Immunosuppressive interactions of viral diseases in poultry

Abstract

Poultry can be exposed to different kinds of immunosuppressive agents that impair health and welfare by destroying innate and acquired immunity leading to diminished genetic potential of poultry for efficient production. Immunosuppression is a condition characterised by humoral and cellular immune dysfunction that leads to increased susceptibility to secondary infections and vaccine failure. Immune dysfunction at the humoral level is largely due to change in soluble factors mediated by complement or chemokines for innate immunity or due to alterations in antibodies or cytokines for adaptive immunity. In contrast, immune dysfunctions at cellular levels include alterations in neutrophils, monocyte/macrophage, and natural killer cells for innate immunity or changes in B or T lymphocytes for adaptive immunity. In poultry, stress-induced immunosuppression is manifested by failure in vaccination, and increased morbidity and mortality of flocks. Immunosuppressive agents can have cytolytic effects on lymphocyte populations leading to atrophied and depleted lymphoid organs. Immunosuppression can be due to infectious agents or non-infectious agents or due to a combination of them. At present, several modern cellular and molecular approaches are being used to determine the status of the immune system during stress and disease. Comprehensive methodologies for the evaluation of immunosuppression by combined non-infectious and infectious aetiologies have not found general application. Currently, investigations are being developed in order to detect genetic expression of immunologic mediators and receptors by microarray technology. It is likely that this new technique will initiate the development of new strategies for the control and prevention of immunosuppression in poultry. A long term immunosuppression preventive approach involves genetic selection for resistance to immunosuppressive diseases. In general, intervention approaches for immunosuppressive diseases largely rely on minimising stress, reducing exposure to infectious agents through biosecurity, and increasing immune responses by vaccination against immunosuppressive agents.

Introduction

Poultry is constantly exposed to immunosuppressive stressors and infectious diseases. Animals housed in artificial habitats are confronted by a wide range of potentially provocative stressors (Glaser and Kiecolt-Glaser, 2005; Shini et al., 2010). The stress response is a complex and multifaceted mechanism. It involves a series of behavioural, physiological, metabolic, and immunological reactions that are used by the body to redistribute the demands placed on it, adapt to them, and survive (Shini et al., 2010). The environment is a composite of potential stressors that may adversely affect animals living in captivity and includes all their living conditions. Therefore, the ability of birds to maintain homeostasis largely depends on its physiological response and severity of the stressors (Horning et al., 2003; Shini et al., 2010). In addition, management and ecological factors such as confinement, climatic and seasonal fluctuations, poor feeding, and worm infestations are associated with stress and reduced immune response. Stressful factors have been reported to cause functional and morphological changes in chickens (Horning et al., 2003; Umar et al., 2014; 2015a).

Immunosuppression has great economic importance in the poultry industry, because affected flocks are susceptible to secondary infections, respond poorly to vaccines and do not perform as well as non-affected birds (Calcagni and Elenkov, 2006). Innate and acquired immunity interact with each other and their response can be mediated either by soluble factors (e.g. cytokines) or cell-to-cell contact. Infections may lead to immunosuppression by destroying immune cells or changing their equilibrium. Immunosuppression can be induced by a variety of events, including as infectious and non-infectious agents. Non-infectious immunosuppressive agents can cause pre- and post-hatch stress, nutritional stress, mycotoxicosis, housing and medicinal stress. Chick handling, transportation from hatchery to farm, delayed time to first feed intake, and suboptimal brooding may impose additional physiologic stresses (Glaser and Kiecolt-Glaser, 2005). Stresses such as ammonia and excess heat, change the cellular counts of circulating heterophils and decrease circulating lymphocytes in an additive manner.

Genes determine functions of the neuroendocrine system in controlling animal coping strategies and productivities. Selective breeding of chickens for genetic or phenotypic features associated with specific behavioural and physiological characteristics has become a major tool to improve bird's well-being. Genetic resistance is the intrinsic property of the chicken host to resist immunosuppressive agents. Genetic selection for resistance to immunosuppressive diseases has shown promising results (Hoerr, 2010; Cazaban, 2015; Rehman et al., 2016).

Several years of studies have been dedicated to the investigation of genetic resistance against other immunosuppressants. Selective breeding may produce chickens that are resistant to immunosuppressive agents, but should not change production traits and should not increase the susceptibility to ailments or other pathogens. There is a need to study the interaction between genetic factors, domestic behaviours, and hormonal homeostasis in birds in response to immunosuppressive agents that is critical in preventing immunosuppression and welfare problems in the poultry industry. Some of the important immunosuppressive agents are infectious bursal disease virus (IBDV), Marek's disease virus (MDV), chicken infectious anaemia virus (CIAV), Orthoreovirus, Metapneumovirus, mycotoxins, and ammonia (Hoerr, 2010; Couteaudier and Denesvre, 2014; Kegne and Chanie, 2014; Umar et al., 2014; 2016). In commercial poultry flocks, immunosuppression may be manifested clinically in a number of ways depending on age, the infectious agent, or genetic background. In general, flock performance is typically affected, with increased incidence of secondary infections, poor feed conversion, reduced protective response to commonly used vaccines, and increased carcass condemnation rates at the processing plant have been observed in immunosuppressed poultry (Sahin et al., 2015).

In this paper, the current knowledge on infectious immunosuppressive agents in poultry is reviewed and immune measures that can be used to detect reduced immunity are discussed. Possible interventions and other possible novel methods for the control of immunosuppression in poultry are included.

Infectious immunosuppressive agents

infectious bursal disease and chicken anaemia virus

Infectious bursal diseases virus (IBDV, Birnaviridae) is horizontally transmitted and infects all ages of chickens. The cells of the specific and innate immune systems are temporarily affected by IBDV. The main target cells for IBDV replication are the actively dividing B-lymphocytes, causing rapid necrosis and depletion of lymphocytes from the bursal follicles (Mahgoub et al., 2012; Kegne and Chanie, 2014). IBDV specifically destroys plasma cells in the BF leading to an impaired humoral response. The virulence of the IBDV-strain, genetic background, age of the chickens and residual maternal antibodies may influence the outcome of IBDV immunopathogenesis (Sharma et al., 2000; Kegne and Chanie, 2014). The BF is principally involved, but lymphoid tissues elsewhere (spleen, caecal tonsil, proventriculus) are also affected. The primary response to IBDV infection occurs as an influx of T lymphocytes (Kim and Sharma, 2000). Intrabursal T cells and T cell-mediated responses are important in viral clearance and are necessary to promote recovery from infection (Tables 1 and 2).

Table 1 Common lymphotropic infections associated with immunosuppression (Cazaban, 2015 ).

Table 2 Agents responsible for atrophy of some immune organs (Cazaban, 2015).

X = the agents are causing atrophy in organs.

Cytotoxic T lymphocytes help to limit viral replication but promote bursal tissue damage and delay tissue recovery, possibly through the release of cytokines and cytotoxic effects (Sharma et al., 2000). The magnitude of the T-cell responses in the BF during IBDV infection is influenced more by the virulence of the IBDV than the viral load in the tissue (Poonia and Charan, 2004). The local T-cell events in the bursa alone may not be indicative of a rapid and protective immune response. Acute infection may occur as clinical or subclinical disease, and mortality is a feature of virulent strains of IBDV. During acute infections, chickens develop systemic disease and may develop lesions in the lymphoid tissues, liver, and kidney, in association with circulating immune complexes.

Chickens that survive acute infections clear the viral infection and bursal follicles are repopulated with IgM+ B lymphocytes (Kim and Sharma, 2000; Mahgoub et al., 2012). As the virulence of the IBDV strain increases, the follicular repopulation or restitution decreases. For individual follicles, two sequelae are observed histologically: large reconstituted follicles with numerous lymphocytes in the cortex and medulla, and small, poorly developed follicles with a poorly discernible cortex and medulla (Withers et al., 2005; Cazaban, 2015). Poor humoral response has been observed in chickens with undifferentiated follicles, possibly due to lack of peripheral B cells. Neonatal chickens that survive acute infections are immunosuppressed despite repopulation of the bursa with B cells. When IBD occurs in chickens at 14 days of age or younger, B lymphocyte seeding of secondary lymphoid centres is curtailed, resulting in a permanently defective humoral immunity, and leaving the chicken susceptible to secondary infections (Naqi and Millar, 1979). In chickens older than 14 days, IBD causes transient depression of systemic antibody production and, with necrosis of plasma cells in the Harderian gland, diminished mucosal immunity. Cell-mediated immunity, heterophil and macrophage functions are also transiently depressed.

Advances in protecting young chickens from high mortality and permanent immunosuppression have been realised through breeder hen immunisation, allowing transfer of high levels of maternal antibodies, and vaccination of the young chicken with attenuated strains of IBDV (Hoerr, 2010). IBD still occurs in broilers, young breeders and commercial layer pullet replacements, but the window of susceptibility is 20 to 30 days of age. In this scenario, the severity and duration of the transient immunosuppression are likely factors for the impact of IBD and the effectiveness of vaccination programs (Kegne and Chanie, 2014). As the response and recovery from acute IBDV infection require intact immune responses, affected poultry are vulnerable to other immunosuppressive agents and disease. IBD interacts with infectious bronchitis by interfering with effective immunisation, enabling persistent respiratory infections by infectious bronchitis virus (IBV), and increasing the severity of nephropathogenic bronchitis. IBD reduces or delays the secretory antibody to IBV in tears, creating an environment for IBV replication in the Harderian gland and shedding infectious virus (Toro et al., 2006; Kegne and Chanie, 2014). IBDV interacts with NDV whereby it increases the severity of NDV outbreaks, interfering with the antibody response to immunisation, reducing the resistance to NDV challenge, and prolonged duration of shedding during infection (Joiner et al., 2005).

Under circumstances where there is early onset of IBD, chickens have an increased risk for developing Marek's disease (MD). Exposure to IBD at less than one week of age increases susceptibility to MD (Hoerr, 2010). In chickens, IBDV infection leads to persistence of reovirus in tissues, and lower antibody response to reovirus compared to reovirus alone (Moradian et al., 1990). IBD is associated with increased severity of bacterial diseases of chickens. IBD and severe enteritis are predisposing conditions for a severe outbreak of subcutaneous clostridial infection (gangrenous dermatitis) involving Clostridium perfringens and C. septicum in broilers. IBD increases the general susceptibility of Salmonella typhimurium (ST) and decreases the humoral immune response to ST infection in broilers (Kegne and Chanie, 2014). In poultry, IBDV and other immunosuppressors are usually followed by E.coli infections. IBD can enhance the chances of E.coli infection without a predisposing factor. Chickens with IBD are more susceptible to Staphylococcus aureus, which can express as gangrenous dermatitis. In outbreaks of coccidiosis, IBD and bursal atrophy are associated with an increase in lesion severity and mortality and apparent failure of anticoccidial drug treatments (Hoerr, 2010; Kegne and Chanie, 2014).

Chicken anaemia virus (CAV, Circoviridae) is vertically transmitted from hens suffering from acute infections during egg production, and is transmitted to the developing embryo (Wani et al., 2014). When the chick hatches, CAV targets haemocytoblasts in the bone marrow and lymphocytes in the thymus, resulting in aplastic anaemia, thrombocytopenia, leukopenia, and thymus depletion in chickens from seven to 14 days of age (Dhama et al., 2008; Umar et al., 2014). During acute infections, virus is shed to penmates through the digestive tract and feather dander. Chicks with aplastic anaemia are immunosuppressed and susceptible to adenovirus infections and bacterial disease (Hoerr, 2010: Umar et al., 2014). Chicken infectious anaemia is now largely controlled by ensuring that hens are exposed to CAV prior to the onset of lay. The hen develops antibody to CAV that is transferred to and protects chicks, even though the hen may continue to shed CAV through the reproductive tract (Wani et al., 2014). As a result of broad application of this basic control, blue wing disease is observed less frequently in broiler and layer production today.

The rapid development of antibodies and the lack of clinical signs are suggestive of age-related resistance to aplastic anaemia. Subclinical immunosuppression by CAV in chickens older than two weeks of age is now recognised as economically important to production (McNulty et al., 1991). With the decline of maternal immunity, chickens again become susceptible to infection by CAV that is ubiquitous in poultry environments (Dhama et al., 2008). In older chickens, the bone marrow is spared, but the thymus is infected with CAV and depleted of lymphocytes by apoptosis. The chickens have impaired T lymphocyte and macrophage activities, and lose bactericidal capability for up to four weeks post-infection. Maternal immunity can be low in a percentage of chicks during the first week, and is typically depleted by 10 to 14 days of age, but can persist up to 20 days of age (Hoerr, 2010). Depending on the level of exposure, oral intake of CAV, as occurs with natural exposure, may require 10 to 14 days to induce peak virus load in the thymus that correlates with the severity of atrophy. Thus, chickens with declining maternal immunity become susceptible to CAV at 20-30 days of age, and may be younger if maternal antibody protection is marginal. The thymus depletion is a subclinical event, but broilers with CAV-depleted thymuses develop secondary infections, including gangrenous dermatitis, coccidiosis, and viral and bacterial infections of the respiratory tract (Wani et al., 2015). Chickens co-infected with IBDV have increased susceptibility to CAV infection at older ages, are more likely to develop contact infections by CAV, have higher mortality rates and prolonged acute phase prior to recovery or mortality (Umar et al., 2014). In chickens aged 35-40 days IBDV infection enhances CAV infection by inhibiting virus-neutralising antibody to CAV, which prolongs CAV viraemia, and there is an increased presence of CAV in the distal intestine. In broilers, IBDV infection delays the recovery of the thymus from CAV-induced lymphocyte depletion (Hoerr, 2010).CAV does not replicate in the bursa but it can cause atrophy of the bursa follicles. In broiler production, the onset of bursal and thymus atrophy, consistent with IBD and CIA, respectively, suggests that these two diseases are sequential or concurrent subclinical immunosuppressive risk factors underlying the presenting clinical diseases (Toro et al., 2006). The pathogenesis of MD can be enhanced or inhibited by CAV, depending on the titre of the challenge dose of MDV.

Early CAV infection results in decreased protection from vaccination for either ND or laryngotracheitis, and combined CAV and IBDV challenge reduced vaccine protection for fowl pox (Hoerr, 2010; Wani et al., 2015). CAV-infected chickens had more severe respiratory reaction to attenuated NDV vaccine, manifested as a reduction in growth. In broiler flocks in Alabama, peak isolation of cycling vaccine strains of IBV coincides with the onset of bursal atrophy attributed to IBDV infection at 20-30 days of age, and thymus atrophy was attributed to subclinical CAV infection that emerged at 30-40 days of age (Dhama et al., 2008; Wani et al., 2014). CAV infection is a risk factor for bacterial infections, due to reduced bactericidal capabilities of macrophages that can last for up to 28 days. CAV is associated with staphylococcal infections and gangrenous dermatitis (Umar et al., 2014). Increased medication cost linked to bacterial infections is one factor associated with CAV causing decreased performance and profitability of broilers (Dhama et al., 2008; Hoerr, 2010; Umar et al., 2014).

marek's disease and enteric disease viruses

MDV, or Gallid herpesvirus 2 (GaHV-2) is the etiological agent responsible for MD in chickens, a multifaceted disease most widely recognised by the induction of a rapid and extensive malignant T-cell lymphoma (Couteaudier and Denesvre, 2014). MD, named after the Hungarian veterinary pathologist Marek over 100 years ago, is a major disease affecting poultry health worldwide according to the data from the World Organisation for Animal Health (OIE), although the data are difficult to obtain because it is not a notifiable disease.

MD results in substantial economic losses, estimated at more than 1 billion per year (Biggs and Nair, 2012; Couteaudier and Denesvre, 2014; Hu et al., 2014). The history of MD vaccination reflects the ongoing emergence of virulent strains of MDV capable of breaking through protection afforded by once-effective vaccines. Infection by MDV causes lymphocytolytic infection and atrophy of the BF and thymus (Heidari et al., 2010), lymphopenia, and reduced humoral immune response (Biggs and Nair, 2012; Haq et al., 2013). The degree of immunosuppression is indicative of virulence of the emerging strains (Hoerr, 2010; Couteaudier and Denesvre, 2014; Hu et al., 2014). Vaccines that protect against lymphoma formation by virulent MDV have a sparing effect on immunosuppression but may not provide complete protection (Heidari et al., 2010; Haq et al., 2013). MD-induced immunosuppression is not easy to measure due to the continuous circulation of CAV and IBDV in the environment. An indirect but practical approach to this is that immunosuppression by MD should be expected to accompany emerging problems with lymphomas, skin tumours, or other syndromes (Biggs and Nair, 2012; Couteaudier and Denesvre, 2014). Despite knowledge of molecular and cellular mechanisms of immunity against MD, there are still several gaps in this area. There is limited understanding of the process and dynamics of T cell-mediated responses to MDV. Moreover, new information with respect to T cell responses to tumour-specific or tumour-associated antigens is lacking. Further research on the molecular mechanisms of disease, genetic resistance, vaccine-induced protection and evolution of virulence will be needed to develop more sustainable control strategies in the coming years.

enteric diseases

Several viruses have been identified in recent years in the intestinal contents of chickens and turkeys with enteric problems, which have been observed in commercial farms worldwide. The economic impact of enteric virus infections on the poultry industry has been evaluated and ranges from insignificant effects to those that are severe and cause devastating losses (Nuñez and Ferreira, 2013; Mettifogo et al., 2014). Enteric diseases tend to predominantly affect young birds, however, they may occur in all age groups, which increases susceptibility to other diseases, decreases feed conversion efficiency and prolongs the time to market. At present, no specific treatment exists, and commercially available vaccines have not yet been developed for any of the viruses that are involved in this disease. In the past, enteric disease has been called ‘pale bird syndrome’ and ‘helicopter wing disease’ characterised by poor growth and retarded feather development. These symptoms are observed alongside less frequent clinical signs including diarrhoea, increased mortality, and pancreatic and lymphoid atrophy. Enteric diseases seem to be the most acceptable name for this clinical manifestation because it most appropriately reflects the consistency of clinical findings and indicates that these cases are probably caused by the same infectious agents (Alavarez et al., 2014; Mettifogo et al., 2014).

Viral enteritis syndromes in chickens and turkeys (runting stunting syndrome (RSS); poultry enteritis and mortality syndrome (PEMS)) involve one or more etiologic viruses, and contributing management issues, such as short down time between flocks. Astrovirus, rotavirus, reovirus, parvovirus and others have been identified in young broilers (aged seven to 14 days) and young turkeys exhibiting the signs of diarrhoea and growth reduction (de Wit et al., 2011), although pathogenesis is not known. Interference with normal feeding behaviour and interrupted nutrition could contribute to atrophy of the lymphoid tissues (Bidin et al., 2011). In some cases, liver histology showed bile duct proliferation in one or more chicks per case, suggestive of contributions to immunosuppression by hepatotoxicity from natural toxins. Lymphocyte depletion in lymphoid tissues occurs in turkeys with PEMS (Jindal et al., 2010). Turkey enteric coronavirus is associated with PEMS and turkeys inoculated with coronavirus and E. coli developed lymphocyte necrosis and depletion in the bursa of Fabricius (Kang et al., 2012). Thymus atrophy is associated with PEMS, and it occurs in turkeys inoculated with Astrovirus (Grgić et al., 2011). A small round virus, possibly an enterovirus or Astrovirus, can replicate in lymphoid tissues of turkeys, causing lymphocyte necrosis and depletion of lymphoid organs, and corresponding reductions in lymphocyte subpopulations in circulating blood (Hoerr, 2010). Virulent strains of avian Reovirus cause atrophy of lymphoid tissues and interfere with humoral immunity (Kang et al., 2012). Reoviruses can replicate in monocytes but not in lymphocytes; thus, the lymphoid atrophy is not caused by reovirus tropism specifically for lymphocytes in contrast to IBDV, CAV, MDV, and some of the other enteric viruses. Goose parvovirus is the etiology of a fatal hepatitis in young geese (Derzsy's disease) (Brown et al., 1995). A variety of parvovirus related to, but distinct from, goose Parvovirus causes degenerative rhabdomyopathy in Muscovy ducklings (Poonia and Charan, 2004). Chickens and turkeys can be infected by parvoviruses, which may have a role in naturally occurring enteric infections (Zsak et al., 2008). Despite extensive research, knowledge of enteric viruses is minimal and information regarding their role in enteric diseases is scarce. This justifies the importance of further studies on enteric viruses to explore them and their role in enteric diseases.

avian leukosis and reticoendotheliosis viruses

Retroviruses (REV) cause a variety of illnesses such as leukaemia, acquired immunodeficiency syndrome and neurodegenerative diseases in humans and animals (Nakamura et al., 2014). Avian leukosis of chickens and reticuloendotheliosis of turkeys, chickens, and other avian species cause tumours, immunosuppression, reduced productivity, and other production related problems in affected flocks (Payne and Nair, 2012). In chickens, replication competent REV strain A and chicken syncytial virus have been shown to cause runting syndrome and bursal atrophy. Chickens with tolerant infections by REV become immunodepressed (Payne and Nair, 2012), which may contribute to the development of acute leukaemia by inhibiting the proliferation of cytotoxic cells directed against the tumour cell antigens (Hoerr, 2010; Nakamura et al., 2014). Chicks infected with REV and challenged with NDV developed more severe clinical signs and had reduced antibody responses and prolonged recovery time. A myeloblastosis strain of avian leukosis virus (ALV) capable of inducing osteopetrosis caused atrophy of lymphoid organs, and decreased macrophage function and bacterial clearance. An erythroblastosis strain of ALV caused thymus atrophy and decreased T-cell competencies (Payne and Nair, 2012). In some countries, it has now been eradicated from commercial primary broiler breeder flocks by rigorous application of well-tested eradication programmes, but in other countries it remains and has spread to layer flocks. New threats to the poultry industry due to viral mutation can thus be expected.

other opportunistic immunosuppressive pathogens

ND causes lymphocyte necrosis and depletion from lymphoid organs, and apoptosis of peripheral blood lymphocytes and mononuclear cells, which may increase susceptibility to secondary bacterial infections (Umar et al., 2015c). Cases of pneumovirus infections in turkeys are characterised by concurrent bacterial and viral co-infections. Pneumovirus replicates and causes cytopathology in the upper respiratory epithelial cells, causing impairment of protective clearance mechanisms (Catelli et al., 1998). The mechanism of immunosuppression appears to involve innate respiratory immunity more than acquired immunity. Pneumovirus infection augments bacterial infection of the lung and air sacs, and with more invasive bacterial infection, there is deeper (bronchial) infection by the pneumovirus (Hoerr, 2010). Avian pneumovirus infections render turkeys more susceptible to infections by E. coli, Ornithobacterium rhinotracheale (ORT) and Bordetella avium; exacerbate infections by Chlamydophila psittaci and increase the severity to avian paramyxovirus challenge (Marien et al., 2005). Chickens with pneumovirus infections become more susceptible to E. coli and ORT (Majo et al., 1997). This can manifest as swollen head syndrome with rhinitis, sinusitis, facial cellulitis and oedema, and inflammation of the cranial air spaces. E. coli infections alone can induce marked lymphocyte depletion from bursa and thymus in chickens (Hoerr, 2010).

Immunological evaluation of immunosuppression

Despite the extensive research on immunosuppressive diseases, a limited number of methods are available for accurate evaluation. Most of the available methods rely on T or B cells function as exemplified by lymphocytes activation and proliferation, cytokines production, leukocytes count and expression, and specific antibody production ( Table 3 ).

Table 3 Immunological evaluation of immunosuppression in poultry.

*The list of tests is an update of the panel recommended by Dietert et al. (1994). It is suggested that a combination of several tests may help assess immunosuppression; specific immune assays from this panel might be more suitable for field or research studies, or both (Shini et al., 2010).

For diagnosis, detection of immunosuppression largely focuses on detecting specific diseases by serologic surveillance, isolation, or molecular detection of the etiologic agent, or presumptive lesions identified at necropsy or by histopathology. Assays to assess immune-competency are mainly used, however, a broader approach could assess combined environmental and infectious contributions of immunosuppression (Dietert et al., 1994; Haq et al., 2013). The heterophil:lymphocyte ratio in the circulating blood is a general indicator of stress and may provide general assessment of immunocompetency (Hocking et al., 2002).

Measurements of chicken's interferon-alpha and interferon-gamma mRNA have been proposed as a general assessment of immune function. Both are increased by subclinical infections with either IBDV or CAV (Lamont et al., 2014; Wani et al., 2015), and could be assessed following a brief laboratory challenge by inactivated Newcastle virus (Hoerr, 2010). Interleukin-2 has been proposed as a general measure of cell mediated competency in the chicken's intestine. Whole-blood lymphocyte stimulation assay is a relatively simple assay of circulating T-cell responses and has application for the evaluation of coccidiosis outbreaks and of vaccine efficacy. Functional assessment of peripheral blood monocytes has been proposed to assess immune function of cattle, including ingestion of Staphylococcus aureus, antibody-dependent cell-mediated cytotoxicity, and chemiluminescent assays. While these proposals have existed for some time, none has seen consistent application or gained common acceptance. A functional genomics approach to the study of avian innate immunity uses cDNA microarray, which is capable of testing for over 4000 genes (Lamont et al., 2014). The avian innate immunity microarray contains 25 avian interleukin, chemokine and cytokine elements. The array also contains elements for several innate immune pathways, including genes involved in the toll-like receptor pathway, avian interferon/antiviral response pathway genes, and genes involved in apoptosis, antigen presentation, and the oxidative burst. The avian specific microarray can test for global gene expression patterns in a number of immunologically relevant tissues and in chickens, turkeys and ducks. The array has been evaluated for its ability to monitor the avian immune response to both bacterial (avian pathogenic E. coli) and viral (avian influenza) pathogens. Analysis of cytokine and chemokine gene expression following coccidiosis revealed that the primary response to coccidiosis is robust in immunocompetent chickens (Hoerr, 2010). This procedure holds the potential for characterising specific immune response deficiencies occurring as a result of some of the immunosuppressive conditions described herein.

Immunosuppression interventions

The ultimate goal is the prevention of economic losses due to immunosuppressive viral infections. This will be achieved mainly through strict biosecurity to prevent exposure to the causes of immunosuppressive diseases, and increasing the resistance to challenge from immunosuppressive agents through carefully chosen vaccination strategies (Umar et al., 2015c) and genetic selection. Treatments against viral diseases in poultry are not available so far. Vaccines are still the best choice to producing specific immune responses effective in the prevention of diseases; however, vaccines are not available for most of the avian bacterial and parasitic infections. On the other hand, immunosuppression remains a hidden threat that can lead to vaccine failure and disease outbreak (Shini et al., 2010). With recent cloning of avian cytokine genes, their utilisation as therapeutic agents for viral diseases in poultry as well as for vaccine adjuvants has become more feasible in the future (Umar et al., 2015b). Furthermore, immunomodulatory products have been shown to reduce the severity of some virus-induced lesions and consequently the immunosuppression (Umar et al., 2015d).

Currently, MD, IBD and CIA are three major immunosuppressive diseases in poultry. MD outbreaks are under control by the use of vaccines. However, control of IBD and CIA is still a big challenge largely due to their subclinical nature and are detected along with secondary diseases during diagnosis. Today, with ever-expanding flock and farm sizes, litter is reused due to economic and environmental constraints, cleaning and disinfection become seasonal events rather than between flocks and there is variable application of all-in all-out management. These factors all contribute to an environment where challenge from ubiquitous immunosuppressive agents is virtually certain. MD, IBD, and CIA are not unique to these environments, but they are a constant challenge due to the ability of many animals to replicate and shed large quantities of virus.

Immunosuppression has historically cost the poultry industry in increased mortality and in performance factors during rearing, and it has negatively impacted the processing of chickens due to associated health problems. Strategies to control immunosuppression in broilers and commercial layers are largely based on vaccination programs for breeders and broiler progeny, and management to minimise stress during rearing (Fussell, 1998). The genes of the major histocompatibility complex (MHC) encode proteins that are essential in the functioning of the immune system. The MHC antigens of chickens are cell-surface glycoproteins of three different classes: class I (B-F), class II (B-L), and class IV (BG), which are essential in the regulation of cell-cell interactions. A second histocompatibility complex of genes occurs in the chicken, Rfp-Y, composed of MHC class I and class II genes. Immunogenetic selection for specific resistance to immunosuppressive infectious diseases has yielded variable results.

The strong association of genes for resistance to MD (Hoerr, 2010; Haq et al., 2013) has resulted in the application of genetic selection. Selection for specific immunogenetic traits is a challenge because selection for disease resistance must be balanced with production (Zekarias et al., 2002). Immunogenetic associations for resistance to IBD are equivocal (Hudson et al., 2002), but differences occur in the antibody response of various MHC haplotypes to IBDV infection and to inactivated IBDV vaccines (Juul-Madsen et al., 2006; Hoerr, 2010). CAV infects cells bearing class II antigens in bone marrow and T-helper lymphocytes in thymus. An MHC effect on resistance to CAV infection was not detected, but induced mutations on CAV had an effect on virus replication, cytopathogenicity and down regulation of MHC class I genes in infected cells (Joiner et al., 2005; Umar et al., 2014; Wani et al., 2015).

Immunogenetic control of antibody responsiveness is complex as multiple loci of MHC genes are involved. As an example of the complexity of these associations, the selection of chickens with high-antibody responses had no effect on genetic resistance to E. coli infection or Eimeria tenella coccidiosis. The MHC has been primarily identified with genetic control of immune response and disease resistance, but lesser characterised genes outside of the MHC also regulate immunoresponsiveness. The existence of toll-like receptors in chickens and turkeys provides a genetic basis for selection for enhanced innate immunity. Understanding the interaction of adjuvants with immunogenetics may lead to improved vaccines (Yin et al., 2006).

Conclusions

Currently, stress due to immunosuppressive agents causes increased mortality, morbidity and productivity losses in the poultry industry. Despite our knowledge of molecular and cellular mechanisms of immunity against viral diseases, there are still several gaps in this area. We have a limited understanding of the process and dynamics of T cell-mediated responses to viral diseases. There are no research-based strategies for controlling stress and biological cost in poultry. New methods, such as early administration of cytokines, vaccines, or both, have only just started to demonstrate an increase in viability and disease resistance of chickens. On the other hand, modifying and improving the environment of the bird, in other words, minimising the frequency or severity of the stressor, could increase production, reduce indices of disease, and enhance the quality of life for birds.

It should be noted that, in many cases, disease problems associated with intensive production systems are often husbandry problems. Fundamental improvements in husbandry practice, such as adequate conditions and supplies of food, fresh water, natural housing material, and construction will give hens the space needed for movement that will reduce aggressive behaviour (i.e. stress). As for other animal production systems, eliminating all stressors is impossible. One solution is to improve the ability of the bird to cope through genetic selection (e.g. developing selected breeding lines of laying hens that display far less aggression than their commercial counterparts) while maintaining industry standard egg production and physiological status. Characterisation of the new emerging immunosuppressive agents (e.g. enteric viruses), their pathogenesis single and/or in combination and methods for prevention and control should be the target of future studies.