discontinuation of antibiotics leads to return of the infection, and possibly an alteration in the bacterial populations in the tissue,” she explains.

Horse owners want to know if staphylococcal infection in cellulitis is the same as MRSA (methicillin-resistant Staph aureus), because many people are familiar with that term and are afraid of this type of infection that is resistant to many antibiotics. “A cellulitis infection is not necessarily MRSA, but we can’t always determine the actual pathogen because there is usually no exudates or other material to culture—there’s no draining wound or accessible pocket of infection to sample,” says Sprayberry.

“A diagnosis of cellulitis is often clinical (determined by visual examination of the horse, sometimes using ultrasound) rather than one we can confirm with laboratory culture. We typically initiate treatment on the presumption that it is staph, and treatment often involves penicillin or a related drug to begin with and some anti-inflammatory medication such as phenylbutazone or Banamine,” says Sprayberry.

Costa says that, ideally, the choice of antimicrobial should be made based on the microbiological culture of a sample collected by the veterinarian. “Unfortunately, not all cases will be cultured. Most studies, however, have reported successful culture and targeted therapy, which improves the chance of a positive outcome,” says Costa.

In cases where culture is attempted, a fair number will show a certain bacterium that can be isolated and identified—and then the veterinarian can determine what drugs it would be sensitive to. Then you won’t be using an antibiotic that might not work. “In the horse, if you use antibiotics blindly, there is always some risk of causing colitis, which can lead to additional complications and expenses. So whenever possible, a sample should be collected and a culture attempted,” explains Costa.

There might be some areas of fluid under the skin, and the veterinarian may ultrasound the leg, to find the best spot to take the sample. Even though it will be expensive to have your veterinarian do diagnostic tests, if you compare the initial investment against the length of time you’ll need to treat the horse, it will be cheaper to find out at the beginning what you should use for treatment, than merely guessing and maybe having to treat the horse longer because the first antibiotic did not work.

Length of treatment (how long the horse needs to be on antibiotics) will vary. “If you get the right antibiotic at the right dose from the beginning, there will be greater chance of clearing it up more quickly,” says Costa. “The horse needs to be on antibiotics for at least a week, and often longer. Severely affected horses should be given injectable antibiotics (and sometimes IV antibiotics), rather than oral. Then you know exactly how much the horse is getting,” she says.

It is important to be aggressive with early treatment. “A high number of horses with cellulitis will later develop laminitis, either because of the inflammatory process that’s already going on, or overloading the good leg because of the lameness. Therefore, many veterinarians who treat a horse with cellulitis will initiate frog support and other preventative measures to protect against laminitis, even though there may not be any sign of laminitis at that point,” says Costa.

Many people feel that the horses with fever, with more severe signs of pain and inflammation, changes in their blood work, etc. are more likely to have laminitis develop on down the road. “Thus it’s important to get a good assessment of the horse from the beginning, to know how aggressive you should be in treating that horse,” she says.

Some horses have recurring episodes of cellulitis. “Some of these horses die, if the treatment is not aggressive. There are many horses that are treated successfully on the farm and never go to a referral hospital. But the ones that don’t respond quickly, or from the beginning have fever, their treatment should be very aggressive—to help make sure that they can return to normal function,” she says. There is always some risk of a poor outcome with cellulitis, and horse owners need to be aware of this.

“Cellulitis can lead to fibrosis of the affected area—as the inflammation continues—and cause local scarring of connective tissues and lymphatic channels beneath the skin,” says Sprayberry. “This can impair the flow of blood and lymph through the channels that return blood from the extremities back to the general circulation. When the collapsible lymphatic vessels and small veins are impaired and cannot transport fluid, swelling (which may be semi-permanent or permanent) may result,” she says.

In these instances the swelling persists as a cosmetic issue, even after the infection is no longer there. Physical therapy measures are thus very important to help maintain circulation during and after the infection. Cold hosing and limited walking can also be beneficial.

“An episode of cellulitis can become a chronic or recurring problem,” says Sprayberry. “You may get it under control initially but have episodic flare-ups later. It is important to treat the condition early and aggressively to help prevent a recurrence, but there are many instances in which the horse does get treated promptly and the inflammation still returns later,” she says.

“The problem is that staph organisms don’t necessarily create a simple abscess in the tissue. Some of the toxins produced by these bacteria in the tissue include coagulase, hyaluronidase and toxic shock syndrome toxin. These extend the inflammation in the soft tissue in a way that enables the pathogen to grow in linear tracts along tendons and muscles below the skin, and the toxins may access the circulation and cause fever, laminitis and other sites of infection,” says Sprayberry.

An abscess, by contrast, is a fenced-off, localized infection. “The organism’s propensity for causing this diffuse kind of cellulitis infection, without concentrating into a pocket or abscess (which could be drained), makes this condition very challenging to resolve completely,” she explains.

Horse owners often wonder how to tell the difference between cellulitis and lymphangitis, since both of these types of infections cause limb swelling.

“It may take biopsy and histologic evaluation of the tissue specimen to tell these conditions apart with certainty,” says Sprayberry. “Lymphangitis is inflammation or infection of the lymphatic vessels beneath the skin, so the swelling looks very much the same,” says Sprayberry.

“Lymphangitis involves the lymphatic vessels, which are deeper under the skin, and they become cord-like (which can be felt on palpation). Also the swelling tends to travel higher up the leg—above the knee or hock,” says Costa.

In both cases, generally only one leg is affected. “A lot of times, however, lymphangitis infection will actually open up and drain some sort of pus. There are some diseases—which are exotic in the U.S. and which are very worrisome in terms of bio-security—that will cause lymphangitis with ulcerations and drainage. That’s why it is important to have your veterinarian evaluate the case at the very beginning and monitor its progression. All cases of lymphangitis start off with signs of cellulitis,” says Costa.

If the inflammation travels above the knee or hock and there is a lot of thickening, or you think a case of cellultis is very severe, you need to have your veterinarian examine the horse to see if it might be lymphangitis. With cellultis there may be fluid under the skin, but if the swelling breaks open and starts draining pus from little nodules that pop up, you are dealing with lymphangitis.

“In early stages they can look the same. Lymphangitis may be more severe, in a sense, because it is deeper,” she explains. “It’s not necessarily worse than a bad case of cellulitis, but a lot less common. There are some diseases that cause lymphangitis, and they must be treated very specifically. Some may be fungal, and some with a draining tract are zoonotic (which means other animals and humans can get it, too) such as sporotrichosis. If there’s a draining tract, you need to have your veterinarian do a thorough examination, collect samples, and make sure of the proper diagnosis. The last thing you want is for people to become exposed to a serious disease,” says Costa. Lymphangitis may start off looking like cellulitis but you may soon realize that it’s also affecting the lymphatics. In these instances the treatment must be even more aggressive.

Lymphangitis has also been associated with certain bacterial infections. “One of the most common forms of lymphangitis, especially in the western part of the U.S., is infection with Corynebacterium pseudotuberculosis (the cause of pigeon fever),” says Sprayberry. “This also causes damage to and scarring of the lymphatic vessels. With this condition, areas of ulceration (open sores) in the skin may be evident, or the limb may just be grossly swollen with the skin intact. Both cellulitis and lymphangitis are infectious, but in most cases not thought to be contagious. Because of the staphylococcal association, however, it is a good idea to wear Latex gloves when handling a limb in which there is wound drainage or in which the swelling is severe enough to cause serum weeping through the skin,” she says.

Just as with cellulitis, it is most common for only one limb to be affected with lymphangitis, and it is often a hind limb, for reasons that have not yet been determined. “They can both be challenging to treat, and may recur at unexpected intervals. They often leave some cosmetic disfiguration such as a permanently thickened limb,” she says.

The limb swelling can impair the horse’s gaits and movement to some degree. “In the acute phase, with both types of infection, the horse may be quite lame, but afterward there may be just some mild residual stiffness or lameness, depending on the degree of swelling.” Lameness, from mild to moderate, may be ongoing.

Again, physical therapy is important. “Cold water hosing, bandaging to maintain light pressure against the swelling, and using Game Ready, Aqua-Tread or other types of therapies to promote circulation to the area can also help. I have also used iontophoresis treatment, in which antibiotics are transferred across the skin into the infected area (this is accomplished by electro-osmosis, using electrical current to move ions of soluble salts into the tissues of the body). Anything to decrease the swelling and promote blood circulation into the area, and keep fluid moving up and down the limb is very important,” says Sprayberry.

Horse owners also need to be aware that laminitis, severe enough to progress to founder and loss of the horse’s life, can be a sequel to either of these problems. “It can be a factor during the acute phase of the infection when it is still developing. The horse may become septic; bacteria, bacterial toxins, or both, can enter the circulation and injure the laminar tissues in the feet,” she explains.

“And, if the affected limb stays very painful for a protracted length of time, the horse may develop a mechanical laminitis in the unaffected foot because of excessive weight-bearing. The digital pulses in all four feet should be checked several times daily in horses that are in the acute phase of a cellulitis or lymphangitis infection. Detection of increasing pulse intensity in the feet may warrant a protective course of immersion of the feet in a bath of ice and water to help protect the metabolically active laminae from the circulating toxin,” says Sprayberry.

“In our practice, we have also incorporated hyperbaric oxygen treatment into the management regimen in a number of horses in which the cellulitis or lymphangitis has become a chronic-active or recurrent problem. We see this type of limb swelling in many broodmares, and in sport horses. The owners become vigilant in observing the horse, and as soon as the limb swelling begins, the horse is started on a course of antibiotics, corticosteroids, anti-inflammatoaries and sometimes hyperbaric oxygen therapy, and this seems to help—along with judicious enforced exercise,” she says.

“Sometimes the infection is resolved and doesn’t return, but in a certain percentage of cases, the resulting limb swelling becomes something that we have to manage rather than cure. As with all serious medical conditions, the chances for a favorable outcome are maximized with prompt diagnosis and treatment.”

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