It's a painful, disfiguring disease that may strike horses as early as 2 years of age, then over time cause formation of large nodules that interfere with normal pastern movement, permanent skin ulceration and lameness, eventually leading to the animals' early demise.
Photo 1: A moderate to severe case of chronic progressive lymphedema, with swelling of the leg and formation of pastern folds and numerous skin nodules that are eroded. (PHOTOS: COURTESY OF DR. VERENA AFFOLTER)
The disease is chronic progressive lymphedema (CPL), and it affects large draft breeds with heavily feathered lower extremities. Early on, the disease may appear to be what often is referred to as a therapy-resistant pastern dermatitis, "scratches" or "mud fever," but over time, in addition to skin lesions typical for scratches, affected horses have vascular and lymphatic-vessel changes.
That was found during skin-lesion biopsies submitted to the dermato-pathology service at the University of California-Davis, as reported by Verena Affolter, DVM, PhD, Dipl. ACVP, a professor of dermatopathology, Department of Pathology, Microbiology and Immunology at UC-Davis' School of Veterinary Medicine, in "The Horse Report," published by the school's Center for Equine Health (Vol. 19, No. 4, October 2001).
"The progressive disease is characterized by progressive swelling, fibrosis, marked hyperkeratosis and formation of skin nodules of the distal limbs," says Hilde DeCock, DVM, formerly Affolter's colleague in the same department and now working and teaching in Belgium.
Photo 2: This is a more severe form of CPL with marked swelling and oozing of the skin surface. The partially regrown feathering is covering some of the nodules.
The disease is seen in primarily in Shires, Clydesdales and Belgian draft horses, while other draft breeds are not affected, and certain genetic lines are more readily prone to develop clinical signs, indicating at least a partial genetic etiology and a hereditary component, the colleagues explain.
Although CPL is seen around the country and the world in the three draft breeds, only the researchers at UC-Davis and their colleagues in Ghent, Belgium, are conducting ongoing studies.
They are exploring the genetics and control of the disease, seeking a definitive cause and a method to successfully manage afflicted horses.
The genetic studies include evaluation of the elastin gene, which is currently studied in Leuven, Belgium, and the vascular endothelial growth factor receptor 3 (VEGFR-3), which is studied at UC-Davis by Danika L. Bannasch, DVM, PhD, associate professor in genetics, Department of Population Health and Reproduction. The FOXC2-gene, associated with distichiasis and lymphedema in humans, was evaluated by Bannasch and found to not be associated with equine CPL.
Photo 3: The skin surface has marked hyperkeratosis and the epidermis is markedly thickened, Secondary infections are common, as shown here with an inflammation of the hair-follicle opening.