Acute Glomerulonephritis in Cats: Symptoms, Causes & Treatment

Introduction

Acute glomerulonephritis is a serious and often sudden kidney emergency that demands fast veterinary attention. When treated promptly, however, some cats can return to a normal life.

Acute glomerulonephritis is a sudden, immune-mediated inflammation of the kidney's microscopic filtering units, called glomeruli. It is triggered when antigen-antibody complexes (clusters of foreign invaders bound to the body's defense proteins) become lodged in the glomerular walls ^[1]. These trapped immune complexes activate the body's inflammatory response, damaging the delicate glomerular filtration barrier and allowing protein, and sometimes blood, to leak into the urine ^[1].

Unlike the chronic form of the disease, which progresses slowly over months or years, the acute form strikes abruptly. It frequently results in acute kidney injury (AKI), a sudden and dangerous decline in kidney function that requires urgent care ^{[2, 3]}. Many cases are idiopathic, meaning no underlying trigger can be identified, but infections, inflammatory diseases, or cancer can also be the root cause ^{[1, 4]}.

This guide walks you through everything cat owners need to know: the symptoms, causes, classification, diagnosis, treatment, dietary management, complications, prognosis, quality-of-life considerations, and long-term care for a cat with acute glomerulonephritis.

Symptoms and First Signs to Watch For

Sudden swelling of the paws, face, or abdomen, or any acute loss of vision, is a red-flag emergency that warrants immediate veterinary care. Because symptoms can appear and worsen within hours, knowing what to look for is critical.

Some cats show no outward signs at all, and the only early clue is an elevated amount of protein detected on a routine urinalysis (a lab test of the urine) ^[4]. When symptoms do appear, they can include sudden swelling (edema) of the paws, face, hocks (ankles), or scrotum, or a fluid-filled abdomen called ascites ^[1].

Respiratory difficulty can arise when fluid accumulates in the chest cavity (pleural effusion) or when a blood clot becomes lodged in the arteries of the lungs (pulmonary thromboembolism) ^{[4, 1]}. Another alarming sign is sudden blindness, which can be the first recognized symptom of severe systemic hypertension (dangerously high blood pressure), causing retinal detachment (separation of the light-sensing layer at the back of the eye) or bleeding inside the eye ^[1].

Non-specific signs often include:

• Lethargy and reduced activity
• Poor appetite and weight loss
• Intermittent vomiting
• Increased thirst (polydipsia) and urination (polyuria) ^{[1, 4]}

A true emergency is the "saddle thrombus," a blood clot that blocks the main artery supplying the hind legs, causing sudden paralysis of the rear limbs. A clot to the lungs causes acute, severe respiratory distress ^{[1, 4]}. Because acute glomerulonephritis can deteriorate rapidly, recognizing that sudden difficulty breathing, hind limb weakness, or acute loss of vision may signal thromboembolism (a traveling blood clot) or a hypertensive emergency, and seeking immediate veterinary care, can be life-saving ^[1].

Causes and Risk Factors

Although several infections and immune diseases can trigger acute glomerulonephritis, in many affected cats no underlying cause can be identified. Despite this, treating any underlying disease is still a top priority.

Immune complex formation occurs when the body produces antibodies against persistent antigens (foreign substances that trigger an immune response) circulating in the bloodstream. These antigen-antibody complexes become lodged in the glomeruli, sparking inflammation ^[1]. Common triggers include:

• Viral infections: Feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), and feline infectious peritonitis (FIP) are well-documented triggers ^{[1, 4]}.
• Bacterial infections: Pyometra (uterine infection), endocarditis (heart valve infection), chronic skin infections, gingivitis, and tick-borne diseases like ehrlichiosis and Lyme disease ^{[1, 4]}.
• Immune-mediated disorders: Systemic lupus erythematosus (SLE) or immune-mediated polyarthritis (joint inflammation triggered by the immune system) are less common but possible ^{[4, 1]}.
• Cancers: Lymphosarcoma and mast cell tumors are known triggers ^[1].

In many cats with glomerulonephritis, no underlying cause can be identified and the condition is considered idiopathic ^[4]. However, immunofluorescence studies show that immune-complex glomerulonephritis (ICGN) is identified in 58% (31/53) of proteinuric cats and in 74% (31/42) of cats with protein-losing nephropathy (PLN), indicating that most cases demonstrate immune-complex deposition even when the triggering antigen cannot be identified ^[5]. Young adult male cats are significantly over-represented, with a median (middle) age at diagnosis of approximately 3–4 years ^[5]. Any breed can be affected, but unlike dogs, where familial (inherited) glomerulopathies are well documented in several breeds, no inherited glomerulonephritis syndromes have been recognized in cats ^[2]. Any chronic condition that persistently stimulates the immune system has the potential to cause the disease ^[4].

How Acute Glomerulonephritis Is Classified

Veterinarians classify acute glomerulonephritis in three complementary ways: by cause, by histopathology (the pattern of damage seen under the microscope), and by clinical course. Each lens helps guide treatment and predict outcomes.

By cause, the condition is either primary (idiopathic) when no predisposing disease can be found, or secondary when triggered by an underlying infection, inflammatory condition, or tumor ^[1].

By histopathology (after a renal biopsy, a tissue sample taken from the kidney), the most common pattern in cats is membranous glomerulonephritis (MGN). MGN is characterized by immune complex deposits on the outer (subepithelial) side of the glomerular basement membrane, the thin filtering layer of the kidney ^[5]. The second most common pattern is membranoproliferative glomerulonephritis (MPGN), where deposits sit beneath the endothelial cells (the cells lining the inside of the tiny kidney blood vessels) and mesangial cells (specialized support cells in the kidney's filtering tufts) multiply ^[5].

Other histologic forms include proliferative patterns with mesangial or endocapillary proliferation (excess cell growth inside the filtering vessels), crescentic glomerulonephritis (a severe form in which scar-forming cells fill the glomerular space), and sclerotic forms like focal segmental glomerulosclerosis (scarring of parts of some glomeruli).

By clinical course, the disease can be acute (abrupt onset with AKI), acute-on-chronic (an acute exacerbation in a cat with pre-existing chronic kidney disease), or it can progress to chronic kidney disease with persistent proteinuria (excess protein in the urine) ^{[3, 1]}.

How Vets Diagnose This Condition

Diagnosis often begins with a simple urine test, but reaching a definitive answer usually requires more advanced workup, sometimes including a kidney biopsy. Because affected cats can deteriorate quickly, prompt diagnosis is essential.

The first clue is often significant proteinuria detected on a routine urinalysis. A urine protein:creatinine ratio (UPC), a test that compares the amount of protein to creatinine (a normal waste product) in the urine, is then measured to quantify the protein loss and gauge its severity ^[1]. Bloodwork evaluates kidney function (creatinine, blood urea nitrogen [BUN], and SDMA, a newer marker of kidney function) and frequently reveals azotemia (abnormally high levels of nitrogen waste products in the blood due to failing kidneys), hypoalbuminemia (low blood albumin, a key protein), hypercholesterolemia (high cholesterol), and non-regenerative anemia (low red blood cell count that the body cannot easily replace) ^{[1, 6]}.

Blood pressure measurement is recommended for all cats with glomerulonephritis to identify systemic hypertension, following a standardized protocol to ensure accuracy ^{[3, 4]}. An abdominal ultrasound allows visualization of kidney size, shape, and internal architecture, and helps rule out obstructions, kidney infections (pyelonephritis), or tumors ^[6].

Definitive diagnosis requires a renal biopsy performed under ultrasound guidance. The tissue is submitted for light microscopy (examining thin slices of tissue under a microscope), special stains (PAS, Jones methenamine silver), immunofluorescence for IgG, IgM, IgA and lambda light chain (lab techniques that highlight immune proteins), and transmission electron microscopy (an ultra-high-magnification imaging technique) to confirm immune complex deposition and classify the subtype ^{[5, 1]}. A biopsy is the only reliable method to distinguish glomerular proteinuria from tubular proteinuria and to tell immune-complex glomerulonephritis apart from other glomerulopathies such as amyloidosis (a disease in which abnormal protein deposits clog the kidneys) ^[5]. Antithrombin III activity may be assessed to gauge the risk of blood clots, since antithrombin III is a natural blood-clotting inhibitor that is lost through damaged kidneys ^[1]. In practice, a high clinical suspicion for immune-complex glomerulonephritis arises in young, male cats with markedly elevated proteinuria and compatible acute signs ^[5].

Treatment Options for Acute Glomerulonephritis

Treatment success depends on speed. The first priority is to identify and eliminate any underlying trigger, while supporting the kidneys and reducing protein loss through medication and diet.

Eliminating the trigger is the first priority: antibiotics for bacterial pyometra, appropriate management of FeLV or FIV, and surgical removal of tumors when possible ^[1]. When no treatable cause is found or the trigger cannot be removed, immunosuppressive therapy is indicated for immune-complex glomerulonephritis, ideally after biopsy confirmation, or when strongly suspected ^{[5, 2]}.

Common immunosuppressive protocols include:

• Prednisolone (a steroid that calms the immune system)
• Chlorambucil (a chemotherapy drug that suppresses immune cells)
• Ciclosporin (cyclosporine) (a drug that blocks specific immune cell activation)
• Mycophenolate (a drug that dampens immune cell proliferation)

In a small retrospective study (15 cats with survival data, 9 receiving immunosuppression and 6 not), cats receiving immunosuppressive medications had a median survival time of 244 days compared with 17 days for untreated cats. However, this difference should be interpreted with caution because of the very small sample size and likely selection bias — untreated cats may have been sicker or had more advanced disease. The original authors concluded that further studies are needed to determine the true effect of immunosuppression on outcomes in cats with ICGN ^[5]. Angiotensin-converting enzyme (ACE) inhibitors such as benazepril or enalapril are prescribed to reduce proteinuria and control hypertension, regardless of whether immunosuppressive drugs are used ^{[4, 5]}. Low-dose aspirin has been recommended by some veterinarians to reduce glomerular inflammation and clot formation in cats, but no proven benefit on preventing thromboembolism in cats with glomerulonephritis has been established, and its use in cats should only be considered under direct veterinary supervision. Owners should never administer aspirin (or any medication) without explicit dosing instructions from their prescribing veterinarian ^[1].

Cats presenting with acute kidney injury require hospitalization, intravenous fluid therapy, and close monitoring of urine output, electrolytes (minerals like sodium and potassium essential for body function), and body weight ^{[3, 5]}. Diuretics such as furosemide are used cautiously to manage life-threatening ascites or pulmonary edema, while carefully avoiding dehydration ^[1]. If hypertension persists despite ACE inhibition, additional antihypertensives like amlodipine are added ^[5]. Empirical antibiotic therapy (giving antibiotics before test results confirm an infection) may be used if a concurrent infection is suspected, but the regimen should be revised to target identified pathogens as soon as culture and sensitivity results are available ^[5].

Diet and Nutrition: What to Feed (and What to Avoid)

The right diet can meaningfully reduce protein loss and support kidney function. Sticking to a prescription renal diet is one of the most important things you can do at home.

A moderately protein-restricted therapeutic renal diet is recommended once the cat develops azotemia (IRIS stage 2 or later) ^{[1, 2]}. In non-azotemic cats with glomerular proteinuria, the mainstays of management are angiotensin-converting enzyme (ACE) inhibitors or angiotensin-receptor blockers (e.g., telmisartan); premature protein restriction risks muscle wasting without proven benefit. Dietary sodium restriction, however, is appropriate regardless of azotemia status to help control hypertension ^{[1, 2]}. Supplementation with omega-3 fatty acids, commonly given as fish oil, is advised to dampen glomerular inflammation and potentially lower proteinuria ^{[1, 2]}.

What this means for your cat: Choose the prescription renal diet your veterinarian recommends once your cat develops azotemia, keep portions measured, and resist the urge to supplement with extra protein. Because dietary sodium restriction is recommended for cats with glomerulonephritis regardless of azotemia status, salty snacks and table scraps should be avoided to help control blood pressure and prevent undermining the prescribed diet ^[1].

Providing a palatable, calorie-dense diet is crucial to prevent weight loss and muscle wasting. Appetite stimulants or temporary feeding tubes may be necessary for cats with reduced food intake. Transition to the prescribed renal diet should be gradual to ensure acceptance, and offering small, frequent meals can help maintain intake in cats with reduced appetite.

Complications and Long-Term Risks

The most dangerous complications are blood clots, hypertensive crises, and progression to chronic kidney failure. Knowing what to watch for at home can mean the difference between life and death.

• Thromboembolism: Cats can form blood clots that travel to the lungs (pulmonary embolism), causing acute respiratory distress, or lodge at the aortic bifurcation (the point where the main artery splits toward the hind legs), creating a "saddle thrombus" that results in sudden hind limb paralysis. Both are life-threatening emergencies ^{[1, 4]}.
• Systemic hypertension: May lead to retinal detachment, intraocular hemorrhage (bleeding inside the eye), and acute blindness. If undetected, it can also damage the brain, heart, and kidneys ^[1].
• Nephrotic syndrome: A constellation of signs caused by massive protein loss, including severe hypoalbuminemia, widespread edema (limbs, face), ascites, and pleural effusion (fluid around the lungs) that compromises breathing ^[4].
• Progressive chronic kidney disease (CKD): Ongoing inflammation and scarring cause irreversible loss of functional filtering units (nephrons), leading to azotemia, uremia (buildup of toxins in the blood normally cleared by the kidneys), and anemia, requiring long-term renal management ^[5].
• Renal scarring: Glomerulosclerosis (scarring of the filtering units) and tubulointerstitial fibrosis (scarring of the kidney's supporting tissue) may develop even if acute inflammation subsides, reducing future kidney reserve ^[5].

Common laboratory abnormalities include non-regenerative anemia, hypercholesterolemia, and hypoalbuminemia, all of which can independently decrease quality of life ^[5].

Prognosis: Can Acute Glomerulonephritis Go Away?

The honest answer is: sometimes yes, sometimes no, but treatment makes a real difference. Survival times range widely depending on how early the disease is caught and how the cat responds to therapy.

The course of acute glomerulonephritis is variable. Some cats with normal kidney function at diagnosis and prompt treatment experience complete resolution of proteinuria and return to normal health ^[1]. A proportion follow a smoldering course with persistent proteinuria but stable kidney function for months to years ^[1]. In progressive cases, renal function declines to end-stage kidney failure; median survival times from diagnosis range from approximately 3 to 14 months, though some cats live over a year with treatment ^[5].

Poorer prognosis is associated with:

• Severe azotemia at presentation (markedly elevated BUN and creatinine)
• Nephrotic syndrome
• Thromboembolic events
• Underlying malignancy

Cats that receive immunosuppressive therapy for confirmed immune-complex glomerulonephritis have significantly longer survival than those that do not ^[5]. Early detection of kidney disease allows implementation of care pathways that can slow disease progression, improving both clinical outlook and quality of life ^[5].

Pain, Quality of Life, and End-of-Life Decisions

Cats are skilled at hiding pain, which makes careful quality-of-life monitoring essential. Open communication with your veterinarian is the best way to navigate these tough decisions.

Cats with acute glomerulonephritis may exhibit lethargy, anorexia, vomiting, and respiratory distress from fluid accumulation. While they may not cry out, pain from thromboembolic events can be severe ^{[1, 4]}. Utilizing a validated feline quality-of-life scale (a checklist that tracks a cat's comfort, appetite, mobility, and happiness over time) that assesses parameters such as appetite, mobility, social interaction, and comfort can help owners and veterinarians objectively monitor well-being.

When medical management no longer keeps a cat comfortable and quality of life is poor, humane euthanasia should be considered ^[6]. Open discussions about realistic goals, financial considerations, and palliative options are essential for informed end-of-life decisions, and veterinarians can assist owners in understanding their cat's quality of life and formulating an end-of-life care plan.

Living With and Managing Your Cat's Kidney Health

Most of the long-term management happens at home. Consistency with medication, diet, and observation is what gives cats the best chance of a stable, comfortable life.

At home, monitor for early warning signs: swelling of the limbs, face, or abdomen, increased breathing effort, pale gums, changes in appetite, and any episode of lameness or sudden blindness ^[1]. Administer all medications exactly as prescribed, and never discontinue immunosuppressants or antihypertensives abruptly without first consulting your veterinarian.

Attend all scheduled recheck appointments for blood pressure checks, renal panels, UPC measurements, and antithrombin III monitoring to assess treatment effectiveness and disease progression ^[1]. Provide a quiet, low-stress environment, ensure fresh water is always available, and strictly adhere to the prescribed renal diet, avoiding any dietary indiscretions.

Know the emergency signs:

• Sudden difficulty breathing
• Paralysis of the hind legs
• Acute loss of vision
• Collapse

Any of these require immediate veterinary attention ^[1].

Similar Conditions That Can Look Like Acute Glomerulonephritis

Several other kidney and systemic diseases can mimic the signs of acute glomerulonephritis, and only thorough diagnostics can tell them apart.

• Renal amyloidosis: Deposition of abnormal amyloid protein in glomeruli causes severe proteinuria and nephrotic syndrome. It is differentiated from immune-complex glomerulonephritis by special Congo red staining (a dye test that highlights amyloid deposits) on biopsy ^{[2, 5]}.
• Focal segmental glomerulosclerosis (FSGS) and other primary sclerotic glomerulopathies can present with proteinuria and renal dysfunction; renal biopsy with electron microscopy reveals podocyte foot-process effacement (flattening of the tiny cellular projections that help filter the blood) without immune complex deposits, distinguishing them from immune-mediated glomerulonephritides such as MGN or MPGN.
• Tubulointerstitial nephritis (TIN) can cause proteinuria and azotemia but typically with a lower UPC and primary tubular damage on biopsy; absence of immune deposits helps rule out glomerulonephritis ^[5].
• Acute tubular necrosis (ATN) from toxins, ischemia (loss of blood flow), or severe infections leads to acute uremia and sometimes mild proteinuria, but histopathology shows tubular epithelial cell death rather than immune-mediated glomerular injury.
• Congestive heart failure can produce ascites and pleural effusion, but proteinuria is typically minimal or absent, and echocardiography (ultrasound imaging of the heart) reveals underlying cardiac dysfunction.
• Systemic hypertension secondary to hyperthyroidism or idiopathic causes can result in retinal detachment that mimics hypertensive retinopathy from glomerulonephritis, although urinalysis typically shows little or no proteinuria.

What We Still Don't Know: Gaps and Current Research

Veterinary research into acute glomerulonephritis is still catching up to what we know about the disease in humans and dogs. Several key questions remain unanswered.

The optimal immunosuppressive drug combination, dosage, and duration for feline immune-complex glomerulonephritis have not been determined; current practice is extrapolated from canine studies and small retrospective case series ^[5]. The true prevalence of acute glomerulonephritis in the general cat population is unknown, and because affected cats may be asymptomatic and renal biopsy is frequently declined, many cases likely go undiagnosed ^{[1, 5]}.

Whether early blockade of the renin-angiotensin-aldosterone system (RAAS), the body's hormonal blood-pressure and fluid-balance pathway, alone or combined with immunosuppression, alters long-term outcomes more effectively has not been rigorously studied in cats ^[5]. Randomized controlled trials, the gold standard for proving a treatment works, are needed to confirm whether aggressive immunosuppression in the acute phase can prevent progression to chronic kidney disease ^[5]. The efficacy of antiplatelet agents (aspirin, clopidogrel) for thromboembolism prevention in feline glomerulonephritis remains anecdotal; controlled studies are lacking ^[1].

Research into urinary biomarkers (substances in urine that signal early kidney damage) could allow earlier, non-invasive diagnosis and monitoring, but these are not yet clinically available for cats ^[5]. In idiopathic acute glomerulonephritis, the specific antigens driving immune complex formation are usually unknown, and identifying them could open avenues for targeted immunotherapy. Long-term quality-of-life studies and validated pain assessment tools specific to cats with kidney disease are currently lacking, limiting objective evaluation of treatment success.

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