The appropriate literature has been reviewed for the purpose of defining the phenomenon of stress in lactating dairy cattle, establishing a baseline concept of lactation stress and emphasizing the most significant aspects of the natural mammary defence mechanisms. Data on the general adaptation syndrome (GAS) make it clear that stress is essentially the rate of wear and tear of the biological system affected by a stressor either eliciting stress of the organism as a whole or partly so. Owing to the variety of stressors which may affect the dairy cow at physiological and pathological levels, a definition of stress in the broad sense is indicated. This is essential from the point of view of the anti-homeostatic effects (metabolic and immunological) of lactation stress, aggravated by anti-homeostatic effects elecited by superimposed other types of stress (e.g. heat stress). The lactating cow, as a ruminant in a state of sustained stress, requires a special profile of hormonal mediators. In high yielding cows, for example, acute and sustained heat stress promotes increased activities of prolactin, progesterone and catecholamines. Compared with the mainly glycogenic/glycogenolytic metablolism of non-ruminant mammals, the lipogenic/lipolytic and glycogenic/glycogenolytic metabolism of the dairy cow depends on hormonal mediators which differ from those of the former not so much in their nature but in their magnitude and ratios. Stressors induce the development of GAS reactions in the dairy cow. These enable the cow to create and maintain homeostasis of its integrated 3 main physio-pathological systems and thus to endure the stressor(s). The cow's compensating adjustments to a stressor are therefore the effects of stress. This means that natural lactation is the effect of the lactation stress induced by the cow's progeny (i.e. the natural lactation stressor). Artificial lactation stressors (e.g. removal of milk by hand and machine) may affect the lactation stress in magnitude but not necessarily in nature. Likewise, a range of behavioural, physiological, lactational and lacteal changes related to other stressors are the effects of different types of stress. Lactation stress, like other types of stress, shows 3 stages of development, i.e., an overcompensating alarm phase (= lactogenesis), resistance phase (= galactopoiesis) and exhaustion phase (= regression). They facilitate adjustments of the cow's homeostasis from the level of involutional homeostasis (= no lactational activity) to that of lactational homeostasis. Like other tissues in a state of stress, the lactating mammary epithelium requires a greatly increased supply of glucose. This causes the glucose metabolism of the lactating cow to become so precarious that in the secretory mammary epithelium glucose is being reserved for specific key functions, such as balance of energy and electrolytes, and formation of lactose, NADPH and citrate. The secretory epithelium is therefore particularly susceptible to fluctuations of its oxydative glucose metabolism, which, in turn, enables the cow readily to control the number of secretory cells, their level of secretory activities and the integrity of the secretory epithelium. For that reason, mammary regression, unphysiological both in magnitude and timing, if stimulated in dairy cattle by stressful conditions superimposed on normal lactation stress, is of primary importance as a counter-measure to galactopoiesis, and the main predisposing factor to mastitogenic infection. Mammary regression may be considered the cow's inherent, non-specific response to stressors affecting it and/or its lactating udder. Stressful conditions, superimposed on the lactation stress, promote increased intramammary activities of adrenalin, which contribute to unphysiological mammary regression. Adrenalin also affects the bacterial trapping component, reverse pumping component and other components of the proposed 3 core systems of the natural defence mechanisms of the lactating bovine udder. This unfavourable effect of adrenalin on udder health may be further aggravated by various hormonal mediators in the plasma and milk. From the data reviewed it is evident that stress may elicit significant lactational and lacteal fluctuations jeopardize the anti-microbial efficacy of the natural defence mechanisms of the udder and increase the risk of unphysiological mammary regression and subclinical and clinical mastitogenic udder infections. It is clear therefore that stress may be of eminent importance to the 3 major determinants of bovine udder health, namely, intramammary integrity, somatic cellular defence and bacterial challenge.
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