Gout is a metabolic disorder characterized by hyperuricemia and deposition of monosodium urate crystals in tissues. Unlike mammals, birds lack the enzyme uricase (urate oxidase), making uric acid the end product of purine metabolism.
Overview and Clinical Importance
Gout is a metabolic disorder characterized by hyperuricemia and deposition of monosodium urate crystals in tissues. Unlike mammals, birds lack the enzyme uricase (urate oxidase), making uric acid the end product of purine metabolism. This physiological difference makes birds highly susceptible to urate crystal deposition when renal function is compromised.
Avian gout represents one of the most significant metabolic diseases in both commercial poultry and pet birds, causing substantial economic losses with mortality rates reaching 15-50% in affected flocks. Understanding the pathophysiology, clinical presentation, and management is essential for the NAVLE.
| Feature |
Visceral Gout (Acute) |
Articular Gout (Chronic) |
| Onset |
Acute; rapid progression |
Chronic; slow progression |
| Location |
Serosal surfaces: pericardium, liver capsule, peritoneum, air sacs, kidneys |
Joints: synovial membranes, tendon sheaths; primarily feet and wing joints |
| Pathogenesis |
Rapid renal failure with sudden hyperuricemia; counterpart to uremia in mammals |
Long-term hyperuricemia; chronic granulomatous reaction to urate crystals (tophi) |
| Kidney Status |
Usually severely damaged with visible urate deposits |
Often grossly normal; primary metabolic defect |
| Common Causes |
Infectious (IBV, ANV, CAstV), dehydration, nephrotoxins, high calcium diet |
Genetic defects, excessive dietary protein, chronic metabolic derangement |
| Prognosis |
Poor; often fatal within 36-48 hours |
Guarded; chronic management possible |
Classification of Avian Gout
Avian gout is classified into two primary forms based on anatomical location of urate crystal deposition:
High-YieldRemember: VISCERAL = VITAL ORGANS (pericardium, liver, air sacs); ARTICULAR = APPENDAGES (joints, especially toes and wings). Visceral gout is associated with acute renal failure; articular suggests chronic hyperuricemia.
| Agent |
Mechanism |
Key Features |
| Nephropathogenic IBV |
Replicates in renal tubular epithelium causing interstitial nephritis |
Coronavirus; vertical transmission; most common infectious cause |
| Avian Nephritis Virus |
Astrovirus causing interstitial nephritis and tubulonephrosis |
Two serotypes; primarily affects young chicks; fecal-oral transmission |
| Chicken Astrovirus |
Kidney tropism leading to visceral gout |
Biii subgroup associated with gout; up to 40% mortality |
| Goose Astrovirus |
Causes visceral gout in goslings and ducklings |
Emerging; two clades (GoAstV-1, GoAstV-2); up to 61% mortality |
Etiology and Risk Factors
Avian gout is multifactorial, arising from factors that either increase uric acid production or decrease excretion:
Infectious Causes
NAVLE TipYoung broiler chicks (less than 2 weeks old) with sudden high mortality and white visceral deposits: think IBV or astrovirus first. Nephropathogenic IBV can be vertically transmitted from breeders.
Nutritional Causes
Toxic and Management Causes
- Mycotoxins: Ochratoxin A, aflatoxins, citrinin, oosporein are nephrotoxic
- Aminoglycosides: Gentamicin is particularly nephrotoxic; owls and cockatoos sensitive
- NSAIDs: Diclofenac causes visceral gout - devastated vulture populations
- Dehydration: MOST CRITICAL factor - reduced water allows urate crystallization
- High brooding temperature: Reduces chick water intake in first week
- Water deprivation: Equipment malfunction, frozen lines, improper drinker height
| Factor |
Mechanism |
Critical Values |
| Excess Calcium |
Precipitates as calcium-sodium-urate crystals in renal tubules |
Greater than 3% dietary calcium in non-laying birds |
| Low Phosphorus |
Phosphorus acidifies urine and prevents crystal formation |
Low available phosphorus with high calcium most dangerous |
| High Protein |
Increases purine metabolism and uric acid production |
Greater than 30% crude protein |
| Vitamin A Deficiency |
Squamous metaplasia of renal tubular epithelium |
Long-term deficiency required |
| Excess Vitamin D3 |
Increases intestinal calcium absorption causing hypercalcemia |
Hypervitaminosis D3; problematic in juvenile macaws |
Pathophysiology
Avian Uric Acid Metabolism
- No uricase: Birds cannot convert uric acid to allantoin, making them susceptible to hyperuricemia
- Tubular secretion: Uric acid eliminated primarily by tubular secretion (70%), independent of GFR
- No urinary bladder: Urates expelled directly from cloaca as white component of droppings
- Solubility threshold: Gout develops when uric acid exceeds approximately 600 micromol/L
- Renal reserve: Greater than 70% of renal function must be lost before hyperuricemia develops
- A bird with no renal function typically dies within 36 hours from urate accumulation
| Organ |
Lesion Description |
| Pericardium |
White, gritty deposits; heart appears frosted |
| Liver |
Liver capsule covered with chalky white material |
| Kidneys |
Swollen, pale, mottled with snowflake pattern; urate deposits in tubules |
| Ureters |
Distended with white urate crystals; may contain staghorn calculi |
| Air Sacs |
White granular deposits on membranes |
Clinical Signs and Findings
Visceral Gout Clinical Presentation
- Non-specific signs: Anorexia, depression, lethargy, ruffled feathers
- Polydipsia/polyuria: Increased water intake and watery droppings
- White feces: Staining of feathers around cloaca with white fecal material
- Sudden death: Often presents as sudden mortality without premonitory signs
- Flock pattern: High mortality (15-50%) in young chicks, especially first 2 weeks
Articular Gout Clinical Presentation
- Joint swelling: Enlarged, warm, painful joints (feet, hocks, wing joints)
- Lameness: Reluctance to walk; shifting weight between feet
- Tophi formation: Hard nodules around joints (pea to broad bean size)
- Joint aspirate: White, milky, gritty material when joints incised
- Perching behavior: Prefers flat surfaces over perches due to foot pain
| Method |
Findings |
Notes |
| Serum Uric Acid |
Elevated (normal 2-15 mg/dL); gout birds may reach 44 mg/dL+ |
Indicates greater than 70% renal dysfunction; not affected by hydration |
| Cytology |
Needle-like, non-staining crystals from joint aspirate |
DO NOT use formalin; use polarized light for confirmation |
| Gross Necropsy |
White chalky deposits on serosal surfaces; kidney changes |
Definitive diagnosis; pathognomonic appearance |
| Viral Testing |
RT-PCR for IBV, ANV, CAstV from kidney |
Important for flock prevention strategies |
Gross Pathology and Lesions
Visceral Gout Lesions
Pathognomonic finding: White, chalky, powdery deposits (resembling powdered sugar) on serosal surfaces
High-YieldDifferentiate urate deposits from purulent exudates! Urate = WHITE and CHALKY with gritty texture. Inflammatory exudates = YELLOW and FIBRINOUS or CASEOUS.
| Treatment |
Dosage |
Notes |
| Fluid Therapy |
5-10% body weight IV/IO; maintenance 50 mL/kg/day |
MOST IMPORTANT; restores renal perfusion; helps flush urates |
| Allopurinol |
10-30 mg/kg PO q12-24h (pet birds) |
XO inhibitor; CONTROVERSIAL - may induce gout in raptors at 50 mg/kg |
| Colchicine |
0.01-0.04 mg/kg PO q12-24h |
Reduces inflammation; use with allopurinol in severe cases |
| Urine Acidification |
Ammonium chloride, DL-methionine in water |
Dissolves urate crystals; prevents stone formation |
| Analgesics |
Meloxicam 0.5-1 mg/kg PO q12-24h |
Articular gout is painful; NEVER use diclofenac |
Histopathology
CRITICAL: Uric acid crystals dissolve in formalin. Fix tissues in absolute alcohol (95-100%) to preserve crystals.
- Crystal appearance: Feathery, needle-like, acicular crystals; basophilic spherical masses
- Polarized light: NEGATIVE birefringence (yellow parallel, blue perpendicular to polarizer)
- Inflammatory reaction: Granulomatous inflammation with heterophils, macrophages, giant cells
- Renal lesions: Urate cylinders in tubules, interstitial nephritis, tubular necrosis
Diagnosis
NAVLE TipSerum uric acid is more reliable than creatinine/BUN for renal function in birds because it reflects tubular secretion independent of GFR. However, normal uric acid does not guarantee healthy kidneys.
Treatment
Important: Visceral gout is often diagnosed at necropsy and is generally not treatable. Treatment focuses on articular gout and supportive care.
High-YieldFluid therapy is the CORNERSTONE of gout treatment. Never use diclofenac in birds - it causes visceral gout.
Prevention and Control
Poultry Flock Prevention
- IBV vaccination with nephropathogenic strains (breeder and commercial flocks)
- CAstV/ANV vaccination in breeder schedules (maternal antibody protection)
- Maintain proper Ca:P ratio (approximately 2:1); avoid layer diet in non-layers
- Keep crude protein at appropriate levels (less than 24% for growers)
- Ensure constant water availability - check drinker function daily
- Optimal brooding temperature; minimize transport time without water
- Mycotoxin management - screen raw materials for ochratoxin A