The Impact of Heat Stress on Immune Status of Dairy Cows

Heat stress (HS) is well known to negatively influence dairy cow’s wellbeing, health and productivity. Heat stress is a multi-billion-dollar global problem. It impairs cow’s performance during summer when cows are exposed to high ambient temperatures, direct and indirect solar radiations, and high humidity. While significant developments have been achieved over the last few decades to mitigate the negative impact of HS, such as physical modification of the environment to protect the cows from solar radiation and heat, or help them dissipate it to the environment. Still remains a significant challenge for the dairy industry, compromising dairy cattle health and welfare, as well as farmer and industry economy. In such a scenario, it is essential to have a good understanding of how the immune system of dairy cattle responds to HS, and identify the variable responses among the cows. This understanding will help to better cope with heat stress, through nutritional and management improvements, and in parallel, identify heat-resilient dairy cows through genetic improvements to support sustainable dairy cattle production.

Heat stress (HS) is a complex phenomenon, which triggers a variety of cow’s response mechanisms that have negative impact on their welfare and production. While these negative effects are well established and known to be associated with production responses, recent studies show that HS leads to negative effects on the immune response of the dairy cow and so, indirectly influence cow’s performance. Heat stress negatively affect cow’s immune system via the hypothalamic–pituitary–adrenal axis, as well as a shift in the adaptive immune function, which weakens cow’s immune function.

There are sufficient data demonstrating the impact of increased temperature and humidity on endocrine responses to HS in dairy cattle, especially changes in concentration of hormones like estrogen, prolactin and cortisol, which also provide an indication of the likely impact on the immune system.

Mastitis represents one of the primary pathogen-induced diseases of cattle during lactation, and there is evidence that heat stress exacerbates the occurrence of the disease due to its effects on the immune function, resulting in reductions in yield and milk quality, and in parallel, increase in medicines use and in cow’s culling rate. Likewise, a linkage of uterine diseases with heat stress has also been observed for decades. It was found that the percentage of cows with a retained placenta and metritis doubled in the summer relative to the winter. Data collected from cows presenting retained placenta and metritis, indicate higher incidence in summer, regardless of bacterial load, and greater persistency of the negative impacts of such diseases on performance, when they occur in the summer vs. the winter months.

There are at least four possible ways through which heat stress affects cow’s immune function:

1. Heat stress reduces the release of the hormone estrogen, which has an immune-enhancing effect, by modulating B cell function.

2. Heat stress suppresses the synthesis and release of cytokines, which have important roles in chemically induced tissue damage repair, and the modulation of immune reactions.

3. As heat stress intensity increases, the phagocytic capacity of the cells declines, indicating a functional decline in immune system resilience.

4. Heat stress causes a reduction in feed intake, which causes depression of the thyroid gland activity and leads to decreased thyroid hormone levels. This can significantly affect the function of the immune system, since thyroid hormones have an important role in autoimmunity and pathogen clearance.

The reduction in immunity under heat stress conditions can occur in the pre and post-natal period of the newborn offspring, in the dry period and in the lactation. In the dry period, which can be characterized also as the “non-lactating interval between successive lactations” period, we can observe in dairy cattle elevated pathogenic and metabolic disease incidences. New intra-mammary infections peak around dry-off and calving, relative to all other phases of lactation. The incidence rates vary with the season but are typically elevated in the summer season in comparison with winter. The potential for greater pathogen loading is enhanced with the high ambient temperature of summer and negatively affects cow’s immune function. It was observed in dry periods, corresponding to the summer months, the increased incidence of mastitis, respiratory disease, and displaced abomasum, suggesting that high ambient temperatures in summer reduced the immune competence of cows, and may alter the response to a post-partum pathogen challenge, especially in the beginning of lactation.

Among the factors that affect thermal stress-immunity relationship, we can include the following:

1. Pathogen type

Viruses are generally less sensitive to the change of temperature while survival and proliferation of bacteria can be linearly enhanced with a temperature rise.

2. Breed of animal and level of production

Holstein cows are impacted significantly less than Brown Swiss, which indicates some genetic variability in resilience to heat stress. High yielding cows generate more metabolic heat, to be dissipated to the environment, are more sensitive to heat stress and then, can suffer from impaired immune system function earlier.

3. Duration of exposure to HS conditions

The immune function is suppressed and the severity of infection increases as the duration of heat is lengthened.

Advances in management and nutritional strategies help to mitigate the negative effects of HS on the cows, and improve their performance under HS conditions. Heat mitigation strategies such as providing shade, proper use of fans and sprinklers for direct cooling the cows, or use fogging or evaporative panels to cool the barns (especially in dry climates), have the potential to help cows maintain normal body temperatures, a key requisite to prevent the impairment in their immune system function. Nutritional practices, as providing the cows a free access to cool and clean water and fresh food all day around, improve forage quality, increase diet energy and protein density, can all allow cows maintain good performance and immune system under heat stress conditions. Recently, some nutritional additives appear in the market, supposed to improve cow’s immune system, mostly under HS conditions. Farmers should check and evaluate their effectiveness and the convenience of their use to reach the goals they were developed for.

In conclusion, the impact of heat stress on the immune responses of dairy cows can be reduced by adopting appropriate amelioration strategies, among them, nutritional interventions and implementation of heat mitigation strategies, adapting them to each farm special conditions. This is important in all cow’s life cycle, but especially when cows are in the dry period and in early lactation. In parallel, the use of current animal selection methods and the development of climate resilient breeds may all support, together with the management and nutritional practices above mentioned, the sustainability and profitability of cow’s production systems in the future.

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