Heat stress: an integrated approach

Heat stress: an integrated approach

At VIV MEA 2018 Dr. Josje Hakker, Technical Manager at Kepro, explained the integrated approach to heat stress during a seminar. Below you will find more information about the subject and the slides of the presentation during the seminar. 

Genetic selection for fast growth – like in broiler chickens – has decreased the heat tolerance of the animals. Male birds are more sensitive to high temperatures than female birds. Laying hens are more heat tolerant than broilers, but heat stress will decrease egg production and egg shell quality, because of reduced calcium and bicarbonate availability.


Image 1 - Temperature and humidity index for poultry

Heat stress

Heat stress occurs when ambient temperature exceeds the upper critical temperature (image 1), and the animal needs to make extra efforts to release excess heat from the body, by means of vasodilatation and evaporation. Panting is a good way to release heat from the body by evaporation, but this also causes loss of water, acids and minerals, leading to alkalosis, disturbed electrolyte balance, and dehydration. During periods of heat stress, feed intake usually declines, leading to insufficient uptake of essential nutrients, vitamins, minerals and amino acids. On the other hand, the requirements for these nutrients is often higher during stressful events and the subsequent recovery period (image 2 and 3).

Image 2 - High temperatures and relative humidity require additional actions to release heat from the body
Image 3 – Physiology of heat stress in chicken

In case of chronic heat stress, the integrity of the epithelial cells of the gut wall is threatened. The functionality of the tight junctions decreases and this leads to the so-called ‘leaky gut syndrome’: The risk of inflammation is then imminent, because toxins and pathogens have more easy access
to the blood. Also, digestion and absorption of nutrients is hampered when the gut wall is damaged (image 4).

Image 4 - Tight junctions that are damaged by heat stress lead to leaky gut syndrome

Stress: mechanisms and effects

Short term stress responses are – in general – useful for the animal, but negative side effects may impair animal health status and performance. Anti-inflammatory responses to acute stress may aggravate the symptoms of viral or bacterial infections and delay recovery.

Stress affects the immune system and the digestive system. In response to stress factors signals are released to the hypothalamus. This activates the HPA axis (Hypothalamus - Pituitary gland -Adrenal gland) and as a result, cortisol is released (image 5). The hormone cortisol causes many effects in the body, like sympathetic nervous system activation increasing blood sugar for energy purposes. Cortisol also increases anti-inflammatory effects that include the inhibition of certain inflammatory mediators that are important in innate immunity. The stress hormone cortisol can increase the formation of free radicals in the body, which can be extremely destructive. Free
radicals react with oxygen and trigger oxidative chain reactions. When cortisol levels in the blood remain high for too long, acute stress may progress into chronic stress, affecting blood pressure,
impairing fertility, and causing chronic inflammations. Cortisol in general has a negative effect on the immune system, resulting in a lower level of total circulating antibodies, decreased number of lymphocytes, and a decreased humoral immune response.

Image 5 - Effects of stress factors on the central nervous system and the endocrine hormone system

What can be done against heat stress?

First of all, preventive measures are required: Optimise the climatic conditions in the animal facility by installing insulation, ventilation, fans, air conditioning and/or cooling systems. Check the behaviour of the animals to ensure that they are comfortable. In poultry, panting and lifting of the wings are signs of heat stress.

Also, dietary measures may be taken: avoid the use of excess protein (formulate diets with free amino acids instead of crude protein), and replace carbohydrates by fat as energy source. In broilers, provide the feed during the cooler hours of the night, leaving the lights on to encourage feed intake. Feed laying hens in late afternoon, and provide extra calcium, to facilitate the egg formation during the night. Drinking water should be fresh, clean and cool. Warm drinking water reduces feed intake. Animals may require extra vitamins, minerals and bicarbonate during periods
of heat stress. Kepro offers a variety of supplements that will help your animals to get through stressful episodes.

Vitamins, minerals and other supplements

Zinc has antibacterial properties and supports gut health. Zinc is a cofactor for different enzymes and it is important for cellular immunity, normal growth and maintenance of bone tissue, feathers and appetite. Extra zinc during heat stress reduces the harmful effects of oxidative stress on the animal. Provision of extra sodium during episodes of heat stress may improve the immune response by balancing the Na:Cl ratio in the body.

Vitamin C supplementation during periods of stress causes reduced synthesis of cortisol and alleviates many of the harmful effects of stressful stimuli such as high environmental temperature.

Vitamin E is important as an antioxidant in blood and on a cellular level, in the maintenance of the integrity of membranes, as a detoxifier, in fertility and in the immune response, both on a cellular and humoral level. Supplementation of the diet of laying hens with vitamin E alleviates the negative effects of heat stress on production performance. Vitamin E is also a potent antioxidant, important in oxidative stress risk situations, like high ambient temperature.

Anti Stress Forte wsp contains all the vitamins and minerals mentioned above, and is especially designed to support pigs and poultry in periods of heat stress. Anti Stress Forte wsp increases the resistance of pigs and poultry to heat stress. Phytogenic products, Keprofix Oral, may be useful to improve gut wall integrity. In case of inflammation and fever, Salicyl Forte wsp can be applied to reduce body temperature and to increase water intake. Stress Aid wsp, Powervit wsp and Vitamino Trace Oral are supplements containing vitamins, minerals and amino acids, useful for the prevention and treatment of heat stress effects.

In summary

Heat stress results in lack of appetite, decreased feed intake, damage to gut wall integrity, and increased risk of infection. The combination of a weakened immune system and a weakened first line of defence (intestinal lining), caused by heat stress, poses significant threats to animal performance and health. Preventive measures should be taken to avoid harmful effects of heat stress on animal performance and health. These measures include...

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