A syndrome of sudden death has been reported in most areas of the world that raise broilers intensively. It is most commonly seen in healthy, fast-growing broiler chickens, and approximately 60%–80% are males. Those affected die suddenly, with a short, terminal, wing-beating convulsion. Many affected broilers just “flip over” and die on their backs. The condition is uncommon or unrecognized when low-density feed is used. Incidence can be minimized by slowing growth rate in broilers particularly during the first 3 weeks of life.
The cause of sudden death syndrome in broiler chickens is unknown, but it is thought to be a metabolic disease related to carbohydrate metabolism, lactic acidosis, loss of cell membrane integrity, and intracellular electrolyte imbalance. Recent studies link this disease to cardiac arrhythmias. The modern broiler, which has been selected for growth rate and feed conversion efficiency, has a predisposition to cardiac arrhythmias. One study found the prevalence of arrhythmias to be much higher in broilers (27%) than leghorns (1%), but it is not clear whether this predisposition is dietary or genetic. Stress is the most likely trigger of cardiac arrhythmias in broilers, which predisposes the bird to death from ventricular fibrillation. The prevalence in a rapidly growing healthy broiler flock is typically 0.5%–4%.
Broilers show no premonitory signs of sudden death syndrome. They appear healthy and may be feeding, sparring, walking, or resting, but suddenly extend their necks, gasp or squawk, and die rapidly with a short period of wing beating and leg movement, during which they frequently flip onto their backs. They also may be found dead on their sides or breasts.
Sudden death syndrome may occur as early as day 7 and may continue until 10–12 weeks in roaster flocks. Mortality usually peaks between days 12 and 28, although it may peak as early as day 9. If growth is restricted early, it may peak only after day 28. Mortality of 0.25%–0.5% per day can occur for 1–3 days.
There are no specific gross lesions for sudden death syndrome. Recent studies indicate that affected birds have characteristic microscopic lesions in cardiomyocytes and subendocardial Purkinje cells in the heart, and this may help in diagnosis.
Dead birds are well fleshed, have an empty or partially filled crop, and have feed in the GI tract. The abdomen is distended because the bird is fat and the intestines are filled with ingesta, indicating peracute death. The muscles are mottled red and white as a result of focal congestion, and the organs are moderately to severely congested. There may be small hemorrhages in the liver and kidney. Although the ventricles of the heart are contracted, there is no sign of hypertrophy, and the atria are dilated and blood filled. The lungs are congested and frequently edematous; however, pulmonary edema increases with time after death and is not prominent in broilers that are examined within a few minutes of death. The gallbladder may be small or empty, because feed intake is normal up until the time of death.
Sudden death syndrome should be suspected in well grown and otherwise healthy-looking broilers found dead on their backs. That position is rare in death from other causes except cardiac tamponade, asphyxia, and ascites syndrome. The syndrome is also the likely cause when dead birds that are otherwise in good condition are found lying on their sides or breasts randomly throughout the pen.
Diagnosis is supported by necropsy findings if there is a lack of obvious pathology (ie, a digestive tract filled with ingesta, contracted ventricles, dilated and blood-filled atria, lung congestion, and edema). The presence of characteristic microscopic lesions in the heart helps to support the diagnosis. Affected cells have vacuolated sarcoplasm, cytoplasmic eosinophilia, and nuclear pyknosis.
The condition called sudden death syndrome in Australia in broiler breeders coming into production is a different disease; it is reported to be caused by potassium deficiency. Similar mortality, caused by a combination of high environmental temperature and hypophosphatemia or by acute hypocalcemia, has been reported in North America.
The incidence of sudden death syndrome can be minimized by slowing the growth rate of broilers, particularly during the first 3 weeks of life. Growth rate can be moderated by controlling nutrient intake. This can be accomplished by reducing the number of hours of light per day, reducing the energy and protein level in the diet, or limiting the amount of feed provided.