Dr Micaela Pinto, technical specialist at Chemuniqué, says ascites is linked to insufficient cardiovascular and pulmonary output required to meet metabolic demands.
“Ascites is essentially right-sided heart failure. It develops when there is an imbalance between the bird’s oxygen demand and the heart and lungs’ ability to supply enough oxygen.
“Modern broilers have been genetically selected for rapid muscle growth, but the pulmonary and cardiac systems have not expanded at the same rate. As birds gain weight quickly, lung capacity becomes relatively smaller, creating a physiological bottleneck. When oxygen demand exceeds supply, the heart works harder to push blood through the lungs, increasing pressure in the pulmonary arteries.
“Over time, the right ventricle enlarges and eventually fails, causing pulmonary hypertension, which, in turn, forces fluid to leak from the blood vessels in the liver into the abdominal cavity,” she explains.
Several management and environmental factors can trigger or exacerbate the development of ascites, including rapid growth, cold stress during brooding, high altitude, poor ventilation, respiratory disease, and exposure to air contaminants.
Prevention hinges on growth, air quality, and temperature
Pinto explains that, although rapid growth is a key driver of ascites, growth should not be restricted at all in the first seven days of life. During this period, the immune organs, skeletal system, and cardiovascular system develop rapidly and require full support.
Careful early-life management is therefore essential to underpin these processes and set up a robust broiler for the entire grow-out period.
After day seven, producers can use lighting programmes to add one to two hours of darkness per day, particularly at high altitude, to slow down growth. This reduces feeding time and activity, lowering oxygen demand.
Feed management options include using mash instead of pellets to slow intake, reducing nutrient density, or applying controlled feeding programmes where appropriate.
Ventilation plays an equally critical role. Poor air quality reduces oxygen availability and damages the respiratory tract, compounding the risk of ascites. The Ross Broiler Management Handbook 2025 advises keeping carbon dioxide (CO2) levels below 3 000 parts per million (ppm) and ammonia ideally below 10 ppm.
Ventilation rates should be based on total house biomass, not just bird numbers. As birds grow, fan run time must increase to meet oxygen demand. As a guideline, producers should supply a minimum of 0,09m³ of air per hour for a 50g chick, rising to around 1,45m³/h for a 2kg bird.
Regular fan maintenance is essential, as blocked inlets, worn belts, and dirty shutters can drastically reduce airflow. CO2 monitors can also provide a simple yet practical indicator of air quality.
Pinto notes that cold stress in young broilers significantly increases the risk of ascites later on, as it raises metabolic oxygen demand to maintain body temperature, which can impair cardiovascular development and function.
Producers should follow proper temperature step-down programmes as birds age and avoid sudden drops that raise metabolic demand.
No cure, only an early warning
There is no cure for ascites once it becomes clinical. Pinto stresses that by the time visible signs appear, the condition is already irreversible.
“Any bird with abdominal distension or blue combs and wattles is in terminal heart failure. The fluid build-up further restricts breathing, creating a downward spiral, and death usually follows within days,” she explains.
Early warning signs include reduced activity, slightly darkened combs and wattles, lower feed and water intake, and birds that appear smaller or stunted compared with their pen mates. Advanced signs include a fluid-filled abdomen, laboured breathing, reddened abdominal skin, and sudden death.
Post-mortem examinations typically reveal clear yellow fluid in the abdominal cavity; an enlarged, congested liver; a flaccid heart with a dilated right ventricle; fluid around the heart; and congested lungs.
Ascites causes significant global broiler losses through mortality, carcass condemnations, and reduced performance. Pinto notes that South African producers operating at higher altitudes, particularly in Highveld regions of between 900m and 1 500m above sea level, face an increased baseline risk.
Genetics also play a role, with some breeding companies increasingly selecting against ascites susceptibility by measuring blood oxygen saturation.
“Given that treatment is impossible, prevention is always the most cost-effective approach. Investment in good housing, ventilation, and management pays for itself many times over compared with the cost of an ascites outbreak,” she adds.
For producers, paying close attention to early growth, air quality, and temperature management remain the most reliable defence against this costly condition.
For more information email Dr Micaela Pinto at [email protected].
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