Chronic Progressive Lymphedema (CPL) is a condition that describes impaired lymph flow in the lower legs. Primarily observed in draft horse breeds, CPL causes swelling and skin inflammation on affected legs. [1]

Horses with the condition are also at risk of lesions and secondary bacterial and parasitic infections developing on their legs because of poor lymph flow.

A veterinarian can typically diagnose CPL based on a clinical examination. There is no cure for CPL. However, intensive management strategies can slow the progression of the disease and improve the quality of life of affected horses.

CPL was recognized in 2003, although skin lesions associated with the condition were initially reported in the early 20th century. [1] CPL is believed to be caused by a combination of genetic and environmental factors.

What is Chronic Progressive Lymphedema?

A systemic disease of the lymphatic system, CPL occurs due to the accumulation of lymph fluid in the lower legs of affected horses. [2]

CPL is a debilitating condition that causes secondary recurrent bacterial and parasitic infections due to poor lymphatic drainage and blood circulation. [2]

Recurrent infections promote the development of skin lesions that can extend up the leg to the knees or hocks. These infections typically increase the amount of lymphedema in the affected legs. [2]

Decreased lymph flow in the legs of horses with CPL may initially go unnoticed if heavy hair (feathering) is present on the legs. The tissue damage caused by CPL is progressive and can result in lameness, severe disability, disfigurement, and potentially premature death. [2]

Chronic progressive lymphedema differs from chronic pastern dermatitis, a condition that occurs due to primary infection with microorganisms.

Treatments of skin infections do not resolve the underlying lymphedema associated with CPL.

Lymphatic System Dysfunction and CPL

The role of the equine lymphatic system is to maintain fluid balance in the tissues, support immune function in the skin, and remove cellular waste products.

Protein-rich lymph fluid circulates throughout the body through lymph vessels and lymph nodes.

Elastin, an extracellular matrix protein, is necessary to move lymph fluid throughout the body. Abnormal elastin metabolism and impaired elastin quality in the skin are believed to be associated with CPL. [2]

If lymph flow is compromised, oxygen supply to tissues decreases and metabolic waste products accumulate within the body. Poor lymph flow (lymph stasis) impairs the skin’s immune response to pathogens and jeopardizes the integrity of the skin barrier.

In advanced cases of CPL, there may be complete lymph stasis due to lymphatic dysfunction, lymph vessel dilatation, and the formation of excess fibrous connective tissue (fibrosis). [2]

Prevalence of CPL

Horses that develop CPL typically don’t show detectible signs of the disease before the age of two. [2]

CPL affects breeds including: [3]

  • Belgian draft horses
  • German draft horses
  • Shires
  • Clydesdales
  • Gypsy Vanners
  • English Cobs
  • Friesians
  • Percherons

The number of horses affected by CPL is unknown. However, a study of 161 Belgian draft horses found that 82% had CPL. [4]

Another study involving 912 horses of six different breeds of German draft horses older than 2.5 years determined a prevalence of CPL ranging from 47.5 to 96.1%. [4]

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Signs of CPL

The severity of CPL correlates with how severely lymphatic clearance is delayed. [1] Early signs of CPL include:

  • Swelling in the lower legs
  • Soft pitting edema

Advanced signs of CPL include:

  • Firm swelling in the legs
  • Folds in the skin due to swelling and tissue damage
  • Lesions (ulcers)
  • Fibrosis that results in nodule formation
  • Poor hoof growth
  • Thrush
  • Scaling skin
  • Granulation tissue
  • Discharge of exudate (pus) due to secondary bacterial overgrowth from the crevices of skin folds
  • Loss of shape of the lower leg
  • Skin folds on the neck and the trunk
  • Impaired movement due to nodule development and skin folds on the limbs

Secondary Infections

Horses with CPL are prone to the following types of recurring infections due to reduced lymph flow which disrupts the normal barrier function of the skin.

Bacterial Infections:

Skin infections in horses with CPL may involve bacteria including various species of Staphylococcus and Dermatophilus congolensis. [2] Heavy feathering traps moisture and bacteria that propagate infection.

Parasitic Infections:

Chorioptes bovis, a type of skin mite that causes chorioptic mange, is common in horses with CPL. [2]

Feathered horses with CPL are prone to overproducing keratin (hyperkeratosis) in their skin. [1] This abnormal thickening of the outer layer of the skin is associated with the development of skin crusts that feather mites feed on.

Mites are a trigger for CPL as they can cause scarring and damage to the skin and superficial lymph vessels.

Causes of CPL

The cause of CPL is currently unknown. However, a combination of the following factors is believed to be involved in the development of the condition.

Biological Abnormalities

Reduced and or Poor-Quality Elastin:

Organized in a three-dimensional network in the skin, elastin surrounds lymphatic vessels and functions like an elastic sponge to facilitate lymph transport throughout the body.

Horses with CPL have an abnormal quantity and/or quality of elastin in their skin which results in impaired lymph uptake and clearance from the tissues.

A study that compared skin samples from horses with CPL and those without the condition determined that affected horses had increased amounts of dermal elastin in their lower legs and neck, whereas the non-affected horses of a susceptible breed had decreased amounts. [5]

Antibodies to elastin have been noted in the blood serum of horses with CPL. [6]

Although the quantity of dermal elastin increases in the neck and leg skin of horses with CPL, the architectural structure of this elastin is less organized compared to horses without the condition.

It has been proposed that the body forms new elastin to compensate for a lack of elastic support from the original elastin.

Abnormal Desmosine:

An amino acid that cross-links elastin fibers, desmosine is reduced in the skin of the neck and lower legs of horses with CPL. However, desmosine levels have been found to increase when lesions develop. [2]

Genetic Influence

No specific genetic markers or mutations are known to be present in horses with CPL, although research sections of DNA sequences may be associated with the condition. [7]

Genetics are believed to play a role in the development of CPL in certain breeds of draft horses. The high rate of occurrence in some breeds suggests a genetic predisposition for the condition.

CPL is known to occur in Belgian draft horses after continuous selected breeding aimed at producing offspring with dense feathering and heavier legs. [2]

A study of 28 Friesian horses found that affected horses had a significantly increased gaskin length, thus suggesting a genetic phenotype may be associated with the development of CPL. [1]

Environmental Triggers

The environmental conditions in which horses with CPL live affect the progression of their disease. [2] Sandy and muddy environments are believed to promote the development of lesions, whereas clean rubber surfaces do not. [2]

Lesions occur more often in horses used for breeding and food production compared to those that are used for work or riding. [1]

Diagnosis

A veterinary diagnosis of CPL is typically based on clinical presentation and consideration of breed disposition for the disease. Diagnosis typically involves a physical examination and diagnostic testing.

The early stages of the disease are typically evident when palpating the lower limbs. Clipping of the feathers is often required to identify lesions.

The following diagnostic testing strategies are less commonly used to diagno