Physiology of stress in poultry
Naeem Tahir
Poultry Science Department

University of Agriculture Faisalabad
[email protected]
Homeostasis mechanisms maintaining the constant internal environment in the body thereby keeps the normal physiological function of the animal / birds. A deviation from normal condition is called the stress. Generally the term “Stress” is used to describe the detrimental effects of variety of factors on the health and performance of poultry. Birds have limited body resources for growth, reproduction, response to environmental changes and defense mechanism (Rosales, 1994). Under the stress conditions, there is redistribution of body resources including energy and protein at the cost of decreased growth, reproduction and health (Beck, 1991; Brake, 1987; Gross and Siegel, 1987). Under the long term stress condition or repeated stress, birds became fatigued and weak. These conditions lead to birds starvation and infectious disease (Dohms, 1990; Freeman, 1987). Domestic birds are subjected to frequent stress factors, and therefore it is important to have an effective management program to minimize their effects on the performance and health of the birds, identifying and managing factors that causes stress in birds is a critical part of a successful poultry production. Under this paper physiological mechanisms of stress, common causes of stress, types of stress, effect of stress in poultry (physiological indicator of stress) and future course of stress managemental practices in birds are discussed.

Physiological mechanism of stress regulation
Exposure of birds to stress is an inevitable event in poultry husbandry, when the threshold level of stress is crossed it results in distress to birds. Then the birds show stress syndromes, which are classified into three stages.
Stage of alarm reaction (Neurogenic system).
Stage of resistance or adaptation (Endocrine system).
Stage of exhaustion.

1. Neurogenic (sympatho- adrenal) system (Short-term regulation of stress): This system consists of sympathetic (post ganglionic) nervous system and adrenal medullary tissue. It controls the rapid response to the animal i.e. fight or flight or alarm (emergency) reaction (Cannon, 1929). This reaction lasts only a short time. It is characterized by increased rates secretion of the catecholamine from the adrenal medulla. These catecholamines prepare the bird for "Fight or Flight" reaction and commanding a rapid release of glucose in blood, depletion of liver glycogen, increased peripheral vasomotor activity, altered ventilation rate and increased neural sensitivity (Selye, 1950; Siegel. 1980). Catecholamines also stimulate the activity of hepatic adenyl cyclase, the enzyme required for the production of cAMP (Robinson and Sutherland, 1971). cAMP regulates the number of energy reaction (physiological processes) and directly increases the formation of antibody (Braun et al. 1971).
2. Endrocrine system (Long-term regulation of stress) : Involvement of endocrine system in stress regulation is called the 'stage of resistance'. This system is comprised of hypothalamus-pituitary adrenal axis (HPA). It is characterized by adrenal cortical hypertrophy and increased synthesis and release of adrenal glucocorticoids, known as corticosterone in bird (Siegel, 1971, 1980). Activation of the HPA is a longer-term adjustment by the animal to the surrounding changes. Selye (1936) called it General Adaptation Syndrome (GAS).
The endocrine mechanism of stress regulation is started with the stimulation of hypothalamus and release of ACTH from anterior pituitary, which causes the increase of adrenal cortical steroid secretions. Continuous stimulation to adrenal cortex leads to chronically high levels of corticosteroid hormone. This hormone is responsible for the formation of glucose from body's reserve of carbohydrates, lipid and proteins. Corticosteroids contribute to many of the disease associated with long-term stress, such as, cardiovascular and gastronistestinal disease, hypercholesteraemla, metabolic rearrangements and antibody suppression. (Siegel, 1985).
Other hormones:

i) Glucagon: The α cells of the pancreas are the source of glucagon, are stimulated in alarm response in both mammals and birds (Freeman. 1980).
ii) Thyroid hormone: Hormone produced by thyroid glands are also involved in stress regulation (Klandorj et al. 1978).

3. Stage of exhaustion:
Finally, if the bird does not recover from the stressor and the availability of body reserves and hormones from the adrenal gland are inadequate, a third or exhaustion phase leads to fatigue of the homeostatic mechanisms and death (Brake, 1985; Freeman,1987; Maxwell,1993).

Types of stress:
It was suggested to keep growing poultry in houses containing different climatized sections for resting and for activity (locomotion, water, food intake) because birds are able to choose their optimum temperature area in relation to their needs (Tzschentke and Nichelmann, 2003, Tzschentke, 2004). In addition, it must be an acknowledged that even in state of the art facilities, there are common sources of stress, which can be grouped under, one or more of the following categories (Freeman, 1987).
Climatic stress (extreme heat and cold, high humidity) (Chancellor and Glick, 1960; Regnier and Kelley, 1981).
Environmental stress (bright light, wet litter, poor ventilation) (Chancellor and Glick ;1960 Regnier and Kelley, 1981).
Nutritional stress (shortages of nutrients, feed intake problems) (Ben-Nathan et al. 1981; Gingerich, 1992; Glick et al. 1981).
Physiological stress (rapid growth, process of maturing sexually) (Freeman, 1987; Mauldin, 1992).
Physical stress (catching, immobilization, injections, transport (Jones et al.1988; Gregory et al.1992).
Social stress (overcrowding, poor body weight uniformity) (Gross and Siegel, 1981; Craig, 1992; Guhl, 1958).
Psychological stress (fear, harsh care takers) (Beuving et al.1981).

Pathological stress- Exposure to infectious agents is a common source of stress, however challenges may not result in overt disease. When sub-clinical infections due to poor bio-security and sanitation persist, excessive activation of the immune system will result in a condition known as immunological stress. This condition results in a series of changes in nutrient metabolism induced by mediators of the immune response.
In addition to the categories of stress mentioned above, all the possible types of stressors can be broadly classified under two categories (a) avoidable stressors (b) unavoidable stressors

Avoidable stressors Un-avoidable stressors
Overcrowding Extreme weather
Poor ventilation Handling
Wet litter Vaccination
Toxins in feed Transportation
Starvation Rapid growth
High ammonia level Debeaking
Dehydration Lighting
Poor management Medication
Effective stress management involves complete elimination of avoidable stressors and minimizing the load of unavoidable stressors on the birds.
High ambient temperature in the tropics, like that of ours in India accompanied by high relative humidity is one of the most important stressor. Birds are more susceptible to high environmental temperature than low environmental temperature due to absence of sweat glands in the feathered body, fatty nature and high body temperature (40.1 0 C to 41.6 0C). The degree of susceptibility to tropical heat stress is higher in broilers than layers. Among broilers males are more susceptible to heat stress than females (Marin, Good layers housed in cages are more susceptible than poor layers reared on deep litter.

Physiological indicator of stress in poultry

Several workers have reviewed the effect of stressors in fowl (Brown 1967, Freeman, 1971, 1976). They found the following indicators of stress in birds:
Atrophy of the thymus and atrophy of the bursa of fabrics in young birds, enlargements of the anterior pituitary and the adrenal glands. Depletion of the adrenal cholesterol. A rise level of plasma corticosterone, insulin or glucagon.
Increased reliance on glucose as an energy source.
Hypoglycemia (increased glucose utilization).
Decreased growth and increased muscle degradation.
Release of acute-phase cytokines (monokynes and lymphokynes)
Impaired growth of cartilage and bone.
Synthesis of specific heat shock proteins.
Decreased voluntary feed intake (anorexia)
Increased body temperature
Changes in the level of plasma metabolites (e.g. glucose, tryglyceride, non-estrified fatty acids and lactate). Epinephrine content in yolk of donor hens also serves as a very good tool to reflect stress load in layer stock. Changes in the numbers of circulating leucocytes profiles (heterophil: lymphocyte ratios and basophil and eosinophil numbers).

Excess fat deposition in the abdomen (abdominal fat pad).
Ascites (water belly) in high producing broilers.
Common causes of stress in birds
Some of the most common causes of stress in poultry as categorized by Rosales (1984) are summarized as follows :
Poor brooding conditions (low temperatures, cold water)
Contaminated premises (built-up litter, early exposure to various disease agents)
High stocking density (limited feeder and dringker space)
Temperature exteremes (cold and heat)
Handling, weighing, vaccination, grading and transport (pain, physical damage)
Beak trimming (handing pain)
Lack of body weight uniformity (magnified differences in the packing order)
Rapid growth (Strict nutrient demand)
Quantitative feed and water restrictions (frustration, hunger)
Postvaccinal reactions (reduced feed intake, fever)
Feed quality problems (variation in nutrient content)
Long or uneven feed distribution (split feeding)
Sex separate feeding (pressure to restrict body weight gains)
Harsh caretakers (poor husbandry)
Inadequate ventilation (deterioration of the air quality)
Clinical or subclinical diseases (reduced feed intake, fever, pain)
Poor litter conditions (wet and cold)
Sexual maturity and onset of egg production (drastic stimulation with feed and light)
Work carried out by CARI, Izatnagar:
Most of the research work on stress and its management in poultry has been conducted in the temperate climatic conditions. Reports are also available on the stress management in the tropical climatic conditions. The Physiology & Reproduction Division of this Institute has also contributed in this regard. Work carried out by Moudgal et al (1991) indicated that immobilization stress for 30 min. daily caused the apparent decline in egg production. To assess whether any inter-relationship exists between the duration of rapid growth of ovarian follicles and egg production under condition of stress and during advanced age, work was carried out by Moudgal et al (1990) in this regard. They have also established the relationship of serotonin (5-HT) and epinephrine with stress physiology (Moudgal et al 1990, 1992). Effect of starvation and high cage density on semen characters of healthy broiler cocks was investigated by Mohan et al (1993). They investigated that semen volume, sperm concentration and angiotensin-converting enzyme activity (ACE, EC. showed a significant (P<0.05) decrease on the 7 th day followed by cessation of semen ejaculation on the 15 th day of starvation. The influences of high cage density on sperm concentration and ACE activity were not detected by the 24 th week of age. However, these parameters reduced significantly (P<0.05) at 27 th and 30 th week of age in comparison to control birds. Thus, both starvation and high cage density were found to be associated with the deterioration of semen quality.
Catecholamines have been well implicated in mediating stressful conditions. Alteration of catecholamines metabolic biosynthesis through dietary L-dopa fortification in laying quails and then observation on its production traits were assessed. Increased level of L-dopa showed trends of lowering adrenal and liver weight, but increasing dopamine, nor-epinephrine and epinephrine concentration in egg yolk as well as egg number (Tyagi et al 1996).

Future course of stress managemental practices in birds
Research work in the area of stress physiology should be directed in future in the following directions:
Need good indicator of physiological stress : The physiological indicator of stress such as atrophy of the thymus and bursa of fabrics in young birds, enlargements of the anterior pituitary and the adrenal glands are good indicator of stress but there are inherent problems with their detections. These organs cannot be weighed in live birds and require slaughter of the animal. Therefore, a suitable technique of physiological indicator of stress is currently needed.
Suitability of the technique : Blood sampling of bird for hormone itself can be associated with techniques themselves. Suitability of technique is also an important factor. Practical problems can be associated with techniques themselves. Some technique may not be suitable for mammalian species but may be suitable for avian species.

Mechanism of stress : To study the mechanism of stress (climatic and environmental stress) laboratories should be strengthen with some specific facilities like climatic chamber.

Suitable managemental practices: The production efficiency of poultry is severely affected by various kind of stressors. It has significant effect on economic production of poultry. Appropriate managemental practices can be investigated to reduce the different type of stress in poultry and getting the best returns from them. The research work should begin right from construction of poultry shed, along with feeding, disease control and other managemental options.
Identification of a physiological cue initiating the vicious cycle of events in birds that are under stress.
Amelioration of stress in birds at physiological level - Development of a anti-stress kit for birds.
The goal of poultry scientist should be to strike a balance between the hypo-and hyper-stress and to find as much eustress as possible and to minimize distress. While devising strategies for stress control it is necessary to eliminate all avoidable forms of stressors and maintaining unavoidable stressors under control. Thus, the ultimate aim of successful poultry husbandry is not eliminate stress but to maintain it at optimum level for good production efficiency. A targeted multi-disciplinary futuristic approach is advocated so that the problem of stress is well tackled.
Multifarious efforts should be made to develop suitable technology to overcome the problem of follicular atresia as this is one of the main channels responsible for the drop in egg production under stressful conditions.

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