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Friday, 03 December 2010 16:45

DOC – WHAT IS LYMPHANGITIS IN THE HORSE?

Written by  A.J. Neuman DVM
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The answer to that question is very easy because Stedmans Medical Dictionary defines the condition as an inflammation of the lymph vessels and lymph nodes in the horse’s body. Now you really do not know much about lymphangitis after reading the definition in the dictionary. Lymphangitis is a very complex condition, as seen in the horse, and has many causes.

I just wrote an article, for The Draft Horse Journal, describing ventral abdominal edema as it occurs in the pregnant mare. This article sparked a number of requests to define lymphangitis and discuss its causes and cure, if there should be one. To discuss lymphangitis in this fashion is simply not that simple.

I strongly suspect that a good number of those people who are asking about lymphangitis have had a horse or mule with a swollen limb or two and their vet has told them they have an animal with lymphangitis. Perhaps the treatment that was employed at the time had little or no effect on the condition. To effectively treat a case of lymphangitis, the parties involved should first determine the cause and then formulate a treatment or method to remove the cause.

In every case of lymphangitis we see edema. “Dependent” edema is associated with many disease processes and will occasionally occur in a horse just from lack of exercise. Almost every horseman has seen a horse’s legs and hocks swell when it is confined to a stall. The edema or swelling will disappear after the animal has been allowed to exercise. This is a simple case of “dependent” edema.

How does the lymphatic system work? At the cellular level there is a relatively high hydrostatic pressure at the end of the arterial capillaries combined with a low osmotic pressure. These conditions promote the movement of water into the intercellular spaces. This fluid will contain many nutrients which the cells need to maintain their life. The fluid is then removed by lymphatic drainage and is absorbed into the ends of the venous capillaries. This process is aided by the muscular movements of the host animal. Any process which causes the rate of fluid entering the intercellular space to exceed that of it leaving, will cause edema formation.

In horses and mules this edema formation often occurs in the limbs, ventral abdomen, thorax, and neck and head. This is known as “dependent edema,” as opposed to an edema occurring from some local cause, such as a bee sting or blow, which is known as local edema.

When “dependent edema” occurs in an animal it is almost always a clinical sign of a generalized edema. “Local edema” will occur when the cause limits the edematous area to a single portion of the victim’s body.

The most common cause of cases of generalized edema is the stalling of the horse and, therefore, limiting the amount of exercise the animal will receive. “Filling” or “stocking” of the rear legs, and or hocks, is often seen in horses, especially those that are in “show shape” and well fed, when they are stalled for relatively long periods of time. That is why exercise for show stock is very important when they are confined in stables, at a show, for several days at a time.

Some infectious diseases will cause an acute protein-losing enteropathy with dependent edema occurring in the victims. A list of the diseases (and conditions) which cause edema in this fashion are Salmonellosis, Clostridial diarrhea, Potomac fever and heavy parasitism by endoparasites.

Occasionally a disease of the horse or mule will affect the vascular system causing the blood vessels to leak water; thereby causing a drop in the osmotic pressure in the vessel itself. This will lead to a generalized edema.

Diseases that produce this form of vasculitis include Equine viral arteritis, Equine infectious anemia, Equine ehrlichiosis and our old enemy Purpura hemorrhagica.

Other causes of dependent edema in the horse are acute and chronic heart failure. These conditions are not common in the equine but they can and have occurred. Edema of the legs, abdomen and lungs is often seen in horses and humans suffering from these heart conditions.
These are not all of the causes for a generalized or dependent edema but they are responsible for most cases.

Trauma and infection are the two most common causes of lymphangitis resulting in local edema. These swellings usually involve the legs, head or neck. A small localized swelling can expand and obstruct venous and lymphatic drainage causing the whole area or limb to become swollen and edematous. In many of the cases involving local infections of the limbs, head and neck, the lymphatic vessels and regional lymph nodes become involved. This results in a rapid swelling of the affected tissues.

There are many bacterial and fungal agents which can cause this type of lymphangitis and edema. When this condition is seen in one or more legs, the whole limb may become swollen and edematous in a short time. I have seen some of these legs as hard as a piece of wood and swollen tremendously. They looked like a stovepipe. In fact, that is the term used by horsemen to describe the situation: “Stovepipe legs.” Vigorous early treatment to resolve the infection and edema is very important. It is generally assumed that if the edema lasts seven days or longer, fibrosis will start to occur in the interstitial spaces and permanent swelling of the limb will result. The leg will become hard and “corded” to the touch and lose some of its function. It has been my experience that no treatment will restore these limbs to their proper size and use once they have reached this condition. Therefore it is very important to determine the cause of the edema and treat it at once.

There are several infectious agents which can cause lymphangitis that either have been eradicated from the U.S. or are found only in certain areas.

Contagious glanders or “farcy” is a rare disease that has been eradicated from the U.S. and Western Europe. It is now found only in parts of Asia. The lymphatic infection is characterized by the formation of many nodules containing pus which looks much like honey. This disease will also infect man.

Another disease, which we do not have in this country, is Epizootic lymphangitis. It is caused by a yeast-like fungus, which forms skin nodules that erupt with a thick pus, resembling cream. This disease, too, is limited to areas in Asia and Africa.

Sporotichosis is a fungal agent which is found in Europe, India and the U.S. It is widespread and enters the area through superficial wounds and then spreads through the lymphatic system. It will cause nodular eruptions of pus and “cording” of the involved tissue.

A plant parasite, Pythium insisiosum, which is found in stagnant water in tropical and subtropical areas, will cause a rapid edema and death or necrosis of subcutaneous tissues which it has infected. The lesions will develop rapidly giving the condition an appropriate name, “swamp cancer.”

One of the most common causes of multiple edematous lesions, found on horses or mules, are those caused by insect bites or stings. Mosquitoes and other biting gnats and insects will attack an animal by swarms. Bites or stings about the head can cause severe local swelling and difficulty in breathing.

In certain areas of the country, snake bites will occur often on the head of a horse or mule. According to the species of snake and its venom, the amount of local edema will vary. In most cases there are not enough toxins in the venom to kill the bitten animal; however, infection at the site may lead to its death.

I sincerely hope that reading this column has helped you to understand a little bit about lymphangitis and its two forms; the generalized or dependent edema versus the localized edematous type. The next time you discover a “swelling” on your horse or mule, try to identify it and treat the source as soon as possible. You just might save your animal's life or prolong its years of usefulness.
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