Well Being Assessment Physiological Criteria

Katherine Albro Houpt

Cornell University, Ithaca, New York, U.S.A.

INTRODUCTION

There is no single valid measure of stress (or well-being). Nevertheless, we can use physiological variables to assist in validation. The hormone most often used for measuring well-being is cortisol, the product of the mammalian adrenal cortex. One also can measure the levels of hormones and metabolites that are affected by cortisol. The sympathetic nervous system is the other major source of reactions to stress, pain, or fright.

SYMPATHETIC NERVOUS SYSTEM

There are two components to the sympathetic nervous system neural and hormonal. The most rapid response is neural. Centers in the diencephalon (the hypothalamus, primarily) are stimulated by the frightening event, and therefore, the sympathetic pathways in the spinal cord and then the nerves of the sympathetic chain are stimulated. The neurotransmitters released by the sympathetic nerves are norepinephrine and epinephrine (adrenaline and noradrenaline are alternative names). The structures innervated by the sympathetic nerves are the blood vessels, the hair follicles, the heart and lungs, and the gastrointestinal tract. The action on the gastrointestinal tract is primarily negative: Secretion and motility are inhibited. The actions on the heart are to increase the frequency and strength of contraction and to dilate the bronchioles of the lungs. The pupils of the eyes dilate. The hair stands on end (piloerection).

Any of these reactions can be measured to assess welfare. The hormones norepinephrine and epinephrine are very quickly degraded, so blood samples need to be taken quickly and the blood kept cold and processed quickly. The hormones can also be measured in saliva, which is a less invasive method, but still involves restraint of the animal. For this reason, it is more practical and probably more valid to measure the results of sympathetic stimulation, for example, heart rate. There are heart rate monitors that can be attached to the animal with a chest band. These can be retrieved later to determine any change in heart rate or in variability of heart rate.

There can be confounding factors in any measure of stress. For example, ceiling effects can make interpretation difficult. A ceiling effect occurs when the response is already high and cannot be any higher physiologically. Branding is used for identification of beef cattle in the United States. Although the modern techniques of micro-chipping would also make identification possible, what the rancher needs is a symbol, unique to his ranch, that is visible from a distance. There are two methods of branding: hot-iron branding and freeze branding. Hot-iron branding destroys the hair follicles and creates a scar. Freeze branding does not destroy the hair follicles, but causes the hair to regrow white rather than pigmented. These brands are somewhat harder to read than hot-iron brands, but presumably are more humane. When the responses of beef cattle to the two types of branding were compared, the heart rate and catecholamine levels were high following both procedures. The explanation is that the restraint necessary to brand the animals was extremely stressful to all the cattle, so their response was maximal. In other words, there was a ceiling effect. When the comparison of branding methods was repeated using dairy cattle, hot-iron branding caused higher heart rate and more avoidance than freeze branding.[1] Dairy cattle are much more accustomed to the presence of humans, to restraint, and to being handled than are most beef cattle.

HYPOTHALAMIC-PITUITARY ADRENAL AXIS

Stress to the animal leads to stimulation of those hypothalamic neurons that produce corticotropin releasing factor (CRF).[2] This is carried in the hypothalamic pituitary portal system to the anterior pituitary, where it stimulates release of adrenal corticotropic hormone (ACTH). This, in turn, stimulates release of the adrenal cortical hormones, in particular cortisol (in mammals) and corticosterone (in birds). The mineral corticoids aldo-sterone may also be released to a lesser degree. This hormonal cascade will take some time (minutes to hours), in contrast to the more rapid neural activity of the sympathetic nervous system. One important question is how much does an animal's cortisol level have to rise before we should consider the animal stressed. Barnett and Hemsworth[3] have suggested that a 40% increase indicates stress. One could use any increase above the normal range for the particular laboratory and species.

There are pitfalls in the use of cortisol (or any other physiological measurement), not because cortisol is not an indicator of stress, but because of confounding circumstances. For example, veal calf welfare is frequently questioned, so measuring cortisol was assumed to be a valid measure. As expected, when the calves were first placed in veal crates their cortisol was elevated, but several weeks later their cortisol was lower than age-matched calves that were housed in pens.[4] The controls had higher cortisol than the confined calves, probably because they had to be chased and caught before the blood samples were taken.

The method of obtaining the sample is important. If blood is taken by direct venipuncture and several attempts have to be made before the vein is punctured, the cortisol may be high for that reason. Taking blood from the anterior vena cava of a supine pig is much more likely to be stressful than taking it from the jugular vein of a horse habituated to handling and injections. Preplacement of an indwelling vascular catheter avoids some of those problems. There is a definite circadian rhythm of cortisol secretion, so that morning cannot be used as a control for afternoon. In fact, loss of the rhythmicity is another sign of stress. Twenty minutes should be allowed after the stressor for cortisol to rise.

Cortisol can be measured in other body fluids. Salivary cortisol can be collected easily by putting a cotton-tipped applicator in the animal's mouth. Urinary cortisol can be measured, but creatinine must be measured also in order to control for concentration of the urine. A low cortisol concentration in dilute urine could represent a higher plasma level than a higher level of cortisol in concentrated urine. Fecal cortisol has been measured successfully and is particularly useful when the well-being of free-ranging or wild species is to be evaluated. One advantage of measuring fecal cortisol is that cortisol production over a matter of hours is represented, rather than cortisol at a single point in time, as with a blood sample.

The actions of cortisol on the rest of the body can also be measured and used to evaluate welfare. Cortisol has effects on the liver, the fat depots, and the immune system. The hormone stimulates gluconeogenesis. Gluconeogen-esis is the deamination of amino acids, freeing glucose for immediate energy. The ammonia produced forms urea, and urea can be measured as a sign of stress. In this case cortisol stimulation more urea is produced, but levels may be high because less is excreted. Impaired excretion would indicate a kidney problem. Therefore, when a high level of urea is detected, renal health should be evaluated before stress is diagnosed. Renal function can be measured from the specific gravity of the urine, from the presence or absence of protein in the urine, and by the ratio of urea to creatinine, a compound that rarely varies in plasma concentration.

Under the influence of cortisol, fatty acids are metabolized rather than forming more adipose tissue. These two actions, gluconeogenesis and antilipogenesis, complement the actions of the adrenal medullary hormones that stimulate glycogenolysis and lipolysis.

One of the major actions of cortisol is the reduction of inflammation, and inflammation is reduced by suppression of the immune system. The number and type of white blood cells can be measured. There are several types of white blood cells, including neutrophils and lymphocytes. The lymphocytes are the antibody-producing cells, and these are the cells suppressed by cortisol. The ratio of neutrophils to lymphocytes can therefore be used as a measure of stress. The fewer the lymphocytes, the more likely the animal is secreting more cortisol and is stressed. One can also measure the activity of white blood cells rather than simply the number of cells. Some of these measures are mitogen-induced lymphocytic proliferation and natural killer-cell cytotoxicity. These have been used to assess well-being, but the results are often inconsist-ent.[5]

Suppression of the immune system is the most dangerous effect of cortisol. Although the swelling and pain of inflammation will be decreased, the white blood cells that cause these signs will not be protecting the body from invasion by bacteria or viruses. Antibodies will not form complexes with foreign antigens, and bacteria will not be destroyed by phagocytosis. The result of suppression of the immune response is illness. The respiratory or gastrointestinal pathology (shipping fever) seen in newly mixed or transported animals is a result of stress-induced immunosuppression.

The adrenal glands are not the only ones stimulated by stress. Thyroid-stimulating hormone is released from the pituitary and stimulates release of thyroxine from the thyroid gland. Thyroxine increases metabolic rate and, therefore, calorigenesis. Carbohydrate stores will be utilized first, and then fat stores.

Cortisol is a useful measure of some kinds of stress, but not others. For example, cortisol increases when horses are transferred from one environment to another and when they are trailered, but chronic deprivation of water or exercise does not cause cortisol to rise or the response of cortisol to ACTH to change. Fortunately, there are other physiological values that can be used. Examples include plasma protein, which can be used to assess the effects of furosemide. Furosemide is a drug frequently administered to race horses, ostensibly to prevent exercise-induced pulmonary hemorrhage. However, it also improves the animal's performance, because the horse is 10 20 kilograms lighter in weight as a consequence of diuresis. If a horse is treated with furosemide, the loss of fluid from the circulation causes an increase in plasma protein. If horses are given limited amounts of water, as in mares used for estrogen production, they have normal plasma protein but an elevated osmotic pressure.[6]

The most recently used physiological measure of well-being is acute phase proteins. These are haptoglobins, a glycoprotein of the alpha-2-globulin fraction by hapto-cytes in response to stress, ACTH, and cortisol. They are elevated following castration of piglets and after transporting older pigs for more than 3 hours.

CONCLUSION

The animal whose well-being is compromised responds with a variety of physiological changes. These can be used, in combination with behavioral measures, to help us determine the optimum housing, social grouping, and transport of farm animals.

REFERENCES

1. Lay, D.C., Jr.; Friend, T.H.; Bowers, C.L.; Grissom, K.K.; Jenkins, O.C. A comparative physiological and behav ioral study of freeze and hot iron branding using dairy cows. J. Anim. Sci. 1992, 70, 1120.

2. Dantzer, R.; Mormede, P. Stress in Domestic Animals: A Psychoneuroendocrine Approach. In Animal Stress; Moberg, G.P., Ed.; American Physiological Society: Bethesda, MD, 1985; 81 95.

3. Barnett, J.L.; Hemsworth, P.H. The validity of physiolog ical and behavioral measures of animal welfare. Appl. Anim. Behav. Sci. 1990, 20, 177 187.

4. Stull, C.; McDonough, P. Multidisciplinary approach to evaluating welfare of veal calves in commercial facilities. J. Anim. Sci. 1994, 72, 2518 2524.

5. McGlone, J.J.; Salak, J.L.; Lumpkin, E.A.; Nicholson, R.I.; Gibson, M.; Normal, R.L. Shipping stress and social status effects on pig performance, plasma cortisol, natural killer cell activity, and leukocyte numbers. J. Anim. Sci. 1993, 71 (4), 888.

6. Houpt, K.A.; Houpt, T.R.; Johnson, J.L.; Erb, H.N.; Yeon, S.C. The effect of exercise deprivation on the behaviour and physiology of straight stall confined pregnant mares. Anim. Welf. 2001, 10, 257 267.

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