Effects of Thermal Environment on Thermoregulation and Haematologic Responses of the Domestic Rabbit to Nutritional Changes

Abstract

This study was performed to investigate effects of thermal environment on thermoregulatory and haematologic responses of the domestic rabbit to nutritional changes. Four experiments were conducted in summer and winter. Experiment 1 investigated effects of seasonal changes in the thermal environment on the diurnal pattern of thermoregulation and blood composition. In both seasons, the rectal temperature (Tr) significantly increased with rise in ambient temperature during the day. The increase was more during winter. Haemoglobin (Hb) concentration and packed cell volume (PCV) values showed high levels at 0800 hrs and progressively lower levels at 1200 hrs and 1600 hrs, irrespective of the season. The total white blood cell (WBC) count was the lowest at 1600 hrs during summer. Winter values did not reveal a diurnal pattern. The mean cell haemoglobin concentration (MCHC) significantly decreased at 1600 hrs during summer. The mean cell volume (MCV) showed the highest values at 1600 hrs and the mean cell haemoglobin (MCH) concentration showed the lowest values at 1200 hrs during winter. In both seasons, lymphocyte percentage showed high values at 0800 hrs and 1600 hrs. The highest percentage of heterophils was obtained at 1200 hrs. Blood glucose level was the highest at 1200 hrs and during summer, whereas during winter, the highest level was obtained at 1600 hrs. Urea concentration was significantly higher at 1200 hrs and 1600 hrs in both seasons. In Experiment 2, effects of seasonal changes in the thermal environment on the responses of thermoregulation and blood composition to water restriction and rehydration were investigated. Tr significantly increased in water restricted state, irrespective of the season, whereas the respiration rate (RR) significantly decreased in water- restricted state during summer. The highest values of PCV and Hb were measured in water-restricted state in both seasons. Blood glucose level was significantly lower in animals subjected to dehydration during winter and it was significantly higher in summer. Urea and creatinine concentrations were significantly higher only in the state of water restriction during summer. Following rehydration, on day 3, the animals reestablished the normal hydration values. The effect of seasonal changes in the thermal environment on the responses of thermoregulation and blood composition to food restriction was investigated in Experiment 3. During summer, Tr was significantly lower in food-restricted animals at 0800 hrs, but during winter, Tr was significantly lower in food-restricted animals at 0800 hrs and 1400 hrs. RR values at 0800 hrs and 1400 hrs were significantly lower in food-restricted animals in both seasons. Food restriction resulted in a significant reduction in the mean body weight only during winter. The values of PCV, Hb and total WBC count were significantly lower in food-restricted animals, irrespective of the season. Plasma total protein was significantly lower in food-restricted animals in both seasons, whereas, concentrations of albumin and globulins were not significantly affected in both seasons. In food- restricted animals, plasma glucose level was significantly higher in summer but in winter, the glucose level significantly decreased. Urea and creatinine concentrations were not affected by food restriction during summer, but the concentrations were significantly higher in winter during food restriction. The plasma levels of Na and Ca were not significantly affected by food restriction in both seasons. Following refeeding, on day 3, the animals almost reestablished the normal feeding values. In Experiment 4, thermoregulatory, haematologic as well as certain histopathological responses to acute exposure to solar radiation were studied in ad libitum fed and watered, water-restricted and food-restricted rabbits. Tr reached peak values in water-restricted group before food restricted group, indicating that the state of body hydration was important in development of heat stroke. The increase in PCV following exposure to solar radiation was significant only with water restriction. WBC count significantly decreased during acute heat exposure, irrespective of nutritional treatment. Plasma concentrations of glucose, urea and creatinine significantly elevated in all groups in response to acute heat exposure. The main histopathological changes associated with acute heat exposure were oedema, haemorrhage and congestion in the brain and congestion in the liver. From the present investigations, it might be concluded that there are marked circadian and circannual patterns of body temperature and blood composition in mammals. Accordingly, when a critical assessment of physiological and pathological states of animals are required in mammals, these changes should be considered. It was also clear that the responses to the levels of water and food intake might be influenced by seasonal changes in the thermal environment. Mammals might be more susceptible to heat stroke when water intake is restricted and the restriction of food intake might be beneficial and reduce incidence of heat stroke in hot environment.