Fatty liver hemorrhagic syndrome (FLHS), also known as fatty liver disease and hepatic lipidosis, is one of the most common noninfectious causes of mortality in laying hens. The disease is characterized by sudden death of the bird, resulting from liver rupture and internal bleeding. In Queensland, Australia, 74% of deaths in caged laying hens is caused by FLHS. The primary organ affected is the liver. FLHS is most often seen in obese, actively laying adult hens. FLHS is very similar to nonalcoholic fatty liver disease (NAFLD) in humans. Thus, chickens have been used often in research studies as an animal model for this disease.
The liver is a vital organ, and is responsible for multiple metabolic roles. When a chicken develops FLHS, it means that their normal liver cells are gradually accumulating fat which causes them to no longer be work efficiently and over time these liver cells may be destroyed. As the cells die, they are replaced with scar tissue--or fibrous connective tissue. Affected birds may appear as if they suddenly became ill, however in reality, the condition has most likely been building up until the bird's liver and other organs can no longer compensate, resulting in the clinical signs.
The onset of FLHS in chickens is influenced by several nutritional, metabolic, environmental, genetic, and hormonal factors. However, it is most strongly believed to be linked with nutrition, as 97% of the affected birds were obese. Influencing factors that cause increased risk for chickens to develop FLHS include:
- Low-protein high-fat (LPHF) diet: Chickens on a low-protein (less than 17.5% Crude Protein) diet are more at risk of developing FLHS. Risk is further increased if the diet is also high in fat (greater than 3.5% Fat). This is related to the fact that birds have a unique method of removing lipids from the liver. The removal of lipids from the liver occurs in the form of lipoprotein, which is dependent on the availability of the protein moiety and phospholipids components. Thus, when birds have a protein deficiency, usually related to inadequate amino acids, it results in the buildup of lipids and the formation of fatty liver.
- High-energy diet: Chickens on a high-energy feed with corn (maize) as the predominate ingredient are more at risk of developing FLHS. Incidences of FLHS were reduced in laying hens that were fed a low-energy barley diet, especially during summer months.
- Increased age: Older hens are more likely to develop FLHS than young hens, regardless of their diet. In addition, older hens tend to have more severe accumulations of liver fat content than young hens.
- Hormone imbalance: Increased estrogen levels are an important factor in the accumulation of fat in the liver. Laying hens are predisposed to hormonal imbalances involving estrogen levels, due to fluctuations involved in the egg laying process.
- Heat stress: Exposure to excessive hot weather conditions has been known to increase the risk of sudden death in hens with FLHS.
Signs that a hen has FLHS are very difficult to identify while the bird is alive. However it is usually distinguishable during necropsy. FLHS-affected chickens develop gross lesions of the liver that are characterized by enlarged, friable, soft, and varying in color from tan to yellow to orange. Hemorrhaging is usually present in varying amounts. Sometimes blood clots are observed in the abdominal cavity, originating from the liver. The liver color and amount of hemorrhaging present usually is in direct correlation with the amount of fat accumulation in the liver.
|Liver Status||Descriptive Characteristics|
|Normal||Small, smooth, brilliant and wine red in color.|
|Level 1 - Mild||Paler, slightly more granular and less brilliant.|
|Level 2 - Moderate||Dark brown colored, granular, and enlarged liver.|
|Level 3 - Severe||Yellowish, putty-colored, ochre, without brilliance, and swollen.|