Amanda Mansz   

 Animal Health Laboratory, University of Guelph, Guelph, ON. 

AHL Newsletter 2024;28(3):21.

Following the euthanasia of a 7-year-old female Clydesdale horse with progressively worsening lameness and weight loss, the animal was submitted to the AHL for postmortem. On gross examination, markedly expanding and encircling the pastern, fetlock and extending up the cannon bone, the skin surface of all four limbs was massively thickened by multifocal to coalescing, firm/fibrous, variably-sized (2-3 mm up to 5 cm) raised, partially haired or alopecic nodules. There was frequent dermal ulceration and deep folds filled with purulent exudate and crusted material (Fig. 1). The hoof surfaces were heavily cracked and irregular with an undulating and uneven contour. The tip of the tail was markedly expanded (to a 17 cm diameter) by firm, fibrous tissue with an alopecic, black dermal surface, and few 1-2 cm surface ulcers. There was mild bruising and excoriation/ulceration with flaky yellow crusting of the haired skin encircling the vulva, extending ventrally to the inguinal region and the medial surface of the thighs. CT imaging and sagittal sectioning of the limbs revealed a severe deep digital flexor tendinopathy of the left forelimb, multi-limb osteoarthrosis of the interphalangeal joints and pedal osteitis.

Microscopic lesions of the limbs, tail tip and perivulvar/inguinal regions included marked hyperkeratosis, surface necrosis and ulceration with formation of deep necrotic crevasses between skin folds and marked superficial to deep dermal fibrosis. There was prominent nested proliferation of superficial, middle and deep vascular plexi with thickening of vessel walls by smooth muscle hyperplasia, accompanied by thick rings of peripheral fibrosis. Lymphatic vessels were mildly to markedly dilated and tortuous with variable perilymphatic fibrosis and inflammation (Fig. 2).

Gross and histological findings were all compatible with a diagnosis of a severe case of chronic progressive lymphedema (CPL).  Previous terms for this condition included “chronic pastern dermatitis” and “chronic proliferative pastern dermatitis”, but because of the clinical and histological similarity to the human counterpart “non-filarial chronic lymphedema” (or elephantiasis nostras verrucosa), the condition was renamed “chronic progressive lymphedema in horses”. CPL is a debilitating condition where there is buildup of lymph fluid in the lower legs, progressive swelling with associated skin folds, nodules and ulcerations. Secondary bacterial, fungal, or parasitic infections often complicate and aggravate the lesions and lead to advanced progression of this disease. Many affected horses also develop poor hoof growth. The dermal lesions can limit the horse’s movement and result in lameness due to the inappropriate alignment of the articular bones, which is the potential cause for the changes to the phalangeal bones and tendinopathy diagnosed in this case. 

CPL is more prevalent in certain bloodlines, therefore, a genetic component is suspected. Belgian draft horses are the most frequently affected; however, other breeds including Shires and Clydesdales, Gypsy Cobs and Gypsy Vanners, Friesians, the American Belgian, German draft horse breeds, the Percheron, and other large breed draft horses have been documented to be diagnosed with CPL.

Dysfunction of the lymphatic system, progressive fibrosis of the tissues, and a disruption of the elastin matrix in the skin all appear to be components of the pathogenesis, however, the chronology of these changes and exact mechanisms of this disease remain uncertain.   AHL

Figure 1. Gross lesions of the distal limb compatible with chronic progressive lymphedema.

a. Marked dermal thickening with multifocal to coalescing, firm, fibrous, partially-haired or alopecic nodules with frequent dermal ulceration, and deep folds filled with purulent exudate and crusts. b. A cross section of the cutaneous nodules demonstrates marked dermal fibrosis.

Figure 2. Histologic lesions compatible with chronic progressive lymphedema. H&E stain.

a. Hyperkeratosis with thick crusts and formation of deep necrotic crevasses between skin folds 10x. B. Marked deep dermal fibrosis and proliferation of nested vascular plexi with surrounding fibrosis. 20x. C. proliferative vascular nest with surrounding fibrosis and mild inflammation. 40x.

Reference

1.Brys M. Claerebout E, Chiers K. Chronic progressive lymphedema in Belgian draft horses: Understanding and managing a challenging disease. Veterinary Sciences. 2023;10(5):347.