This is a sudden onset of severe leg swelling, presumed to be caused by a bacterial infection low in the limb. It usually occurs in a single hind limb. The infection involves the lymph vessels, which are responsible for the movement of fluid from the tissues back from the limb toward the heart.

In a normal healthy animal, clear fluid (plasma) diffuses from the blood into the tissues. It is the job of the lymphatic system to pick this fluid up and transport it back into the circulation thought a series of thin walled lymph vessels with one-way valves. Unlike blood vessels, movement of fluid through these vessels is not caused by blood pressure. It relies on the movement of the tissues to push fluid through.

If these lymph vessels are blocked, then the limb rapidly swells. In lymphangitis, bacteria are thought to gain access through a break in the skin, colonizing the lymph vessels and inciting severe inflammation, which causes blockage of the vessels and a vicious cycle of pain, swelling and inflammation. Most often, the condition is seen in horses that have chronic inflammation of the lower limb (the so-called pastern dermatitis syndrome), dermatitis of other origin, or a healing wound.

Diagnosis is usually presumed when other conditions have been ruled out. A horse with a sudden onset of massive swelling of a hind limb, often right up into the groin, usually with a fever, has this condition unless proven otherwise. In most cases, there is a scab or crust low on the limb that allowed bacterial access.

Treatment is aggressive antibiotic and anti-inflammatories. Bandaging is often used, along with cold water therapy. Hand walking may help as this helps move lymph and infected material back into the circulation, thereby decreasing swelling.