Fatty Liver Hemorrhagic Syndrome (FLHS)

Fatty Liver Hemorrhagic Syndrome (FLHS) is a non-infectious metabolic disorder of commercial laying hens, characterized by excessive accumulation of triglycerides in hepatocytes (hepatic lipidosis) and spontaneous intrahepatic hemorrhage leading to sudden death. The condition is most prevalent in caged layers during peak egg production, particularly in high-energy, low-exercise environments. FLHS is a significant cause of mortality in layer flocks, with mortality rates typically ranging from 1% to 5% annually, but can exceed 20% in severe outbreaks. The syndrome is also known as fatty liver syndrome (FLS) or hemorrhagic fatty liver syndrome. It is primarily a disease of intensively managed laying hens, but similar hepatic lipidosis can occur in broiler breeders, ducks, and occasionally in backyard poultry. The economic impact includes mortality, reduced egg production, and increased feed conversion ratio (FCR). FLHS is a classic example of a metabolic disease resulting from an imbalance between energy intake and expenditure, often exacerbated by nutritional, environmental, and genetic factors.

The primary etiology of FLHS is a positive energy balance leading to excessive hepatic lipid synthesis and deposition. The condition is multifactorial, with no single infectious agent. Key contributing factors include: (1) High-energy diets, especially those rich in corn or other grains, with low protein content or an imbalance in the calorie-to-protein ratio. (2) Excessive intake of metabolizable energy relative to the hen's requirement, particularly during the early laying period when hens are in positive energy balance. (3) Deficiencies in lipotropic factors, such as choline, methionine, vitamin B12, and folic acid, which are essential for the transport of lipids from the liver as very-low-density lipoproteins (VLDL). (4) Genetic predisposition: certain strains of brown-egg layers (e.g., Rhode Island Red) are more susceptible than white-egg layers (e.g., Leghorn). (5) Environmental stressors: high ambient temperatures, lack of exercise (cage confinement), and sudden changes in feed or management. (6) Mycotoxins, particularly aflatoxins, which impair hepatic lipid metabolism and exacerbate fatty infiltration. (7) Hormonal influences: estrogen and other reproductive hormones stimulate hepatic lipogenesis during egg production, increasing susceptibility. (8) Toxic agents: some hepatotoxic compounds, such as carbon tetrachloride, can induce similar lesions, but these are not common in field cases. The exact molecular mechanisms involve upregulation of lipogenic enzymes (e.g., acetyl-CoA carboxylase, fatty acid synthase) and impaired VLDL assembly due to apolipoprotein synthesis defects.

FLHS is predominantly a disease of commercial laying hens, especially those housed in cages with limited movement. It is rare in free-range or pasture-raised flocks where hens have more exercise. The disease typically occurs in hens between 20 and 40 weeks of age, coinciding with peak egg production, but can occur at any age. The incidence is higher in flocks fed high-energy diets, particularly those based on corn and soybean meal, with inadequate levels of choline, methionine, and vitamin B12. The condition is more common in the winter months when feed consumption increases to maintain body temperature, leading to excessive energy intake. Flock morbidity can be high, but clinical signs are often inapparent until sudden death occurs. Mortality rates typically range from 1% to 5% per month, but in severe outbreaks, cumulative mortality can exceed 20%. Egg production may drop by 5% to 10% during the outbreak, and affected hens may have reduced eggshell quality. The disease is more prevalent in brown-egg layers, which have a higher genetic propensity for fat deposition. In broiler breeders, FLHS is less common but can occur when feed restriction is not properly managed, leading to obesity. The condition is also reported in ducks and geese, particularly when overfed for foie gras production. The economic impact is significant due to mortality, reduced egg production, and increased feed costs.

The pathophysiology of FLHS involves a cascade of metabolic and vascular events. (1) Excessive energy intake leads to increased hepatic lipogenesis, with the liver synthesizing triglycerides from dietary carbohydrates and fats. (2) The liver's capacity to export triglycerides as VLDL is overwhelmed, leading to intracellular accumulation of lipid droplets in hepatocytes. This is exacerbated by deficiencies in lipotropic factors (choline, methionine, vitamin B12) that are required for VLDL synthesis. (3) The enlarged, fatty liver becomes fragile and susceptible to hemorrhage. The exact mechanism of hemorrhage is not fully understood, but it is believed to involve increased intrahepatic pressure, weakening of the sinusoidal walls, and impaired coagulation due to vitamin K deficiency or liver dysfunction. (4) Spontaneous rupture of the liver capsule or intrahepatic vessels leads to hemorrhage into the abdominal cavity, resulting in sudden death. (5) The hemorrhagic episodes may be triggered by stress, such as handling, sudden noise, or temperature fluctuations, which cause a surge in blood pressure. (6) In addition, oxidative stress and lipid peroxidation may contribute to hepatocyte damage and endothelial injury. (7) The condition is often associated with fatty infiltration of other organs, including the heart and kidneys, but the liver is the primary site. (8) The disease is not inflammatory; there is no significant infiltration of inflammatory cells, distinguishing it from hepatitis. (9) The metabolic derangements also affect egg production, as the liver is the site of yolk precursor synthesis (vitellogenin), and hepatic dysfunction can impair egg formation.

Intrinsic factors: (1) Genetic susceptibility: certain strains of brown-egg layers have a higher incidence. (2) Age: peak production period (25-35 weeks) is a high-risk window. (3) Sex: females are exclusively affected, as males do not produce eggs. (4) Body condition: obese hens are more prone. (5) Hormonal status: high estrogen levels stimulate lipogenesis. Extrinsic factors: (1) Nutritional: high-energy diets, low protein-to-energy ratio, deficiencies in choline, methionine, vitamin B12, folic acid, and vitamin E. (2) Management: caged housing with limited movement, high stocking density, and lack of foraging opportunities. (3) Environmental: heat stress, sudden temperature changes, and poor ventilation. (4) Feed form: pelleted diets may increase feed intake and energy consumption. (5) Mycotoxin contamination: aflatoxins and other hepatotoxic mycotoxins. (6) Water quality: poor water intake can affect metabolism. (7) Vaccination and handling stress: any stressor can precipitate hemorrhage in susceptible birds. (8) Lack of exercise: caged hens have minimal physical activity, contributing to positive energy balance.

Clinical signs in FLHS are often subtle and may go unnoticed until sudden death occurs. Affected hens may appear obese, with a pale comb and wattles due to anemia from chronic blood loss. They may show a drop in egg production, reduced feed consumption, and lethargy. Some hens may have diarrhea or pasty vents. In acute cases, hens may be found dead without prior signs, often with a pale comb and an enlarged, pale liver at necropsy. In less acute cases, hens may show respiratory distress, cyanosis, or abdominal distension due to hemorrhage. Neurological signs are rare but can occur if hemorrhage compresses nerves. The disease is often diagnosed at necropsy, as clinical signs are non-specific. Flock history may reveal a recent increase in mortality, particularly in well-fleshed hens. Egg production may decline by 5-10%, and eggshell quality may be poor. In severe cases, mortality can reach 20% or more. It is important to note that FLHS is a diagnosis of exclusion, and other causes of sudden death, such as heat stress, peritonitis, or toxicity, must be ruled out.

Differential diagnoses for FLHS include: (1) Fatty Liver and Kidney Syndrome (FLKS) in broilers: a metabolic disease of young broilers characterized by fatty liver and kidneys, but it occurs in younger birds and is associated with biotin deficiency. (2) Hepatic lipidosis due to aflatoxicosis: aflatoxin poisoning causes similar fatty liver changes, but also includes bile duct proliferation and fibrosis, and can be differentiated by feed analysis for aflatoxin. (3) Hemorrhagic syndrome due to vitamin K deficiency: causes widespread hemorrhages, but the liver is not typically fatty. (4) Bacterial septicemia (e.g., E. coli, Pasteurella multocida): presents with systemic signs, and necropsy reveals fibrinous peritonitis, pericarditis, and splenomegaly, with positive bacterial cultures. (5) Avian leukosis (lymphoid leukosis): causes tumorous enlargement of the liver, but histopathology shows lymphomatous infiltration. (6) Marek's disease: can cause visceral lymphomas, but also neurological signs and nerve enlargement. (7) Toxic hepatopathy (e.g., carbon tetrachloride, copper toxicity): history of exposure and characteristic lesions. (8) Egg yolk peritonitis: causes abdominal distension and peritonitis, but the liver is not typically fatty. (9) Heat stress: causes sudden death in layers, but necropsy shows congestion and no fatty liver. (10) Fatty liver hemorrhagic syndrome must also be differentiated from other causes of sudden death in layers, such as aortic rupture or cerebral hemorrhage.

The diagnostic approach for FLHS involves a stepwise process: (1) Flock history: evaluate age, production stage, diet, housing, and recent mortality patterns. (2) Clinical observation: note any signs of obesity, pale combs, or sudden death. (3) Gross necropsy: perform a thorough post-mortem examination of affected birds. Key findings include an enlarged, pale, friable liver with hemorrhages or blood clots in the abdominal cavity. The liver may be yellowish and greasy, and the capsule may be ruptured. (4) Histopathology: collect liver samples in 10% neutral buffered formalin for microscopic examination. Histological findings include diffuse hepatocellular vacuolation (lipid), sinusoidal congestion, and areas of hemorrhage. (5) Blood chemistry: measure serum levels of liver enzymes (AST, ALT), cholesterol, triglycerides, and bile acids. Elevated liver enzymes and hyperlipidemia are supportive. (6) Feed analysis: test feed for mycotoxins (aflatoxin, fumonisin) and nutrient levels (energy, protein, choline, methionine). (7) Rule out infectious causes: perform bacterial cultures of liver and other organs to exclude septicemia. (8) Molecular diagnostics: PCR for viral agents (e.g., avian leukosis virus) if tumors are suspected. (9) Imaging: radiography or ultrasound may be used to detect hepatomegaly, but are not commonly used in field diagnosis. (10) Response to treatment: if dietary modifications and lipotropic supplementation improve the condition, it supports the diagnosis.

Serology: Not typically used for diagnosis, but can rule out infectious causes. Molecular diagnostics: PCR for viral agents (e.g., avian leukosis virus) if tumors are suspected. Microbiology: Bacterial cultures of liver and other organs are negative for significant pathogens. Mycology: Feed analysis for mycotoxins, especially aflatoxin, which can be present at levels >20 ppb. Blood chemistry: Elevated serum levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), and gamma-glutamyl transferase (GGT). Hypercholesterolemia and hypertriglyceridemia are common. Bile acids may be elevated. Hematology: Anemia may be present due to chronic blood loss. Coccidiosis lesion scoring: Not applicable. Feed assays: Nutrient analysis may reveal high metabolizable energy (>2,900 kcal/kg) and low choline (<1,000 mg/kg) or methionine (<0.35%).

Radiography: May show hepatomegaly, but is not specific. Ultrasonography: Can detect increased echogenicity of the liver due to fat infiltration, but is rarely used in poultry practice. Gross necropsy photography: Essential for documentation. The liver appears enlarged, pale, and greasy, with subcapsular hemorrhages or blood clots. The abdominal cavity may contain free blood. Other organs may show pallor due to anemia.

Gross necropsy findings: The liver is enlarged (often 2-3 times normal size), pale yellow to tan, and greasy. The capsule is tense and may be ruptured. Hemorrhages may be present on the surface or within the parenchyma. The abdominal cavity may contain blood or blood clots. The bird is often obese, with abundant abdominal fat. Microscopic histopathology: Diffuse hepatocellular vacuolation (macrovesicular steatosis) is the hallmark. Hepatocytes are distended with large lipid droplets, displacing the nucleus to the periphery. There may be sinusoidal congestion and areas of hemorrhage. In chronic cases, there may be mild fibrosis. No significant inflammatory infiltrate is present. Special stains, such as Oil Red O on frozen sections, can confirm lipid content. Electron microscopy would show lipid droplets and mitochondrial changes, but is not routinely performed.

Treatment of FLHS focuses on dietary modification and supportive care. (1) Immediate dietary changes: Reduce the metabolizable energy content of the feed by decreasing the amount of corn or fat, and increase the protein content to achieve a calorie-to-protein ratio of approximately 140-160. (2) Supplement lipotropic factors: Add choline chloride (1,000-2,000 g/ton of feed), methionine (0.1-0.2% of diet), vitamin B12 (0.01-0.02 mg/kg), and folic acid (1-2 mg/kg). (3) Increase fiber content: Add oats or wheat bran to dilute energy and increase satiety. (4) Provide vitamin E and selenium as antioxidants (vitamin E 100-200 IU/kg, selenium 0.3-0.5 mg/kg). (5) Vitamin K supplementation (2-4 mg/kg) may help reduce hemorrhage risk. (6) In severe cases, feed restriction may be necessary to reduce energy intake, but this must be done carefully to avoid a drop in egg production. (7) Ensure adequate ventilation and reduce stress. (8) If mycotoxins are present, use a mycotoxin binder (e.g., aluminosilicate) at 0.5-1% of the diet. (9) Monitor flock closely for further mortality. (10) In some cases, antibiotics may be prescribed to prevent secondary bacterial infections, but they are not effective against FLHS itself.

The prognosis for FLHS is generally good if dietary modifications are implemented early. Mortality typically decreases within 2-4 weeks after correction of the diet. Egg production may recover to pre-outbreak levels within 4-6 weeks, but may not fully recover if the liver damage is severe. The long-term prognosis is favorable if the underlying nutritional imbalances are corrected. However, if the condition is left untreated, mortality can continue and may lead to significant economic losses. In severe cases, the flock may be culled if mortality exceeds 20%. The prognosis is worse in flocks with concurrent diseases or mycotoxin exposure.

After an outbreak of FLHS, it is important to monitor the flock closely. (1) Weekly mortality checks: Record mortality rates and necropsy any dead birds to confirm resolution. (2) Egg production monitoring: Track daily egg production and egg quality. (3) Feed consumption: Monitor feed intake to ensure it is appropriate for the production stage. (4) Body condition scoring: Assess body weight and fat deposition periodically. (5) Blood sampling: Periodically measure liver enzymes and lipid levels to assess recovery. (6) Feed analysis: Re-analyze feed to ensure nutrient levels are correct. (7) Review management practices: Evaluate housing, ventilation, and stress factors. (8) Implement a biosecurity plan to prevent other diseases. (9) Consider a vaccination program review if other diseases are a concern. (10) Document the outbreak and use it as a learning tool for future prevention.

Pearls: (1) FLHS is a diagnosis of exclusion; always rule out infectious causes. (2) The liver is the key organ; a pale, friable, fatty liver with hemorrhage is pathognomonic. (3) Sudden death in well-fleshed, high-producing hens should raise suspicion. (4) Dietary history is crucial; high-energy, low-protein diets are a red flag. (5) Choline and methionine supplementation is a cost-effective preventive measure. (6) Brown-egg layers are more susceptible; adjust management accordingly. (7) Necropsy should be performed promptly to avoid autolysis, which can obscure lesions. Pitfalls: (1) Misdiagnosing FLHS as a viral or bacterial disease, leading to unnecessary antibiotic use. (2) Overlooking mycotoxin contamination, which can exacerbate the condition. (3) Failing to consider the role of stress in precipitating hemorrhage. (4) Implementing drastic feed restriction, which can cause a sharp drop in egg production. (5) Not monitoring body condition, allowing obesity to develop. (6) Ignoring the impact of environmental temperature on feed intake. (7) Using outdated feed formulations that do not meet the hen's requirements.

There is no specific drug treatment for FLHS; management is primarily nutritional. However, supportive therapies may include: (1) Vitamin K (menadione sodium bisulfite) at 2-4 mg/kg of feed for 7-10 days to support coagulation. (2) Vitamin E (dl-alpha-tocopheryl acetate) at 100-200 IU/kg of feed for 2-4 weeks as an antioxidant. (3) Selenium (sodium selenite) at 0.3-0.5 mg/kg of feed for 2-4 weeks. (4) Choline chloride at 1,000-2,000 g/ton of feed for 4-6 weeks. (5) Methionine (DL-methionine) at 0.1-0.2% of the diet for 4-6 weeks. (6) Vitamin B12 (cyanocobalamin) at 0.01-0.02 mg/kg of feed for 4-6 weeks. (7) Folic acid at 1-2 mg/kg of feed for 4-6 weeks. (8) In cases with secondary bacterial infections, antibiotics such as amoxicillin (10-20 mg/kg body weight orally twice daily) or oxytetracycline (10-20 mg/kg body weight) may be used, but only under veterinary supervision and with appropriate withdrawal times. (9) Mycotoxin binders (e.g., hydrated sodium calcium aluminosilicate) at 0.5-1% of the diet can be used if aflatoxin is present. (10) Electrolytes and vitamins in drinking water may be used to reduce stress. All drug uses must comply with local regulations and withdrawal times.

FLHS has been extensively studied in poultry science. Key research includes: (1) Early studies by Couch (1956) and others identified the role of high-energy diets and lipotropic factors. (2) A study by Squires and Summers (1993) demonstrated that dietary choline and methionine supplementation reduced liver fat content in laying hens. (3) Research by Maurice et al. (1994) showed that genetic selection for low abdominal fat reduced the incidence of FLHS. (4) A field study by Shini et al. (2003) reported that increasing dietary fiber reduced mortality from FLHS in caged layers. (5) A meta-analysis by Leeson and Summers (2005) summarized the nutritional factors influencing FLHS. (6) The AAAP Avian Disease Manual provides guidelines for diagnosis and management. (7) The Merck Veterinary Manual includes a section on FLHS. (8) Recent studies have explored the role of oxidative stress and antioxidants in FLHS. (9) A study by Chen et al. (2018) investigated the effects of betaine on liver lipid metabolism in laying hens. (10) Overall, the consensus is that FLHS is a preventable metabolic disorder that requires careful nutritional management and monitoring of body condition.