“Doc, stick a needle in it and let some of that fluid out, would ya?” I spent several years working on the race track. Every so often one of the horsemen would say these words as we were looking at a horse with a big fat leg. Swollen legs are always a concern, and sticking a needle in one to drain it is almost never the solution.
Below the knee and hock, a horse is pretty much trimmed down. The powerful leg muscles end at the knee or hock and have tapered into long, strong tendons. There is no fat to speak of, even in a roly-poly overweight horse.
You can usually grab a handful of skin in places on the upper body, but never in the lower legs. Beneath that tight skin the outlines of the tendons, bones, and joints are sharp and clear. For this reason, any swelling in the lower legs is immediately noticeable. This is true for 100 pound foals and one ton draft horses. Equines are apt to get all kinds of bumps and swellings on their legs. Splints, bowed tendons, and swollen joints are easy to identify if you know your anatomy. In this article I am ignoring all of these localized bumps and will be talking about the leg that is “stocked up.” A stocked up leg is one that is swollen from the knee or hock down and is big all around.
At its worst, the leg looks like a stove pipe. It is hard to tell where the ankle begins and ends. From the coronary band up, the leg circumference may be as much as 3 times bigger normal. In other cases the swelling is mild, and barely noticeable.
A leg that is stocked up is filled with edema, which is a collection of excess fluid. However, if you were to try to drain the fluid with a needle, it wouldn’t work. You would never get more than a drop or two. The fluid is in the spaces between the cells, and not just under the skin in a big pool. If you push your finger into an edematous leg, it will leave a dent. Edema can occur anywhere in the body, but partly because of gravity it tends to collect in low spots.
There are several causes of big legs, but the most common is due to a problem with the lymphatic system. You may be familiar with the fact that arteries carry blood from the heart and veins return it. Veins were not designed to handle all of the return. In the lower leg the lymph vessels are almost as important as veins in getting fluids back up. Large protein particles, bacteria, and contaminants are picked up by the lymphatic system and are filtered out in the lymph nodes. The lymph vessels generally follow the course of the veins and have many one way valves which prevent the fluid from draining back down.
The lower leg has no extra room and the fragile lymphatic vessels are right under the skin. Anything that interferes with the movement of the lymph up the leg will cause it to accumulate. When the lymph is not moving, the leg starts to fill. The most dramatic filling of a leg occurs when the lymphatic system becomes infected. This condition is called lymphangitis. It can start with any break in the skin, including a simple scrape or even scratches. If enough bacteria enter a wound, the lymph system, which is built to handle infection, may be overwhelmed. The vessels become swollen, choked off, and unable to carry the fluid. The lymph leaks out of the vessels and collects quickly, sometimes in just a few hours. The swelling that results can be alarming. The horse may run a fever and go off feed, and the leg is often extremely sensitive.
If lymphangitis is not treated, the edema may reach all the way up the leg to the body and sometimes spread down the other leg. In extreme cases, because of the pressure, the skin may break open and let the fluid seep out. If the horse is treated early and intensively, the response is usually good. An animal with lymphangitis should be seen by a veterinarian. Antibiotics alone are often not enough to correct the problem.
Mechanical blockage of the lymph vessels may also cause a backup of fluid, even if there is no infection. The most common example of this is a scar across the front of the hock joint. This is usually seen within a week after a wire cut. The scarring goes deep and beneath the skin it acts like a dam across the lymph vessels. The barrier blocks the fluid from getting back up the leg. Other conditions such as an enlarged suspensory may push against the skin and also retard the upward flow.
I have known several brood mares with swollen hind legs. The swollen udder can cause a blockage of the lymph return up the leg. Whenever possible we try not to medicate pregnant brood mares. Often moderate exercise will take the swelling down considerably, and, of course once the mares are nursed the problem is over.
Any horse that is stocking up should be examined by your veterinarian so that the source can be diagnosed and treated. Big legs are one of those situations where early treatment may make a big difference and prevent a temporary problem from becoming permanent.
–David A. Jefferson, DVM