Clinical, hematological and pathological responses in severely heat-stressed steers with special reference to the threshold value for survival.

To find out a critical point of physiological responses of steers which would finally lead to death due to heat stress, four Holstein steers weighing about 300 kg were housed in an artificial climatic room and exposed to each of the following room temperature and relative humidity: 35 degrees C and 70 per cent, 36 degrees C and 80 per cent, 38 degrees C and 70 per cent, and 42 degrees C and 60 per cent. Sequential clinical and hematological examinations were carried out during thermal loads on the steers, and pathological examinations after their death or slaughter. All the steers manifested panting, rise of rectal temperature, reduction or disappearance of peristalsis, hypersalivation, dysfunction of the skin reflex, and remarkable decrease in white blood cell count and pressure of CO2 (Pco2) value of jugular blood. At the end of heat stress, an animal would be survived if it meet the following conditions: (1) rectal temperature was below 42.7 degrees C, (2) the respiration rate did not decrease to one half (about 100/minute) of the maximum rate during the exposure period, (3) peristalsis and skin reflex disappeared almost completely, (4) the white blood cell count did not decrease to less than 50 per cent of the normal level, and (5) the Pco2 value of venous blood was maintained at a level of more than 15 mmHg. Cloudy discoloration with a boiled-meat appearance was noticed in the skeletal muscle of the hind part of the body in two dead cases. No significant changes were detected in any other steer slaughtered several days after release from the thermal stress.