Due to the characteristic appearance of affected limbs, cutaneous lymphangitis is commonly referred to as “big leg” or “fat leg.” | Photo: Courtesy Dr. Kennon Keckler

The lymphatic system is an important component of the cardiovascular system and consists of lymphatic vessels, lymph nodes, tonsils, spleen, and thymus. Lymph, a clear colorless fluid, is formed from fluid loss that occurs during normal nutrient exchange in capillary beds. The lymphatic vessels transport lymph to regional lymph nodes for filtration to aid in immunologic detection of microorganisms, toxins, and foreign material. Once filtered, the vessels once again transport the lymph to large veins, which ultimately return it back into the circulatory system to replenish the fluid lost from the capillaries.

Lymphatic disease can occur when lymph vessels become inflamed, leaky, and/or blocked. Cutaneous lymphangitis—inflammation of the skin’s lymphatic vessels—is fairly uncommon in horses, does not exhibit age, sex, or breed predilections. It can develop from both infectious and non-infectious causes. Clinically, cutaneous lymphangitis in horses can manifest as a swollen limb, skin abnormalities characterized as multiple skin nodules that can abscess or develop draining tracts, and/or lameness. Cutaneous lymphangitis typically affects the distal (lower) portion of a single hind limb, between the hock and hoof. Due to the characteristic appearance of affected limbs, the disease is commonly referred to as “big leg” or “fat leg.”

Infectious cutaneous lymphangitis in horses has traditionally been associated with poor hygiene and insect transmission of microorganisms. It is sporadically diagnosed in horses, but sometimes occurs simultaneously in multiple horses on the same farm. Infection of the lymphatic system develops following contamination of skin wounds by various bacteria, most commonly Corynebacterium pseudotuberculosis (the causative agent of ulcerative lymphangitis/pigeon fever). However, pure or mixed infections with other bacteria, such as Staphylococcus spp, Streptococcus spp, Trueperella pyogenes, Rhodococcus equi, Pasteurella haemolytica, Pseudomonas aeruginosa, Fusobacterium necrophorum, Actinobacillus equuli, and Burkholderia mallei (the cause of glanders) can also result in cutaneous lymphangitis. Additionally, pathogenic fungi such as Sporothrix spp (the cause of sporothricosis) or Histoplasma farciminosum (the cause of epizootic lymphangitis) also have been associated with lymphatic system infection. The U.S. is currently free from glanders and epizootic lymphangitis