Liver: A Master Organ for Overall Health

The liver is a multi-functional organ and the largest gland of the body. It is an accessory organ of digestive system and serves both exocrine and endocrine functions. Liver is located in the anterior end of the body cavity of birds. Lobules, the functional unit of the liver are composed of hepatocytes and non-parenchymal cells (endothelial cells and Kupffer cells). Hepatocytes contributes major portion of the liver volume (80%) and performs many functions. Kupffer cells are macrophages that phagocytise pathogens, cell debris and damaged red blood cells that may enter via the hepatic portal blood. The major functions of liver are – 

1. Metabolism of carbohydrates, proteins, fats, vitamins, and minerals 
2. Fat mobilization & biotransformation
3. Removal of waste products 
4. Detoxification
5. Storage

Liver is the source of bile which plays an important role in fat digestion and absorption. Liver is a prime site for storage of fat, fat-soluble vitamins (A, D, K and E) and water-soluble vitamins (B1, B2 and B12), glycogen, and some minerals like iron (Fe) and copper (Cu). The precursors of egg yolk (Very Low-Density Lipoprotein and Vitellogenin) are synthesized in the liver and transported to the ovary through blood circulation. The liver is also involved in the activation of vitamin D3 to stimulate calcium absorption for eggshell formation. 

Factors affecting liver health:

Metabolic disorder: Fatty liver haemorrhagic syndrome:

Excess lipid accumulation in the liver is known as fatty liver or steatosis. FLHS is a significant disease of caged layer hens characterized by excessive accumulation of fat in the liver and abdominal cavity, liver rupture, haemorrhage, reduced egg production, increased FCR, reduced eggshell quality, and higher mortality rates. The kidneys are also pale and swollen. The actual cause of the disease is still unclear. Some of the common causes of FLHS in birds include high energy diets, positive energy balance, increased lipogenesis in the liver, biotin deficiency, aflatoxin and lack of physical activity Heat stress and a high level of oestrogen in the body are also considered predisposing factors of FLHS in layers.

FLHS is mainly managed through diet regulation. Substituting carbohydrates with supplemental fat in the diet, while keeping energy content constant seems to be beneficial. Additives like fat mobilizers (e.g., choline chloride) can also be incorporated in the diet of the bird.  

Mycotoxins:

Mycotoxins are toxic metabolites of fungus. Mycotoxicosis usually occurs when fungi grow in grains and feed which could be before harvest or during storage. Hundreds of mycotoxins have been identified up to this date and many of them are classified as pathogenic. When two or more mycotoxins are present in the feed, they may have additive or even synergistic effects. There is a category of toxins known as masked toxins. These mycotoxin derivatives are undetectable by conventional analytical techniques because their structure has been changed in the plant or during feed processing. Feeding of the grains containing fungus-producing toxins causes moderate to severe liver damage. Mycotoxicosis induce signs like reduced egg production, fertility, hatchability and depressed growth. Also increased feed conversion ratio, bruising, downgrading, and immunosuppression. This severely affects the economy of a farm. Control of mycotoxicosis is possible through management practices like proper ventilation, proper cleaning of feeders, use of mould inhibitors and toxin binders etc.  Currently use of binding agents other than clays are also coming into practice. Inclusion of liver tonic in liquid during severity and regular dosage in feed helps in managing mycotoxicosis efficiently.

Bacterial infections:

Several bacterial infections affect the poultry liver. However, some diseases like fowl cholera, fowl typhoid and tuberculosis produce significant lesions in liver. 

In fowl cholera, the liver may be enlarged with small haemorrhages on the surface and often develops multiple necrotic foci. In fowl typhoid, lesions in the older bird may include a swollen, friable, and often bile-stained liver with or without necrotic foci. Tuberculosis is a chronic disease with white, firm masses of various sizes called as caseous necrosis in the liver.

Antibiotics and liver tonics can be used for treatment purposes. 

Viral diseases:

The liver is affected in most of the viral affections in one way or another. Viral diseases which affect the liver the most are inclusion body hepatitis (IBH) in which enlarged, pale, friable liver is observed, Marek’s disease (tumours are observed in liver) and Avian lymphoid Leucosis (nodules are observed). As no definitive treatment for viral infections is available use of Liver tonics constituting immune strengthening properties or Immuno-strengthener helps in quick and better recovery of birds. 

Role of some common feed additives on liver:

Optimization of function and overall health of liver can be done by changes in feed such as inclusion of additives, enzymes or dietary fibres.

1. Phytobiotics: Extracts from plants like Andrographis paniculata, Picrorhiza kurrooa, Boerhavia diffusa, Eclipta alba, Solanum nigrum etc. are hepatoprotective and in addition to that provide anti-viral, anti-inflammatory and anti-bacterial benefits.
2. Fat mobilizers: these can be incorporated into the diet of birds to increase the mobilization of cholesterol from liver which in turn reduces level of fat. Some of the fat mobilizing agents commonly used in poultry are Tri Choline Citrate, Choline chloride, inositol and sorbitol. 
3. Amino acids: incorporation of methionine and lysine improves the amino acid balance and consequently promotes growth performance by enhancing feed efficiency, increases protein synthesis and decreases fat synthesis. 
4. Antioxidants: free radical oxidation of the liver cells can be reduced or prevented by incorporation of antioxidants like selenium, vitamin C and vitamin E.

Conclusion:

Role of liver in health is irreplaceable. To keep the bird healthy proper functioning of the liver is absolutely essential. Factors like mycotoxins, bacteria, viruses and metabolic diseases severely affect the economy of farms thus prevention of these conditions is the best way forward. Incorporation of the phytogenics compounds, vitamins, minerals, amino acids, antioxidants, fat mobilizers etc. individually or in combination has proven to be beneficial in preventing liver damage as well as better recovery in disease conditions. Regular usage of liver supplements in feed helps in prevention of diseases as well improves feed efficiency in birds.

References:

Berthiller, F., Crews, C., Dall’Asta, C., Saeger, S. D., Haesaert, G., Karlovsky, P., & Stroka, J. (2013). Masked mycotoxins: A review. Molecular nutrition & food research, 57(1), 165-186.

Zaefarian, F., Abdollahi, M. R., Cowieson, A., & Ravindran, V. (2019). Avian liver: the forgotten organ. Animals, 9(2), 63.

Krafts, K. P. (2010). Tissue repair: The hidden drama. Organogenesis, 6(4), 225-233.

Bunchasak, C. (2009). Role of dietary methionine in poultry production. The Journal of Poultry Science, 46(3), 169-179.