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 temperature (T-a) and by supplying a pellet form of diet. Weight gain, hematocrit, hemoglobin, and plasma thyroxin (T-4) and triiodothyronine (T-3) concentrations were measured weekly for each bird, and comparisons were made between birds that eventually died from ascites and those that did not. Mortality from ascites amounted to 24.3 and 24.2% in Trials 1 and 2, respectively. Weight gain did not differ between ascitic and healthy chickens up to approximately 2 wk before death but was significantly lower in the ascitic broilers 1 to 2 wk before death. Hematocrit was significantly higher in broilers with ascites with the exception of ascitic broilers that died at the age of 7 wk (Trial 1). In ascitic broilers, T-4 and T-3 concentrations declined 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 concentration is one of the triggers of the syndrome initiation. This question requires further investigation. It can be concluded that a high rate of weight 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.