Association between weight gain, blood parameters, and thyroid hormones and the development of ascites syndrome in broiler chickens

The present study examined the association between thyroid hormones and the development of ascites on one hand and the ability to predict ascites from growth rate and hematocrit on the other hand. Ascites syndrome was induced ill broiler chickens in two trials by exposing the chicks to low ambient t emperature (T-a) and by supplying a pellet form of diet. Weight gain, hemat ocrit, hemoglobin, and plasma thyroxin (T-4) and triiodothyronine (T-3) con centrations were measured weekly for each bird, and comparisons were made b etween birds that eventually died from ascites and those that did not. Mort ality from ascites amounted to 24.3 and 24.2% in Trials 1 and 2, respective ly. Weight gain did not differ between ascitic and healthy chickens up to a pproximately 2 wk before death but was significantly lower in the ascitic b roilers 1 to 2 wk before death. Hematocrit was significantly higher in broi lers with ascites with the exception of ascitic broilers that died at the a ge of 7 wk (Trial 1). In ascitic broilers, T-4 and T-3 concentrations decli ned significantly during the week of death. The present findings raise the question of whether the association between low levels of thyroid hormones and the development of ascites is one of the physiological responses in the syndrome cascade, or whether the failure to maintain thyroid hormones conc entration is one of the triggers of the syndrome initiation. This question requires further investigation. It can be concluded that a high rate of wei ght gain is not always a good predictor of ascites development. Hematocrit and thyroid hormones can provide a good indication but only during the last week of life, and not in all cases. None of these parameters, however, can predict the development of ascites at an early age.