Ascites syndrome in broiler chickens results from inadequate cardiac and pulmonary capacity relative to rapid growth, causing fluid accumulation and death. It represents a production-related welfare problem linked to genetics.
Ascites represents a welfare consequence of selecting for extreme growth rates without proportionate selection for cardiac and pulmonary capacity. The birds that die from ascites do not do so suddenly - they experience progressive respiratory distress as fluid accumulates in the abdomen, reducing lung capacity and causing oxygen deprivation. This is a genuinely suffering death that affects millions of birds annually. The welfare solution is the same as for other growth-related broiler welfare problems: slowing growth rates through breed selection reduces ascites substantially. The resistance from sectors of the broiler industry to adopting slower-growing breeds therefore perpetuates not only lameness but also ascites as predictable, avoidable welfare problems built into the production system.