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.
Energy
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.
Protein
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
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.
Retinal
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……
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