Prevention is better than cure, as they say. Many nutrients, energy, amino acids, vitamins and minerals - play different but significant roles in the immune response and so can contribute to keeping birds in good health, without the need for medication.
Presently, the aim of commercial poultry breeding is to achieve higher body weight and maximum egg production per unit of feed intake. However, there is a negative correlation between production and immunity in chickens as a result of the conflict between production and immunity, i.e. maturation and function oh the immune system. Accomodation of all the physiological demands within the limited resources, i.e. nutrients, available to birds may be the factor responsible for the negative relationship between performance traits and immunity. The genotypes with the maximum bodyweight exhibit lower immunity, as indicated by E. coli lesion score and cellular immunity antibody titres, compared to those having lower body weights. Therefore, the possibility of breakdown of the immune system in commercial chicken crosses is more evident nowadays than before.
In addition to genetic selection, certain non-genetic factors like dietary nutrient concentration also modulate the expression of the genes responsible for immuno-responsiveness by altering the maturity of the immune system and magnitude of antibody production.
Defence mechanism in chickens
Under intensive farming conditions, the poultry environtment contains ubiquitous micro-organisms that continuously challenge the bird;s immune system. Generally, the invading pathogen will be attacked by antibodies, whichs wil neyutralise, weaken and inactiveate the pathogen and finally, phagocytic cels will engulf the invader. The mechanism is quite effective in controlling extra-cellular phatogens, such as bacteria. For the intracellular pathogens-viruses-cell-medicated immunity (CMI) plays a key role. The CMI protects the host by destroying the cells that harbour the pathogen with the help of cytotoxic T-lymphocytes. Againts invading pathogens, the immune system produces a variety of compounds like acute phase protein (APP), proteolytic and hydrolytic enzymes, oxygen radicals and nitrogen derivatives, which destroy the invader or infective cells.
Nutrient recommendations are typically developed using indices of productivity such as growth, egg production and feed efficiency. The criteria for adequacy of immunocompetence are often ignored. Nutrients also influence the maturity of the immune system and magnitude of the antibody. During the acute phase of the immune response, the greatest nutritional need is for the synthesis and release of APP by the liver. The process requires more energy and amino acids than are normally needed for responding leucocytes. Interactions among various nutrients and imbalace or toxicity of nutrients lead to disturbances in normal physiology of the bird, with consequent immunosuppresiaon in chickens.
Variations on concentration of energy in the diet modulate the immune response in birds, probably due to the change in intake of nutrients, wich influence the immunity. Energy intake regulates the acitvity of the immune cells and activity of certain hormones, e.g. thyroxin, corticosteroids, growth hormones, glucagons, catecholamines, wich influence immunity. Variation in the level and composition of dietary fat also influence the immune response in chickens by altering the structure of the cell membrane and modulating the synthesis of prostaglandins. Mortality associated with E. coli and Mycobacterium tuberculosis was reduced by increasing the level of fat from 3% to 9% of the diet. Antibody titre against sheep red blood cells (SRBC) antigen was markedly increased with supplemental tallow at 6% in the chick diet. Higher levels of unsaturated fatty acids enhance immune function by stimulating macrophages.
The growth of bursa and thymus are relatively faster than the bird’s body growth. Therefore, it is important to supply the required quantity of protein, particularly during the early growth phase. Deficiency of protein at this stage leads to the improper development of lymphoid organs. Several research workers have suggested that there is a higher amino acid requirement for immunity than for growth. However, the influence of level of protein in diet on severity of disease depends on the type of infective organism. The lesion score to E. coli inoculation dcreased with the increase in the protein level (18, 20.5 and 23%) in broiler diets. With coccidiosis, the mortality decreased from 32% to 8% in chickens fed protein-deficient diets compared to those fed a normal protein level.
High dietary protein increases the activity of trypsin in the chicken gut. A high level of trypsin in the gut leads to a faster release of coccidia from oocysts, which will aggravate the disease symptoms.
Dieatary methionine levels in exces of those required for maximum growth are essential for maximising immunity. Methionine is required by the thymus-derived- T-cell function. Methionine deficiency produces severe lymphocyte depletion and atrhopy of the bursa and an increased suspectibility to Newcastle disease coccidiosis.
Cystine supplementation also stimulates cellular and humoral immunity (70 to 84% as effective as methionine)
Deficiency (16 to 50%) of branched-chain amino acids, i.e. isoleucine, leucine, and valine, reduces the antibody titres againts SRBC in broilers.
Immunoglobulins contain a high concentration of valine and threonine. A deficiency of either of these amino acids reduces the immune response in chickens. A higher ratio between leucine to valine + isoleucine reduces immunity due to structural antagonism between the three amino acids. The absorption of valine and isoleucine are inhibited by a high leucine content din the diet.
Increasing the dietary concentration of lysine improved the haemagglutination and agglutinin titres, and IgG and IgM levels.
Arginine is a substrate in the synthesis of nitric oxide, a cytotoxic product that is helpfu in phagcytic activity of macrophages and kills bacteria and intracelluar parasites.
Vitamins act as co-factors in several metabolic functions in immune reactions and therefore, deficiencies of vitamins cause impairmentt of immunity. Generally, higher levels of vitamins than the current recommendations will increase the immune response.
This vitamins Is important for maintaining the cellularity of the lymphoid organs and epithelial tissues and for enhancing both cellular and humoral immunity. Vitamin A helps in maintaining the mucous membrane of natural orifices in healthy condition to prevent the invasion of microorganisms. Vitamin A directs differentiation and development of B-lymphocytes. The concentration of vitamin A in the diet modulates the expression of retinoic acid receptors on lymphocytes in chickens.
The production of immunosuppressive agents (hydrocortisones) is reduced with higher levels of vitamin A in the diet. Furthermore, deficiency of vitamin A causes keratinisation of basal cells of the bursa and impairment on the response of T-lymphocytes. Therefore, deficiency of vitamin A impairs immunity by producing defective T, B-lymphocytes, impaired phagocytosis and reduced resistance to infection. Increased morbidity due to Newcastle disease virus has been reported due to a deficiency of vitamin A in the diet. The requirement of vitamin A for maximum immunity, i.e. lymphoid organ weight, was higher than for the bodyweight gain in the chicken. An increase in vitamin A from 12850IU to 42850 or 74045IU/kg decreased mortality due to E. coli, and CRD in chickens and increased the rate of clearance of the pathogen from the blood. However, the benficial effect of higher levels of vitamin A depends on the concentration of other fat-soluble vitamins in the diet. An excessive level of vitamin A interferes with the utilisation of vitamins D and E.
The administration of 60IU of vitamin A per chick per day during a severe attack of coccidiosis reduced mortality from 100% to almost zero. However, practical chick and young layer diets should contain 4000 and 2000UI/kg, respectively. To minimize stress damage and also to prevent immune suppresion, dietary vitamin A levels shoul be increased to ten tomes the normal requirement. A combination of vitamin A (14000IU/kg) and zinc (65mg/kg) has been shown to enhance growth and both humoral and CMI immunity in chickens.
TO BE CONTINUE……