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.

Pulmonary vascular pressure profiles in broilers selected for susceptibility to pulmonary hypertension syndrome: age and sex comparisons.

Broilers that are susceptible to pulmonary hypertension syndrome (PHS, ascites) have an elevated pulmonary arterial pressure (PAP) when compared with PHS-resistant broilers. Two distinctly different syndromes, pulmonary arterial hypertension and pulmonary venous hypertension (PVH), both are associated with increases in PAP. Pulmonary arterial hypertension occurs when the right ventricle must elevate the PAP to overcome increased resistance to flow through restrictive pulmonary arterioles upstream from the pulmonary capillaries. In contrast, PVH is commonly caused by increased downstream (postcapillary) resistance. The sites of resistance to pulmonary blood flow are deduced by making contemporaneous measurements of the PAP and the wedge pressure (WP) and calculating the transpulmonary pressure gradient (TPG) (TPG = PAP - WP). We obtained PAP and WP values from 8-, 12-, 16-, 20-, and 24-wk-old anesthetized male and female broilers from a PHS-susceptible line. Pressures were recorded as a catheter was advanced through a wing vein to the pulmonary artery and onward until the WP was obtained. In addition to sex and age comparisons of vascular pressure gradients, the data also were pooled to obtain 3 cohorts for broilers having the lowest PAP values (n = 52; range: 12 to 22.9 mmHg), intermediate PAP values (n = 63; range: 23 to 32.9 mmHg), and highest PAP values (n = 62; range: 33 to 62 mmHg) independent of age or sex. Within each of the age, sex, and PAP cohort comparisons, broilers with elevated PAP consistently exhibited the hemodynamic characteristics of pulmonary arterial hypertension (elevated PAP and TPG combined with a normal WP) and not PVH (elevated PAP and WP combined with a normal or reduced TPG). Susceptibility to PHS can be attributed primarily to pulmonary arterial hypertension associated with increased precapillary (arteriole) resistance.

Expression of inducible nitric oxide synthase in lungs of broiler chickens following intravenous cellulose microparticle injection.

Intravenous microparticle (MP) injection is a patented method used to select broilers with a robust pulmonary capacity and improved resistance to pulmonary hypertension syndrome (PHS, ascites). Injected MP become entrapped in the terminal pulmonary arterioles where they elicit an increase in pulmonary arterial pressure attributable to vascular occlusion and focal thrombocyte aggregation. Within 2 to 48 h postinjection perivascular mononuclear cell aggregates begin to form around MP-occluded vessels. Nitric oxide (NO) has been shown to modulate the pulmonary arterial pressure response to MP entrapment, but a role of NO during the more chronic (2 to 48 h) focal inflammatory response has not been evaluated. In this study we determined the time-course of inducible nitric oxide synthase (iNOS) expression in the lungs of MP-injected broilers from PHS-resistant (RES) and PHS-susceptible (SUS) lines. Four-week-old broilers (10 broilers/line per time point) were injected i.v. with a minimally lethal dose of MP, and the right lung was collected at 0 h (no MP) and 2, 24, and 48 h postinjection. Immunohistochemistry revealed that macrophage infiltration increased over time in both lines and was higher in the RES line than the SUS line (P<0.0001) at all time points. Nicotinamide adenine dinucleotide phosphate diaphorase staining showed nitric oxide synthase activity around MP-occluded vessels and in the perivascular mononuclear cell aggregates. Relative iNOS expression in lung tissue was examined by 2-step reverse transcription PCR. Lines differed in relative iNOS mRNA expression only at 24 h (P<0.001; RES > SUS line). For the RES line iNOS mRNA expression increased consistently from 0 to 48 h, but for the SUS line iNOS mRNA expression increased at 2 h, decreased to baseline at 24 h, and increased again by 48 h. The decline in iNOS expression in the SUS line between 2 and 24 h coincides with the interval when most of the MP-induced mortality occurs, which suggests that NO synthesized by iNOS may contribute to lower MP-induced mortality in the RES line when compared with the SUS line.

An inadequate pulmonary vascular capacity and susceptibility to pulmonary arterial hypertension in broilers.

Broilers are susceptible to pulmonary hypertension syndrome (PHS; ascites syndrome) when their pulmonary vascular capacity is anatomically or functionally inadequate to accommodate the requisite cardiac output without an excessive elevation in pulmonary arterial pressure. The consequences of an inadequate pulmonary vascular capacity have been demonstrated experimentally and include elevated pulmonary vascular resistance (PVR) attributable to noncompliant, fully engorged vascular channels; sustained pulmonary arterial hypertension (PAH); systemic hypoxemia and hypercapnia; specific right ventricular hypertrophy, and right atrioventricular valve failure (regurgitation), leading to central venous hypertension and hepatic cirrhosis. Pulmonary vascular capacity is broadly defined to encompass anatomical constraints related to the compliance and effective volume of blood vessels, as well as functional limitations related to the tone (degree of constriction) maintained by the primary resistance vessels (arterioles) within the lungs. Surgical occlusion of 1 pulmonary artery halves the anatomical pulmonary vascular capacity, doubles the PVR, triggers PAH, eliminates PHS-susceptible broilers, and reveals PHS-resistant survivors whose lungs are innately capable of handling sustained increases in pulmonary arterial pressure and cardiac output. We currently are using i.v. microparticle injections to increase the PVR and trigger PAH sufficient in magnitude to eliminate PHS-susceptible individuals while allowing PHS-resistant individuals to survive as progenitors of robust broiler lines. The microparticles obstruct pulmonary arterioles and cause local tissues and responding leukocytes to release vasoactive substances, including the vasodilator NO and the highly effective vasoconstrictors thromboxane A(2) and serotonin [5-hydroxytryptamine (5-HT)]. Nitric oxide is the principal vasodilator responsible for modulating (attenuating) the PAH response and ensuing mortality triggered by i.v. microparticle injections, whereas microparticle-induced increases in PVR can be attributed principally to 5-HT. Our observations support the hypothesis that susceptibility to PHS is a consequence of anatomically inadequate pulmonary vascular capacity combined with the functional predominance of the vasoconstrictor 5-HT over the vasodilator NO. The contribution of TxA(2) remains to be determined. Selecting broiler lines for resistance to PHS depends upon improving both anatomical and functional components of pulmonary vascular capacity.

Maternal antibody transfer from dams to their egg yolks, egg whites, and chicks in meat lines of chickens.

Maternal antibodies are transferred from hens to the chicks via the egg. To gain insight into maternal antibody transfer and endogenous production of antibodies in broiler chicks, total IgY, IgA, IgM, as well as anti-Newcastle disease virus (NDV) and anti-infectious bronchitis (IBV) antibody levels were examined in the dams' plasma, egg yolks, egg whites, and chicks' plasma on d 3, 7, 14, and 21. Blood was collected from 39-wk-old breeder hens (line 1, n = 17; line 2, n = 21). Fertile eggs were used for antibody extraction from the egg yolks and egg whites (4 to 5 eggs/dam) and for hatching. Unvaccinated chicks (4 to 5 chicks/dam) were reared in a HEPA-filtered room and were bled on d 3, 7, 14 and 21. Based on ELISA methods, plasma levels of IgY and IgM were higher (P < 0.0001), and those of IgA were similar (P = 0.31), in line 2 compared with line 1. Egg yolk IgY and IgA, as well as egg white IgY, IgA, and IgM levels were higher in line 2 compared with line 1 (P < 0.0001). Independent of line of chicken, the percentage dam-to-chick (3 d) plasma transfer of IgY was estimated to be approximately 30%, with that for IgM and IgA less than 1%. Chicks synthesized IgM first, followed by IgA and IgY. Anti-NDV and anti-IBV antibodies were detected in the dams' plasma, egg yolks, and in the chicks' plasma on d 3 and 7, with line 2 having higher anti-IBV and lower anti-NDV levels than line 1 in all samples (P < 0.0001). In summary, IgY levels, total or antigen-specific, in the dams' plasma or eggs were found to be a direct indicator of maternal antibody transfer to the chicks' circulation, with an expected percentage transfer of approximately 30%. This knowledge, together with the observed time course of endogenous antibody production in broiler chicks, may find direct application in formulating strategies for protecting chicks, especially during the first few weeks of age when their immune system is not yet fully functional.