Merck Manual

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Pet Owner Version

Amyloidosis in Horses


Ian Rodney Tizard

, BVMS, BSc, PhD, DSc (Hons), DACVM, Department of Veterinary Pathobiology, College of Veterinary and Biomedical Sciences, Texas A&M University

Reviewed/Revised May 2019 | Modified Oct 2022

Amyloidosis is a condition that occurs when amyloid, a substance composed of abnormally folded protein, is deposited in various organs of the body. Some types of amyloidosis are hereditary. Others occur as a result of various infections, cancers, or other inflammatory or immune-related conditions. However, the cause is often unknown.

Amyloid can be deposited throughout the body, or in just one specific area. This causes damage by displacing normal cells. The disease can become fatal if amyloid is deposited into the tissue of critical organs, such as the kidneys, liver, or heart. All domestic mammals may develop amyloidosis, and aged animals commonly have minor deposits of amyloid without signs or problems. Tumor-like amyloid nodules and amyloid deposits under the skin have been reported in horses.

There are several types of amyloid, and the classification of amyloidosis is based on which amyloid protein is involved. Deposits of AA amyloid can result from chronic inflammatory diseases, chronic bacterial infections, and cancer. The amyloid is usually deposited in organs such as the spleen or kidneys. The animal may not show any signs. If AA amyloid is deposited in the kidneys, it can lead to a buildup of protein and result in kidney failure. AL amyloid is another common form of amyloid protein. AL amyloid tends to be deposited in nerve tissue and joints.

Because of its wide distribution and stealthy onset, amyloidosis is difficult to diagnose. However, your veterinarian might suspect amyloidosis if your horse has a chronic infection or inflammation and develops kidney or liver failure. Amyloid can be identified in tissue samples with a special dye. No specific treatment can prevent the development of amyloidosis or promote the reabsorption of the protein deposits, although eliminating the cause of chronic inflammation may slow amyloid deposition and the progression of the disease.

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