What's Brewing Under Those Feathers?

It is quite probable that many people have never heard of chronic progressive lymphedema. However, if you have spent time with draft horses, chances are much more likely that you are familiar with the condition. This painful, debilitating disease has been identified in Shires, Clydesdales, and Belgians (especially those actually in Belgium). "Percherons, Suffolk Punches, Friesians, and other draft breeds without much feathering on the legs are not usually affected," says Gregory L. Ferraro, DVM, director of the Center for Equine Health at the University of California, Davis. Progressive swelling and thickening of the skin on the lower legs characterizes the condition. Encrusted lesions develop underneath the draft's beautiful feathers (long hairs at the fetlock), and clipping that hair is sometimes the only way to see them.

The clinical signs of lymphedema closely resemble chronic lymphedema or elephantiasis nostras verrucosa in humans. In these human conditions, the swelling is caused by a malfunctioning lymphatic system and a compromised immune system. It is believed that this might also be the cause of lymphedema in horses. However, to better understand the disease, it will be helpful to briefly review the lymphatic system.

The Lymphatic System

The lymphatic system is a one-way route for interstitial fluid (in the body's tissues) to get to the cardiovascular system. Lymphatic capillaries are present in almost all organs, and they have large, water-filled channels that are permeable to all interstitial fluid constituents, including protein.

The lymphatic system functions to transport body fluids and protect against disease. Lymph, a protein-rich liquid derived from the interstitial fluid, is transported through the lymphatic system to the major vein returning blood to the heart, the vena cava. This process also returns fluid that leaks from capillaries back into the circulatory system.

In addition, lymph vessels from the intestines transport absorbed fat to the vena cava. In the lymphatic system's protection role, foreign particles and lymphocytes (types of white blood cells that fight infection) are carried from infective tissue to the nearest lymph nodes, which are located at various points throughout this network. Accumulation of debris and lymphocytes in lymph nodes accounts for the "swollen glands" often noted in a sick person's throat.

In the circulatory system, the heart pumps blood through the arteries and veins. There is no such pump for the lymphatic system. Instead, skeletal muscle and respiratory pumps move the lymph. Valves increase the pressure and keep the flow going in one direction. Excess fluid also increases the pressure, and when that fluid accumulates, it is termed edema. This familiar "stocking up" condition is common when horses stand for long periods of time. Usually simply getting the horse moving will reduce the swelling.

A Painful Condition

The breakdown of the lymphatic system as it occurs in lymphedema in the horse leads to lymph leaking into lower leg tissue, which in turn causes fibrosis (fibrous scarring) of tissue under the skin. The thickening of the skin causes further blockage and decreased circulation. "This results in a phenomenon known as neovascularization: A process by which the body develops new blood vessels in a futile attempt to provide oxygen to its tissues," explains Ferraro.

Lymphedema often first occurs when the horse is young, and it eventually causes disfigurement, disability, and even premature death. For example, in affected Belgian stallions in Belgium, the horse's life expectancy declines from 20 years to six years because acute lymphedema is often accompanied by secondary infections as a result of the open wounds on the horse's legs. Dark- and white-colored legs are equally affected.

Lesions are similar to pastern dermatitis (also called scratches) that is common in other breeds, but lymphedema lesions do not respond well to treatment. Removing the scabs or even simple movement can cause bleeding, and according to Ferraro, the condition can be quite painful.

The small lesions progressively get larger, and while they often start on the rear of the pastern, they can spread as far up as the knees and hocks. The nodules can range in size from that of a golf ball to a baseball, and they interfere with the horse's natural movement. Thick skin folds form and as the condition worsens, the swelling becomes firm and permanent. This dermatitis is believed to be "a secondary result due to the body's inability to properly supply fluid circulation and oxygenate the skin of the lower leg," states Ferraro.

Searching for Answers

The exact cause of chronic progressive lymphedema is still unknown, but identifying the cause is the key to determining the pathogenesis. A deficiency of or abnormality in elastin (a component of tissue) might be the cause of lymph system degeneration. Since only certain breeds--and animals within that breed--are affected, there might be a genetic link. That link could define the long-term solution.

A study to determine the cause, treatment, and prevention of chronic progressive lymphedema is underway with a collaboration between UC Davis, the University of Ghent in Belgium, and human medical researchers. UC Davis will use Shires and Clydesdales, while the University of Ghent studies Belgians, to confirm that the cause is an abnormality of elastin. Then they will examine the cause and effect relationship between cellular abnormality and disease progression. Why does elastin deficiency lead to a breakdown of the lymphatic system, which in turn causes the progression of clinical signs? Once these questions are answered, there is a better chance of finding a treatment and cure.

Finally, they will examine the genetic profile of the draft horse and how it could be manipulated to avoid the problem. In humans, there are three different genes that cause the inherited form of the disease. For horses, the research is progressing. "We are working on the genetics of the condition and are continuing to study the cellular biology of the disease at this time," says Ferraro. "We hope to have some new scientific articles published this year."

No doubt draft horse owners will be glad to know that answers to this debilitating disease might be coming soon.

About the Author

Stephanie J. Corum, MS

Stephanie J. Corum received a MS in animal science from the University of Kentucky in Lexington. She has worked in various aspects of the horse industry, including Thoroughbred and Arabian racing, for nearly 20 years. More information about her work can be found at www.theridingwriter.com. She has also published the illustrated children's story Goats With Coats. Currently she and her husband own Charisma Ridge, a small horse farm in Maryland, and she competes in dressage.

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