Ascites syndrome (Pulmonary hypertension syndrome) in broiler chickens: Are we seeing the light at the end of the tunnel?

Ascites syndrome in broilers is a cascade of events that result in cardiac anomalies including an enlarged, flaccid heart, and right ventricular hypertrophy, as well as an accumulation of fluid in the abdominal cavity. Ascites was first noticed as a problem for commercial poultry producers, at farms that were located at high altitudes. Within the last 3 decades, however, ascites has become a significant problem for producers everywhere. Ascites fluid accumulation can be caused by many different factors. There are four general categories of dysfunctions that will result in ascites fluid build-up: (1) vascular damage that allows capillaries to leak; (2) blockage of the lymphatic system that prevents drainage; (3) decreased plasma oncotic pressure; and the most frequently seen dysfunction, (4) increased vascular hydraulic pressure. Increased vascular hydraulic pressure can be caused by several types of pathologies including hepatic, cardiac or right atrioventricular, and the most significant, pulmonary hypertension. Pulmonary hypertension accounts for most cases of ascites seen in commercial broilers, yet the hypertension can originate from many different causes. Environmental causes such as altitude, cold stress and incubator environment will affect the incidence of ascites. Management also plays a significant role in ascites development, for example; feed, lighting, air quality and ventilation have all been implicated in ascites development. Anatomy and physiology of the modern broiler also influences the incidence of ascites. Growth rate, oxygen requirements, organ size and capacity, hematological parameters and cellular responses can all determine how resistant or susceptible a broiler is to ascites. All of these factors cause ascites by inducing hypoxia in the bird (Julian, 1987,1988). The bird's demand for oxygen exceeds its cardiopulmonary capacity resulting in pulmonary hypertension. One final area that not only greatly influences, but also has potential to end ascites syndrome, is the area of genetic selection. In the past, a lot of the blame for our current ascites situation has been placed on poultry breeding and genetic selection for rapid growth, high meat yield and good feed conversion. While genetic selection may have exacerbated the ascites problem, it is also our best solution for eliminating this disorder in the near future. The selection of ascites resistant lines of broilers not only provides birds that can be raised at their maximal growth rate, it also provides a population in which to determine the exact molecular basis for ascites and pulmonary hypertension.