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.
Treatment of infectious cutaneous lymphangitis in horses includes appropriate antimicrobials, non-steroidal anti-inflammatory drugs, hydrotherapy, and surgical fluid drainage. Cutaneous lymphangitis can become chronic if left untreated or if treatment is ineffective. Chronic expansion of the subcutis by edematous fluid due to faulty lymphatic vessels can result in the deposition of fibrous tissue and permanent limb disfigurement. This emphasizes the importance of rapid diagnosis and treatment of cutaneous lymphangitis.
Sporadic lymphangitis, also known as “Monday morning leg,” can also result in swollen distal hindlimbs. This condition can develop in horses that are stabled or immobile for extended lengths of time, typically days or more. The cause of sporadic lymphangitis in horses is not well understood, but luckily the condition typically resolves after exercise.
Cutaneous lymphangitis is occasionally diagnosed at the University of Kentucky Veterinary Diagnostic Laboratory. However, the exact frequency of the condition cannot be easily estimated through typical diagnostic submissions, because diagnoses are frequently made by veterinarians in the field and don’t require extensive diagnostic evaluations.