By Dr. Caity Cosentino
Most commonly cellulitis affects only one limb, while ‘stocking up’ or filling in legs occurs in hind limbs or all 4 limbs at once. Horses who stock up should not be lame or painful on palpation of their limbs and the swelling should decrease with exercise. In contrast, a horse with cellulitis is usually extremely painful, with the leg being warm to the touch as well. By definition cellulitis is inflammation and infection of the subcutaneous connective tissues of the horse’s leg. It is possible for cellulitis to occur in other areas of the body, but legs are by far the most common region affected. Horses with cellulitis may also have a fever, increased heart rate, and inappetence that goes along with lameness in the affected leg.
Left untreated the bacterial infection can spread quickly, dissecting its way through the tissues, causing the leg to swell to 2-3 times its normal size. Early diagnosis and treatment are key. Diagnostics often include physical exam and bloodwork along with possible culture and ultrasound of the limb. Radiographs may be taken to rule out bony related causes for the lameness. Often the underlying cause of the infection is not determined, though wounds, scrapes, dermatitis or insect bites may cause enough of a break in the skin to allow bacteria to enter and set up shop. If there is a deep wound, it is valuable to culture it so that antimicrobial therapy can be directed at the bacterial species causing the infection.
A horse with cellulitis will have an elevated serum amyloid A level and an increase in white blood cell count. These values can be monitored to make sure the horse is responding well to treatment. Treatment is aimed at reducing the swelling and inflammation in the leg as well as killing off the bacterial infection that is present. Most horse’s will be started on a course of broad-spectrum antibiotics. If culture results become available, the antibiotics can be changed as needed to target the bacteria with one that they are most susceptible to. Anti-inflammatory drugs, hydrotherapy, bandaging and hand walking can all be helpful at decreasing swelling and making the horse more comfortable.
Severe infections can lead to tissue necrosis (death), loss of skin or serum seeping from the skin, lymphangitis, thrombosis (clotting) of blood vessels and potential laminitis, especially in the opposite limb. Horses who have been dealing with cellulitis more chronically often maintain enlargement of their affected leg permanently and will be more prone to developing cellulitis again in the future as there is often damage to the lymphatic system and circulation of the leg.
Most horses will respond well to early and aggressive treatment of cellulitis. Keeping your horses’ legs clean, dry and free from dermatitis can help reduce the risk of them developing cellulitis, though cannot fully eliminate their chances. Disinfecting brushes, sponges and other grooming equipment eliminates the possibility of these tools becoming fomites for infection. Overall health is also important including a good diet, managing underlying health conditions such as PPID and equine metabolic syndrome and treating all wounds, even the small ones, by cleaning and the use of antibiotic ointment. More serious wounds should be examined and treated by a veterinarian.