Infectious Diseases

The Skinny on ‘Big Leg’

By Paul Basilio

When a horse is presented with diffuse swelling, cellulitis and/or lymphangitis of a limb, the descriptive but not-very-helpful term that is often used is ‘big leg.’ While the presentation is common in clinical practice and the impact on the horse’s welfare is significant, the amount known about the condition is quite limited.

“There are few publications on this condition, and most are case series,” said Harry Carslake, MA, VetMB, PhD, DACVIM, DECEIM, SFHEA, MRCVS, a professor of equine medicine at the Leahurst Equine Hospital at the University of Liverpool, England. “There has been little experimental work involving treatment trials and hardly anything involving what’s going on under the surface. There is a lot that we don’t know about whether this is an infectious disease, whether it’s purely inflammatory or what the underlying cause is.”

To help clinicians gain a better understanding of what’s going on under that big leg, Dr. Carslake gave a breakdown during a presentation at the 2024 BEVA Congress in Liverpool.

Background
Cellulitis and lymphangitis are commonly associated with a bacterial infection of the subcutaneous tissues. In primary cellulitis, no visible wound is present. The prime suspects are a small penetrating injury, transdermal infection via impaired barrier function, and residual infection from a prior episode. When bacteriologic sampling of wounds, draining tracts or an aspirate of subcutaneous fluid is performed, the most common culprits are Staphylococcus and Streptococcus spp.

“When we are presented with these cases, we rarely do culture and sensitivity testing because if we’re not going to actually enter through the skin, there isn’t anything there to take a swab from,” Dr. Carslake said.

For secondary cellulitis, attention should be drawn to the nature and severity of the primary lesion, which is most commonly a skin wound. Differentials include synovial sepsis, trauma, skin conditions, spider or snake bites, clostridial myositis, and chronic progressive lymphedema.

Lymphangitis, or inflammation of the lymphatic vessels, commonly occurs in an area with local inflammation.
“One of the fundamental features of inflammation is loss of function,” he explained. “As a result of the inflammation, these lymphatic vessels lose the ability to drain fluid, so we have this cycle of limb swelling, fluid accumulation and reduced drainage.”

As a result of the decreased drainage, edema occurs. The presence of edema is not necessarily indicative of a serious problem, and edema can be a response to some systemic diseases, such as heart disease. Edema that occurs with cellulitis, however, is usually more painful and does not pit when pressure is applied to the area.

Diagnosis
Cellulitis and lymphangitis predominantly occur in hind limbs, and they’re usually painful. While the horses may be lame on presentation, they’ll often bear weight on the affected leg.

“If you pick up the contralateral leg, they will bear weight,” Dr. Carslake said. “That’s in contrast to things like fractures or septic arthritis, which are differentials for these conditions.”

The limb will be hot to the touch, swelling will be evident, and systemic signs, such as pyrexia and changes on the white blood cell profile, may be present.

Additional diagnostics are typically not performed in these horses. The first choice for most clinicians is to administer antibiotics, so bacteriologic testing would be beneficial. This may be difficult in cases of primary cellulitis, but swabs of ulcers or open wounds may be productive. Dr. Carslake emphasized that cleansing of the surface with sterile, saline-soaked swabs is crucial to get an accurate result, otherwise the test will just show whatever bacteria happened to be on the surface of the horse’s skin at the time.

“If you want to get more invasive, then aspiration of fluid pockets or pus would be optimal,” he said, adding that cleansing the skin prior to aspiration is important. Another option is to examine a biopsy specimen and resect the deep tissue on a sterile surface, but that may be difficult in a barn situation. To avoid complicating the bacteriologic profile, he said that taking the horse off antibiotics for 2 to 3 days before sampling is optimal, but that comes with inherent risks, such as the spread of sepsis.

Treatment and Prognosis
The administration of analgesia is imperative in these horses, both for the animal’s welfare and to help keep the horse moving, as movement increases lymphatic drainage. The sooner the horse is comfortable using the affected leg, the sooner the horse can avoid overloading the opposite limb. NSAIDs and acetaminophen are Dr. Carslake’s primary tools for oral analgesia in these patients. In a hospital setting, IV and epidural analgesia could be options.

“If there is localized cellulitis that is clearly secondary to something like a wound, then my inclination would be to not give glucocorticoids,” he explained. “If there is diffuse swelling of the leg and I suspect inflammation of the lymphatics, then I, personally, would give corticosteroids.”

Considering the adverse effects of these drugs is essential, especially in horses with a history of laminitis or other clinical features of equine metabolic syndrome.

He added that a tetanus booster should also be considered in some cases, and cryotherapy of the affected leg will likely be of benefit. Movement is critical in these horses as well, and a supportive limb bandage may be helpful. Dr. Carslake cautioned that the bandage should be applied tight enough to provide support, but not so tight as to be occlusive.

For antimicrobial therapy, common choices include trimethoprim-sulfamethoxazole or tetracyclines. If a parenteral route is possible, then penicillin/
gentamicin would be a good first choice, pending results of any bacterial culture and susceptibility testing. Long courses—usually a minimum of 2 weeks—are recommended.

Prognosis can vary depending on the severity of the affected limb. Most literature on the subject comes from hospital-based case studies, so the short-term survival rates found in those studies may not be generalizable to those in predominantly primary care clinics. In the studies involving hospitalized cases, the most common reason for euthanasia was laminitis.

“In the only [primary care] paper that’s been published, 16 of 16 horses survived, and all but 1 went back to normal use,” Dr. Carslake said. MeV