A critical assessment of the cellular defences of the avian respiratory system: are birds in general and poultry in particular relatively more susceptible to pulmonary infections/afflictions?

In commercial poultry farming, respiratory diseases cause high morbidities and mortalities, begetting colossal economic losses. Without empirical evidence, early observations led to the supposition that birds in general, and poultry in particular, have weak innate and adaptive pulmonary defences and are therefore highly susceptible to injury by pathogens. Recent findings have, however, shown that birds possess notably efficient pulmonary defences that include: (i) a structurally complex three-tiered airway arrangement with aerodynamically intricate air-flow dynamics that provide efficient filtration of inhaled air; (ii) a specialised airway mucosal lining that comprises air-filtering (ciliated) cells and various resident phagocytic cells such as surface and tissue macrophages, dendritic cells and lymphocytes; (iii) an exceptionally efficient mucociliary escalator system that efficiently removes trapped foreign agents; (iv) phagocytotic atrial and infundibular epithelial cells; (v) phagocytically competent surface macrophages that destroy pathogens and injurious particulates; (vi) pulmonary intravascular macrophages that protect the lung from the vascular side; and (vii) proficiently phagocytic pulmonary extravasated erythrocytes. Additionally, the avian respiratory system rapidly translocates phagocytic cells onto the respiratory surface, ostensibly from the subepithelial space and the circulatory system: the mobilised cells complement the surface macrophages in destroying foreign agents. Further studies are needed to determine whether the posited weak defence of the avian respiratory system is a global avian feature or is exclusive to poultry. This review argues that any inadequacies of pulmonary defences in poultry may have derived from exacting genetic manipulation(s) for traits such as rapid weight gain from efficient conversion of food into meat and eggs and the harsh environmental conditions and severe husbandry operations in modern poultry farming. To reduce pulmonary diseases and their severity, greater effort must be directed at establishment of optimal poultry housing conditions and use of more humane husbandry practices.

Preventing bacterial disease in poultry in the post-antibiotic era: a case for innate immunity modulation as an alternative to antibiotic use

The widespread use of antibiotics in the poultry industry has led to the emergence of antibiotic-resistant bacteria, which pose a significant health risk to humans and animals. These public health concerns, which have led to legislation limiting antibiotic use in animals, drive the need to find alternative strategies for controlling and treating bacterial infections. Modulation of the avian innate immune system using immunostimulatory compounds provides a promising solution to enhance poultry immune responses to a broad range of bacterial infections without the risk of generating antibiotic resistance. An array of immunomodulatory compounds have been investigated for their impact on poultry performance and immune responses. However, further research is required to identify compounds capable of controlling bacterial infections without detrimentally affecting bird performance. It is also crucial to determine the safety and effectiveness of these compounds in conjunction with poultry vaccines. This review provides an overview of the various immune modulators known to enhance innate immunity against avian bacterial pathogens in chickens, and describes the mechanisms involved.

Co-expression of surfactant protein A and chicken lung lectin in chicken respiratory system

Chicken surfactant protein A (cSP-A) and chicken lung lectin (cLL) are C-type lectins that play important roles in pulmonary host defense responses. Herein, we explored the localization of cSP-A and cLL in the chicken respiratory system. Six tissues from 30-days-old SPF chickens were used to quantify the expression of cSP-A and cLL using the quantitative real-time reverse transcriptional polymerase chain reaction (qRT-PCR) and fluorescence multiplex immunohistochemistry staining (fluorescence mIHC staining). Results showed that cSP-A and cLL mRNA were highly expressed in lungs compared to other tissues. cSP-A mRNA expression levels in all tissues were higher compared with cLL expression levels as analyzed using qRT-PCR. Fluorescence mIHC co-expression of cSP-A and cLL were mainly detected in lung parabronchial epithelia, and mucosal epithelia of larynx, trachea, syrinx, bronchus and air sac, with cSP-A showing a stronger positive staining compared with cLL. cLL is expressed on both mucosal surfaces, some individual lung epithelial cells and cartilage cells, while cSP-A is mainly restricted to mucosal surfaces of the respiratory tract. These histological findings may be useful for understanding the biological significance of this pulmonary lectins in future studies.

Development of immunization trials against Pasteurella multocida

Pasteurellosis is one of the most important respiratory diseases facing economically valuable farm animals such as poultry, rabbit, cattle, goats and pigs. It causes severe economic loss due to its symptoms that range from primary local infection to fatal septicemia. Pasteurella multocida is the responsible pathogen for this contagious disease. Chemotherapeutic treatment of Pasteurella is expensive, lengthy, and ineffective due to the increasing antibiotics resistance of the bacterium, as well as its toxicity to human consumers. Though, biosecurity measures played a role in diminishing the spread of the pathogen, the immunization methods were always the most potent preventive measures. Since the early 1950s, several trials for constructing and formulating effective vaccines were followed. This up-to-date review classifies and documents such trials. A section is devoted to discussing each group benefits and defects.

The avian lung-associated immune system: a review

The lung is a major target organ for numerous viral and bacterial diseases of poultry. To control this constant threat birds have developed a highly organized lung-associated immune system. In this review the basic features of this system are described and their functional properties discussed. Most prominent in the avian lung is the bronchus-associated lymphoid tissue (BALT) which is located at the junctions between the primary bronchus and the caudal secondary bronchi. BALT nodules are absent in newly hatched birds, but gradually developed into the mature structures found from 6-8 weeks onwards. They are organized into distinct B and T cell areas, frequently comprise germinal centres and are covered by a characteristic follicle-associated epithelium. The interstitial tissue of the parabronchial walls harbours large numbers of tissue macrophages and lymphocytes which are scattered throughout tissue. A striking feature of the avian lung is the low number of macrophages on the respiratory surface under non-inflammatory conditions. Stimulation of the lung by live bacteria but not by a variety of bacterial products elicits a significant efflux of activated macrophages and, depending on the pathogen, of heterophils. In addition to the cellular components humoral defence mechanisms are found on the lung surface including secretory IgA. The compartmentalisation of the immune system in the avian lung into BALT and non BALT-regions should be taken into account in studies on the host-pathogen interaction since these structures may have distinct functional properties during an immune response.

Gene expression profiling of avian macrophage activation

Through the process of phagocytosis, the macrophage is responsible for the clearance and destruction of both intracellular and extracellular pathogens. When stimulated, macrophages undergo a process of activation involving an increase in size and motility, enhanced phagocytic, bactericidal, and tumoricidal activity, and up-regulation of several cell-surface markers. One well characterized method of mammalian macrophage activation involves the Toll-like receptor (TLR) pathway. TLRs are surface molecules that function as direct receptors for microbial components. Binding of ligand to TLRs results in activation of transcription factors that regulate genes involved in microbial killing, apoptosis, and antigen recognition, as well as pro- and anti-inflammatory cytokines and chemokines. We have constructed a 4906-element (14,718 spot) avian macrophage-specific cDNA microarray (AMM). The AMM contains 16 of the approximately 44 genes identified within the mammalian TLR pathway. This array was used to examine the transcriptional response of avian macrophages to Gram-negative bacteria and their cell wall components and to evaluate the contribution of the avian TLR pathway to that response. Of the elements on the AMM, 981 (20%) exhibited significant (greater than two-fold, p < 0.01) changes in expression during phagocytosis of Escherichia coli and 243 (5%) exhibited significant expression changes during exposure to lipopolysaccharide (LPS). A unique set of overlapping elements (154), were observed to exhibit significant changes in expression for both phagocytosis and LPS stimulation, representing a set of core response elements. Of these elements, 63% were commonly induced, while 32% were commonly repressed. Both LPS and bacteria were found to induce NFkappabeta and several end products of the TLR pathway.

Pasteurella multocida Infection in Heterophil-Depleted Chickens

The present study was aimed at elucidating the role of heterophil granulocytes during the initial infection with Pasteurella multocida subsp. multocida in chickens. Chickens (17 and 19 wk old) were depleted of their heterophil granulocytes by 5-fluorouracil treatment. When the heterophil blood counts were significantly reduced, the birds were inoculated intratracheally with 1.8-4.3 x 10(4) colony-forming units of P. multocida. Twelve, 24, or 48 hr postinoculation, the birds were euthanatized and examined for macroscopic and histologic lesions in the lungs. Bacterial invasion was determined by culture of P. multocida from the spleen. Recruitment of heterophils into the respiratory tract during infection was found to contribute considerably to the lung lesions in chickens and was found to mediate tissue damage, possibly allowing a more rapid systemic spread of P. multocida. However, during progression of the infection, the heterophil-mediated necrosis in chickens seemed to stimulate giant cell demarcation of infected lung tissue, which coincided with the clearance of P. multocida from the spleen, thus hampering further invasion. Consequently, heterophil activation plays a dual role for the outcome of a P. multocida infection in chickens, where it initially seems to promote invasion and systemic spread but subsequently helps limit the infection by giant cell formation and bacterial clearance.

The effect of concurrent infections with Pasteurella multocida and Ascaridia galli on free range chickens

Pasteurella multocida and Ascaridia galli are observed with high prevalences in free range chickens in Denmark, but the impact is unknown. A study was carried out to examine the interaction between A. galli and P. multocida in chickens and the impact on production. Five groups, each with 20 18-week-old Lohmann Brown chickens were infected. Group 1 was orally infected with 1000+/-50 embryonated A. galli eggs. Group 2 received 10(4) cfu P. multocida intratracheally. Group 3 was infected with A. galli and subsequently with P. multocida. Group 4 was infected with P. multocida followed by A. galli. Group 5 was the control. The study ran for 11 weeks where clinical manifestations, weight gain and egg production were recorded. Excretion of P. multocida was determined on individual basis and blood smears were made for differential counts. At the end of the study pathological lesions and the number of adult worms, larvae and eggs in the faeces were recorded. The birds were more severely affected when infected with both pathogens compared to single infections with A. galli or P. multocida, respectively. A lower weight gain and egg production was observed with dual infections. A. galli infection followed by a secondary P. multocida infection resulted in more birds with pathological lesions and continued P. multocida excretion. In conclusion a negative interaction between A. galli and P. multocida was observed and it is postulated that free range chickens are at higher risk of being subjected to outbreaks of fowl cholera when they are infected with A. galli.

Nonspecific cellular defense of the avian respiratory system: a review

The normal, steady-state, avian respiratory system has very low numbers of residing avian respiratory phagocytes (ARP). Birds must rely heavily on the influx of ARP to defend against infectious agents. The system is refractory to elicitation by inert stimulants, but responds efficiently to replicating bacteria, with very rapid influx of large numbers of activated ARP (polymorphonuclear neutrophils, heterophils, and macrophages) with increased phagocytic proportions and capacities. The numbers subside within a few a days. Activated ARP act in a non agent-specific manner: Pasteurella multocida-activated ARP can defend against a severe Escherichia coli airsacculitis. Parenteral routes of stimulation generally are not, respiratory routes are very, efficient in activating ARP. Heterophils are the most efficient in defensive reactions, such as oxidative burst, production of nitric oxide and killing of bacteria. Respiratory viruses may stimulate, but also may diminish some of the defensive functions of ARP. This is also true for attenuated, modified live virus vaccines. These vaccines must be used carefully in the presence of subclinical bacterial, mycoplasmal infections. Published literature on non-specific cellular defense of the avian respiratory system is very limited, particularly about interactions among multiple infectious agents and the system.

Effects of the lipopolysaccharide-protein complex and crude capsular antigens of Pasteurella multocida serotype A on antibody responses and delayed type hypersensitivity responses in the chicken

The effects of the lipopolysaccharide-protein complex (LPS) and crude capsular antigen (CCA) prepared from Pasteurella multocida serotype A isolated from a duck in the Philippines, on antibody responses to sheep red blood cells (SRBC) and Brucella abortus (BA) and delayed type hypersensitivity (DTH) responses to bovine serum albumin (BSA) in the chickens were studied. Chickens injected subcutaneously with LPS and CCA at 1 and 2 weeks of age and immunized intravenously with the mixed antigens of SRBC and BA, at 3 and 4 weeks of age showed significantly increased antibody responses against both SRBC and BA, when evaluated at 7 days after each immunization. In addition, these chickens sensitized intramuscularly with the emulsion of BSA in complete Freund's adjuvant at 5 weeks of age, and then injected into the wattle with BSA at 7 weeks of age also showed significantly increased DTH responses against BSA, when evaluated at 24 and 48 hr after challenge. These results indicate that LPS and CCA of P. multocida serotype A have a property enhancing humoral and cell-mediated immune responses.

Effects of four antigenic fractions of Pasteurella multocida serotype A on phagocytosis of chicken peripheral blood leukocytes

The effects of four antigenic fractions of Pasteurella multocida serotype A isolated from a duck in the Philippines on the phagocytic activity of chicken peripheral blood leukocytes were studied by a flow cytometer. These fractions were the lipopolysaccharide-protein complex (LPS), crude capsular antigen (CCA), ribosomal fraction (RS) and outer cell layer (OCL). Among these four antigens, only CCA but not LPS RS and OCL, significantly increased the phagocytic activities of mononuclear cells (MNC) and polymorphonuclear cells (PMN). This result indicates that CCA has an immunological property enhancing the phagocytic activities of MNC and PMN.

Oxygen radicals and nitric oxide production by turkey respiratory macrophages

The influence of different induction protocols on the recovery of elicited turkey respiratory macrophages (RM), and on their oxygenation activity and nitric oxide (NO) production was examined. RM were induced in three week old specific pathogen free turkeys with Sephadex G-50, Thioglycollate broth, and an emulsion of incomplete Freund's adjuvant (IFA), supplemented either with Mycoplasma hyorhinis grown in Modified Channock broth (IFA-M. hyorhinis) or with Modified Channock broth (IFA-Broth). The RM were recovered by lavage of the lungs and air sacs and were purified by centrifugation through a Percoll suspension. Their oxygenation activity was evaluated in luminol-enhanced chemiluminescence assays, following stimulation with Zymosan A. The NO production was evaluated by incubating the RM with lipopolysaccharide (LPS) from Salmonella enteritidis for 24 or 48 hours. The number of recovered RM was slightly, but not significantly lower for Sephadex G-50 and IFA-Broth than for Thioglycollate broth and IFA-M. hyorhinis. RM elicited with Sephadex G-50 and IFA-Broth showed a significantly higher oxidative burst response to Zymosan A, compared to the Thioglycollate and IFA-M. hyorhinis elicited RM. Although all elicited RM showed a high NO production upon stimulation with LPS, no significant differences were seen in the NO production of the RM obtained following the different induction treatments. Our results point out that care should be taken when applying elicited RM for in vitro assays, as distinct levels of oxygenation activity were obtained using different induction protocols.

Stimulation of avian respiratory phagocytes by Pasteurella multocida: effects of the route of exposure, bacterial dosage and strain, and the age of chickens

The effects of the route of exposure (intratracheal [IT], drinking water [DW] and aerosolization [AS]), the age of chickens, and the dose of two vaccine strains of Pasteurella multocida (CU and M-9) on the number, phagocytic proportion and capacity of macrophages, granulocytes and lymphocytes (collectively avian respiratory phagocytes [ARPs]) were analyzed. Administration of P. multocida via the DW even at very high dose failed to stimulate ARPs. In contrast, administration of both strains of P. multocida either IT or by AS resulted in rapid and highly significant increases in the numbers of ARPs. 10 x 10(9) colony forming units (cfu) of aerosolized P. multocidaCU strain activated ARPs maximally in both young (3-6 weeks of age) and old (4-6 months of age) chickens. Old chickens responded in dose dependent manner to 20 x 10(9), 8 x 10(9), and 4 x 10(9) cfu of aerosolized P. multocida CU strain. Young chickens responded significantly only to 8 x 10(9) CU organisms. The M-9 and CU strains had limited differences in inducing migration of ARPs into the respiratory system of chickens or elevating phagocytic proportions and capacity of ARPs. The results indicate that the analyzed factors influence the response of ARPs to P. multocida to various degrees.