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Cellulitis in Horses

Cellulitis refers to a disseminated infection of the subcutaneous tissue, which is the tissue found just beneath the skin. In horses, cellulitis often develops rapidly, seemingly over a number of hours. More common in the hindlimbs than the forelimbs, cellulitis is recognized by a rapid swelling of the limb, often accompanied by heat, pain with touching, and lameness. Typically, a single limb is affected. 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. Swelling may include the entire limb or the bottom half of the limb. Fever often accompanies acute (rapid onset) cases of cellulitis, with temperatures ranging from 102F to 105F. While a wound can cause cellulitis by allowing introduction of bacteria into the subcutaneous tissue, the presence of a wound is not required for the development of cellulitis. In these cases, the cause is unknown. A small scratch or nick can allow bacteria to gain entry into the subcutaneous tissue, so the size of the wound does not correlate with the likelihood of developing cellulitis.

The most common predisposing factors to the development of cellulitis in horses include wounds on the limbs, frequent clipping of the limbs, frequent bathing, and shared bathing tools. Infrequently, cellulitis may develop following joint injection, arthroscopy, or other surgery on the limb. Staphylococcus is the bacteria most commonly isolated from limbs with cellulitis, but several others have been involved as well. Ultrasound is often used by veterinarians to support a diagnosis of cellulitis, and to identify any fluid pockets that may be present under the skin. Bacterial culture can be performed on fluid retrieved from such a pocket, which helps to guide veterinarians in selecting appropriate antimicrobials.

Treatment of cellulitis varies with the severity of the case and is best directed by your veterinarian. Cases with very sudden onset, severe cases and those with fever often benefit from intravenous antibiotics. Giving antibiotics by this route provides high blood concentrations and therefore excellent delivery of the drug to the affected tissues. Milder cases may be managed with oral antibiotics. Two to 8 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. In addition to antibiotics, treatment with anti-inflammatories, bandaging, cold hosing and limited walking are also utilized.

Complications of cellulitis include sloughing of skin overlying the infected tissue and laminitis (founder) in the supporting limb(s). These complications can be fatal if severe.

Many horses have some residual swelling of the affected limb permanently, and some of these horses will have recurrent bouts of active cellulitis. Depending on the athletic use of the horse, this may affect performance. Rapid recognition of cellulitis by the horse owner and prompt institution of treatment by the veterinarian is critical to the outcome. Delaying treatment for as little as 48 hours may significantly impact the outcome. In addition, aggressive therapy, in the form of intravenous medications is often necessary to treat aggressive forms of cellulitis.

Article Courtesy of:

Phoebe A. Smith, DVM, DACVIM
Riviera Equine Internal Medicine & Consulting
Santa Ynez, California

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