R. F. Wideman

Pathophysiology of heart/lung disorders: pulmonary hypertension syndrome in broiler chickens

Pulmonary hypertension syndrome (PHS, ascites) in broiler chickens provides an outstanding example of the pathophysiological interplay between the lungs and heart. Recent research has confirmed the hypothesis that PHS susceptible broilers have an inherent potential to outgrow their pulmonary vascular capacity. Definitive proof that pulmonary hypertension (PH) is initiated as a consequence of an excessive pulmonary arterial resistance was obtained through measurements of pulmonary wedge pressures. Definitive proof that a pulmonary diffusion limitation causes the hypoxaemia characteristic of susceptible broilers was demonstrated by the rapid return to full arterial blood oxygenation in pre-ascitic broilers when they were provided with 100% oxygen to breathe. Experiments involving acute and chronic unilateral pulmonary occlusion provided definitive proof that the entire pathogenesis of PHS can be replicated by reducing the pulmonary vascular capacity. These experiments conclusively demonstrate that the resistance of broilers to PHS depends upon the capacity of their pulmonary vasculature to accept the requisite cardiac output at blood flow rates and pressures sufficiently low to avoid triggering PH and systemic hypoxaemia. The chronic unilateral pulmonary artery occlusion technique was successfully applied to develop a broiler line that is highly resistant to PHS induced by fast growth and exposure to rigorous cool temperatures, demonstrating for the first time that a dominant gene codes for a highly significant proportion of the PHS susceptibility. We now possess the knowledge necessary to create broiler lines capable of maintaining extremely rapid growth under rigorous field conditions while retaining very low susceptibility to PHS.

Pulmonary arterial hypertension (ascites syndrome) in broilers: A review

Pulmonary arterial hypertension (PAH) syndrome in broilers (also known as ascites syndrome and pulmonary hypertension syndrome) can be attributed to imbalances between cardiac output and the anatomical capacity of the pulmonary vasculature to accommodate ever-increasing rates of blood flow, as well as to an inappropriately elevated tone (degree of constriction) maintained by the pulmonary arterioles. Comparisons of PAH-susceptible and PAH-resistant broilers do not consistently reveal differences in cardiac output, but PAH-susceptible broilers consistently have higher pulmonary arterial pressures and pulmonary vascular resistances compared with PAH-resistant broilers. Efforts clarify the causes of excessive pulmonary vascular resistance have focused on evaluating the roles of chemical mediators of vasoconstriction and vasodilation, as well as on pathological (structural) changes occurring within the pulmonary arterioles (e.g., vascular remodeling and pathology) during the pathogenesis of PAH. The objectives of this review are to (1) summarize the pathophysiological progression initiated by the onset of pulmonary hypertension and culminating in terminal ascites; (2) review recent information regarding the factors contributing to excessively elevated resistance to blood flow through the lungs; (3) assess the role of the immune system during the pathogenesis of PAH; and (4) present new insights into the genetic basis of PAH. The cumulative evidence attributes the elevated pulmonary vascular resistance in PAH-susceptible broilers to an anatomically inadequate pulmonary vascular capacity, to excessive vascular tone reflecting the dominance of pulmonary vasoconstrictors over vasodilators, and to vascular pathology elicited by excessive hemodynamic stress. Emerging evidence also demonstrates that the pathogenesis of PAH includes characteristics of an inflammatory/autoimmune disease involving multifactorial genetic, environmental, and immune system components. Pulmonary arterial hypertension susceptibility appears to be multigenic and may be manifested in aberrant stress sensitivity, function, and regulation of pulmonary vascular tissue components, as well as aberrant activities of innate and adaptive immune system components. Major genetic influences and high heritabilities for PAH susceptibility have been demonstrated by numerous investigators. Selection pressures rigorously focused to challenge the pulmonary vascular capacity readily expose the genetic basis for spontaneous PAH in broilers. Chromosomal mapping continues to identify regions associated with ascites susceptibility, and candidate genes have been identified. Ongoing immunological and genomic investigations are likely to continue generating important new knowledge regarding the fundamental biological bases for the PAH/ascites syndrome.

Dietary arginine: metabolic, environmental, immunological and physiological interrelationships.

Arginine is an essential amino acid for chickens due to the absence of a functional urea cycle in birds. Arginine plays critical roles in metabolic pathways associated with growth and immune-competence. Likewise, as a precursor of nitric oxide synthesis, arginine is important as the key vasodilator that opposes the onset of pulmonary hypertension in broiler (meat-type) chickens. Dietary arginine levels in commercial broiler diets meet NRC recommendations. However, this review shows that NRC recommendations may not be adequate to support maximal growth, support arginine-depleting immune responses, and prevent the onset of pulmonary hypertension in broilers reared under rigorous environmental conditions. Dietary composition is highly important and broiler performance may suffer when insufficient levels of arginine are included in the diet. Fortification of broiler diets with supplemental arginine may be necessary under such circumstances.

A wire-flooring model for inducing lameness in broilers: Evaluation of probiotics as a prophylactic treatment

Bacterial chondronecrosis with osteomyelitis (BCO) is the most common cause of lameness in commercial broilers. Bacteria entering the blood via translocation from the respiratory system or gastrointestinal tract spread hematogenously to the proximal epiphyseal-physeal cartilage of rapidly growing femora and tibiae, causing BCO. We tested the hypothesis that rearing broilers on wire flooring should increase the incidence of BCO by persistently imposing additional torque and shear stress on susceptible leg joints. We also tested the hypothesis that probiotics might attenuate bacterial translocation and thereby reduce the incidence of BCO. In 5 independent experiments using 4 commercial lines, broilers grown on wire flooring developed lameness attributable predominately to BCO. The fastest-growing birds were not necessarily the most susceptible to lameness on wire flooring, nor did the genders differ in susceptibility in the 2 experiments that included both male and female broilers. The pathogenesis of BCO is not instantaneous, and accordingly, many broilers that did not exhibit lameness, nevertheless, did possess early pathognomonic lesions. These subclinical lesions were equally likely to develop in the right or left leg. The lesion status of the proximal femoral head did not determine the lesion status of the ipsilateral or contralateral proximal tibial head and vice versa. Broilers reared on wire flooring consistently had higher incidences of lameness than hatch-mates reared on wood-shavings litter. Adding probiotics to the diet beginning at 1 d of age consistently reduced the incidence of lameness for broilers reared on wire flooring. These experiments indicate that probiotics administered prophylactically may constitute an alternative to antibiotics for reducing lameness attributable to BCO. Rearing broilers on wire flooring provides an important new research model for investigating the etiology, pathogenesis, and treatment strategies for BCO.

Effects of supplementation of canola meal-based diets with arginine on performance, plasma nitric oxide, and carcass characteristics of broiler chickens grown at high altitude

A total of 300 male broilers (Ross 308) were exposed to cool conditions at high altitudes to study the effects of dietary Arg supplementation on performance and physiological and zootechnical variables. A corn-soybean meal (SBM) and a corn-canola meal (CM) diet were formulated for the starting (1 to 3 wk of age) and growing (3 to 6 wk of age) stages according to NRC recommendations. Two additional diets were prepared by supplementing 0.2 and 0.4% l-Arg to the corn-CM diet. Substitution of CM for SBM caused a significant (P < 0.05) reduction in weight gain and feed intake and resulted in impaired feed:gain. Supplementing Arg in the CM diet restored the feed and weight losses to a significant extent so that a significant difference was found between CM diet and CM + 0.4% Arg in terms of weight gain for the growing (3 to 6 wk) stage and the entire study (1 to 6 wk; P < 0.05). Total plasma nitric oxide (NO) concentration analyzed by nitrate plus nitrite assay was measured in the treatment groups. A significant (P < 0.05) decrease in plasma NO level was observed by substituting CM for SBM in the diet. Supplementing the CM diet with Arg increased the plasma NO level above that of SBM group. Carcass and breast yields were significantly decreased (P < 0.05) as a result of substituting CM for SBM. The substitution of CM for SBM, however, significantly (P < 0.05) increased the proportions of thighs and heart. The right ventricular weight:total ventricular weight ratio and ascites mortality showed a significant (P < 0.05) increase when SBM was replaced by CM in the diet. Fortification of the CM diet with Arg eliminated the significant difference in the right-to-total ventricular weight ratios when compared with the SBM diet. In conclusion, feeding CM to broiler chickens raised at high altitude caused reduced growth performance and predisposed the birds to pulmonary hypertension and ascites, which were partly restored by Arg supplementation.

Use of a Hypobaric Chamber as a Model System for Investigating Ascites in Broilers

In a series of experiments, broilers were reared at simulated altitudes of 3000, 3500, and 5000 meters utilizing a hypobaric chamber. The purpose was to evaluate the chamber as a model for ascites syndrome and to further study the pathophysiology of this disease. Results show that this model can effectively reproduce the disease as it is seen in the field. Birds reared at the simulated altitude of 5000 meters had a statistically significant increase in EKG amplitude, packed cell volume, and heart-to-body-weight and lung-to-body-weight ratios when compared with controls reared at 366 meters. Additionally, there was a trend for birds dying of ascites to have larger-amplitude EKGs than survivors when reared at 5000 meters.

Bone circulatory disturbances in the development of spontaneous bacterial chondronecrosis with osteomyelitis: a translational model for the pathogenesis of femoral head necrosis.

This review provides a comprehensive overview of the vascularization of the avian growth plate and its subsequent role in the pathogenesis of bacterial chondronecrosis with osteomyelitis (BCO, femoral head necrosis). BCO sporadically causes high incidences of lameness in rapidly growing broiler (meat-type) chickens. BCO is believed to be initiated by micro-trauma to poorly mineralized columns of cartilage cells in the proximal growth plates of the leg bones, followed by colonization by hematogenously distributed opportunistic bacteria. Inadequate blood flow to the growth plate, vascular occlusion, and structural limitations of the microvasculature all have been implicated in the pathogenesis of BCO. Treatment strategies have been difficult to investigate because under normal conditions the incidence of BCO typically is low and sporadic. Rearing broilers on wire flooring triggers the spontaneous development of high incidences of lameness attributable to pathognomonic BCO lesions. Wire flooring imposes persistent footing instability and is thought to accelerate the development of BCO by amplifying the torque and shear stress imposed on susceptible leg joints. Wire flooring per se also constitutes a significant chronic stressor that promotes bacterial proliferation attributed to stress-mediated immunosuppression. Indeed, dexamethasone-mediated immunosuppression causes broilers to develop lameness primarily associated with avascular necrosis and BCO. Prophylactic probiotic administration consistently reduces the incidence of lameness in broilers reared on wire flooring, presumably by reducing bacterial translocation from the gastrointestinal tract that likely contributes to hematogenous infection of the leg bones. The pathogenesis of BCO in broilers is directly relevant to osteomyelitis in growing children, as well as to avascular femoral head necrosis in adults. Our new model for reliably triggering spontaneous osteomyelitis in large numbers of animals represents an important opportunity to conduct translational research focused on developing effective prophylactic and therapeutic treatments.

Plexiform Lesions in the Lungs of Domestic Fowl Selected for Susceptibility to Pulmonary Arterial Hypertension: Incidence and Histology

Plexiform lesions develop in the pulmonary arteries of humans suffering from idiopathic pulmonary arterial hypertension (IPAH). Plexogenic arteriopathy rarely develops in existing animal models of IPAH. In this study, plexiform lesions developed in the lungs of rapidly growing meat‐type chickens (broiler chickens) that had been genetically selected for susceptibility to IPAH. Plexiform lesions developed spontaneously in: 42% of females and 40% of males; 35% of right lungs, and 45% of left lungs; and, at 8, 12, 16, 20, 24, and 52 weeks of age the plexiform lesion incidences averaged 52%, 50%, 51%, 40%, 36%, and 22%, respectively. Plexiform lesions formed distal to branch points in muscular interparabronchial pulmonary arteries exhibiting intimal proliferation. Perivascular mononuclear cell infiltrates consistently surrounded the affected arteries. Proliferating intimal cells fully or partially occluded the arterial lumen adjacent to plexiform lesions. Broilers reared in clean stainless steel cages exhibited a 50% lesion incidence that did not differ from the 64% incidence in flock mates grown on dusty floor litter. Microparticles (30 μm diameter) were injected to determine if physical occlusion and focal inflammation within distal pulmonary arteries might initiate plexiform lesion development. Three months postinjection no plexiform lesions were observed in the vicinity of persisting microparticles. Broiler chickens selected for innate susceptibility to IPAH represent a new animal model for investigating the mechanisms responsible for spontaneous plexogenic arteriopathy. Anat Rec, 2011.

Physiologic and electrocardiographic changes occurring in broilers reared at simulated high altitude.

An experiment was conducted to determine whether differences in the electrocardiograms (EKGs) of broilers reared at simulated high altitude from the day of hatch can be used to predict which birds are developing ascites. In three replicate experiments, conducted with 100 broilers per replicate, birds were reared at a simulated altitude of 3000 meters or at ambient atmospheric pressure. Lead I, II, and III EKGs were obtained from all birds on days 0, 14, 28, and 42. No consistent significant differences were seen on day 0 in the amplitude of the R or S wave or total amplitude of the QRS complex when broilers that developed ascites while being reared at simulated high altitude were compared with unaffected birds reared at simulated high altitude and with birds reared at ambient atmospheric pressure. On days 14 and 28, the average amplitude of the S wave and the total amplitude of the QRS complex were significantly higher in the ascites group than in the two other groups. Packed cell volumes were significantly higher in birds reared at simulated high altitude at all sampling days (days 14, 28, and 42) than in those reared at ambient atmospheric pressure, and they were significantly higher in the ascites group on day 28 than in the two other groups. Birds in the ascites group weighed significantly less than the two other groups by day 14, and this trend persisted.

Differential expression of vasoactive mediators in microparticle-challenged lungs of chickens that differ in susceptibility to pulmonary arterial hypertension.

Pulmonary hypertension syndrome (PHS; ascites) in fast growing meat-type chickens (broilers) is characterized by the onset of idiopathic pulmonary arterial hypertension (IPAH) leading to right-sided congestive heart failure and terminal ascites. Intravenous microparticle (MP) injection is a tool used by poultry geneticists to screen for the broilers that are resistant (RES) or susceptible (SUS) to IPAH in a breeding population. MPs occlude pulmonary arterioles and initiate focal inflammation, causing local tissues and responding leukocytes to release vasoactive mediators such as serotonin (5-HT), endothelin-1 (ET-1), and nitric oxide (NO). RT-PCR was used to examine the differences between RES and SUS broilers in terms of gene expression of ET-1, ET receptor types A and B (ET(A) and ET(B)), the serotonin transporter (SERT), serotonin receptors (5-HT(1A), 5-HT(2A), 5-HT(1B), 5-HT(2B)), endothelial NO synthase (eNOS), and inducible NOS (iNOS) in the lungs of these broilers before (0 h) and after (2, 6, 12, 24, and 48 h) MP injection. In SUS broilers MP injection elicited higher (P < 0.05) pulmonary expression of 5-HT(1A), 5-HT(2B), and ET-1, which promote vasoconstriction and proliferation of pulmonary arterial smooth muscle cells (PASMC). In RES broilers the MP injection elicited higher expression of eNOS, iNOS, and ET(B), which promote vasodilation and inhibit PASMC proliferation. These observations support the hypothesis that the resistance of broiler chickens to IPAH may be due to the higher expression of vasoactive mediators that favor enhanced vasodilation and attenuated vasoconstriction during MP injection challenges to the pulmonary vasculature.