Review Article

Optimizing Poultry Production: Nutritional Interventions, Management Practices and Diagnostic Approaches for Fatty Liver Disease in Layers

Bilal Javid*, Hafiz Muhammad Bilal Akhtar, Tahira Kamal and Muhammad Naeem Riaz

National Institute for Genomics and Advanced Biotechnology (NIGAB), National Agricultural research Centre Park Road, Islamabad, Pakisktan.

Received | August 16, 2024; Accepted | October 9, 2024; Published | December 26, 2024

*Correspondence |Bilal Javid, National Institute for Genomics and Advanced Biotechnology (NIGAB), National Agricultural research Centre Park Road, Islamabad, Pakisktan; Email: [email protected]

Citation | Javid, B., H. M. B. Akhtar, T. Kamal and M. N. Riaz. 2024. Optimizing Poultry Production: Nutritional Interventions, Management Practices and Diagnostic Approaches for Fatty Liver Disease in Layers. Pakistan Journal of Agricultural Research, 37(4): 340-354.

DOI | https://dx.doi.org/10.17582/journal.pjar/2024/37.4.340.354

Keywords | Fatty Liver Disease, Role of Nutrition, Diagnosis, Management, Prevention

Copyright: 2024 by the authors. Licensee ResearchersLinks Ltd, England, UK.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Introduction

Poultry farming plays an important role in meeting essential protein demands through the production of meat and eggs worldwide. This practice not only addresses nutritional needs but also serves as a significant source of revenue for small-scale farmers (Farooq et al., 2022). The poultry industry is the most rapidly expanding agricultural subsector, especially in emerging countries, and it is expected to continue growing in the future as a result of rising incomes, urbanization, and population growth, which will increase the need for meat and eggs (Sriboonyong et al., 2022). The poultry industry contributed significantly to the country’s economic growth by the production of 2362 tons of chicken meat and 25212 million eggs from a total of 2065 million poultry birds (Economic Survey of Pakistan 2023-24) The poultry sector in Pakistan encounters a range of disease obstacles, comprising infectious diseases like Newcastle disease, fowl typhoid, and salmonella, (Reynolds and Barry. 2022; Mahmood and Sabir 2021) as well as non-infectious diseases such as nutritional deficiencies, nematode infestation, and fatty liver syndrome among poultry (Murwani et al., 2022; Sajjad et al., 2021). Fats and oils play very important role in digestion process of birds, as they stimulate the release of important digestive substances like enzymes, bile, and mucus, facilitating the breakdown and absorption of nutrients in the digestive tract (Zhang et al., 2008). Fatty Liver Syndrome primarily affects commercial caged layers and is characterized as a dietary and metabolic disorder due to less exercise and confinement as compared to free rage birds or birds which are kep in furnished cages with proper place to move and express behavioral characteristics (Trott et al., 2014). Additionally, there is an abundance of fat deposited in abdominal cavity. This disease primarily affects commercial layers poultry (caged), which results in decreased egg production, death along with financial losses at large scale (Julian, 2005). Liver rupture causing internal bleeding can be the cause of a laying hen’s sudden death (Butler, 1976; Shini et al., 2020). When the birds remains alive, it can be quite challenging to detect the indications of this metabolic disorder. When necropsy is done, the liver may be found to be putty-colored and yellowish, with numerous different types of hemorrhages and occasionally big blood clots originating from the liver in the abdominal cavity (Shini et al., 2006).

Fatty liver disease can cause a notable drop in egg production and trigger sudden mortalities, leading to substantial financial setbacks in the poultry sector and this condition is characterized by significant fatty degeneration and the buildup of fat within the liver’s parenchymal cells (Marchesini et al., 2003). FLD has the ability to impair hepatocyte mitochondria, consequently affecting the β-oxidation process of fatty acids. (Wei et al., 2008). Furthermore, there exists a strong association between Fatty liver disease and peroxisome proliferator-activated receptors (PPARs), which performs a vital role in regulating β- oxidation. Elevate levels of free fatty acids (FFAs) present in hens’ livers during the late egg-laying phase may disrupt biofilm integrity via the generation of free radicals (Sambasiva and Reddy, 2004; Zhou et al., 2008). Furthermore, a lot of saturated fatty acids (SFAs) have been shown to be harmful to hepatocytes directly as well as indirectly through causing cellular damage. (Kohjima et al., 2015). Moreover, FFAs elevated plasma level can trigger toll-like receptor 4 activation on macrophages as well as adipocytes, leading to the initiation of an inflammatory reaction within the fatty liver. (Yuan, 2001). When Kupffer cells in liver become activated, they have the ability to release significant quantities of cytokines, such as TNF-α, IL-1, IL-6, and IL-8.a (Rutkowski et al., 2009). TNF-α stimulates the release of FFAs in tissue which are in peripheries and the synthesis of fatty acids in liver, while Interleukin-1 and Interleukin-6 similarly delay the transport and secretion of triglycerides. (Navasa et al., 1998). FFAs disrupt lipid metabolism and glycogen synthesis in primary laying hen hepatocytes by altering the AMPK signaling pathway. This leads to oxidative stress, cell proliferation arrest, and apoptosis (Huang et al., 2022). Due to customer concerns about hen welfare, husbandry practices in the egg industry have changed. Consequently, different methods for producing eggs have been implemented over the past 20 to 30 years. Despite this shift, cage production systems remain the dominant method for laying hens in many countries. (Mench et al., 2011).

Roughly 85% of layers in the world were housed in traditional cage setups in 2011. (Windhorst et al., 2011). According to studies conducted in 2018, the usage of cage systems, mostly conventional cages, was the leading method of egg production in all major egg-producing countries and regions worldwide, The EU-15 being the exception. (Mench and Rodenburg et al., 2018). In China, Japan, and the United States, the three countries that produce the most eggs in the world, caged hens provide around 90% of the eggs produced. In the remaining four top countries in terms of poultry egg prduction (Turkey, India, Russia, and Mexico), this figure reaches 100%. (Mench and Rodenburget al., 2018). Research aimed at examining the impact of diet on fatty liver disease in layers found that birds fed higher levels of poultry oil (3% and 4.5%) had increased hemorrhagic liver scores. The study concluded that a high-energy diet is the most typical reason for Fatty liver disease in caged layers. Ultimately, Fatty liver disease is a disorder of lipid metabolism (Liaqat et al., 2023). There are 5 freedom of animal welfare which comprise (1) freedom from thirst and hunger; (2) freedom from discomfort (3) freedom from pain, injury, or disease; (4) freedom to exhibit typical behavior; and (5) freedom from fear and distress (McCausland 2014). To understand animal welfare, three major science-based frameworks have been employed (Fraser, 2003; Hemsworth et al., 2015). Among them are: Affective state: animal subjective sensations, biological functions: an animal’s capacity to adapt to its surroundings and determine whether its needs are met and natural living: an animal’s capacity to live in accordance with its nature and exhibit typical behaviors (Broom, 1986; EFSA, 2005; Fraser, 2003; Hemsworth et al., 2015). Lack of exercise in traditional cages can lead to metabolic issues in hens. According to Duncan (2001), paralysis in caged hens around their maximum production is known as “cage layer fatigue” and is caused by spinal cord compression, fracture of the thoracic vertebrae, and weakening of the bones. When comparing traditional cages to methods that provide more opportunities for movement and behavioral expression, non-infectious diseases including osteoporosis and fatty liver are more prevalent. (Weitzenbürger et al., 2005; Kaufman-Bart, 2009; Lay et al., 2011; Widowski et al., 2013). One of the most important metabolic conditions called fatty liver is commonly observed in poultry which are kept in traditional cages (EFSA, 2005; Jiang et al., 2014). It commonly results in liver damage or rupture and unexpected mortality. Elevated ambient temperatures, a high degree of stress, and inactivity are the key factors believed to lead to the occurrence of fatty liver (EFSA, 2005). The severe fat buildup in the abdominal cavity and the visceral areas is the first thing that is noticed during a post-mortem examination the liver is swollen, spherical, and extremely delicate. Its color changes from pale-brown to yellow due to fat buildup (Meijering, 1979). A suspected case of fatty liver disease based on postmortem examination was observed at local layer poultry farm at Lodhran, Punjab Pakistan. The Figure 1 shows the pathological signs of fatty Liver disease in layer bird. The labeled Figure 1 shows the liver color changed from normal to pale- yellow and underlying accumulated fat is also indicating fatty liver disease. Figure 2 is showing the enlarged liver size with excessive amount of fat which is suspected indicative of fatty liver disease. Different types of cages have different impact on chicken health, growth and stress level as there are 3 major types of housing system for poul

 

 

try, firstly is the cage free system, secondly, conventional cage system and thirdly, the most important is furnished cage system. Compared to cages, cage-free setups typically provide more opportunities for locomotion and proper movement. The movement level of the bird is usually high due to dispersed resources. Stocking densities need to be low enough to allow for freedom of movement and expression of behavior; otherwise, birds could be restrictive in terms of movement (Leone and Estevez, 2008; Lay et al., 2011). Hens kept in cage-free systems may face more fractures during the laying season than hens kept in furnished cage systems during flight or fall on objects like feeders, drinkers etc. (Lay et al., 2011; widowski et al., 2013). The conventional cage system allows insufficient movement and exercise which can worsen bone fragility and muscle weakness in birds (Webster, 2004; Widowski et al., 2013). Hens frequently develop disuse osteoporosis as a result of the severe behavioral limitation they endure in traditional cages, which further weakens their bones (LayWel, 2006). Out of all the housing systems, conventional caged hens have the lowest bone strength and the highest rate of fractures (Widowski et al., 2013). Compared to conventional cages, furnished cages typically provide greater mobility. This system specifically provides more space, perches, nests, and a scratch area. Because of this, hens’ behavior is more varied and unrestrained than it would be in conventional cages, and their physical condition is improved. Birds are allowed to engage in some of their most highly driven behaviors, however some issues may be faced while using cage systems which include insufficient delivery of litter in cage, limited movement, less or no ground scratching and wing flapping for very short time due to cage walls (Appleby et al., 2002; Lay et al., 2011). Another drawback to use cage system when hens are highly motivated to carry out a variety of natural behaviors, such as perching and identification of a nesting site and housing restrictions hinder hens from carrying out these behaviors, they may become frustrated and experience emotional distress, which may manifest as typical back-and-forth pacing behavior (Fraser et al., 2013), and Stress may be developed due Hysteria and feather pecking (lay et al., 2011).

Mechanism of Fatty Liver Disease (FLD) development

Molecular mechanism such as Insulin resistance is involved in the development of FLD by down grading of insulin receptor substrate 2 (IRS-2), over expression of Sterol regulatory element binding proteins-1 (SREBP-1), upregulation of de novo lipogenesis (DNL), and inhibition of β-oxidation of fatty acids which further promotes hepatic lipid accumulation (Wree et al., 2016). Triglycerides are the primary byproduct of hepatic de novo lipogenesis, and the liver is the primary location for the synthesis of phospholipids and cholesterol (Piotrowska et al., 2011). FLD is developed in layers when there is any disruption in lipid metabolism and layer birds develop this metabolic disease due to a combination of factors including hormones, toxic compounds, nutritional habits, diet, genetics, and environmental conditions (Guo et al., 2012). One of the main characteristics of FLHS is insulin resistance, which is necessary for lipotoxicity, oxidative stress, and the inflammatory cascade to be activated (Del et al., 2018). An important study showed that FLD is markedly raised the levels of Cidea and Cidec mRNA expression in liver and adipose tissue and these two lipid droplet-associated proteins are crucial in hepatic lipid accumulation and

increased steatosis (Peng et al., 2019). AMPK signaling pathway is found to play a critical role in the development of FLD in laying hens. The pathway was associated with increased mRNA expression of lipid synthesis-related genes, such as ACC, FAS, GPAT, and cholesterol synthesis-related genes, HMGR, and HNF4α, and decreased expression of fatty acid oxidation-related gene CPT1 (Gao et al., 2019). A study using 30-week-old laying hens demonstrated that exogenous estrogen (E2) administration led to elevated liver weights, notable liver modifications, and an increased risk of Fatty Liver Hemorrhagic Disease, particularly when the hens were given an unrestricted diet compared to those with restricted feed intake (Shini and Agim 2014). Zhu et al. (2021) suggested a practical model to describe the genetic pathways to understand pathogenesis of Fatty liver disease in chickens. Furthermore, they brought attention to CNTF, SOCS3, and IL6 in the Jak-STAT signaling cascade as possible treatment targets for chicken FLHS. To cause hepatic lipid deposition, Zhang et al. (2011) discovered that dietary carbohydrates greatly elevated the transcript levels of the genes SREBP-1c and ChREBP mRNA, which are connected to fat synthesis. Additionally, they significantly increased the activity of FAS (fatty acid synthase) and ACC (acetyl CoA carboxylase). Reduced protein intake prevents the liver from properly moving fat out, which results in F. Low protein diets also do not supply enough protein to make apolipoproteins. One reason why laying hens develop fatty liver could be due to a diet low in protein (Zhang et al., 2011). FFAs interferes with AMPK signaling, which impacts oxidative stress, energy metabolism, and cellular functions (Huang et al., 2022). Figure 6 demonstrates the interference of free fatty acids with AMPK signaling pathway.

Role of Dietary Intake in Fatty Liver Disease

Nutrition (carbohydrates, fats and proteins) plays very important role as predisposing factor for the fatty liver disease in layers. There have also been reports that genetic, hormonal, and environmental variables contribute to poultry FLHS (Hansen and Walzem, 1993). It is clear that nutrition has a significant role in the onset of fatty liver disease, even though it is caused by a variety of causes, either separately or in combination in poultry because 97% of the affected birds had significant fat deposition or were considered overweight (Trott et al., 2014). Certain dietary components may have a role in the advancement of fatty liver disease, according to other studies (Walzem et al., 1993; Cherian et al., 2002; Yousefi et al., 2005) such as maize, when fed throughout the summer as opposed to the winter, a high energy maize feed was observed to cause an increased rate of Fatty Liver Disease than barley feed which is low in energy. poultry fed diets rich in calories, poor in linoleic acid, and low in choline had higher liver weights and hemorrhagic scores (Pearson et al., 1981; Yousefi et al., 2005). High energy diet is also a predisposing factor It has been observed after different experiments that birds which were effected with fatty liver disease had history of taking high energy adlibitum poultry feed (Polin and Wolford, 1976; Polin and Wolford, 1977; Haghighi and Polin, 1982; Choi et al., 2012). The use of oil in poultry feed causes sever liver problems in layers. The important study investigated the impact of poultry oil on liver health and fatty liver syndrome in 120 Lohmann Single Comb layers. Birds fed diets with increasing poultry oil (0%, 1.5%, 3.0%, 4.5%) for 20 weeks showed significant adverse effects on liver health, with the highest oil level causing severe liver damage and elevated plasma lipid profiles. The study concluded that high levels of poultry oil (4.5%) negatively affect liver health, leading to fatty liver syndrome (Liaqat et al., 2023). Fats and oils are crucial for bird digestion by promoting the secretion of digestive substances and maintaining gut cell integrity. However, excessive intake can disrupt lipid metabolism, leading to Fatty Liver Syndrome due to excessive liver fat deposition (Zhang et al., 2008). High energy diets either from fat or carbs were found to cause FLS, while the group that used fat as an energy source had an average high hemorrhagic score (Rozenboim et al., 2016; Zhang et al., 2008). Rozenboim et al. (2016) reported that layers fed a meal high in energy and poor in protein had higher liver hemorrhage scores. Layer birds’ liver had higher levels of fat and dry matter when 4.5% poultry oil was fed. The elevated fat content of the liver was linked to the existence of fatty liver disease in layer birds (Choi et al., 2012; Zhang et al., 2008). The primary symptoms seen in studies at greater levels of poultry oil are fatty degeneration and hemorrhages, which are crucial indicators of FLS in layer hens (Trott et al., 2014). Increased consumption of feed and positive energy can lead to the advancement of Fatty Liver Syndrome in layer birds (Shini et al., 2020). Laying hens may naturally be predisposed to Fatty Liver sundrome due to low protein diets with greater ME from fat. (Rozenboim et al., 2016). The laying hens fed maize as an energy source had a significantly higher incidence of FLD compared to those fed wheat. The use of oats as a protein source significantly reduced liver fat, the liver fat content and triglyceride levels were 30 to 50% higher in the maize/soybean diet group versus the barley/soybean diet group. The use of oats as a protein source significantly reduced liver fat (Cross et al., 1987). According to reports, laying hens’ liver fat content will significantly increase when 1.5 mg/kg of aflatoxin is added to their feed. Additionally, FLHS is also brought on by the T2 toxin found in Fusarium oxysporum. Many toxic compounds, including aflatoxin, which can seriously affect laying hens liver and create problems with the metabolism of lipids in liver cells, ultimately resulting in fatty liver, can be produced by moldy feed (Huang et al., 2022). Figure 7 demonstrates the onset of metabolic disease (FLD) due to deficiencies of important nutrients in poultry feed.

Diagnostic Tests for Fatty Liver Disease in Layers

Liver function tests reveal the liver’s capacity for clearance, metabolism, and synthesis (Jaensch, 2000). Bile acids, plasma albumin concentrations, or clotting factor assays can be used to evaluate synthesizing capacity of liver. It is impossible to evaluate overall liver function with just one test, given the liver’s intricate roles in metabolism, excretion, and defense. Therefore, a series of laboratory tests are typically used together to identify liver issues, determine their severity, monitor the disease’s progression, and aid in diagnosing the cause and guiding treatment. (Hochleithner et al., 2006). The early diagnosis of fatty liver disease in laying hens is determined by many biochemical tests such as serum cholesterol, serum calcium, aspartate aminotransferase (AST), lactate dehydrogenase (LDH) and creatine kinase (CK). The average levels of serum cholesterol, serum calcium, lactate dehydrogenase (LDH), creatine kinase (CK), and aspartate aminotransferase (AST) were found to be 139.4 ± 87.2 (mg/dL), 24.5 ± 5.4 (mg/dL), 1238.3 ± 475.2 (IU/L), 1107.3 ± 422.8 (IU/L), 53.6 ± 23.1 (IU/L) in the Non-FLHS flock, and 210.2 ± 173.2 (mg/dL), 174.3 ± 53.5 (IU/L), 25.2 ± 4.1 (mg/dL), 1104.9 ± 472.9 (IU/L), 1694.9 ± 691.3 (IU/L), in the FLHS flock. With the exception of CK, the activities of serum cholesterol, aspartate aminotransferase (AST), and LDH were considerably greater in the FLHS flock compared to the Non-FLHS flock (So et al., 2009). Hepatocyte damage is reflected by serum ALT activity, a very sensitive and moderately selective preclinical and clinical indication of hepato-pathology (Ozer et al., 2008). The findings imply that measuring enzyme activity in birds that are suggestive of liver damage—specifically, AST, LDH, and GDH glutamate dehydrogenase—is a useful method for identifying FLHS (fatty liver-hemorrhagic syndrome) in a flock of layers (Diaz et al., 1999). Metabolomics is a novel technology that has been utilized in recent years to study and characterize the general alterations in endogenous small molecule metabolites brought on by inside and outside stimuli at a particular time and under a particular set of conditions (Zira et al., 2013; Yu et al., 2020). There are two major analytical platforms of metabolomics which are Mass spectrometry (MS) and nuclear magnetic resonance (NMR) (Patel and Ahmed, 2015). GC-TOF-MS, noted for its high separation efficiency and detection sensitivity, is an effective metabolomics technique for diagnosing and assessing FLHS in laying hens by identifying key biomarkers such as glutamate and oleic acid through ROC curve analysis (Guo et al., 2021). For the purpose of diagnosing FLHS in laying hens, all biochemical analyses were carried out. These analyses included: lipid metabolism indices total triglycerides (TGs), total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C); liver function indices, which included aspartate aminotransferase and alanine aminotransferase; liver antioxidant capacity indices, which included glutathione peroxidase (GSH-Px), malondialdehyde (MDA) and glutathione peroxidase (GSH-Px); and mitochondrial function indices, which included hydrogenase (CAT), citric acid synthase (CS), carnitine palmitoyl transferase-I (CPT-1), cytochrome C oxidase (COX), and coupling protein 2 (UCP2) (San et al., 2023). Indicators of fatty liver disease are employed in layers to diagnose certain plasma enzymes at particular concentrations. The important enzymes include Aspartate Aminotransferase (AST), Lactate dehydrogenase (LDH), Glutamate dehydrogenase (GDH) (Diaz et al., 1999). Some Important diagnostic tests from different published articles are given in Table 1. Microscopic image of normal liver (Figure 3) is histologically different from fatty liver in which hepatocytes are distended with fat vacuoles (Figure 4).

Risk factors associated with fatty liver disease in layers

Outbreaks of fatty liver disease in layers are linked to different factors such as environment, genetics, and nutrition. Diets high in calories but low in protein are the main cause of this disease. (Diaz et al., 1999; Choi

 

 

et al., 2012; Rozenboim et al., 2016). Typically, FLHS is more common in birds in the middle and late stages of egg production as well as those with excess body weight (BW). Furthermore, liver is the primary organ involved in metabolic process of lipids in birds, and a variety of fatty liver diseases are caused by gradual alterations in hepatic lipid metabolism. Laying hens with Fatty Liver disease have increased triglyceride synthesis (TG) and disrupted very low-density lipoprotein (VLDL) production and secretion, which may be caused by an excess of fatty acid supply or hindered oxidation in the liver. (Dong and Tong, 2019; Gao et al., 2019). Based on production-related statistics, aged layers have a higher risk of FLHS, and their body weight is consistently greater than recommended levels (Dong and Tong, 2019). Other studies have also demonstrated that laying hens’ fatty liver illness is mostly caused by an excessive accumulation of liver fat, which is brought on by an abnormal build-up of fat caused by an issue in the liver’s fat metabolism. Estrogen level effectively caused Fatty Liver disease in hens, with characteristic and systemic effects on liver, coming from a disruption in the metabolism of fat and long-term low-grade inflammation (Dong and Tong, 2019; Shini et al., 2019; Zhuang et al., 2019; Shini et al., 2020). A high body temperature inhibits the thyroid gland’s ability to secrete thyroid hormones and weakens lipolysis, both of which are risk factors in the development of fatty liver disease (Huang et al., 2022). Lipid metabolism can be changed by hormones

 

Table 1: Some important diagnostic tests for the diagnosis of fatty liver disease in poultry are given in table 1 with references.

S.N.	Diagnosis	Diagnostic/Biochemical tests	Reference
1	Lipid Metabolism	Total triglycerides (TGs) Total Cholesterol (TC) High-Density Lipoprotein Cholesterol (HDL-C) Low-density lipoprotein cholesterol (LDL-C)	San et al., 2023
2	Mitochondrial Function	Hydrogenase (CAT) Citric acid synthase (CS) Carnitine palmitoyl transferase-I (CPT-1) Cytochrome C oxidase (COX) Coupling Protein 2 (UCP2)
3	Liver Function Tests	Aspartate Aminotransferase(AST) Alanine aminotransferase (ALT)
4	Liver Antioxidant Capacity	Glutathione peroxidase (GSH-Px) Malondialdehyde (MDA) Mlutathione peroxidase (GSH-Px)
5	Lipid Metabolism, Energy Metabolism, Amino acid metabolism	Serum Mrtabolomic Profiling based on Gas chromatography time-of-flight mass spectrometry (GC-TOF-MS)	Guo et al., 2021
6	Lipid Profile	Total Cholestrol (TC) High Density Lipoprotein (HDL) Low density lipoprotein (LDL) Triglycerides (TG)	Liaqat et al., 2023
7	Blood Serum chemistry	Serum Cholesterol Serum calcium Aspartate aminotransferase Lactate dehaydrogenase Creatinine kinase	So et al., 2009
8	Plasma Enzymes	Aspartate Aminotransferase (AST) Lactate dehydrogenase (LDH) Glutamate dehydrogenase (GDH)	Diaz et al., 1999

 

 

such as estrogen thyroid hormone, cortisol, and other hormones, which can impact the development of fatty liver disease. The majority of studies have demonstrated that laying hens affected with fatty liver disease have much greater estrogen levels than regular laying chickens. On the other hand, estrogen increases fat synthesis in the laying hens’ liver and decreases the ability of fatty acids in the liver’s mitochondria to oxidize. Exogenous estrogen can cause male chicks’ livers to accumulate fat. The average liver weight of the estrogen group was much larger than that of the normal group, and the serum concentrations of TC and TG were also significantly higher (Choi et al. (2012). Hens with fatty liver disease have markedly reduced serum thyroid hormone levels, which may contribute to the condition due to the hormone’s role in stimulating lipolysis and boosting metabolic rate to reduce body fat. (Zhu et al., 2021). Figure 5 demonstrates the major risk factors of FLD (fatty liver disease in Poultry). Some genes are also associated with fat development in poultry liver which are ACC (Acetyl-CoA Carboxylase) FAS (Fatty Acid Synthase) PGC-1α (Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1-alpha) PPARγ (Peroxisome Proliferator-Activated Receptor Gamma) SREBP-1c (Sterol Regulatory Element-Binding Protein 1c) and PLIN1 (Perilipin 1) and these genes act differently in different chicken breeds (Kang et al., 2022). 23 target genes are important for FLD and these genes, such as HAO1, ABCD3, and BLMH, are controlled by both DNA methylation and (long non coding RNAs) lncRNAs (Tan et al., 2020). Intestinal microbes have been identified as a crucial component of the enteric-liver circulation, indicating a strong correlation between the intestinal flora and the development of fatty liver (Federico et al., 2016).

Management and Control of Fatty Liver Disease

Taurine can control fat metabolism in the liver, decrease fat buildup in the liver, prevent oxidative stress, and improve mitochondrial dysfunction hence protecting laying hens from FLHS (San et al., 2023). Taurine, one of the limited free amino acids, earned

 

 

its name from being initially discovered and isolated from cattle bile. Taurine demonstrates numerous physiological and pharmacological impacts, such as antioxidant, stabilizing cell membranes, detoxification, regulating osmotic pressure, and contributing to the development of the brain and retina (Roysommuti and Wyss, 2014). According to Cross et al. (1987), the degree of damage to liver caused by hemorrhages and liver fat content in fatty liver disease in laying hens were considerably decreased when oats were used as a protein source. It has been observed that addition of components such alfalfa, wheat bran, or fermentation leftovers greatly lowers liver cholesterol levels (Maurice et al., 1978). Energy intake is therefore crucial, but it doesn’t seem to be the only nutritional component influencing the buildup of lipids in birds. Several dietary components have been shown to have implications for liver lipid levels that cannot be explained in the context of dietary energy content. For example, it was found that the occurrence of subclinical fatty liver disease and liver lipid levels decreased in that order with isocaloric diets based on maize, wheat, or barley (Perason et al, 1978). Thus, a positive energy balance seems to be a contributing component to FLHS, and it has been found that environmental and nutritional factors that impact this also affect the buildup of liver fat. Thus, consumption of lower-energy diets can diminish the fat content in the liver. Exercise also plays a role in energy balance; chickens kept in cages were found to have higher liver lipid contents than hens kept in floor pens. It has also been demonstrated that hens kept in cages can have lower liver fat levels when pushed to exercise (Whitehead, 1979). Energy balance can also be impacted by environmental temperature; birds housed at higher temperatures have been shown to have higher liver lipid levels (Perason and Butler, 1978). Studies have indicated that the fatty liver syndrome is most likely to occur in summer season (Ivy and Nesheim, 1973). Lipid metabolism’s homeostasis is destroyed by excessive triglyceride buildup in the liver and fatty acid oxidation inhibition (Miao et al., 2021). By functionally preventing the buildup of extra lipid in the liver, osteocalcin (OCN) lowers the possibility of FLD (Miao et al., 2021; Lin et al., 2020). Osteocalcin protects chickens from fatty liver disease (FLHS) by activating the JNK and AMPK signaling pathways, down-regulating PPARα signaling to mitigate liver damage from various factors, and stimulating pancreatic β-cells to enhance insulin synthesis and reduce insulin resistance (Tu et al., 2023). Probiotic are also a good source to mitigate the FLD in poultry (Gao et al., 2021).

Conclusion and Recommendations

Fatty liver disease represents a complex and multifactorial disorder in layer poultry, with substantial implications for production efficiency and bird welfare. Understanding the etiology, pathogenesis, risk factors, diagnostic methods, and management strategies associated with FLD is crucial for mitigating its negative impact on the poultry industry. Implementing preventive measures, early detection, and targeted interventions are essential for optimizing production performance and promoting the welfare of layer poultry affected by FLD. Future research should be focused on innovative approaches, including modulation of the gut-liver axis and precision nutrition, holds promise for improving outcomes and addressing the challenges posed by FLD in layer poultry production. Accurate diagnosis of FLD in layer poultry is crucial for timely intervention and management. Diagnostic methods encompass a combination of clinical assessment, biochemical assays, imaging techniques, and histopathological examination. Clinical signs such as decreased egg production, lethargy, and hepatomegaly provide initial clues, while biochemical assays measure serum biomarkers indicative of liver dysfunction. Imaging modalities such as ultrasonography and computed tomography facilitate non-invasive evaluation of liver morphology and lipid content. Histopathological examination of liver biopsy specimens remains the gold standard for confirming FLD diagnosis and assessing histological changes associated with disease severity. There may be additional risk factors responsible for fatty liver disease in poultry that have not been discussed in published research. The different type of housing system have different impact on poultry production and occurrence of diseases but comparatively, furnished cage system have positive impact in terms of higher production, desirable welfare and less chances of disease occurrence. This review offers a thorough understanding of the gaps in the diagnostic literature as well as other management-related concerns pertaining to fatty liver disease in poultry, which could be beneficial in reducing the issue within the poultry industry.

Future Directions

Emerging research on the gut-liver axis and the role of gut microbiota in FLD pathogenesis offers promising avenues for novel therapeutic interventions. Modulating gut microbiota composition through probiotics, prebiotics, or dietary additives may help prevent FLD development and improve liver health in layer poultry. Furthermore, advances in genetic selection for FLD resistance and precision nutrition strategies tailored to individual bird requirements hold potential for enhancing production efficiency and welfare outcomes. Continued research efforts aimed at elucidating the underlying mechanisms of FLD and evaluating novel interventions are essential for addressing this significant challenge in layer poultry production.

Acknowledgments

I would like to extend my heartfelt appreciation to the senior colleagues at the National Institute for Genomics and Advanced Biotechnology, NARC, Islamabad, for their exceptional guidance and unwavering support during the writing and analysis of this review paper. Their profound expertise and valuable insights have greatly enhanced the manuscript’s quality and depth. Additionally, I am deeply grateful to my coauthors for their collaborative spirit and substantial contributions, which have greatly enriched the review’s content. This work owes much to their collective dedication and encouragement.

Novelty Statement

This review aims to fill critical research gaps in the understanding of fatty liver disease (FLD) in poultry by providing a comprehensive synthesis of current knowledge. It offers insights into various diagnostic tests, examines the impact of different housing systems on poultry health, and elucidates the pathogenesis and associated risk factors of FLD. Additionally, the review highlights effective management strategies, serving as a vital resource for researchers and poultry practitioners dedicated to improving animal welfare and health outcomes.

Authors Contributions

Bilal Javid: Idea and conceptualization, write up of original article.

Hafiz Muhammad Bilal Akhtar: Collection of relevant data, Visualization and arrangement of data.

Tahira Kamal: Proof reading of review paper.

Muhammad Naeem Riaz: Technical support and guidelines.

Conflict of Interest

The authors declare that there is no conflict of interests regarding the publication of this article.

Ethics Approval

Not applicable

References

Appleby, M.C., Walker, A.W., Nicol, C.J., Lindberg, A.C., Freire, R., Hughes, B.O. and Elson, H.A. (2002). Development of furnished cages for laying hens. Br. Poul. Sci., 43(4), 489-500. https://doi.org/10.1080/0007166022000004390

Assi Husain, H., Sadeghi, A. and Karimi, A. (2023). Effects of chicory, turmeric, artichoke and sage powder in high energy and low protein diets on yield, egg quality and fatty liver status in laying hens. Anim. Sci. J., 36(138), 19-38.

Broom, D.M. (1986) Indicators of poor welfare. British Vet. J. 142: 524-526. https://doi.org/10.1016/0007-1935(86)90109-0

Butler, E. J. (1976). Fatty liver diseases in the domestic fowl—A review. Avian Pathology, 5(1), 1-14.

Cherian, G., Holsonbake, T.B., Goeger, M.P. and Bildfell, R. (2002). Dietary CLA alters yolk and tissue FA composition and hepatic histopathology of laying hens. Lipids, 37(8), 751-757. https://doi.org/10.1007/s11745-002-0957-4

Choi, Y.I., Ahn, H.J., Lee, B.K., Oh, S.T., An, B.K. and Kang, C.W. (2012). Nutritional and hormonal induction of fatty liver syndrome and effects of dietary lipotropic factors in egg-type male chicks. Asian-Australas. J. anim. Sci., 25(8), 1145. https://doi.org/10.5713/ajas.2011.11418

Cross, K.E., Dodds, P.F., Noble, R.C., McCartney, R. and Connor, K. (1987). Effects of age and diet on the lipid content and composition of gallbladder bile, liver and serum in laying strains of hen. Br. Poult. Sc., 28(4), 577-584. https://doi.org/10.1080/00071668708416993

Del Campo, J.A., Gallego, P. and Grande, L. (2018). Role of inflammatory response in liver diseases: Therapeutic strategies. World j. Hepatol., 10(1), 1. https://doi.org/10.4254/wjh.v10.i1.1

Diaz, G.J., Squires, E.J. and Julian, R.J. (1999). The use of selected plasma enzyme activities for the diagnosis of fatty liver-hemorrhagic syndrome in laying hens. Avian Di., 768-773. https://doi.org/10.2307/1592746

Dong, X. and Tong, J. (2019). Different susceptibility to fatty liver-haemorrhagic syndrome in young and older layers and the interaction on blood LDL-C levels between oestradiols and high energy-low protein diets. Br. Poul. Sci., 60(3), 265-271. https://doi.org/10.1080/00071668.2019.1571164

Duncan, I.J. (2001). The pros and cons of cages. World’s Poult. Sci. J., 57(4), 381-390. https://doi.org/10.1079/WPS20010027

EFSA (2005) Welfare aspects of various systems for keeping laying hens. EFSA J. 197: 1-23. https://doi.org/10.2903/j.efsa.2005.197

EFSA (2005). Welfare aspects of various systems for keeping laying hens. EFSA Journal 197: 1-23.

Farooq, U., Mustafa, R., Khalid, M.F., Auon, M., Mahmood, U., Wahaab, A. and Hussain, J. (2022). Supplementation of herbal seeds to improve the growth performance and digestion in Japanese quail (Coturnix coturnix Japonica). Agrobiol Rec., 10, 19-25. https://doi.org/10.47278/journal.abr/2022.021

Federico, A., Dallio, M., Godos, J., Loguercio, C. and Salomone, F. (2016). Targeting gut-liver axis for the treatment of nonalcoholic steatohepatitis: translational and clinical evidence. Transl. Res., 167(1), 116-124. https://doi.org/10.1016/j.trsl.2015.08.002

Fossum, O., Jansson, D.S., Etterlin, P.E. and Vågsholm, I. (2009). Causes of mortality in laying hens in different housing systems in 2001 to 2004. Acta Veterinaria Scandinavica, 51, 1-9. https://doi.org/10.1186/1751-0147-51-3

Fraser, D. (2003). Assessing animal welfare at the farm and group level: the interplay of science and values. Anim. Welf., 12(4), 433-443. https://doi.org/10.1017/S0962728600026038

Fraser, D., Duncan, I. J., Edwards, S. A., Grandin, T., Gregory, N. G., Guyonnet, V., & Whay, H. R. (2013). General principles for the welfare of animals in production systems: the underlying science and its application. The Veterinary Journal, 198(1), 19-27.

Gao, X., Liu, P., Wu, C., Wang, T., Liu, G., Cao, H. and Guo, X. (2019). Effects of fatty liver hemorrhagic syndrome on the AMP-activated protein kinase signaling pathway in laying hens. Poult. sci., 98(5), 2201-2210. https://doi.org/10.3382/ps/pey586

Gao, X., Liu, S., Ding, C., Miao, Y., Gao, Z., Li, M. and Song, S. (2021). Comparative effects of genistein and bisphenol A on non-alcoholic fatty liver disease in laying hens. Environ. Pollut., 288, 117795. https://doi.org/10.1016/j.envpol.2021.117795

Government of Pakistan, Ministry of Finance. (2023). Economic survey of Pakistan 2022-2023. https://finance.gov.pk/survey/chapter_24/Highlights.pdf

Guo Xiao Quan, G.X., Cao Hua Bin, C.H., Hu Guo Liang, H.G., ZhangCaiYing, Z.C., Li Hao Tang, L.H., Chao HongFeng, C.H. and Li Lin, L.L. (2012). Effect of high-energy low-protein diet supplemented with biotin on fat metabolism of laying hens.

Guo, L., Kuang, J., Zhuang, Y., Jiang, J., Shi, Y., Huang, C. and Guo, X. (2021). Serum metabolomic profiling to reveal potential biomarkers for the diagnosis of fatty liver hemorrhagic syndrome in laying hens. Front. Physiol., 12, 590638. https://doi.org/10.3389/fphys.2021.590638

Haghighi-Rad, F., and Polin, D. (1982). Lipid alleviates fatty liver hemorrhagic syndrome. Poultry Science, 61(12), 2465-2472

Hansen, R.J. and Walzem, R.L. (1993). Avian fatty liver hemorrhagic syndrome: a comparative review. Adv. Vet. Sci. Comp. Med., 37, 451-468.

Hemsworth, P.H., Mellor, D.J., Cronin, G.M., and Tilbrook, A.J. (2015). Scientific assessment of animal welfare. N.Z. vet. J., 63(1), 24-30. https://doi.org/10.1080/00480169.2014.966167

Hochleithner, M., Hochleithner, C. and Harrison, L.D. (2006). Evaluating and treating the liver. Clin. avian med., 1, 441-450.

Huang C, Li Y, Tang R, Feng, C. and Gao, X. 2022. Pathogenesis and nutritional regulation of fatty liver hemorrhage syndrome in laying hens. In: Abbas RZ, Khan A, Liu P and Saleemi MK (eds), Animal Health Perspectives, Unique Scientific Publishers, Faisalabad, Pakistan, Vol. I, pp: 214-220. https://doi.org/10.47278/book.ahp/2022.28

Huang, C., Gao, X., Shi, Y., Guo, L., Zhou, C., Li, N. and Guo, X. (2022). Inhibition of hepatic AMPK pathway contributes to free fatty acids-induced fatty liver disease in laying hen. Metabolites, 12(9), 825. https://doi.org/10.3390/metabo12090825

Ivy, C.A. and Nesheim, M.C. (1973). Factors influencing the liver fat content of laying hens. Poult. Sci., 52(1), 281-291. https://doi.org/10.3382/ps.0520281

Jaensch, S. (2000, July). Diagnosis of avian hepatic disease. In Seminars in avian and exotic pet medicine (Vol. 9, No. 3, pp. 126-135). WB Saunders. https://doi.org/10.1053/ax.2000.7140

Jiang, S., Hester, P.Y., Hu, J.Y., Yan, F.F., Dennis, R.L. and Cheng, H. W. (2014). Effect of perches on liver health of hens. Poult. sci., 93(7), 1618-1622. https://doi.org/10.3382/ps.2013-03659

Julian, R.J. (2005). Production and growth related disorders and other metabolic diseases of poultry–a review. The Vet. J., 169(3), 350-369. https://doi.org/10.1016/j.tvjl.2004.04.015

Kang, X., Amevor, F. K., Zhang, L., Shah, A. M., Zhu, Q., Tian, Y. and Zhao, X. (2022). Study on the Major Genes Related with Fat Deposition in Liver and Abdominal Fat of Different Breeds of Chicken. Braz. J. Poult. Sci., 24(01), eRBCA-2020. https://doi.org/10.1590/1806-9061-2020-1373

Kaufmann‐Bart, M. and Hoop, R.K. (2009). Diseases in chicks and laying hens during the first 12 years after battery cages were banned in Switzerland. Veterinary record, 164(7), 203-207. https://doi.org/10.1136/vr.164.7.203

Kohjima, M., Enjoji, M., Higuchi, N., Kato, M., Kotoh, K., Yoshimoto, T. and Nakamuta, M. (2007). Re-evaluation of fatty acid metabolism-related gene expression in nonalcoholic fatty liver disease. Int. j. mol. Med., 20(3), 351-358. https://doi.org/10.3892/ijmm.20.3.351

Kohjima, M., Yoshimoto, T., Enjoji, M., Fukushima, N., Fukuizumi, K., Nakamura, T., & Nakamuta, M. (2015). Hepcidin/ferroportin expression levels involve efficacy of pegylated-interferon plus ribavirin in hepatitis C virus-infected liver. World Journal of Gastroenterology: WJG, 21(11), 3291.

Lay Jr, D.C., Fulton, R.M., Hester, P.Y., Karcher, D.M., Kjaer, J.B., Mench, J.A. and Porter, R.E. (2011). Hen welfare in different housing systems. Poult. Sci., 90(1), 278-294. https://doi.org/10.3382/ps.2010-00962

Leone, E. H.and Estevez, I. (2008). Use of space in the domestic fowl: separating the effects of enclosure size, group size and density. Anim. Behav., 76(5), 1673-1682. https://doi.org/10.1016/j.anbehav.2008.08.004

Liaqat, S., Yousaf, M., Ahmad, F. and Saleemi, M.K. (2023). Impact of poultry oil supplementation on liver health and fatty liver syndrome in caged layers. Pak. J. Agric. Sci., 60(3). https://doi.org/10.21162/PAKJAS/23.111

Lin, Chiao-Wei, Ting-Wei Huang, Yu-Ju Peng, Yuan-Yu Lin, Harry John Mersmann, and Shih-Torng Ding. “A novel chicken model of fatty liver disease induced by high cholesterol and low choline diets.” Poult. Sci., 100, no. 3 (2021): 100869. https://doi.org/10.1016/j.psj.2020.11.046

Mahmood, M.S. and Sabir, R. (2021). Preparation and evaluation of avian influenza (H9) and Newcastle disease (thermostable i-2 strain) bivalent vaccine for commercial poultry. Agrobiol. R., 3, 17-23. https://doi.org/10.47278/journal.abr/2020.017

Marchesini, G., Bugianesi, E., Forlani, G., Cerrelli, F., Lenzi, M., Manini, R. and Rizzetto, M. (2003). Nonalcoholic fatty liver, steatohepatitis, and the metabolic syndrome. Hepatol., 37(4), 917-923. https://doi.org/10.1053/jhep.2003.50161

Maurice, D.V. and Jensen, L.S. (1978). Influence of diet composition on hepatic lipid accumulation and hemorrhages in caged layers. Poult. Sci., 57(4), 989-997. https://doi.org/10.3382/ps.0570989

McCausland, C. (2014). The five freedoms of animal welfare are rights. J. Agric. Environ. Eth., 27, 649-662. https://doi.org/10.1007/s10806-013-9483-6

Meijering, A. (1979). Fatty liver syndrome in laying hens—An attempt to review. World’s Poult. Sci. J., 35(2), 79-94. https://doi.org/10.1079/WPS19790007

Mench, J.A. and Rodenburg, T.B. (2018). Sustainability of laying hen housing systems. In Adv. Poult. Welf. (pp. 199-225). Woodhead Publishing. https://doi.org/10.1016/B978-0-08-100915-4.00010-5

Mench, J.A., Sumner, D.A. and Rosen-Molina, J.T. (2011). Sustainability of egg production in the United States—The policy and market context. Poult. Sci., 90(1), 229-240. https://doi.org/10.3382/ps.2010-00844

Miao, Y.F., Gao, X.N., Xu, D.N., Li, M.C., Gao, Z.S., Tang, Z.H. and Song, S.Q. (2021). Protective effect of the new prepared at ractylodes macroce phala Koidz polysaccharide on fatty liver hemorrhagic syndrome in laying hens. Poult. Sci., 100(2), 938-948. https://doi.org/10.1016/j.psj.2020.11.036

Murwani, R., Kusumanti, E. and Naumova, E.N. (2022). Areca catechu L. and Anredera cordifolia (Ten) Steenis supplementation reduces faecal parasites and improves caecal histopathology in laying hens. Int. J. Vet. Sci. Med., 10(1), 52-63. https://doi.org/10.1080/23144599.2022.2090732

Navasa, M., Gordon, D.A., Hariharan, N., Jamil, H., Shigenaga, J.K., Moser, A. and Feingold, K.R. (1998). Regulation of microsomal triglyceride transfer protein mRNA expression by endotoxin and cytokines. J. Lipid Res., 39(6), 1220-1230. https://doi.org/10.1016/S0022-2275(20)32546-3

Ozer, J., Ratner, M., Shaw, M., Bailey, W. and Schomaker, S. (2008). The current state of serum biomarkers of hepatotoxicity. Toxicol., 245(3), 194-205. https://doi.org/10.1016/j.tox.2007.11.021

Patel, S. and Ahmed, S. (2015). Emerging field of metabolomics: big promise for cancer biomarker identification and drug discovery. J. pharm. Biomed. Anal., 107, 63-74. https://doi.org/10.1016/j.jpba.2014.12.020

Pearson, A.W. and Butler, E.J. (1978). Environmental temperature as a factor in the aetiology of fatty liver-haemorrhagic syndrome in the fowl. Res. Vet. Sci., 25(2), 133-138. https://doi.org/10.1016/S0034-5288(18)32967-9

Pearson, A.W., Arkhipov, A.V., Butler, E.J. and Laursen-Jones, A.P. (1978). Influence of dietary cereal and energy content on the accumulation of lipids in the liver in fatty liver-haemorrhagic syndrome in the fowl. Res. Vet. Sci., 24(1), 72-76. https://doi.org/10.1016/S0034-5288(18)33101-1

Pearson, A.W., Curtis, M.J. and Butler, E.J. (1981). Bacterial endotoxins and the pathogenesis of fatty liver—haemorrhagic syndrome in the laying hen. Res. Vet. Sci., 31(2), 259-261. https://doi.org/10.1016/S0034-5288(18)32507-4

Peng, G., Huang, E., Ruan, J., Huang, L., Liang, H., Wei, Q. and Huang, J. (2019). Effects of a high energy and low protein diet on hepatic and plasma characteristics and Cidea and Cidec mRNA expression in liver and adipose tissue of laying hens with fatty liver hemorrhagic syndrome. Anim. Sci. J., 90(2), 247-254. https://doi.org/10.1111/asj.13140

Piotrowska, A., Burlikowska, K. and Szymeczko, R. (2011). Changes in blood chemistry in broiler chickens during the fattening period. Folia Biol. (Krakow), 59(3-4), 183-187. https://doi.org/10.3409/fb59_3-4.183-187

Polin, D., and Wolford, J. H. (1976). Various types of diets, sources of energy, and positive energy balance in the induction of fatty liver haemorrhagic syndrome. Poultry Science, 55(1), 325-334.

Polin, D., and Wolford, J. H. (1977). Role of estrogen as a cause of fatty liver haemorrhagic syndrome. The Journal of nutrition, 107(5), 873-886.

Reynolds, D.L. and Simpson, E.B. (2022). Evaluation of ivermectin antiviral activity against avian infectious bronchitis virus using a chicken embryo model. Int. J. Vet. Sci. Med., 10(1), 19-24. https://doi.org/10.1080/23144599.2022.2050077

Rodenburg, T.B., Tuyttens, F.A.M., De Reu, K., Herman, L., Zoons, J. and Sonck, B. (2008). Welfare assessment of laying hens in furnished cages and non-cage systems: an on-farm comparison. Anim. welfare, 17(4), 363-373. https://doi.org/10.1017/S096272860002786X

Roysommuti, S. and Wyss, J.M. (2014). Perinatal taurine exposure affects adult arterial pressure control. Amino Acids., 46, 57-72. https://doi.org/10.1007/s00726-012-1417-5

Rozenboim, I., Mahato, J., Cohen, N.A. and Tirosh, O. (2016). Low protein and high-energy diet: a possible natural cause of fatty liver hemorrhagic syndrome in caged White Leghorn laying hens. Poult. Sci., 95(3), 612-621. https://doi.org/10.3382/ps/pev367

Rutkowski, J.M., Davis, K.E. and Scherer, P.E. (2009). Mechanisms of obesity and related pathologies: the macro‐and microcirculation of adipose tissue. The FEBS j., 276(20), 5738-5746. https://doi.org/10.1111/j.1742-4658.2009.07303.x

Sajjad, M.M., Rasheed, M., Farooq, W., Yasmin, F., Niaz, S. and Yaqub, T. (2021). Recent updates on molecular detection of H9N2 as low pathogenic strain of avian influenza virus from poultry farms of Lahore, Pakistan. Agrobiol. R., 5, 15-20. https://doi.org/10.47278/journal.abr/2020.029

Sambasiva Rao, M. and Reddy, J.K. (2004). PPARα in the pathogenesis of fatty liver disease. Hepatol., 40(4), 783-786. https://doi.org/10.1002/hep.1840400405

San, J., Hu, J., Pang, H., Zuo, W., Su, N., Guo, Z. and Yang, J. (2023). Taurine protects against the fatty liver hemorrhagic syndrome in laying hens through the regulation of mitochondrial homeostasis. Int. J. Mol. Sci., 24(12), 10360. https://doi.org/10.3390/ijms241210360

Shini, S., Shini, A. and Bryden, W.L. (2019). Unravelling fatty liver haemorrhagic syndrome: 2. Inflammation and pathophysiology. Avian Pathol., 49(2), 131-143. https://doi.org/10.1080/03079457.2019.1682119

Shini, S., Shini, A. and Bryden, W.L. (2020). Unravelling fatty liver haemorrhagic syndrome: 1. Oestrogen and inflammation. Avian Pathol., 49(1), 87-98. https://doi.org/10.1080/03079457.2019.1674444

Shini, S., Stewart, G.D., Shini, A. and Bryden, W.L. (2006). Mortality rates and causes of death in laying hens kept in cage and alternative housing systems.

Shini, S., Shini, A., and Bryden, W. L. (2020). Unravelling fatty liver haemorrhagic syndrome: 1. Oestrogen and inflammation. Avian Pathology, 49(1), 87-98.

Shini, A. (2014). Fatty liver haemorrhagic syndrome in laying hens: field and experimental investigations.

So, H.H., Jeon, E.O., Byun, S.H. and Mo, I.P. (2009). Early diagnosis of fatty liver-hemorrhagic syndrome using blood biochemistry in commercial layers. Korean J. Poult. Sci., 36(2), 165-175. https://doi.org/10.5536/KJPS.2009.36.2.165

Squires, E.J. and Leeson, S. (1988). Aetiology of fatty liver syndrome in laying hens. https://doi.org/10.1016/0007-1935(88)90031-0

Sriboonyong, P., Poommarin, P., Sittiya, J., Opanasopit, P., Ngawhirunpat, T., Patrojanasophon, P., and Pornpitchanarong, C. (2022). The utilization of mangosteen pericarp extract for anticoccidial drug replacement in broiler feed. International Journal of Veterinary Science and Medicine, 10(1), 90-99.

Patrojanasophon, P. and Pornpitchanarong, C. (2022). The utilization of mangosteen pericarp extract for anticoccidial drug replacement in broiler feed. Int. J. Vet. Sci. Med., 10(1), 90-99. https://doi.org/10.1080/23144599.2022.2128271

Tan, X., Liu, R., Xing, S., Zhang, Y., Li, Q., Zheng, M. and Wen, J. (2020). Genome-wide detection of key genes and epigenetic markers for chicken fatty liver. Int. J. Mol. Sci., 21(5), 1800. https://doi.org/10.3390/ijms21051800

Trott, K.A., Giannitti, F., Rimoldi, G., Hill, A., Woods, L., Barr, B. and Mete, A. (2014). Fatty liver hemorrhagic syndrome in the backyard chicken: a retrospective histopathologic case series. Vet. Pathol., 51(4), 787-795. https://doi.org/10.1177/0300985813503569

Tu, W., Zhang, Y., Jiang, K. and Jiang, S. (2023). Osteocalcin and its potential functions for preventing fatty liver hemorrhagic syndrome in poultry. Anim., 13(8), 1380. https://doi.org/10.3390/ani13081380

Walzem, R.L., Simon, C., Morishita, T., Lowenstine, L. and Hansen, R.J. (1993). Fatty liver hemorrhagic syndrome in hens overfed a purified diet. Selected enzyme activities and liver histology in relation to liver hemorrhage and reproductive performance. Poult. Sci., 72(8), 1479-1491. https://doi.org/10.3382/ps.0721479

Webster, A.B. (2004). Welfare implications of avian osteoporosis. Poult. Sci., 83(2), 184-192. https://doi.org/10.1093/ps/83.2.184

Wei, Y., Rector, R.S., Thyfault, J.P. and Ibdah, J.A. (2008). Nonalcoholic fatty liver disease and mitochondrial dysfunction. World j. gastroenterol., WJG., 14(2), 193. https://doi.org/10.3748/wjg.14.193

Weitzenbürger, D., Vits, A., Hamann, H. and Distl, O. (2005). Effect of furnished small group housing systems and furnished cages on mortality and causes of death in two layer strains. Br. Poult. Sci., 46(5), 553-559. https://doi.org/10.1080/00071660500303206

Whitehead, C.C. (1979). Nutritional and metabolic aspects of fatty liver disease in poultry. Vet. Q., 1(3), 150-157. https://doi.org/10.1080/01652176.1979.9693738

Widowski, T.M., Classen, H., Newberry, R.C., Petrik, M., Schwean-Lardner, K., Cottee, S.Y. and Cox, B. (2013). Code of practice for the care and handling of pullets, layers, and spent fowl: poultry (layers): review of scientific research on priority issues. Can.: Lacombe, AB.

Windhorst, H.W. (2011). The changing global egg industry. World, 16, 0-4.

Wree, A., Mehal, W.Z. and Feldstein, A.E. (2016, February). Targeting cell death and sterile inflammation loop for the treatment of nonalcoholic steatohepatitis. In Seminars in liver disease (Vol. 36, No. 01, pp. 027-036). Thieme Medical Publishers. https://doi.org/10.1055/s-0035-1571272

Xiong, X., Sheng, X., Liu, D., Zeng, T., Peng, Y. and Wang, Y. (2015). A GC/MS-based metabolomic approach for reliable diagnosis of phenylketonuria. Anal. Bioanal. Chem., 407, 8825-8833. https://doi.org/10.1007/s00216-015-9041-3

Yousefi, M., Shivazad, M. and Sohrabi-Haghdoost, I. (2005). Effect of dietary factors on induction of fatty liver-hemorrhagic syndrome and its diagnosis methods with use of serum and liver parameters in laying hens. Int. J. Poult. Sci., 4(8), 468-72. https://doi.org/10.3923/ijps.2005.568.572

Yu, Y., Gao, Z., Lou, J., Mao, Z., Li, K., Chu, C. and Chen, F. (2020). Identification of serum-based metabolic feature and characteristic metabolites in paraquat intoxicated mouse models. Front. Physiol., 11, 65. https://doi.org/10.3389/fphys.2020.00065

Yuan, M., Konstantopoulos, N., Lee, J., Hansen, L., Li, Z.W., Karin, M. and Shoelson, S.E. (2001). Reversal of obesity-and diet-induced insulin resistance with salicylates or targeted disruption of Ikkβ. Sci., 293(5535), 1673-1677. https://doi.org/10.1126/science.1061620

Zaefarian, F., Abdollahi, M.R., Cowieson, A.nand Ravindran, V. (2019). Avian liver: the forgotten organ. Anim., 9(2), 63. https://doi.org/10.3390/ani9020063

Zhang, J.W., Chen, D.W., Yu, B. and Wang, Y.M. (2011). Effect of dietary energy source on deposition and fatty acid synthesis in the liver of the laying hen. Br. Poult. Sci., 52(6), 704-710. https://doi.org/10.1080/00071668.2010.547457

Zhang, J., Chen, D. and Yu, B. (2008). Effect of different dietary energy sources on induction of fatty liver-hemorrhagic syndrome in laying hens. Int. J. Poult. Sci., 7(12), 1232-1236. https://doi.org/10.3923/ijps.2008.1232.1236

Zhou, J., Febbraio, M., Wada, T., Zhai, Y., Kuruba, R., He, J. and Xie, W. (2008). Hepatic fatty acid transporter Cd36 is a common target of LXR, PXR, and PPARγ in promoting steatosis. Gastroenterol., 134(2), 556-567. https://doi.org/10.1053/j.gastro.2007.11.037

Zhu, Y., Mao, H., Peng, G., Zeng, Q., Wei, Q., Ruan, J. and Huang, J. (2021). Effect of JAK-STAT pathway in regulation of fatty liver hemorrhagic syndrome in chickens. Anim. Biosci., 34(1), 143. https://doi.org/10.5713/ajas.19.0874

Zhuang, Y., Xing, C., Cao, H., Zhang, C., Luo, J., Guo, X. and Hu, G. (2019). Insulin resistance and metabonomics analysis of fatty liver haemorrhagic syndrome in laying hens induced by a high-energy low-protein diet. Sci. rep., 9(1), 10141. https://doi.org/10.1038/s41598-019-46183-y

Zira, A., Kostidis, S., Theocharis, S., Sigala, F., Engelsen, S.B., Andreadou, I. and Mikros, E. (2013). 1H NMR-based metabonomics approach in a rat model of acute liver injury and