Nature of Disease: A metabolic disorder of highly productive dairy cows resulting from excessive negative energy balance (NEB) at the onset of lactation.
Key Pathological Feature: Excessive deposition of triglycerides (TAG) within hepatocytes (liver cells).
Clinical Impact: Associated with pronounced ketosis, depressed feed intake, decreased productivity, and, in severe cases, liver failure and death.
Timing: Primarily a postpartum disorder, though it often begins developing before and during parturition.
Etiology & Pathogenesis
The Mobilization Trigger: Insufficient dietary energy leads to the mobilization of body fat reserves, releasing nonesterified fatty acids (NEFAs) into the blood.
Hepatic Uptake: The liver retains approximately 15%–20% of circulating NEFAs.
Processing Failure:
NEFAs in the liver are either oxidized (producing ATP or ketones) or esterified into TAG.
In ruminants, the export of TAG as lipoproteins (VLDL) occurs at a very slow rate.
Result: When NEFA uptake exceeds the liver's capacity to oxidize or export them, triglycerides accumulate in the liver cells.
Hormonal Influence: Low insulin and glucose concentrations favor fat mobilization and fatty liver development.
Risk Factors:
Conditioning:Overconditioned (obese) cows (BCS ≥4) are at the highest risk because they experience more pronounced feed intake depression around calving.
Animal Factors: Older, highly productive cows are more susceptible.
Clinical Findings:
Spectrum of Severity: Ranges from mild ketosis to liver coma.
General Signs: There are no pathognomonic (unique) signs; common indicators include depression in feed intake, decreased milk production, and ketosis.
Metabolic Consequences:
Reduced gluconeogenesis (leading to hypoglycemia).
Reduced ureagenesis (leading to hyperammonemia).
Altered endocrine profiles and impaired immune function.
Thresholds: Mild signs appear at TAG contents of ~100 g/kg liver wet weight; liver coma occurs at values ≥300 g/kg.
Diagnosis:
Indirect Assessment (Common Practice): Assessing the severity and duration of NEB through clinical signs and blood/urine metabolites.
Direct Diagnostic Tools:
Liver Biopsy: The gold standard and most reliable method to determine severity.
Copper Sulfate Flotation: A rapid field test to estimate total lipid content from biopsy samples.
Microscopic Evaluation: Estimating the percentage of cell volume occupied by fat.
Biochemical Markers:
High NEFA: (>600 mcM/L) and high beta-hydroxybutyrate (BHB) concentrations.
Low Glucose and Cholesterol: Reflecting impaired liver function and anorexia.
Liver Enzymes: Increased activity of AST, SDH, and GLDH indicates liver cell damage.
Prevention Strategies
Dry Period Management: Focus on optimizing cow well-being and maintaining an ideal BCS of 3–3.5.
Dietary Stability: Avoid rapid diet changes and ensure access to palatable feeds.
Prophylactic Treatments (for at-risk cows): 1) Propylene Glycol: Oral drench (300–600 mL/day) during the final week prepartum, 2)Monensin: An ionophore antibiotic that modulates rumen fermentation to increase propionate synthesis, 3)Glycerol: A palatable alternative to propylene glycol.
Treatment Protocols
Mild Cases: Oral administration of gluconeogenic substances like propylene glycol (250–300 g twice daily) or sodium propionate.
Moderate to Severe Cases:
IV Dextrose: Bolus of 500 mL of 50% solution to trigger an insulin response and interrupt fat mobilization.
Continuous Dextrose Infusion: Reserved for clinical settings to maintain normoglycemia.
Controversial Adjuncts:
Glucocorticoids (Dexamethasone): Debated due to potential lipolytic effects, though they may improve feed intake.
Lipotropic Agents: IV choline, methionine, or Vitamin B12 are sometimes used to aid TAG export, though their effectiveness in ruminants is not definitively proven.