Hepatic Amyloidosis
Basic Information 
Definition
A rare disorder of protein folding in which normally soluble proteins are deposited as insoluble fibrils that progressively disrupt tissue structure and impair function
Epidemiology
Species, Age, Sex
Affected animals tend to be mature, although some may be relatively young (ie, 3–4 years of age).
Associated Conditions and Disorders
• The condition may develop either locally or systemically; the liver is commonly affected in horses with systemic amyloidosis.
• Hepatic amyloidosis occurs after repeated or prolonged inflammatory stimulation and has been most frequently reported in horses used for hyperimmune serum production.
• Hepatic amyloidosis has also been described in horses with severe strongylosis, peritonitis, chronic pleuropneumonia, tuberculosis, and neoplasia.
Clinical Presentation
History, Chief Complaint
• Hepatic amyloidosis typically occurs secondary to recurrent acute or chronic infections, inflammatory diseases, or neoplasia. History and primary complaints are therefore often related to the underlying disease.
• Chronic weight loss is common.
• Horses used for serum production reportedly show few, if any, clinical signs despite severe liver amyloid deposition.
Physical Exam Findings
• Clinical signs related to the underlying disease tend to dominate the clinical picture. Clinical signs directly related to amyloidosis depend on the organ distribution of the amyloid deposition and may be caused by either functional disturbances or mechanical alterations because of organ enlargement.
Etiology and Pathophysiology
• Amyloidosis may be systemic or localized and life threatening or merely an incidental finding. Extracellular amyloid deposition occurs as a result of protein misfolding, disrupting tissue structure and function.
• The amyloidoses are classified by the type of protein deposited; in human medicine, at least 23 different proteins have been implicated.
• AA- and AL-type amyloidosis are most commonly reported in equine cases.
• Both systemic and localized disease occur in horses.
• The protein in AA amyloidosis (secondary or reactive amyloidosis) is derived from serum amyloid A (SAA), which is synthesized by hepatocytes in response to inflammatory stimulation.
• Systemic amyloidosis in horses usually involves AA amyloid.
• Sustained overproduction of SAA is required for development of AA amyloidosis; however, amyloidosis occurs only in a small proportion of patients with chronic inflammatory disorders.
• The protein in AL amyloidosis (primary or immunocytic amyloidosis) is derived from the variable region of immunoglobulin light chains.
• AL amyloidosis is the most common form in humans and is often associated with myeloma.
• AL amyloidosis in horses is usually localized and characterized by firm, circumscribed nodules in the nasal cavity and subcutis.
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