Studies into the causes of a sudden death syndrome (S.D.S.) of broiler breeder chickens

The sudden death syndrome (S.D.S.) emerged as a disease of broiler breeders in the late Seventies and developed into a condition of economic significance in Australia in the early Eighties. Mortalities up to 30% occurred in flocks at the commencement of lay. Visceral congestion was the only post mortem change of note in birds which died suddenly without premonitory signs, however myopathy was seen histologically in recumbent birds which were also present in affected flocks. Low plasma potassium and phosphorus levels were detected in birds in affected flocks, and a severe outbreak of the condition was terminated following potassium supplementation of the water. Blood samples were collected throughout the life of a broiler breeder flock, and it was found that plasma potassium levels were at their lowest levels at 24 to 26 weeks of age, the age of peak mortality rate due to the S.D.S. A basal ration compounded from vegetable protein sources was developed which, when fed to point of lay broiler breeders, resulted in deaths due to the S.D.S. Low plasma potassium levels were recorded in birds fed this ration. The basal S.D.S. inducing ration was supplemented with potassium and phosphorus in one trial, and protein, potassium and phosphorus in another. The higher protein levels did not reduce the incidence of the S.D.S. and may in fact have exacerbated the problem. Supplementation with both potassium and phosphorus was necessary to prevent the disease. It was concluded that a minimum of 0.63% total phosphorus (0.49% available phosphorus) and 0.49% potassium were required in the basal ration to prevent the S.D.S. A linear binomial model was developed based on the data from these two trials and this proved to be able to predict well the death rates due to the S.D.S. with respect to dietary potassium and phosphorus levels. The influence of medullary bone formation on plasma potassium levels was examined. Medullary bone is produced in sexually maturing pullets in response to increased blood levels of oestrogen and testosterone. Oestrogen and oestrogen plus testosterone, administered parenterally to point of lay pullets, led to significant depression in plasma potassium levels, but only those with the combined oestrogen/testosterone treatment produced substantial medullary bone (traces only were present in the oestrogen treated birds). Consequently, the medullary bone formation was not responsible for the drop in plasma potassium levels. The depression in plasma potassium levels was found to be in response to oestrogen administration, partly due to dilution of the plasma with fat and partly due to some other unknown effect. The feeding of purified rations deficient in potassium but complete in other respects, confirmed that adult broiler breeders established in lay have a very low (0.12%) potassium requirement to prevent deaths in short term trials. Point of lay pullets also had a similarly low requirement to prevent mortalities, however they appeared to need more dietary potassium to maintain plasma potassium levels similar to those found in birds fed the diets higher in potassium. A trial in which rations with two levels of phosphorus and four levels of potassium were fed in a factorial arrangement demonstrated that plasma potassium levels were related to dietary potassium (P < .05) and dietary phosphorus (P < .06). Mortalities due to potassium deficiency were greater in birds fed rations with lower phosphorus (P < .05). Comparison of tissue potassium levels between cases of the S.D.S. and frank potassium deficiency are drawn. In the S.D.S, birds die as a result of hypokalaemia and have post mortem signs and heart to body weight ratios indistinguishable from those dying from frank potassium deficiency. It was concluded that the S.D.S. is a disease entity in its own right and is a metabolic disorder rather than a simple potassium deficiency. The hypokalaemia is contributed to by a number of factors. These include dietary levels of potassium, dietary levels of phosphorus, physiological changes taking place in the bird at the time of sexual development and elevated ambient temperatures. The additive effect of these separate entities acting in concert depresses plasma potassium levels below a point compatible with life