Amyloidosis

Amyloidosis is a systemic or localized disease characterized by the extracellular deposition of insoluble, fibrillar protein aggregates known as amyloid. These fibrils are derived from various precursor proteins that undergo misfolding and aggregation, leading to progressive organ dysfunction. In veterinary medicine, amyloidosis most commonly affects the kidneys, but can also involve the liver, spleen, adrenal glands, and gastrointestinal tract. The disease is classified based on the precursor protein involved: reactive (secondary) amyloidosis, associated with chronic inflammation or infection, is most common in dogs and cats, and is typically composed of serum amyloid A (SAA) protein. Primary (immunocyte-derived) amyloidosis, involving immunoglobulin light chains (AL), is rare in domestic animals. Renal amyloidosis leads to progressive proteinuria, nephrotic syndrome, and ultimately chronic kidney disease (CKD) and end-stage renal failure.

The etiology of amyloidosis is multifactorial. In dogs and cats, the most common form is reactive (secondary) amyloidosis, which occurs secondary to chronic inflammatory, infectious, or neoplastic conditions. Chronic infections such as canine leishmaniasis, ehrlichiosis, and feline infectious peritonitis (FIP) are well-documented triggers. Chronic inflammatory diseases like rheumatoid arthritis, inflammatory bowel disease, and chronic dermatitis can also induce amyloidosis. Neoplastic conditions, particularly those associated with chronic inflammation, may also predispose. In certain breeds, such as the Chinese Shar-Pei and Abyssinian cat, there is a genetic predisposition to familial amyloidosis, often without an obvious inflammatory trigger. In these cases, mutations in the SAA gene or other amyloidogenic proteins are suspected. Primary amyloidosis (AL) is rare but can occur due to plasma cell dyscrasias or multiple myeloma. Additionally, localized amyloidosis can occur in the nasal cavity, skin, or other tissues without systemic involvement.

Amyloidosis is relatively uncommon in companion animals but has significant breed and species predilections. In dogs, the Chinese Shar-Pei is notably predisposed to familial renal amyloidosis, often presenting at a young age (1-5 years). Other breeds reported to have increased risk include the Beagle, English Foxhound, and Collie. In cats, the Abyssinian breed has a hereditary form of amyloidosis that primarily affects the liver and kidneys, with onset typically between 1 and 5 years of age. Siamese and Oriental Shorthair cats may also be at higher risk. There is no clear sex predilection, though some studies suggest a slight female predominance in certain breeds. Geographically, amyloidosis is more frequently diagnosed in regions where chronic infectious diseases (e.g., leishmaniasis in Mediterranean countries) are endemic. The overall incidence is low, but it is an important cause of protein-losing nephropathy in predisposed breeds.

The pathophysiology of amyloidosis involves the overproduction and misfolding of precursor proteins. In reactive (secondary) amyloidosis, chronic inflammation leads to sustained elevation of serum amyloid A (SAA), an acute-phase protein produced by the liver. SAA is normally degraded, but in amyloidosis, it is incompletely processed, resulting in the formation of amyloid fibrils composed of N-terminal fragments of SAA. These fibrils deposit in tissues, particularly the renal glomeruli, interstitium, and medullary interstitium. The deposition disrupts normal tissue architecture and function. In the kidney, amyloid deposits in the glomerular mesangium and basement membrane lead to increased permeability and proteinuria. As the disease progresses, glomerulosclerosis, tubular atrophy, and interstitial fibrosis develop, culminating in chronic kidney disease. In familial forms, genetic mutations may lead to the production of amyloidogenic proteins that are inherently prone to misfolding. The deposition of amyloid can also occur in other organs, causing hepatomegaly, splenomegaly, and adrenal insufficiency. The clinical consequences depend on the extent and location of amyloid deposition.

Predisposing factors for amyloidosis include genetic susceptibility, chronic inflammatory or infectious diseases, and certain environmental or management factors. Breed-specific genetic predispositions are significant, particularly in Chinese Shar-Pei dogs and Abyssinian cats. Chronic infections such as leishmaniasis, ehrlichiosis, and FIP are major risk factors in endemic areas. Chronic inflammatory conditions like rheumatoid arthritis, inflammatory bowel disease, and chronic skin infections can also predispose. Age is a factor, as amyloidosis is more common in middle-aged to older animals, except in familial forms where it may occur in young adults. Sex may play a role in some breeds, but evidence is inconsistent. Environmental factors such as poor nutrition, stress, and concurrent immunosuppression may exacerbate the underlying inflammatory state. Additionally, certain medications that induce chronic inflammation or immune stimulation could theoretically increase risk, though specific drug associations are not well-documented.

Clinical signs of amyloidosis are often insidious and progressive, reflecting the underlying organ involvement. In renal amyloidosis, early signs may be subtle and include mild weight loss, lethargy, and polyuria/polydipsia. As proteinuria worsens, signs of nephrotic syndrome may develop, including peripheral edema, ascites, and pleural effusion. In advanced chronic kidney disease, signs include anorexia, vomiting, oral ulcers, and uremic breath. In cats with hepatic amyloidosis, hepatomegaly and jaundice may be prominent. In dogs with familial amyloidosis, signs may include recurrent fever, joint pain, and lameness due to amyloid deposition in joints. Systemic signs such as fever, anorexia, and depression are common, especially if an underlying inflammatory condition is present. In some cases, amyloidosis may be an incidental finding at necropsy. The clinical course can be acute in some familial forms, with rapid progression to renal failure, or chronic and slowly progressive in others.

Differential diagnoses for renal amyloidosis include other causes of protein-losing nephropathy, such as glomerulonephritis (membranous, membranoproliferative, or minimal change disease), focal segmental glomerulosclerosis, and diabetic nephropathy. Chronic kidney disease due to other etiologies (e.g., chronic interstitial nephritis, pyelonephritis, neoplasia) should also be considered. In cats, chronic kidney disease is common, and amyloidosis must be differentiated from other causes of proteinuria. Hepatic amyloidosis should be differentiated from other causes of hepatomegaly and jaundice, such as hepatic lipidosis, cholangiohepatitis, and neoplasia. In Shar-Pei dogs, familial amyloidosis may present with fever and joint pain, mimicking immune-mediated polyarthritis or infectious arthritis. Other systemic diseases such as multiple myeloma, which can cause proteinuria and organ dysfunction, should be ruled out. Definitive diagnosis requires histopathology with Congo red staining.

The diagnostic algorithm for amyloidosis begins with a thorough history and physical examination, with particular attention to breed, age, and any signs of chronic inflammation or infection. Initial laboratory tests include a complete blood count, serum biochemistry profile, and urinalysis with urine protein-to-creatinine ratio (UPC). Persistent proteinuria (UPC > 0.5 in dogs, > 0.4 in cats) with inactive urine sediment is a key indicator of glomerular disease. If proteinuria is confirmed, further diagnostic steps include blood pressure measurement, abdominal ultrasonography, and possibly renal biopsy. Ultrasonography may reveal enlarged, hyperechoic kidneys, but these findings are nonspecific. Renal biopsy is the gold standard for diagnosis, with Congo red staining demonstrating apple-green birefringence under polarized light. In cases where renal biopsy is contraindicated (e.g., coagulopathy), biopsy of other affected organs (liver, spleen, gingiva) may be considered. Genetic testing is available for some breeds (e.g., Shar-Pei) to identify carriers. Additional tests to identify underlying inflammatory or infectious diseases are essential, including serology for leishmaniasis, ehrlichiosis, and FIP.

Laboratory findings in amyloidosis reflect the underlying organ dysfunction. On complete blood count, there may be mild non-regenerative anemia due to chronic disease or renal failure. Leukocytosis may be present if there is an underlying infection or inflammation. Serum biochemistry often reveals hypoalbuminemia due to proteinuria, and hyperglobulinemia may be present in chronic inflammatory conditions. Blood urea nitrogen (BUN) and creatinine are elevated in advanced renal failure. Hypercholesterolemia is common in nephrotic syndrome. Electrolyte abnormalities may include hyperphosphatemia and metabolic acidosis in CKD. Urinalysis typically shows proteinuria (UPC > 1.0 in severe cases), with inactive sediment (few cells, no casts). The urine specific gravity may be isosthenuric (1.008-1.012) in advanced CKD. In hepatic amyloidosis, liver enzyme activities (ALT, ALP) may be elevated, and bilirubin may be increased. Serum amyloid A (SAA) levels may be elevated in reactive amyloidosis, but this is not a routine test. Genetic testing for specific mutations is available for Shar-Pei dogs and Abyssinian cats.

Imaging findings in amyloidosis are nonspecific but can support the diagnosis. Abdominal radiography may show renomegaly or hepatomegaly, but these are not consistent. Ultrasonography is more useful: kidneys may appear enlarged with increased cortical echogenicity, and loss of corticomedullary distinction may be seen. In chronic cases, kidneys may be small and irregular. The liver may be enlarged and hyperechoic in hepatic amyloidosis. Doppler ultrasonography can assess renal blood flow, which may be reduced in advanced disease. Computed tomography (CT) and magnetic resonance imaging (MRI) are rarely used for diagnosis but may be helpful in detecting organomegaly or masses. In cases of suspected pulmonary amyloidosis, thoracic radiography may reveal interstitial or alveolar patterns. Echocardiography may be indicated if cardiac amyloidosis is suspected, though this is rare in animals. Imaging is primarily used to rule out other causes of organomegaly or to guide biopsy.

Cytology and histopathology are essential for definitive diagnosis. Fine needle aspiration (FNA) of the liver, spleen, or kidney may yield amyloid deposits, but sensitivity is low. On cytology, amyloid appears as amorphous, eosinophilic material that may be mistaken for other debris. Histopathology is the gold standard: tissue biopsy (e.g., renal biopsy) stained with hematoxylin and eosin (H&E) shows homogeneous, eosinophilic, extracellular deposits. Congo red staining is confirmatory, demonstrating apple-green birefringence under polarized light. In the kidney, amyloid deposits are typically seen in the glomerular mesangium and along the basement membrane, but can also be found in the interstitium and medullary interstitium. In the liver, deposits are in the space of Disse and along sinusoids. In the spleen, deposits may be in the red pulp or white pulp. Electron microscopy can reveal characteristic non-branching fibrils of 7-10 nm diameter. Immunohistochemistry can identify the specific precursor protein (e.g., SAA, light chains).

Treatment of amyloidosis is challenging and often focuses on managing the underlying inflammatory or infectious disease and providing supportive care. In reactive amyloidosis, aggressive treatment of the underlying condition (e.g., leishmaniasis, ehrlichiosis) may slow or halt amyloid deposition. For familial amyloidosis, there is no specific cure. Supportive care includes management of proteinuria and chronic kidney disease. Angiotensin-converting enzyme inhibitors (ACE inhibitors) such as enalapril (0.5 mg/kg PO q12h) or benazepril (0.25-0.5 mg/kg PO q24h) are used to reduce proteinuria and slow progression of renal disease. Angiotensin receptor blockers (ARBs) like telmisartan (1 mg/kg PO q24h) may be used if ACE inhibitors are not tolerated. Dietary management with a renal diet (low protein, low phosphorus, omega-3 fatty acids) is recommended. In nephrotic syndrome, diuretics such as furosemide (1-2 mg/kg IV/PO q8-12h) may be needed for edema, but careful monitoring is required. In cases of end-stage renal failure, dialysis or renal transplantation may be considered, but these are rarely available in veterinary practice. Colchicine (0.03 mg/kg PO q24h) has been used in some cases to reduce amyloid deposition, but its efficacy is unproven. DMSO (dimethyl sulfoxide) has been used experimentally, but evidence is limited.

The prognosis for amyloidosis is generally poor to guarded, depending on the underlying cause and extent of organ involvement. In dogs with familial amyloidosis, the disease is often progressive and fatal within 6-12 months of diagnosis. In cats with hepatic amyloidosis, the prognosis is also poor, with many cats dying from hepatic rupture or liver failure. In reactive amyloidosis, if the underlying condition can be effectively treated, the prognosis may be better, but renal damage is often irreversible. The degree of proteinuria and the presence of azotemia at diagnosis are negative prognostic indicators. Animals with nephrotic syndrome have a worse prognosis due to the risk of thromboembolism and infections. Early diagnosis and aggressive management of underlying inflammation may slow progression, but most animals eventually develop end-stage renal disease. Median survival times reported in the literature range from a few months to 1-2 years, with some animals living longer if the underlying disease is controlled.

Follow-up for animals with amyloidosis should be structured to monitor disease progression and manage complications. Re-evaluation should be performed every 1-3 months initially, then every 3-6 months if stable. At each visit, a complete physical examination, body weight, blood pressure measurement, and serum biochemistry profile (including BUN, creatinine, albumin, cholesterol) should be performed. Urinalysis and UPC ratio should be repeated to monitor proteinuria. If the animal is on ACE inhibitors or ARBs, renal function and electrolytes should be monitored closely, especially after dose adjustments. In animals with underlying infections, serology or PCR should be repeated to ensure resolution. Imaging (ultrasonography) may be repeated every 6-12 months to assess organ size and architecture. Owners should be educated on signs of disease progression, such as increased thirst, lethargy, vomiting, or edema. In cases of nephrotic syndrome, monitoring for thromboembolic events is important. Adjustments to diet and medications should be made based on laboratory results.

Pearls: 1) In any young Shar-Pei or Abyssinian cat with proteinuria, amyloidosis should be high on the differential list. 2) A UPC ratio > 2.0 with inactive sediment is highly suggestive of glomerular disease, and amyloidosis should be considered. 3) Renal biopsy is the gold standard, but if contraindicated, biopsy of the liver or spleen may be diagnostic. 4) Treating the underlying inflammatory condition is crucial in reactive amyloidosis; resolution of the trigger may halt further deposition. 5) ACE inhibitors are the mainstay of therapy for proteinuria; consider adding an ARB if proteinuria persists. Pitfalls: 1) Do not assume that proteinuria is due to chronic kidney disease without ruling out amyloidosis, especially in predisposed breeds. 2) Avoid the use of corticosteroids in amyloidosis, as they may exacerbate proteinuria and are not effective. 3) Do not delay biopsy; early diagnosis may allow for more effective management. 4) Be cautious with diuretics in nephrotic syndrome, as they can worsen renal function. 5) Do not overlook the possibility of hepatic amyloidosis in cats with hepatomegaly and jaundice; a liver biopsy may be needed.

Drug protocols for amyloidosis focus on managing proteinuria and underlying inflammation. ACE inhibitors: Enalapril (0.5 mg/kg PO q12h) or benazepril (0.25-0.5 mg/kg PO q24h) are first-line for proteinuria. Angiotensin receptor blockers: Telmisartan (1 mg/kg PO q24h) can be used alone or in combination with ACE inhibitors. For nephrotic syndrome, furosemide (1-2 mg/kg IV/PO q8-12h) may be used, but monitor renal function and electrolytes. If hypertension is present, amlodipine (0.1-0.2 mg/kg PO q24h) may be added. For underlying infections, specific antimicrobials are indicated (e.g., allopurinol and meglumine antimoniate for leishmaniasis, doxycycline for ehrlichiosis). Colchicine (0.03 mg/kg PO q24h) has been used to reduce amyloid deposition, but its efficacy is unproven and it may cause gastrointestinal upset. DMSO (dimethyl sulfoxide) at 80-100 mg/kg PO or SC q24h has been used experimentally, but evidence is limited. Supportive care includes antiemetics (e.g., maropitant 1 mg/kg SC q24h) for uremic vomiting, and phosphate binders (e.g., aluminum hydroxide 30-100 mg/kg/day PO divided with meals) for hyperphosphatemia. All dosages should be adjusted based on renal function and individual patient response.

Evidence for the management of amyloidosis in veterinary medicine is limited to case series and retrospective studies. A study by DiBartola et al. (1985) described the clinical and pathological features of renal amyloidosis in dogs, highlighting the breed predisposition in Shar-Peis. Another study by Boyce et al. (1984) reported on familial amyloidosis in Abyssinian cats. More recent studies have evaluated the use of ACE inhibitors in proteinuric renal disease, extrapolating from human medicine. The IRIS (International Renal Interest Society) guidelines recommend ACE inhibitors for proteinuria in dogs and cats. A study by Grauer et al. (2000) showed that enalapril reduced proteinuria in dogs with glomerular disease. There is no consensus on the use of colchicine or DMSO, and these are considered experimental. The ACVIM consensus statement on proteinuria (Lees et al., 2005) provides guidelines for diagnosis and management. Overall, the evidence base is weak, and treatment is largely supportive and based on extrapolation from human medicine.