Invisible Killer in High Temperatures- Heat Stress in Chickens Ⅱ

Invisible Killer in High Temperatures- Heat Stress in Chickens Ⅱ

Review of heat stress in chickens

The concept of stress
Stress is the sum of non-specific responses produced by the body to various adverse external or internal stimuli. It
is the response of animals to internal and external stimuli or challenges. This concept was first proposed in the 1930s by Dr. Selye, a Canadian pathologist and the founder of stress theory.

The main characteristics of stress are:

①   The anterior pituitary gland and adrenal gland become enlarged, and the plasma corticosterone level increases;

②   The thymus, spleen and lymphatic system atrophy, the number of eosinophils and lymphocytes in the blood decreases, and the number of neutrophils increases;

③   Production stagnation or even weight loss;

④   Gastric and duodenal ulcer bleeding.

Hans Selye also called these non-specific changes that have little to do with the stimulus source "General Adaptation Syndrome", also known as General Adaptation Syndrome (GAS). Any stimulus that can cause GAS in the body is called "stressor"

Generally speaking, stress is a dynamic process, which can be roughly divided into three stages: mobilization period (alertness period), resistance period (adaptation period), and exhaustion period. The body also shows different physiological characteristics when it is in different stress stages (NAWAB et al., 2018). When the stress intensity is small, the body can regulate itself by mobilizing various systems throughout the body, thereby resisting stress to a certain extent to maintain body homeostasis. When the stress intensity is high, it exceeds the body's self-regulation range, the body's energy is consumed too much, and various defense methods no longer work. If it continues to develop, it may further lead to disease or death.

The concept of heat stress (HS)

There is a suitable environmental temperature range for warm-blooded animals---the isothermal zone/thermal neutral zone. When in a suitable thermoneutral zone, animals can maintain normal body temperature and consume less energy, which is conducive to rapid growth and development (SLIMEN et al., 2016). In the isothermal zone, the metabolic heat production of the animal body is exactly equal to the heat dissipation. The temperature range in which normal body temperature can be maintained without physical adjustment is called the comfort zone. Generally speaking, the isothermal zone of broilers is 10-32°C, and the comfort zone is 18- 26℃ (Sun Peiming, 2006), isothermal zone and comfort zone will be affected by age, breed, feeding level, etc., so they are only relative values. When the ambient temperature changes, animal body metabolism will also change, and animals in high-temperature environments will suffer from heat stress.

Heat stress generally refers to the physiological response of the animal body's non-specific defense response to heat stress sources. When the ambient temperature exceeds the upper critical temperature of the thermal neutral zone, the heat generated by the body exceeds the heat dissipated to the surrounding environment, causing the energy originally used for growth and development to be used to maintain the body's homeostasis, and exhibits a series of symptoms. A non-specific physiological response, this phenomenon is called heat stress.

Within the upper limit temperature of the comfort zone and the upper limit temperature of the isothermal zone, the animal body can maintain a constant body temperature through some physical means (including but not limited to dilating peripheral blood vessels, erecting the body coat, extending wings or limbs, and accelerating respiratory rate), At this time, animal metabolism and production performance are affected to a lesser extent; when the ambient temperature exceeds the upper limit temperature of the isothermal zone, the animal body needs to reduce heat production through chemical adjustment, triggering a series of endocrine changes, which seriously affects animal metabolism and production performance. Influence (Sun Peiming, 2006). In actual production, the stress reaction caused by the ambient temperature exceeding the upper limit temperature of the isothermal zone is called heat stress.

Heat stress can be divided into two major categories, namely acute heat stress and chronic heat stress.

Acute heat stress:

Chronic heat stress refers to animals being exposed to high temperatures for a short period of time (several hours). Chronic heat stress refers to animals being exposed to high temperatures for a long time (days or weeks) (AKBARIAN et al., 2016);

Chronic heat stress:

It can be divided into (1) cyclic chronic heat stress: animals are in a high temperature environment for part of the day and at a suitable temperature for the rest of the day; (2) constant chronic heat stress: animals are always in a high temperature environment (experiment room status).

Chickens use four ways to discharge excess body heat

Unlike mammals, chickens lack sweat glands. When the ambient temperature is between 28 and 35°C, chickens use non-evaporative heat dissipation (including radiation, conduction and convection) as the main heat dissipation method.

Chickens lose heat non-evaporatively in two ways:

1） Relax and stretch the wings at the side of the body to increase the heat dissipation area;

2） Increase peripheral blood circulation.

When the ambient temperature reaches its body temperature (41°C), the chicken's respiratory frequency increases and mouth breathing occurs to increase evaporative heat dissipation or water evaporation. If mouth breathing fails to suppress the increase in body temperature, the chicken will become lethargic, become comatose, and soon die from respiratory or circulatory failure or electrolyte imbalance.

  Original Source: Huimu Power