Urolithiasis in Dogs

I. Introduction to Canine Urolithiasis

Urolithiasis, commonly referred to as urinary stones or bladder stones, is a significant and painful condition affecting the canine urinary tract. It involves the formation of crystalline concretions (uroliths) within the kidneys, ureters, bladder, or urethra. These stones vary widely in size, composition, and location, ranging from microscopic crystals (crystalluria) that often pass without issue, to large, palpable stones that can cause severe inflammation, chronic infection, and most critically, life-threatening urinary obstruction.

The formation of uroliths is a complex process driven by supersaturation of the urine with specific mineral salts. This supersaturation occurs when the concentration of mineral precursors exceeds their solubility limit, leading to nucleation (initial crystal formation) and subsequent aggregation (growth) into stones. This delicate balance is influenced by numerous factors, including genetics, diet, urinary pH, water intake, anatomical abnormalities, and the presence of bacterial infection.

While often treated as a singular condition, the management and prognosis of canine urolithiasis depend entirely on the type of stone involved. The two most common types are Struvite and Calcium Oxalate, together accounting for approximately 80% of all reported canine uroliths. Understanding the specific composition is paramount, as treatments range vastly from specialized dietary dissolution to immediate surgical removal.

II. Pathophysiology and Mechanisms of Stone Formation

The formation of uroliths is a multi-step process that requires three principal conditions to be met:

1. Supersaturation of Urine

Supersaturation is the primary driver. It means the urine contains an excessive concentration of the solutes that make up the stone (e.g., magnesium, ammonium, phosphate for Struvite; or calcium and oxalate for Calcium Oxalate). This excess concentration can result from:

• Dietary Imbalance: High intake of specific mineral precursors.
• Reduced Water Intake: Leading to highly concentrated urine.
• Metabolic Abnormalities: Such as hypercalcemia (too much calcium in the blood) or defects in amino acid transport (Cystine stones).

2. Nucleation and Growth

Once the urine is supersaturated, the specific salts begin to precipitate, forming tiny initial crystals (nucleation). These crystals aggregate and grow over time in a process facilitated by various factors:

• Lack of Inhibitors: Normal urine contains natural inhibitors (e.g., Tamm-Horsfall mucoprotein, citrate, pyrophosphate) that prevent crystal growth and aggregation. A deficiency or absence of these inhibitors allows crystals to coalesce freely.
• Urinary Stasis: If the urine is not frequently voided (due to infrequent urination or anatomical strictures), the crystals have more time to settle and grow within the bladder.

3. Role of Urinary pH

Urinary pH is critical because the solubility of minerals is highly pH-dependent:

• Alkaline Urine (High pH): Favor Struvite and Calcium Phosphate stone formation. Struvite formation, in particular, relies on the high pH created by bacterial infection.
• Acidic Urine (Low pH): Favors Calcium Oxalate, Urate, and Cystine stone formation, as these minerals are less soluble in acidic environments.

4. Role of Bacterial Infection

Bacterial involvement is overwhelmingly associated with Struvite stones. Certain bacteria, most notably Staphylococcus and Proteus, produce the enzyme urease. Urease breaks down urea (a normal waste product) into ammonia and carbon dioxide. This chemical reaction significantly raises the pH, creating a highly alkaline environment necessary for the rapid precipitation of Struvite (magnesium ammonium phosphate).

III. Types of Canine Uroliths: Specific Causes and Characteristics

Identifying the stone type is the most critical step in management.

A. Struvite (Magnesium Ammonium Phosphate)

• Prevalence: Historically the most common type, though its prevalence has decreased due to improved diagnostics and infection control.
• Cause: In dogs, Struvite formation is overwhelmingly secondary to a Urinary Tract Infection (UTI) caused by urease-producing bacteria (infectious struvite).
• Characteristics: Often smooth or faceted, radiopaque (easily visible on X-ray). They tend to form rapidly in the alkaline environment created by the infection.
• Exceptions: Sterile Struvite (not caused by infection) can occur rarely due to metabolic issues or high concentrations of precursors.

B. Calcium Oxalate (CaOx)

• Prevalence: Currently the most common stone type in many regions, often surpassing Struvite.
• Cause: Primarily metabolic or genetic predisposition, often linked to hypercalciuria (excess calcium in urine) or hypocitraturia (low citrate, an important inhibitor). High blood calcium (hypercalcemia) from primary hyperparathyroidism is a rare but serious underlying cause.
• Characteristics: Extremely hard, rough, multi-faceted, “jack-like” appearance, and highly radiopaque.
• Key Challenge: Unlike Struvite, CaOx stones cannot be medically dissolved and must be removed surgically or procedurally.

C. Urate (Ammonium Urate or Uric Acid)

• Prevalence: Less common overall, but extremely common in certain breeds (Dalmatians).
• Cause: Metabolic defect related to purine metabolism. Purines are broken down into uric acid. Normally, uric acid is converted to allantoin (a soluble compound) by the liver enzyme uricase, and then excreted.
  • In Dalmatians, the liver’s ability to transport uric acid into the cells for processing is impaired, leading to high levels of uric acid in the blood and urine.
  • In other breeds, it can sometimes be associated with severe liver disease (e.g., portosystemic shunt, PSS), where the liver cannot properly metabolize ammonia and uric acid precursors.
• Characteristics: Less radiopaque than Struvite or CaOx; sometimes invisible on standard X-rays, often requiring ultrasound or contrast radiography for detection.

D. Cystine

• Prevalence: Rare.
• Cause: A hereditary metabolic defect (cystinuria) resulting in the inability of the renal tubules to properly reabsorb the amino acid cystine. Cystine is poorly soluble in acidic urine, leading to stone formation.
• Characteristics: Less radiopaque; common in male dogs due to their narrower, longer urethra, which predisposes them to obstruction.
• Affected Breeds: Newfoundlands, Dachshunds, Labrador Retrievers, English Bulldogs, and Australian Cattle Dogs.

E. Other Rare Types

• Silica: Linked to specific diets (often those high in plant proteins or gluten/corn products) or long-term medication use (e.g., antacids containing silicates).
• Calcium Phosphate: Rare, usually associated with highly alkaline urine or certain renal tubular defects.

IV. Clinical Signs and Symptoms

The manifestations of urolithiasis depend heavily on the location of the stone (bladder, urethra, or kidney) and whether it is causing an obstruction.

A. Lower Urinary Tract Signs (Cystitis)

If the stone is floating freely in the bladder, the symptoms mimic a severe UTI, caused by chronic inflammation and irritation of the bladder wall (cystitis).

1. Hematuria: Blood in the urine, varying from microscopic to grossly visible pink/red color.
2. Dysuria/Stranguria: Difficulty or straining during urination, often accompanied by vocalization.
3. Pollakiuria: Increased frequency of urination, often passing only small volumes.
4. Periuria (Inappropriate Urination): Urinating in unusual places (e.g., inside the house) because the dog feels a constant urge or associates outdoor urination with pain.
5. Licking: Excessive licking of the genital area due to discomfort.

B. Life-Threatening Obstruction (Urethral Blockage)

Obstruction occurs when a small stone (or a collection of crystals/sludge) becomes lodged in the narrow urethra, preventing the flow of urine out of the body. This is a severe medical emergency, particularly common in male dogs.

1. Failure to Urinate: The dog strains persistently (often for minutes at a time) but produces little to no urine (anuria).
2. Pain and Restlessness: Extreme discomfort, restlessness, crying, pacing, and reluctance to move.
3. Abdominal Distension: The bladder becomes severely distended and painful upon palpation (a hard, large mass in the abdomen).
4. Systemic Signs (Uremia): If the obstruction is not relieved within 24–48 hours, kidney function rapidly deteriorates, leading to uremia. Signs include vomiting, lethargy, loss of appetite, weakness, and collapse. This can lead to fatal potassium imbalance (hyperkalemia) and heart arrhythmias.

C. Upper Urinary Tract Signs (Kidney/Ureters)

Stones lodged in the kidney (nephroliths) or ureters (ureteroliths) may cause no symptoms if they do not obstruct urine flow. However, they can be a chronic source of pain or infection, and acute obstruction is devastating:

• Flank Pain: Painful abdomen or back (rarely observed).
• Hematuria: Persistent or intermittent blood in the urine.
• Renal Failure: If both ureters are obstructed, the condition leads to acute kidney failure, requiring emergency intervention, dialysis, or bypass procedures.

V. Dog Breeds at Risk (Genetic and Metabolic Predisposition)

While any dog can potentially develop urolithiasis, certain breeds possess genetic or metabolic pathways that significantly increase their susceptibility to specific types of stones.

Breeds Predisposed to Calcium Oxalate (CaOx)

CaOx stones are strongly linked to genetic factors influencing calcium excretion and urinary concentration. These breeds often exhibit higher baseline calcium concentrations in urine or deficiencies in inhibitory substances.

• Miniature Schnauzer: This breed is overwhelmingly predisposed to CaOx, accounting for a disproportionate number of cases. The exact mechanism is still under investigation, but it is believed to involve a systemic metabolic derangement that favors CaOx crystal precipitation, often independent of diet. They frequently require lifelong prophylactic monitoring and dietary management.
• Bichon Frise: Highly represented in CaOx cases, often linked to similar metabolic issues found in Miniature Schnauzers.
• Lhasa Apso & Shih Tzu: Small, toy breeds with a tendency toward concentrated urine and potential genetic defects in calcium handling.
• Yorkshire Terrier, Poodle (Miniature/Toy).

Breeds Predisposed to Struvite

While Struvite is primarily associated with infection, certain female dogs and breeds prone to recurrent UTIs may be overrepresented. There is a specific, rare Sterile Struvite tendency noted in the German Shepherd Dog.

• Cocker Spaniel: Slightly increased risk, often due to recurrent bacterial cystitis.
• Labrador Retriever (Female): Often due to anatomical predispositions or adherence to bacterial colonization.

Breeds Predisposed to Urate

Urate stone formation is fundamentally tied to defects in purine metabolism and uric acid transport.

• Dalmatian: The classic example. Dalmatians possess a genetically fixed defect in the hepatic uptake and transport of uric acid, meaning they cannot convert uric acid efficiently to soluble allantoin. This causes them to excrete high levels of uric acid in their urine (hyperuricosuria), regardless of diet, leading to lifelong risk of urate stone formation.
• English Bulldog: A significant predisposition, often associated with a different, non-Dalmatian type of genetic hyperuricosuria.
• Other Breeds with PSS: Any breed with an untreated congenital Portosystemic Shunt (PSS) risks developing urate stones because the liver bypass prevents the necessary filtering and metabolism of ammonia and uric acid.

Breeds Predisposed to Cystine

Cystine stones are purely due to inherited defects in renal tubular transport of the dibasic amino acids (cystine, lysine, arginine, ornithine). This is an X-linked or autosomal recessive trait depending on the breed.

• Newfoundland: Classic case of a large breed with cystinuria.
• English Bulldog: Also highly represented.
• Mastiff, Miniature Pinscher, Australian Cattle Dog, Dachshund, Irish Terrier, and Scottish Terrier.

VI. Age Predilection: Puppy, Adult, or Older Dogs

Urolithiasis can affect dogs of all ages, but the type of stone often correlates with the age of onset:

1. Puppy/Young Dogs (Under 1 Year)

In younger dogs, urolithiasis is usually associated with:

• Genetic/Metabolic Defects: Cystine stones and Urate stones (especially if caused by a congenital PSS) are typically diagnosed early in life, often within the first two years.
• Struvite: While less common than in adults, Struvite can develop rapidly in puppies with poorly managed UTIs.

2. Adult Dogs (2 to 8 Years)

This is the peak age for the diagnosis of most stone types:

• Calcium Oxalate: CaOx formation is often a long process, resulting in diagnosis in young to middle-aged adult dogs, particularly the susceptible small breeds.
• Struvite: Most infectious Struvite cases occur during adult life, associated with recurrent or chronic bacterial infections.

3. Older Dogs (8+ Years)

Older dogs remain highly susceptible, particularly due to compounding factors:

• Recurrence: Dogs with a history of any stone type are prone to rapid recurrence in later life.
• Concurrent Illnesses: Older dogs often have endocrine disorders (e.g., Cushing’s disease or diabetes mellitus) that can increase water consumption, alter urine composition, and suppress immunity, thereby increasing the risk of UTIs and subsequent Struvite formation.
• Neoplasia: Bladder tumors, while rare, can mimic the signs of urolithiasis and sometimes cause secondary stone formation by obstructing flow or providing a nidus (starting point) for crystallization.

VII. Diagnosis of Urolithiasis

Accurate diagnosis involves a combination of clinical assessment, laboratory work, and advanced imaging.

A. Physical Examination

The veterinarian will palpate the abdomen. In cases of severe cystitis, the bladder may be painful. In cases of urethral obstruction, the bladder will be severely firm, distended, and exceptionally painful—a sign requiring immediate intervention.

B. Urinalysis and Culture

1. Urinalysis (UA): Reveals the presence of crystals (crystalluria), blood (hematuria), white blood cells (pyuria), and protein. High urine specific gravity indicates concentrated urine, a risk factor.
   • Note: Crystalluria alone does not confirm the presence of stones; some normal animals exhibit temporary crystals.
2. Urine pH: Provides clues to the stone type (alkaline suggests Struvite; acidic/neutral suggests CaOx, Urate, or Cystine).
3. Urine Culture and Sensitivity: Essential if infection is suspected. This identifies the specific bacteria and determines which antibiotics will be effective. Crucial for Struvite management.

C. Imaging

Imaging defines the size, quantity, and location of stones.

1. Abdominal Radiography (X-rays):
   • Standard Radiography: Highly effective for radiopaque stones (Struvite, CaOx, Silica). These appear as bright white densities.
   • Limitations: Urate and Cystine stones are often radiolucent (invisible) and will not show up clearly on standard X-rays.
2. Abdominal Ultrasound:
   • Advantages: Excellent for visualizing all stone types, including radiolucent ones (Urate, Cystine). It also allows the veterinarian to assess the thickness and health of the bladder wall, look for large amounts of sludge, and check if stones are present in the kidneys or ureters. Essential for diagnosing urethral obstruction.
3. Contrast Studies (Double-Contrast Cystography):
   • If standard X-rays and ultrasound are inconclusive, contrast dye (positive and negative contrast) can be injected into the bladder. This outlines the bladder wall and helps reveal radiolucent stones as “filling defects” within the dye.

D. Stone Analysis (The Definitive Test)

Once a stone is therapeutically removed (via surgery or passed naturally), it must be sent to a specialized laboratory (e.g., the Minnesota Urolith Center) for quantitative analysis. This is the only way to definitively determine the stone composition, which dictates the long-term preventive treatment. Treating a dog based on presumed composition (e.g., pH or crystal type) is prone to failure and recurrence.

VIII. Treatment Strategies

Treatment is two-fold: immediate management of the clinical crisis and long-term prevention.

A. Emergency Management of Urethral Obstruction

This is a immediate, life-saving measure:

1. Stabilization: Address electrolyte imbalances (especially hyperkalemia) with IV fluids and specific medications (e.g., dextrose, insulin, sometimes calcium gluconate).
2. Decompression: The obstruction must be relieved immediately. This often involves retrohydropropulsion—flushing the stone back into the bladder (where it poses less immediate danger) using a catheter and sterile fluids.
3. Catheterization: An indwelling urinary catheter is often placed for 24–48 hours to ensure patency, monitor urine output, and flush the bladder.

B. Medical Dissolution (Dietary Therapy)

Dissolution is possible only for Struvite, Urate, and Cystine stones under specific conditions. Calcium Oxalate stones must be removed.

1. Struvite Dissolution

• Requires: A specialized, prescription dissolution diet (low in protein, phosphorus, and magnesium) and concurrent, appropriate antibiotic therapy based on culture results.
• Mechanism: The diet makes the urine highly acidic and dilute, dissolving the Struvite matrix. The antibiotics eliminate the source of the urease enzyme.
• Duration: Typically 2–4 months. The stones must be fully dissolved, and treatment should continue for 1 month after radiographic disappearance to eliminate the unseen nidus.

2. Urate Dissolution

• Requires: A low-purine diet (prescription hepatic or urate formulas) and medication (Allopurinol) to inhibit uric acid production.
• Duration: Varies widely, usually several months.

3. Cystine Dissolution

• Requires: A diet that alkalinizes the urine and reduces protein intake. Medications (e.g., Thiola or D-penicillamine) may be used to bind the cystine precursors, increasing their solubility.

C. Surgical and Procedural Removal

If stones cannot be dissolved (CaOx being the primary example), are lodged and causing obstruction, or are too large to pass, removal is necessary.

1. Cystotomy (Bladder Surgery): The most common method. The bladder is surgically opened, the stones are retrieved, the bladder is thoroughly flushed, and samples are collected for culture and analysis.
2. Urethrotomy/Urethrostomy: If a stone is firmly lodged in the urethra and cannot be flushed back, a temporary (urethrotomy) or permanent (urethrostomy) opening may be created in the urethra to remove the stone. Perineal urethrostomy is sometimes performed in male dogs with uncontrollable recurrence risk.
3. Minimally Invasive Techniques (Cystoscopy and Lithotripsy):
   • Urohydropropulsion: A technique used for very small stones (typically 3mm or less) where they are flushed out of the bladder through a catheter while the dog is heavily sedated.
   • Cystoscopic Retrieval: Using a rigid or flexible endoscope to enter the bladder via the urethra, grab small stones with a basket, and remove them.
   • Laser Lithotripsy: Using a laser fiber guided by a cystoscope, the stone is fragmented into tiny pieces that can then be removed or flushed out. This is a highly specialized and expensive procedure often reserved for CaOx stones in hard-to-reach locations.

IX. Prognosis and Complications

The prognosis is generally good if the underlying cause is identified and managed. However, urolithiasis carries significant risks, primarily linked to recurrence and obstruction.

A. Prognosis by Stone Type

• Struvite: Excellent prognosis, provided the underlying infection is completely cleared and the specific dietary plan is rigorously maintained until resolution. Recurrence is high if the infection is not fully eradicated.
• Calcium Oxalate: Guarded prognosis regarding recurrence. Even after surgical removal, the metabolic predisposition remains, and CaOx stones have a very high recurrence rate (up to 50% within 2 years) if long-term prevention is not strict.
• Urate/Cystine: Fair to good prognosis, heavily dependent on owner compliance with strict, often life-long, low-protein/low-purine diets and required medication.

B. Major Complications

1. Recurrence: The single most common complication. Many dogs with metabolic stones require lifelong monitoring.
2. Urinary Obstruction: A life-threatening emergency causing acute kidney injury, hyperkalemia, and potentially bladder rupture if not treated immediately.
3. Chronic Kidney Disease (CKD): Nephroliths (stones in the kidney) can cause chronic inflammation and obstruction over time, leading to irreversible kidney damage.
4. Urethral Strictures: Trauma to the urethra caused by stone passage, chronic catheterization, or Urethrotomy surgery can lead to strictures (narrowing) that predispose the dog to future obstruction.
5. Peritonitis/Sepsis: Rare, but can occur if the bladder ruptures due to severe obstruction or if bacterial infections are overwhelming.

X. Prevention and Long-Term Monitoring

Prevention involves minimizing supersaturation, promoting dilution, and maintaining appropriate urinary pH.

1. Increase Water Intake

Dilute urine is the single most effective preventive measure for all stone types. Higher water intake reduces the concentration of mineral precursors. Strategies include:

• Switching from dry kibble to canned (wet) food.
• Adding water to dry food.
• Encouraging drinking by using water fountains, multiple bowls, or adding flavorings (e.g., low-sodium broth).

2. Dietary Management (Stone Specific)

The cornerstone of prevention is a specialized, prescription therapeutic diet designed to create an unfavorable environment for the specific stone type (detailed below).

3. Frequent Urination

Encouraging more frequent walks and bathroom breaks prevents urinary stasis and allows crystals to be flushed out before they can aggregate.

4. Ongoing Monitoring

Lifelong monitoring is essential, particularly for CaOx, Urate, and complicated Struvite cases:

• Urinalysis and Culture: Every 3–6 months to check pH, crystal type, and screen for infection.
• Diagnostic Imaging (X-ray/Ultrasound): Every 6–12 months. Since CaOx stones can form quickly and quietly, periodic imaging allows for detection while they are still small enough to be removed non-surgically, preventing the need for major surgery or crisis management.

5. Medical Management

Specific supplements or medications are necessary for non-infectious stones:

• Potassium Citrate: Often used for CaOx prevention. It acts as an alkalinizing agent (except in Dalmatians) and increases citrate, a natural inhibitor of CaOx formation.
• Specific Chelators/Inhibitors: For Cystine (Thiola) and Urate (Allopurinol).

XI. Diet and Nutrition: The Cornerstone of Management

Therapeutic diets are not merely general health foods; they are formulated with specific mineral, protein, and pH targets to either dissolve stones or prevent recurrence. These diets should only be used under veterinary guidance.

A. Nutritional Management for Struvite

• Goal: Promote urine acidity (pH < 6.5) and decrease the concentration of magnesium, ammonium, and phosphate.
• Dietary Strategy: Prescription Struvite diets are reduced in protein, phosphorus, and magnesium. The reduced protein limits the building blocks for ammonia and phosphate. The acidic formulation increases the solubility of the Struvite crystals.
• Two Phases:
  1. Dissolution Phase: Highly restricted protein/mineral diet used aggressively for the duration of dissolution.
  2. Prevention Phase (If sterile struvite is a concern): Long-term maintenance with a moderate Struvite prevention diet, though sometimes a regular, high-quality maintenance diet is acceptable once the infection tendency is resolved.

B. Nutritional Management for Calcium Oxalate (CaOx)

• Goal: Promote dilute urine, increase urinary pH (to neutral or slightly alkaline, 7.0–7.5), and reduce urinary calcium and oxalate.
• Dietary Strategy: Prescription CaOx diets are formulated to:
  1. Reduce Calcium and Oxalate Precursors: Moderate protein content (not excessively low) using high-quality sources.
  2. Increase Moisture: High moisture content is essential for dilution.
  3. Sodium Control: Moderate sodium to encourage water consumption without excessive calcium excretion.
  4. Avoid Excess Vitamin D and C: Vitamin D increases calcium absorption; Vitamin C (ascorbic acid) is metabolized to oxalate.
• Supplementation: Potassium citrate is often added orally to the diet to make the urine more alkaline and inhibit crystal growth.

C. Nutritional Management for Urate

• Goal: Reduce the intake of purines and promote alkaline urine (pH 7.0–7.5) to increase uric acid solubility.
• Dietary Strategy: Prescription Urate diets (often labeled as hepatic or liver diets) are highly effective:
  1. Low Purine: Very restricted in purine precursors, meaning limited organ meats, yeast, and certain plant materials. Often rely on eggs or dairy as their main protein source.
  2. Alkalinizing: Formulated to raise the urinary pH.
  3. Low Protein/Low Ammonia: Especially if a PSS is present.

D. Nutritional Management for Cystine

• Goal: Increase solubility of cystine by promoting alkaline urine (pH > 7.5) and restricting protein intake.
• Dietary Strategy: Similar to Urate diets, often utilizing low-protein, alkalinizing formulas. Sodium restriction is also sometimes recommended as high sodium can exaggerate cystine excretion.

XII. Zoonotic Risk (Is Urolithiasis Contagious?)

Urolithiasis itself is not a zoonotic disease; it cannot be transmitted from a dog to a human or vice versa.

The formation of stones is a result of unique metabolic, genetic, or anatomical conditions specific to the individual dog. However, a crucial distinction must be made regarding the cause of Struvite stones:

1. Infectious Struvite: This stone type requires a bacterial infection (UTI) to initiate formation. While the stone itself is not contagious, the bacteria causing the UTI (e.g., certain Staphylococcus or Proteus species) could potentially be shared between pets or, rarely, between pets and immunocompromised humans through direct contact with infected urine, feces, or unhygienic environments.
2. Non-Infectious Stones (CaOx, Urate, Cystine): These are purely metabolic or genetic conditions and pose absolutely no risk of transmission to humans or other animals.

Because urolithiasis treatment requires handling urine and sometimes bloody discharge, standard hygiene measures (washing hands after handling the dog or cleaning up accidents) are always recommended, particularly for immunocompromised individuals, but the stones themselves are not infectious.

CONCLUSION

Urolithiasis is one of the most common and complex urological conditions in veterinary medicine. Successful long-term management requires definitive stone analysis, aggressive intervention for obstruction, and meticulous, often lifelong, commitment to prescribed therapeutic diets and regular veterinary monitoring. Given the high recurrence rates for metabolic stones (Calcium Oxalate, Urate, and Cystine), owner compliance with dietary instructions remains the single most important factor determining the quality of life and longevity of the affected dog. Advances in non-surgical removal techniques, alongside highly specialized prescription diets, continue to improve the prognosis for dogs afflicted with this painful condition.

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