ASCITES IN POULTRY

Research on ascites occurring in meat-type chickens reared at moderate and low attitude has shown that the pathogenesis is similar to that o f the high altitude disease. Pulmonary hypertension (PH) caused by inc reased blood flow or increased resistance to flow in the lung results in right ventricular hypertrophy (RVH), valvular insufficiency, increa sed venous pressure and ascites. The structure of the avian heart, wit h its thin-walled right ventricle and muscular right atrioventricular valve, allows PH to induce heart failure quickly. The sudden increase in pulmonary hypertension syndrome (PHS) in meat-type chickens in the 1980s was associated with a rapid increase in growth rate and feed con version. This was a result of a combination of genetic selection for f ast-growing, heavy broilers with low feed conversion and a more dense, high caloric, pelleted food that supplied all the nutrients required for rapid growth and encouraged a high nutrient intake. PHS in meat-ty pe chickens is usually primary pulmonary hypertension, that is, PH tha t occurs without evidence of prior heart or lung disease that could ac count for the increase in blood flow or resistance to flow that result s in the increased pressure in the pulmonary arteries. The lungs of bi rds are firm and fixed in the thoracic cavity and they do not expand t o draw air into the lung. The blood and air capillaries form a rigid n etwork that allows only minimal expansion of the blood capillaries whe n more blood flow is required. Air is moved through the lung by abdomi nal movement which draws air in and out of the air sacs. The anatomy a nd physiology of the avian respiratory system are important in the sus ceptibility of meat-type chickens to PHS. The small stature of the mod em meat-type chicken, the large, heavy breast mass, the pressure from abdominal contents on air sacs, and the small lung volume compared to body weight, may all be involved in the increased incidence of PHS. Th ere is limited space for blood flow in the avian lung. Factors that in crease blood flow or increase resistance to flow are additive. Increas ed blood viscosity caused by the polycythaemia of hypoxia, or increase d erythrocyte rigidity of high Na+, are more likely to produce PH in f ast-growing than in slow-growing birds. Increased flow due to cold exp osure is also additive. Ascites caused by PH is a production-related d isease at low altitude. It can be prevented easily by restricting grow th rate. It is possible that some meat-type chickens of the phenotype we have created have reached the limit of blood flow through their lun gs and that future improvements in growth rate will only be possible i f the lung and abdominal cavity capacities are enlarged.