Biological consequences of the spatial-differentiated level of the 131I accumulation in sheep thyroid after Chernobyl accident

The aim of this work is to study the ¹³¹I biological effects on sheep at different concentrations of stable iodine in the diet. The problem of the absorbed dose estimation in the sheep thyroid gland (TG) after a radiation accident at the Chernobyl NPP in the conditions of natural micronutrient deficiency is considered. To determine the ¹³¹I critical dose in the sheep TG, leading to its dysfunction and subsequent destruction, complex laboratory studies were performed to refine the compartmental model parameters, based on reliable experimental and theoretical data. Modern technologies are used to model the TG area. The solution of the radiation transport equation is performed by the Monte Carlo technique, which takes into account both the γ - and β-radiation of the ¹³¹I immanent source and the contribution of all secondary radiations.

The studies were carried out on 64 sheep, divided into 10 groups based on the general clinical condition and body weight. The first 5 groups included animals from the Gomel region (32 sheep, iodine content in the daily diet was 0.08 mg/kg ), in the 6–10th groups (32 sheep; 0.43 mg/kg ) – from the Vladimir region. Tests for iodine content in feed and water were performed in the Belarusian Institute of Experimental Veterinary Medicine, Minsk, 1989). For sheep 1–3rd  , 6–8th  groups (9 sheep in the group) once peroral the ¹³¹I was injected with activity: for the 1st  and 6th  groups  3 µCi, for the 2nd  and 7th 15 µCi, from the 3rd  and 8th  – 72 µCi  per capita. The surviving sheep were vaccinated against Rift Valley fever and then exposed to infection with an epizootic strain of the virus of this disease.

The main theoretical result is the conversion factor of the ¹³¹I activity to the average dose rate in thyroid. The main practical result is the evaluation of the lower limit of absorbed dose in the TG (~ 300 Gy), which leads to its destruction. Animals with a reduced content of stable iodine in the diet were characterized by an increased number of cells in venous blood, reduced levels of thyroxine in the serum, altered structure and functional activity of the thyroid and liver. In animals with low levels of iodine nutrition, a large capture of the isotope by the TG was noted, which provided larger (2–5 times) doses. In sheep with iodine deficiency, a decrease in the number of leukocytes, thyroxine levels; survival is reduced. After the ¹³¹I intake, sheep developed a radiation-induced immunodeficiency, but the main mechanisms of the infectious process in animals remained: post-vaccination reactions proceeded without complications, were characterized by antibody formation and immune development.