Presentation, pathology, morphology
Often CPL may go unnoticed by owners until it has progressed significantly to advanced stages. Here is how the UC-Davis Web
site describes it: "The earliest lesions are characterized by skin thickening and crusting. Both often are visible only after
clipping the long feathering covering the pasterns. Secondary infections develop very easily in these horses' legs and usually
consist either of chorioptic mange or bacterial infections. Both dark and white skin on the lower legs is equally affected.
These lesions are consistent with pastern dermatitis, a process also seen in other breeds. ... This disease often progresses
to include massive secondary infections that produce copious amounts of foul-smelling exudates, generalized illness, debilitation
and even death."
Skin lesions typically are milder on the forelimbs and marked to severe on the hind limbs, the UC-Davis group says. The milder
forelimb lesions are predominantly restricted to the caudal pastern region and consist of diffuse hyperkeratosis and mild
diffuse swelling. The first skin folds often are noticed at 2 years of age. Normal movement is impaired and lameness is seen
at about 15 years of age. The more advanced lesions consist of one or two palpably thickened skin folds predominantly in the
rear of the pastern region. With progression, the legs become more edematous, the folds bigger and firmer and there is development
of nodules, which may become quite large and often are the size of golf balls or even baseballs, Affolter explains. Both skin
folds and nodules first develop in the back of the pastern area. With progression, they may extend and encircle the entire
lower leg. The nodules become a mechanical problem because they interfere with free movement and exercise.
Photo 4: The lymphatic vessels are dilated, and their wall is indistinct and shows edema.
Lymphangiography helps demon-strate the tortuous lymphatics in affected legs, as shown by DeCock, Affolter and colleagues,
particularly in the palmar aspect of the metacarpophalangeal joint. Using angiography, a "moderate increase in vascular density
can be found in the palmar/plantar soft tissues of affected metacarpalphalangeal joints," the UC-Davis group notes. Lymphoscintigraphy
demonstrates that the function of lymphatics is impaired.
Several horses were necropsied at UC-Davis, showing lesions were limited to skin and subcutis of affected legs. Internal organs
were within normal limits. In addition to the marked skin lesions with hyperkeratosis, crusting erosion and ulcerations, the
legs are markedly enlarged in affected animals, Affolter says.
The subcutaneous tissue contains not only clear fluid, but also firm fibrosis with numerous tortuous lymphatic vessels embedded
in the tissue of the pastern region. There are numerous fibrous nodules, which occasionally contain inflammation and abscesses.
Morphological evaluation of skin lesions of Shires, Clydesdales and Belgian draught horses with CPL shows progressive thickening
of lymphatic vascular walls associated with chronic dermal edema and fibrosis. By evaluating the tissue by special stains,
De Cock, Leen Van Brantegem (DVM, PhD, docent in pathology, Department of Veterinary Medicine, University of Antwerp, Belgium)
and Affolter demonstrated altered elastin fibers associated with the changes in the lymphatic vessels of the skin.
Photo 5: There are areas of revascularization as an attempt for better perfusion of the fibrotic tissue.
In addition, there is inflammation and neovascularization. Measurements of elastin components, such as desmosin, further illustrates
the altered elastin metabolism, which may include diminished production of elastin, production of altered non-functional elastin
and associated increased elastin degradation.
In her thesis at the University in Ghent, Belgium, Van Brantegem illustrated higher titers of circulating anti-elastin antibodies
in affected horses, which supports the hypothesis that elastin is degraded.
Because severe symptoms do not necessarily present until later in life, CPL-affected younger horses may be bred and therefore
pass on the disease before it is diagnosed. It can significantly reduce the lifespan of affected horses. Some Belgian stallions
have to be euthanized at 6 to 8 years of age because of progressed CPL. In general, normal movement is impaired and lameness
is seen at about 15 years of age.