Haematological changes and plasma fluid dynamics in livestock during thermal stress, and response to mitigative measures

The effects of thermal stress caused by seasonal changes are becoming more severe and frequent with projections indicating increased intensity in the near future as a result of climatic change. Depending on the prevailing season, thermal stress challenges the hypothalamus to activate neuroendocrine changes that induce alterations in plasma fluid and blood cellular parameters. Variation exists in the erythrocytic, leucocytic and thrombocytic responses of different animal species and breeds during thermoregulation and artificial thermal stress amelioration. During heat stress, plasma fluid serves as an efficient thermoregulatory medium to reduce hyperthermia, but with negative consequences on the immune system as well as alteration in erythrocytic and thrombocytic variables. The thermoregulatory responses aimed at making evaporative heat loss more efficient during heat stress may alter the composition and distribution of blood cellular components to an extent that is dependent on the magnitude of heat stress and the thermoregulatory body fluid economy of the livestock. Thermoregulatory body fluid economy, which is influenced by morphology of the skin, hair coat and sweat glands, and respiratory rhythm of heat-stressed livestock may account for the expected decrease or increase in erythrocytic variables. The effects of season on leucocytic parameters of mammalian livestock seem variable with thermal stress accounting for most of the change in the tropics, while both thermal stress and photoperiod have been implicated in the subtropics. It is concluded that understanding the effect of thermoregulation on the dynamics of blood cellular components and plasma fluid in livestock may provide useful information in livestock breeding to select animals that are highly adapted to heat stress, and in adopting preventive measures against immunosuppression and hyper- or hypo-thermia associated with seasonality, thus improving livestock productivity.