Understanding Equine Amyloidosis

Amyloidosis is a rare disease in which abnormal protein, known as amyloid, is deposited within tissue and disrupts normal organ function. Multiple types of amyloidosis occur in humans and domesticated animals, but only reactive systemic amyloidosis (AA) and immunoglobulin-derived amyloidosis (AL) have been described in horses. Reactive systemic amyloidosis develops secondarily to chronic and nonspecific antigenic stimulation. Immunoglobulin-derived amyloidosis occurs due to immunoglobulin light chain deposition during plasma cell dysfunction. Both AA and AL amyloid can be deposited systemically in multiple tissues or locally within a single organ. The University of Kentucky Veterinary Diagnostic Laboratory (UKVDL) USALIMS database was searched for cases of amyloidosis to better understand this disease in horses.

Eleven cases of amyloidosis were diagnosed at the UKVDL from 2010 to 2021. Seven cases were diagnosed in horses that died or were humanely euthanized, and four cases were diagnosed in surgical biopsy submissions. Five cases were identified in Thoroughbreds, two in mixed breeds, and individual cases in a Tennessee Walking Horse, Saddlebred, Quarter Horse, and Warmblood. Horses ranged in age from less than a year up to 16 years. Six cases were diagnosed in mares, four in geldings, and one in a colt.

Animals that were submitted for postmortem examination presented with clinical histories of being found dead (two cases), acute colic (three cases), chronic unresolving abdominal abscessation (one case), or proliferative bone lesions (hypertrophic osteopathy; one case). Amyloid was microscopically identified in multiple tissues (systemic distribution) in all seven horses that died or were euthanized. Amyloid was deposited in the liver and spleen (three cases); liver, spleen, and kidney (two cases); liver, spleen, and lymph node (one case); and liver and kidney (one case). Four cases with liver involvement resulted in hepatic rupture and hemoperitoneum; two of these died naturally and one was euthanized. Co-morbidities in these horses included nodular pulmonary fibrosis, aortic mineralization and aneurysm, chronic peritonitis, bone disease (osteopathy), abdominal Rhodococcus equi abscessation, mesenteric rent with small intestinal entrapment, and chronic lymphangitis (big leg). Of note: Chronic lymphangitis was evident in three of the seven necropsy cases and was the only co-morbidity identified in more than one animal. A chronic inflammatory condition was not overtly identified in one case; this horse died due to a mesenteric rent with small intestinal entrapment.

Four cases of amyloidosis were diagnosed from surgical biopsy submissions. These included three cases of nasal (nasopharyngeal) amyloidosis and one case of cutaneous (skin) amyloidosis. Horses with nasal amyloidosis typically present with clinical signs of epistaxis (nosebleed), difficulty breathing, or decreased athletic performance. Examination of the nasal mucosa can reveal nodular to diffuse thickening of the nasal mucosa due to amyloid deposition. Surgical excision is the treatment of choice for nasal amyloidosis. Horses with cutaneous amyloidosis commonly develop multiple papules, nodules, or plaques in the skin of the head, neck, shoulders, and pectoral regions. Skin lesions are firm, nonpainful, and do not itch. There are no proven effective treatments for cutaneous amyloidosis. Cutaneous and nasal forms are not typically seen with the systemic form and do not develop from chronic inflammation, like the systemic form. A surgical biopsy should be considered if nasal or cutaneous amyloidosis is suspected.

Microscopically, amyloid from all cases consisted of extracellular homogenous eosinophilic material that distorted tissues. The material had an apple-green appearance under polarized light, consistent with amyloid. Granulomatous inflammation was associated with the amyloid in the cutaneous and nasal forms. None of the cases were assessed by proteomic techniques, electron microscopy, or immunohistochemistry.

In summary, equine amyloidosis is an uncommon cause of systemic, cutaneous, and nasal disease. Any horse breed, sex, or age can be affected. The systemic form is associated with chronic disease processes, and the liver, spleen, kidney, and lymph nodes are frequently affected. Nasal and cutaneous forms are not associated with other ongoing diseases and have a good clinical prognosis.


Editor’s note: This is an excerpt from Equine Disease Quarterly, Vol. 31, Issue 2, funded by underwriters at Lloyd’s, London, brokers, and their Kentucky agents. It was written by Alan T. Loynachan, DVM, PhD, Dipl. ACVP, of the University of Kentucky’s Veterinary Diagnostic Laboratory, in Lexington.