5 reasons for swelling, from benign triggers to causes for major concern

Your horse’s sleek, strong legs have remained taut and unblemished throughout (and sometimes despite) his athletic pursuits. Then one day as you offer him breakfast, you notice he has a “fat” leg. Is it cause for alarm or something simple to deal with? Let’s look at the variety of causes for limb swelling.

Initial Assessment Steps

Before you assume the worst, consider that in many cases prompt recognition and treatment can resolve leg swelling. Jump on the situation and contact your veterinarian immediately rather than trying the wait-and-see approach.

First, you’ll want to gather information to share with your veterinarian before making the call. Rosemary Cuming, BVSc MS, MANZCVS, Dipl. ACVIM, of Scone Equine Hospital, in New South Wales, Australia, advises owners to make notes about the whole horse—not just his limbs.

“Pay attention to the horse’s demeanor, appetite, water intake, and fecal output,” she says. “Has the horse shown any signs of illness recently, or has anything changed regarding diet, deworming, medications, blanketing, bandaging, or bedding, for example?”

Matt Randall, DVM, of Collier Equine, in Waller, Texas, urges horse owners to also take the horse’s rectal temperature.

Cuming suggests making a checklist of questions to address. Watch the horse closely when he is standing still and then moving, and ask yourself:

• Does the horse appear comfortable, or is he showing stiffness?
• Is he lame? If so, to what degree?
• Is only one leg or are multiple legs swollen?
• Do you see any drainage or signs of a puncture or other wound?

Feel the affected limb(s), and note any abnormalities:

• Is there heat in the limb?
• Does the horse pull away due to pain following a light touch or firm palpation of the swollen area?
• Do you feel irregularities in the skin or hair, such as scabs and crusts, moist or oozing discharge, or abnormal lumps?
• Is the swelling localized (limited to a specific area), or is there generalized pitting edema (the fluid swelling is over a larger area and the tissue stays depressed after you press on the swelling with your finger)?
• Is the swelling in proximity to or within a synovial structure such as a joint or tendon sheath?

“Fever, lameness, and wounds involving synovial structures definitely increase the potential urgency for veterinary attention to limb swelling,” Randall says.

“Limb swelling can be innocuous or may be indicative of a localized disease process within one or more legs, or due to more serious systemic diseases involving other body systems,” adds Cuming. “Having a clear description of the type and location of swelling, the horse’s overall state of health, any history of previous limb swelling, and any recent management changes is information that helps your veterinarian assess over the phone whether the limb swelling requires immediate veterinary attention or can first be treated symptomatically and monitored.”

Here are five common diagnoses your veterinarian might make.

1. Stocking Up

“Stocking up describes swelling of the lower limbs (below the knees or hocks) in horses due to pooling of fluid (edema) in the tissues,” says Cuming.

Stocking up often occurs when horses undergo a period of restricted movement such as stall confinement, she says. A reduction in the normal physiologic pumping of fluid causes the lower limbs to swell. She likens it to swelling that might develop in your ankles following a long plane ride.

A horse’s lymphatic system is always working to move fluid, proteins, and nutrients around the body. “In the normal horse, fluid, proteins, and nutrients frequently leak out of blood vessels to bathe the cells of the tissues with needed nutrients,” says Cuming. “This fluid, together with cellular debris, dead blood cells, bacteria, and other pathogens (disease- or damage-causing organisms), toxins, and protein molecules, is then removed from the leg via lymphatic vessels, which are thin-walled, valved structures capable of containing large volumes of fluid. The lymph fluid is filtered through the lymph nodes and then returned to the bloodstream via the veins.

“There is no active pump (like the heart) in the lymphatic system, and so pressure and movement in the tissues and organs around lymph vessels influence flow,” she continues. “In horses’ lower legs, movements of the hoof’s digital cushion (the soft tissue beneath the sole that separates the frog and heel bulb from the underlying tendons and bones), the pasterns, fetlocks, ligaments, and tendons play an important role in helping to ‘pump’ lymph fluid up the lymphatic vessels against the force of gravity.” 

While stocking up can occur in any horse, Cuming says veterinarians most commonly see it in older horses or those with previous leg injuries, cellulitis, or lymphangitis (see next section) due to less-efficient circulation. The best thing to do is to get the horse moving, she says. Active exercise with hand-walking, longeing, or riding should resolve at least some of the swelling. Support bandaging also helps reduce swelling rapidly. However, if used too long, bandaging further reduces lymphatic and blood flow and can perpetuate the condition.

2. Cellulitis and Lymphangitis

Significant and alarming limb swelling is often due to cellulitis or lymphangitis. Limb cellulitis typically occurs acutely—the horse seems fine, then becomes lame suddenly, says Cuming. Cellulitis is a diffuse bacterial infection of the skin and subcutaneous tissues that dissects rapidly and extensively through tissue planes. Bacteria such as staphylococcal or streptococcal species enter a break in the skin—sometimes you’ll see a wound, sometimes the infection’s entry point isn’t obvious, such as might occur if leg boots cause microabrasions in the skin, she says.

“The swelling causes significant lameness and fever and is usually in a single limb,” says Randall. “I have seen cases where multiple limbs were involved after too much time standing in nasty water when horses were trapped for days in floodwater after Hurricane Harvey” in 2017.

“The affected limb is characteristically swollen, hot, and very painful to the touch, with an associated moderate to marked lameness,” adds Cuming. “A small number of horses, however, will pull up lame without obvious limb swelling, and you only suspect cellulitis when you feel heat and mild pitting edema in the lame leg. In addition to pain and swelling, cellulitis may also cause a poor appetite, lethargy, elevated heart rate, fever, and oozing of serum from the skin of the swollen leg.”

Rapid recognition and early treatment of cellulitis are important to avoid other serious complications, such as supporting limb laminitis (founder in the opposite or supporting leg), dermal necrosis (skin tissue death, seen as sloughing), recurrent cellulitis, persistent lameness, and vascular thrombosis (clotting of the vessels). Any of these advanced problems can result in chronic debilitation or even the need for euthanasia, says Cuming.

Horses with ulcerative lymphangitis also exhibit lameness, fever, and lethargy, but with an additional clinical sign: Infection and inflammation can result in cording of the lymphatic vessels into chains of hard nodules that turn into abscesses that rupture. This occurs due to bacterial infection, such as Corynebacterium pseudotuberculosis (which causes pigeon fever), which is transmitted by biting flies, or due to deeper penetration by staphylococcal or streptococcal species. Cuming says bacterial infection isn’t always the cause of lymphangitis, though. It can also develop due to genetic anomalies, particularly in draft horses.

In chronic cases of lymphangitis, Randall and Cuming say permanent thickening of the tissue surrounding the lymphatic system can result in persistent swelling and stocking up, resulting in the limb staying thicker and larger in circumference than normal.

3. Infectious Disease

Infectious causes of limb edema tend to involve multiple limbs. “Typically, the horse has a fever and varying degrees of lameness and limb edema,” says Randall.

“Look for swelling involving all four legs, edema in other parts of the body such as under the jaw and in the sheath, and monitor for other clinical signs that your horse is sick rather than injured,” adds Cuming.

Signs of illness include fever, lethargy, decreased appetite, nasal discharge, increased respiratory rate or effort, colic (abdominal pain), or diarrhea.

Strangles, an infectious disease caused by Streptococcus equi, can create a potentially fatal secondary immune syndrome known as purpura hemorrhagica. In these horses, “edema is often quite severe in the limbs, chest, and abdomen, and petechiation (red spots due to small hemorrhages) develops in the mucous membranes and/or the sclera (white) of the eye,” says Randall.

Systemic viral infections can also cause leg swelling due to vasculitis (leaky blood vessels). “These diseases may be mild and self-limiting (resolving on their own) or can be life-threatening,” says Cuming. “Examples include viral infection from equine influenza, equine viral arteritis, equine herpesvirus, or Hendra virus (currently only present in Australia).”

While not infectious, low blood protein concentrations or circulatory compromise related to congestive heart failure or inflammatory bowel disease can also result in multiple limb edema.

4. Injury

Swelling in a single limb is commonly associated with a problem located anywhere from the skin surface or deeper subcutaneous tissues to tendons, ligaments, tendon sheaths, joints, or bones. The many reasons a single limb might swell include:

• Tendon or ligament injury;
• Bony sequestrum formation (when a piece of bone dies and separates from healthy bone) following previous blunt or penetrating trauma to the leg;
• Pastern dermatitis (greasy heel/mud fever/scratches);
• A foot abscess;
• Joint or tendon sheath sepsis; and
• Ringbone.

“Probably one of the biggest tipoffs that swelling is related to an injury is that at least initially, the swelling is localized near a tendon or ligament rather than circumferentially surrounding the entire leg,” says Randall. “Typically there is no fever, but lameness varies significantly depending on the injury and the individual. I’ve seen horses with ruptured superficial digital flexor tendons that walk fine, except for increased fetlock drop. Chronic suspensory branch injuries may have more diffuse and extensive swelling, but it’s usually more fibrous with less edema.”

He says swelling from a foot abscess usually doesn’t advance past the upper pastern. However, at times edema might extend all the way to the carpus (knee) due to constricted blood vessels in the foot, combined with low-grade ascending cellulitis related to the infection.

Similarly, wounds tend to develop localized swelling that might become more widespread due to ­infection.

To differentiate between the many injuries a horse can incur, your veterinarian will perform a thorough physical examination, followed by a lameness exam and appropriate diagnostic tests based on clinical findings. Testing possibilities include radiography (X rays), ultrasound, advanced diagnostic imaging (MRI, CT, nuclear scintigraphy), tissue biopsy, and bacterial culture and antibiotic sensitivity.

5. Windpuffs

Windpuff is a word that simply describes swelling within a digital structure, such as a tendon sheath, bursa, or joint capsule. Randall says many of these are innocuous and mostly cosmetic in nature, especially if they’re similar in both front or hind legs.

“I don’t get too excited if a horse is doing its job without complaint, showing no lameness, and is negative to palpation and flexion of the limb, especially if the effusion (fluid buildup) improves or doesn’t worsen with exercise,” says Randall. “Windpuffs tend to occur secondary to an inflammatory response within the joint or tendon sheath that increases synovial fluid that distends and stretches the synovial structure.”

Just like the loss of elasticity that occurs after you stretch a rubber band, the capsule or sheath often remains stretched and retains some fluid to make a “puff.”

If the effusion is in only one limb or the horse is sensitive to palpation or joint flexion, Randall recommends doing further diagnostic testing. Cuming agrees: “If these swellings are asymmetrical (just in one leg or on one side of the leg) or appear suddenly, grow in size, are hot or painful on palpation, or are associated with a known injury or lameness, then this may indicate an acute injury.”

Examples include a torn ligament or tendon or an acute flare-up of a chronic injury, both of which necessitate prompt veterinarian attention, she says.

Take-Home Message

Many instances of limb swelling are fairly innocuous but can foretell a significant underlying problem. Always get your veterinarian involved to assess if and what needs to be done to address the swelling.