Cardiovascular Diseases of Chickens

DEFINITION Ascites secondary to pulmonary hypertension syndrome (PHS) is one of the most important causes of mortality in broiler chicken flocks. It is associated with rapid growth and a high metabolic rate. OCCURRENCE Ascites occurs worldwide in rapidly growing broiler chicken flocks. HISTORICAL INFORMATION Ascites was first reported in 1968 in broiler chickens raised at a high altitude. However, the incidence of ascites caused by PHS, where broilers are grown at a low altitude, has increased over the past several years and coincides with genetic and nutritional improvements that resulted in better growth rate and feed conversion. ETIOPATHOGENESIS Four pathophysiological mechanisms are recognized to cause ascites: increased hydrostatic vascular pressure, decreased oncotic pressure, increased capillary permeability, and impaired lymphatic drainage. Although numerous chemical toxicities have been reported to cause ascites in broiler chickens through one of these mechanisms, the most common form of ascites in fast-growing broiler chickens is caused by increased hydrostatic vascular pressure. Rapid growth, elevated metabolic rate, and therefore a high oxygen demand impose an increased workload on the heart. This, combined with the insufficient pulmonary capillary capacity of the modern broiler chicken, aggravates the pulmonary hypertension and further precipitates right ventricular hypertrophy. Hypertrophy is soon followed by dilation, right ventricular failure, passive congestion, and then ascites. This process is accelerated in birds because of an anatomical particularity. The right atrioventricular valve is a muscular flap, an extension of the right ventricular wall. Any hypertrophy of the latter affects the valve and its apposition against the septum, facilitating venous regurgitation, passive congestion, and ascites. CLINICAL SIGNS Clinically affected broiler chickens are smaller than normal and depressed with ruffled feathers. Severely affected birds show abdominal distension with reluctance to move, respiratory distress, and cyanosis. LESIONS 1. Hypertrophy and dilation of the right ventricle [Fig. 1; Ascites; AAAP] with or without accumulation of straw-colored ascitic fluid in the peritoneal cavities [Fig. 2; Ascites; NCSU], and a generalized passive congestion are characteristic of ascites secondary to PHS. 2. Hydropericardium, protein clots in the ascitic fluid, and a fibrotic liver [Fig. 3; Ascites; AAAP] may be present in chickens with chronic PHS. 3. Microscopic lesions show generalized passive congestion.