Glomerulonephritis

Glomerulonephritis (GN) is an inflammatory disorder of the renal glomeruli, characterized by immune-mediated injury to the glomerular capillary wall, leading to proteinuria, hematuria, and progressive renal dysfunction. It is a common cause of chronic kidney disease (CKD) in dogs and cats, and can be primary (idiopathic) or secondary to various infectious, neoplastic, or autoimmune conditions. The disease encompasses a spectrum of histopathologic patterns, including membranous, membranoproliferative, and crescentic forms, each with distinct clinical and prognostic implications. Glomerular injury results in increased permeability to proteins, leading to proteinuria, hypoalbuminemia, and often the nephrotic syndrome (proteinuria, hypoalbuminemia, hypercholesterolemia, and edema/ascites). Progression to glomerulosclerosis and tubulointerstitial fibrosis ultimately results in end-stage renal disease.

The etiology of glomerulonephritis is diverse and often involves immune complex deposition or in situ immune complex formation. Common underlying causes include: - Chronic infections: bacterial (e.g., Brucella canis, Borrelia burgdorferi, Ehrlichia canis, Leishmania infantum, Dirofilaria immitis), viral (e.g., feline leukemia virus, feline immunodeficiency virus, canine adenovirus-1, canine parvovirus), fungal (e.g., systemic mycoses), and protozoal (e.g., Babesia, Toxoplasma). - Neoplastic diseases: various tumors, especially those with chronic antigenic stimulation (e.g., lymphoma, mast cell tumor, mammary carcinoma). - Autoimmune diseases: systemic lupus erythematosus (SLE), rheumatoid arthritis, and other immune-mediated disorders. - Drug-induced: certain drugs (e.g., penicillamine, sulfonamides, NSAIDs) can trigger immune complex formation. - Genetic/familial: some breeds have hereditary nephritis (e.g., Samoyed, Bull Terrier, English Cocker Spaniel) with collagen IV mutations. - Idiopathic: many cases have no identifiable underlying cause. The pathogenesis involves deposition of circulating immune complexes (antigen-antibody) in the glomerular basement membrane (GBM) or mesangium, or in situ formation of immune complexes with intrinsic glomerular antigens. This triggers complement activation, recruitment of inflammatory cells, and release of cytokines and growth factors, leading to glomerular injury and proliferation.

Glomerulonephritis is a significant cause of CKD in dogs and cats. It is more commonly diagnosed in dogs than cats. In dogs, the prevalence of proteinuric CKD due to GN is estimated at 0.5-1% of the general population, but it may be higher in certain breeds. Breeds predisposed to immune-mediated GN include the Bernese Mountain Dog, Golden Retriever, and Cocker Spaniel. Hereditary nephritis is seen in Samoyeds, Bull Terriers, and English Cocker Spaniels. In cats, GN is less common but can be associated with feline infectious peritonitis (FIP) or chronic viral infections. There is no strong age or sex predilection, but GN is more common in middle-aged to older animals. Geographic distribution reflects endemic infectious diseases (e.g., heartworm in endemic areas, leishmaniasis in Mediterranean regions).

The pathophysiology of glomerulonephritis involves immune-mediated injury to the glomerulus. Circulating immune complexes (CICs) deposit in the glomerular mesangium or subendothelial space, or antibodies bind to intrinsic glomerular antigens (e.g., GBM antigens, podocyte antigens). This triggers activation of the complement cascade (classical or alternative pathway), leading to the formation of membrane attack complex (C5b-9) and recruitment of neutrophils, macrophages, and platelets. These cells release proteases, reactive oxygen species, and pro-inflammatory cytokines (e.g., TNF-α, IL-1, IL-6), causing damage to the glomerular basement membrane and podocytes. Podocyte injury leads to foot process effacement and proteinuria. In proliferative forms, mesangial and endothelial cells proliferate, and crescents may form in Bowman's space. Chronic injury results in glomerulosclerosis, tubular atrophy, and interstitial fibrosis, leading to progressive decline in glomerular filtration rate (GFR) and renal function. Proteinuria itself is nephrotoxic, causing tubular injury and interstitial inflammation, further accelerating progression.

Predisposing factors for glomerulonephritis include: - Chronic antigenic stimulation: persistent infections (e.g., heartworm, ehrlichiosis, leishmaniasis), neoplasia, or autoimmune diseases. - Genetic predisposition: certain breeds have a higher risk of immune-mediated GN or hereditary nephritis. - Age: middle-aged to older animals are more commonly affected. - Immunosuppression: may predispose to chronic infections that trigger GN. - Environmental factors: exposure to endemic infectious agents. - Drug exposure: certain drugs can induce immune complex formation. - Diet: high protein diets may exacerbate proteinuria and progression. - Concurrent diseases: diabetes mellitus, hypertension, and hyperadrenocorticism can worsen glomerular injury.

Clinical signs of glomerulonephritis vary depending on the severity and stage of disease. Early stages may be asymptomatic, with only proteinuria detected on routine urinalysis. As disease progresses, signs include: - Polyuria and polydipsia (PU/PD) due to impaired renal concentrating ability. - Weight loss, muscle wasting, and lethargy. - Anorexia and vomiting in advanced uremia. - Edema or ascites due to hypoalbuminemia (nephrotic syndrome). - Hypertension (may be present, leading to retinal detachment or neurologic signs). - Thromboembolism (pulmonary or renal) due to hypercoagulability. - In acute severe cases, oliguria or anuria may occur. Physical examination may reveal pale mucous membranes, weak pulses, renomegaly (if chronic), or ascites. In cats, cervical ventroflexion may be seen due to hypokalemia.

Differential diagnoses for glomerulonephritis include: - Amyloidosis: deposition of amyloid in glomeruli; distinguished by Congo red staining on biopsy and presence of underlying chronic inflammation. - Chronic interstitial nephritis: primarily tubulointerstitial disease; proteinuria is usually mild, and histopathology shows interstitial fibrosis. - Acute kidney injury (AKI): sudden onset, often due to toxins or ischemia; may have similar laboratory findings but histopathology shows tubular necrosis. - Nephrotic syndrome due to other causes: e.g., focal segmental glomerulosclerosis, minimal change disease. - Urinary tract infection: can cause proteinuria and hematuria; culture and absence of glomerular histopathology help differentiate. - Diabetes mellitus with nephropathy: glucosuria and hyperglycemia are present. - Hypertension-induced renal damage: usually secondary to GN or other renal disease. - Neoplasia (e.g., renal lymphoma): imaging and cytology/histopathology. - Infectious diseases (e.g., leptospirosis): serology and PCR. - Toxin exposure (e.g., ethylene glycol): history and calcium oxalate crystalluria.

The diagnostic algorithm for glomerulonephritis involves: 1. History and physical examination: identify signs of renal disease, edema, or underlying systemic disease. 2. Baseline laboratory tests: CBC, serum biochemistry, urinalysis with sediment, and urine protein-to-creatinine ratio (UPC). 3. Confirm persistent proteinuria: UPC > 0.5 in dogs and > 0.4 in cats on at least two occasions 2 weeks apart, after ruling out post-renal proteinuria (urinary tract infection, hemorrhage). 4. Assess renal function: serum creatinine, SDMA, and symmetric dimethylarginine (SDMA) for early detection. 5. Blood pressure measurement: rule out hypertension. 6. Screen for underlying infectious, neoplastic, or autoimmune diseases: serology (e.g., heartworm, Ehrlichia, Lyme, Leishmania), PCR, imaging (thoracic radiographs, abdominal ultrasound), and antinuclear antibody (ANA) testing. 7. Renal biopsy: indicated if proteinuria is severe (UPC > 2.0) or if there is rapid progression, to confirm diagnosis and guide therapy. Biopsy should be performed by an experienced clinician with ultrasound guidance and after ruling out coagulopathy. 8. Histopathology: light microscopy, immunofluorescence, and electron microscopy to classify the glomerular lesion.

Laboratory findings in glomerulonephritis include: - Hematology: may be normal; in chronic disease, non-regenerative anemia may be present. Leukocytosis may occur if infection is present. - Serum biochemistry: hypoalbuminemia (albumin < 2.0 g/dL), hypercholesterolemia, elevated BUN and creatinine (if renal failure), hyperphosphatemia, and metabolic acidosis. SDMA may be elevated earlier than creatinine. - Urinalysis: proteinuria (3+ or 4+ on dipstick), hematuria, and casts (granular or cellular). Urine specific gravity may be isosthenuric (1.008-1.012) in advanced disease. - UPC ratio: > 0.5 in dogs and > 0.4 in cats indicates significant proteinuria; > 2.0 is severe. - Blood gas analysis: metabolic acidosis with decreased bicarbonate. - Biomarkers: NT-proBNP may be elevated in cardiac disease, but not specific. Troponin I may be elevated if myocardial injury. SDMA is a renal function marker. - Serology/PCR: for infectious agents (e.g., Ehrlichia, Lyme, Leishmania, heartworm). - Coagulation profile: may show hypercoagulability (increased D-dimers, antithrombin III deficiency).

Imaging findings in glomerulonephritis are often non-specific but may include: - Radiography: abdominal radiographs may show normal or slightly enlarged kidneys. Thoracic radiographs may reveal evidence of thromboembolism (e.g., pulmonary infiltrates) or underlying neoplasia. - Ultrasonography: renal ultrasound may show increased cortical echogenicity, loss of corticomedullary distinction, and reduced renal size in chronic disease. Doppler ultrasound can assess renal blood flow. - Computed Tomography (CT): may be used to evaluate renal architecture and detect masses or infarcts. - Magnetic Resonance Imaging (MRI): not commonly used for renal evaluation but may be helpful in detecting thromboembolism. - Endoscopy: not directly useful for renal disease. - Fluoroscopy: not typically used. - Echocardiography: may be indicated if thromboembolism is suspected or to evaluate for cardiac disease.

Cytology and histopathology are essential for definitive diagnosis: - Fine needle aspirate (FNA) of the kidney: may show inflammatory cells but is not diagnostic for GN. - Histopathology (renal biopsy): light microscopy reveals glomerular hypercellularity, thickening of the glomerular basement membrane, mesangial proliferation, and in severe cases, crescents. Special stains (e.g., Masson's trichrome, Jones methenamine silver) highlight basement membrane changes. Immunofluorescence demonstrates immune complex deposition (IgG, IgM, C3) in a granular pattern. Electron microscopy shows subepithelial or subendothelial electron-dense deposits and podocyte foot process effacement. Histopathologic classification (e.g., membranous, membranoproliferative) guides prognosis and therapy.

Treatment of glomerulonephritis is multifaceted and includes: - Identify and treat underlying cause: e.g., antibiotics for bacterial infections, heartworm treatment, chemotherapy for neoplasia, immunosuppressive therapy for autoimmune diseases. - Reduce proteinuria: use of 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). Angiotensin receptor blockers (ARBs) like telmisartan (1 mg/kg PO q24h) may be used if ACE inhibitors are not tolerated. - Dietary management: renal diets with restricted protein, phosphorus, and sodium, and supplemented with omega-3 fatty acids. - Antithrombotic therapy: low-dose aspirin (0.5-1 mg/kg PO q24h) or clopidogrel (1-2 mg/kg PO q24h) to prevent thromboembolism. - Immunosuppressive therapy: for immune-mediated GN, prednisone (1-2 mg/kg PO q24h) with or without cyclophosphamide (50 mg/m² PO q48h) or mycophenolate mofetil (10 mg/kg PO q12h). - Management of hypertension: amlodipine (0.1-0.2 mg/kg PO q24h) or ACE inhibitors. - Management of uremia: fluid therapy, antiemetics (e.g., maropitant 1 mg/kg SC q24h), phosphate binders (e.g., aluminum hydroxide 30-100 mg/kg/day PO divided), and erythropoietin for anemia. - Supportive care: maintain hydration, correct electrolyte imbalances, and provide nutritional support.

The prognosis for glomerulonephritis varies depending on the underlying cause, severity of proteinuria, and response to therapy. Dogs with mild proteinuria and no azotemia may have a good prognosis with appropriate treatment. However, if the disease progresses to renal failure, the prognosis is guarded to poor. Negative prognostic indicators include severe proteinuria (UPC > 3.5), hypoalbuminemia (< 1.5 g/dL), azotemia at diagnosis, hypertension, and thromboembolism. Median survival times in dogs with proteinuric CKD are reported to be 200-400 days, but with aggressive therapy, some dogs may live longer. Cats with GN generally have a poorer prognosis, especially if associated with FIP.

Follow-up monitoring for glomerulonephritis includes: - Recheck UPC, serum biochemistry, and blood pressure every 1-3 months initially, then every 3-6 months if stable. - Monitor renal function (creatinine, SDMA) regularly. - Adjust ACE inhibitor or ARB dosage based on UPC and blood pressure. - Monitor for adverse effects of immunosuppressive therapy (CBC, liver enzymes). - Repeat imaging (ultrasound) if underlying neoplasia is suspected. - Educate owners on signs of thromboembolism (e.g., acute dyspnea, hindlimb paresis) and seek immediate veterinary care. - Long-term management includes dietary therapy and regular veterinary visits.

Pearls: - Always rule out post-renal proteinuria (e.g., urinary tract infection) before diagnosing glomerular disease. - UPC is essential for monitoring proteinuria; use it to guide therapy. - ACE inhibitors are the first-line treatment for proteinuria; titrate to effect. - Consider renal biopsy early in cases with severe proteinuria or rapid progression. - Screen for underlying infectious diseases, especially in endemic areas. Pitfalls: - Do not use corticosteroids without a definitive diagnosis of immune-mediated GN, as they may worsen proteinuria. - Avoid NSAIDs in patients with renal disease. - Do not overlook hypertension; it can cause further renal damage. - Be cautious with fluid therapy in oliguric patients to avoid volume overload. - Do not discontinue ACE inhibitors abruptly; taper if needed.

Based on Plumb's Veterinary Drug Handbook, the following drug protocols are recommended: - Enalapril: 0.5 mg/kg PO q12h (range 0.25-1 mg/kg). - Benazepril: 0.25-0.5 mg/kg PO q24h. - Telmisartan: 1 mg/kg PO q24h. - Amlodipine: 0.1-0.2 mg/kg PO q24h (max 0.5 mg/kg). - Prednisone: 1-2 mg/kg PO q24h, then taper over 3-6 months. - Cyclophosphamide: 50 mg/m² PO q48h (or 200 mg/m² IV q3 weeks). - Mycophenolate mofetil: 10 mg/kg PO q12h. - Clopidogrel: 1-2 mg/kg PO q24h. - Aspirin: 0.5-1 mg/kg PO q24h (low dose). - Maropitant: 1 mg/kg SC q24h (for nausea). - Aluminum hydroxide: 30-100 mg/kg/day PO divided with meals. - Erythropoietin: 100 U/kg SC three times weekly (if anemic). - Fluid therapy: balanced crystalloids (e.g., Lactated Ringer's) at maintenance (60-80 ml/kg/day) or to correct dehydration. - Dietary: renal prescription diet (e.g., Hill's k/d, Royal Canin Renal) with restricted protein (2.0-2.5 g/kg/day), phosphorus (0.5-1.0 g/100 kcal), and sodium, and supplemented with omega-3 fatty acids (eicosapentaenoic acid 40-80 mg/kg/day). Dosages should be adjusted for renal impairment (e.g., reduce ACE inhibitor dose if creatinine increases >30%). Contraindications: ACE inhibitors are contraindicated in pregnancy and with hyperkalemia. Drug interactions: ACE inhibitors with potassium-sparing diuretics may cause hyperkalemia.

Key evidence-based literature includes: - IRIS (International Renal Interest Society) guidelines for diagnosis and management of proteinuria and CKD. - ACVIM consensus statements on proteinuria and glomerular disease. - Studies by Vaden et al. (2005) on the use of ACE inhibitors in dogs with proteinuria, showing reduction in UPC and slower progression of CKD. - A study by Brown et al. (2013) demonstrated that telmisartan is as effective as benazepril in reducing proteinuria in dogs. - Research on immunosuppressive therapy in immune-mediated GN is limited; a study by Littman et al. (2005) showed benefit of mycophenolate in some cases. - Meta-analyses of dietary therapy in CKD show that renal diets slow progression and improve quality of life. - Consensus recommendations emphasize the importance of early detection of proteinuria and aggressive management to preserve renal function.