Immunosuppression in poultry

Immune system combats the antigens and ensures the good health status of the bird. It identifies almost all disease causing agents, from viruses to parasitic worms and differentiates them from body´s cells and tissues. Immune system of poultry can be divided into non-specific and specific immune mechanisms.

Non-specific immune mechanisms include the innate or inherent ways in which poultry fights diseases. These include genetic factors, body temperature, anatomic features, normal microflora, respiratory tract cilia etc. when the non-specific immune mechanisms are defined, good managemental practices are important in maintaining poultry health is better understood. For example: high concentration of toxins, misuse of antibiotics or poor sanitation may lead to a disturbance of the normal micro flora; poor nutrition may lead to deficiencies which allows the disease causing organisms to penetrate the protective body coverings; selection of disease resistant strains of chickens may prevent or minimize the effects of certain diseases; and others.

Specific immune mechanisms (acquired system), are characterized by specificity and memory. This system is divided into cellular and non-cellular (humoral) components. The non-cellular components consist of immunoglobulin (antibodies) and the cells which produce them.  There are three classes of antibodies which are produced in the chicken after exposure to a disease or vaccination; these include Ig M, Ig G, and Ig A. All these are specific. Ig M is detected after 4-5 days following exposure to a disease organism and then disappears by 10-12 days. Ig G is the most important antibody which is appeared after 5 days following exposure, peaks at 3 to 3.5 weeks, and then slowly decreases. Ig A seems after 5 days following exposure and is found primarily in the mucus secretions of the eyes, gut, and respiratory tract and provides "local" protection to these tissues. B-lymphocytes are the cells which are responsible for the production of antibodies. The cellular components include all the cells that fights with antigens, except those associated with antibody production. The cells associated with this system are the T-lymphocytes. Some T-cells produce lymphokines (over 90 different ones have been identified) and some directly destroy disease organisms.

A chicken may become immune to a disease organism by producing antibodies itself or by obtaining antibodies from another animal. When antigen attacks and chicken produce its own antibodies then the process is called active immunity. This occurs after the bird is exposed to a vaccine or a field disease challenge. Anything which damages the cellular or humoral immune systems lowers the active immunity. When the chick receives pre-made antibodies from the hen through the egg or from any other source then it is called passive immunity.

The organs or cells that are the part of immune system in poultry are:

 Immunosuppression is a state of body when the immune system or humoral (antibodies) and/or cell mediated immu­nity is depressed. Immunosuppression is not a disease and has no clinical sign. Different factors from which it can be recognized are atrophy of lymphoid organs, increased mortality, less uniformity of flock, poor performance, less antibody titer against vaccination and increased chances of bacterial infection. Immunosuppression may be due to infectious agents, sudden change in managemental practices like feeding, watering, temperature, very high or low humidity, improper biosecurity, nutritional deficiencies, or combination of them. Each of the possible cause of immunosuppression must be considered to prevent the loss of production and economic performance.

Immunosuppression in poultry is mainly caused by stress. Managemental practices that induced immunosuppres­sion are topics of financial, public and sci­entific importance. It is the fact that in stressed poultry, losses due to a disease increases. Under commercial conditions, the chicken is exposed to a variety of stressors that may adversely influence the immune system. There are two types of stressor i.e. short-term and long term (chronic) stressors. Due to short term stressors, available nutrients are used towards resist­ance against the bacteria, para­sites and toxins and leaving the bird suscep­tible to various challenges. Long-term stressors make the birds more suscepti­ble to infectious agents challenge.

Poultry demands optimum conditions of temperature, humidity, ventilation etc. to produce efficiently. Any variation in these causes the stress which increases the production of corticosterone ultimately leading to depletion of lymphoid organs like thymus, bursa of fabricius and spleen.  High stocking density, low feeding and watering space etc. also affect the immune system in negative way by impeding the nutrient availability to the body. These stressors change the intestinal microflora which causes poor digestion, watery feces and wet litter which increases the ammonia, humidity and the risk of foot dermatitis. Higher concentration of ammonia (10-20 ppm) causes damage to respiratory epithelium and mucociliary system. Similarly very high or low concentration of humidity also affects the respiratory system adversely. So, chances of respiratory diseases increase which suppress the immune system.

To prevent environmental stressors provide constant and uniform supply of good quality air to birds. Fresh air is required at all stages of production to achieve potential performance from the birds. Proper ventilation can only be achieved if the house has the proper number and capacity of fans, cooling pads and air inlets. With the help of proper ventilation harmful gases, temperature and humidity can be controlled. Increase feeding and watering space and reduction in stocking density of birds help to control immunosuppression.

It is well known fact that well-nourished birds are immunologically more competent and have more resistance to diseases. A change in feed intake leads to a change in nutrient and energy intake. Birds cannot develop their good immune system if they are nutritionally deficient.  However, the commercial poultry feed is not deficient in nutrients and energy. It is well known that immunosuppression in poultry, causes the decrease in feed intake. There are many vitamin, minerals and amino acids whose deficiency in poultry feed cause the immunosuppression. e.g.

Vitamin E is stabilizing the cell mem­branes. Free radicals or oxidative com­pounds can be ingested through poorly stabilized fats, oils and animal byprod­ucts. They are able to damage the cell membranes. The membranes of the rapidly multiplying cells of the immune system are particularly exposed to the consequences of free radicals. Vitamin E is able to remove these oxidative mole­cules in this way it supports the immune system. The most important immunoregulatory mechanism that is linked with vitamin E is the modulation of arachidonic metabolism via cyclooxygenase and lipoxygenase pathways which lead to the synthesis of prostaglandins and leukotrienes, respectively. Vitamin E deficiency causes the increase in no of free radicals which indirectly suppress the immune system. Vitamin C deficiency lowers the speed of healing, increases the effect of stress so, suppress the immune system. Vitamin C is also important for the formation of white blood cells. Vitamin C supplementation also reduces the synthesis of corticosterone during stress conditions. Dose level of vitamin C to use in the feed is usually between 100 and 150 ppm.  Vitamin A seems to be directly involved in antibody production and formation of mucus membranes. Studies have shown that vitamin A deficient birds produced less Newcastle antibod­ies and a lower T-cell response. Deficiency in vitamin A increases the chances of diseases like Newcastle disease, Infectious Bronchitis, Coccidiosis, Necrotic Enteritis, Mycoplasmosis etc. which suppress the immune system directly or indirectly. Without these supplemental vitamins, acceptable performance and carcass characteristics can not be achieved. Vitamins also augment disease resistance under conditions of stress. These vitamins exert their mechanism of action during stress by protecting immune tissue via a lessening of certain hormonal effects and oxidative damage, and improvement of cell differentiation and production.

Dietary arginine concentration is very important in boosting the immune system because its metabolism produces nitric oxide which is a potent immunological modulator and it also increases the nitric acid production from macrophages. Arginine also increases the weight of thymus and functions, increase the lymphocyte mitogenesis, enhance the wound healing and improve immunity against tumors. Too much dietary methionine, than the recommended level will suppress the immune system.

Similarly the deficiency of methionine, valine, threonine, vitamin A, riboflavin, pantothenic acid, pyridoxine, sodium, chloride, zinc and selenium will have a negative impact on the immune system. Thus, these should not be deficient in feed to prevent immunosuppression.

To control the immunosuppression by nutrition it must be kept in mind that poultry feed is not deficient in any amino acid, energy, vitamin and mineral because these play a very important role in functioning of immune system.

It encompasses the practices or measures adopted to minimize the invasion of pathogens on the poultry shed. Biosecurity, vac­cination programs, vaccines and other important factors for the control of im­munosuppressive agents have been extensively researched. Poor hygienic conditions result in pathogenic invasion from outside, which leads to higher pathogenic load on birds; thus cause stress and immunosuppression. So, incidence of immunosuppression can be thwarted by strictly employing the biosecurity measures.

Almost all diseases of poultry can suppress the immune system; however certain diseases whose virus replicates in immune organs like bursa/ thymus, directly influence the immune system. These include Infectious Bursal Disease, Chicken Infectious Anaemia, Marek''''s Disease, Reovirus infection, Newcastle disease and Avian influenza. These diseases by affecting the immune cells or organs in different ways (see Table.1), suppress the immune system.

To control these infections, appropriate vaccination schedule for breeders should be adopted so that their chicks can have approciate maternal antibody levels against these diseases. Similarly, broilers and layers should also be vaccinated against these diseases according to prevalence of diseases in that area. Furthermore, quality of day old chicks should also be monitored with a special focus on mycoplasma and other vertically transmitted infections. In case of prevalence of any above mentioned diseases, a careful diagnosis with the help of postmortem and laboratory test is needed, so that disease can be controlled/treated in time.

Mycotoxins are the metabolites of fungi commonly called molds. They grow and produce toxins in grains and other poultry feed ingredients during growing or harvesting or storage of the crop. It is estimated that about 25% of the world''''s cereals are contaminated with known mycotoxins, while higher per­centages could be contaminated with tox­ins as many toxins are yet to be identified. No region in the world can eradicate these silent killers and their negative impacts on poultry production. There are three types of fungi whose metabolites mainly affect the poultry feed ingredients. These include asper­gillus, penicillium, and fusarium. Major ingredients on which these grow are maize, wheat, rice, and peanut meal.

Aspergillus produces aflatoxin, which impairs the immune cells function by reducing the amount of antibodies following infection or vacci­nation, and by reducing the activity of phagocytes. Aflatoxin is also responsible for lym­phoid depletion and necrosis in the bursa of fabricius, spleen and thymus and hepatotoxicity. Penicillium produces ochratoxin A which is associated with a generalized impaired humoral and cell mediated immune responses. It is also nephrotoxic. Trichotecene is produced by fusarium and it suppresses the immune system in two ways, firstly, it is a strong tissue irritant and alters mucosal membranes integrity. Secondly, it inhibits protein synthesis and consequently interferes with antibody production. Followings measures should be adapted to control the mycotoxins in poultry feed.

Immunosuppression causes poor production and economic losses, which necessitate its prevention for better flock health, vaccination response and performance and uniformity of flock. This can be achieved by controlling the infectious diseases, providing good management, maintaining appropriate environment and by good nutrition. However, further work is required to investigate im­munosuppressive conditions and agents in poultry.