CHAPTER 2: REVIEW OF LITERATURE
2.13 A SSESSING STRESS IN BIRDS
There are various forms of stressors that impact free-range chickens from climatic, environmental, nutritional, physical, to social or physiological factors. Public concern regarding farm animal well-
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being has skyrocketed in the past few years (Quintero-Filho et al., 2010). Many countries presently have laws and welfare codes meant to protect farm animals, including poultry, from distress and fear (Main et al., 2009; Bonafos et al., 2010). Stress in birds, as well as other animals, invokes changes often involving a cascade of physiological adaptive responses (Thaxton and Puvadolpirod, 2000a). Parga et al. (2001) investigated the effect of transporting hawks and falcons on haematological parameters. They noted that transport is usually associated with other stressors like; catching, handling, loading, motion, acceleration, impact, withdrawal of water, fasting, restriction of behaviour, social disruption, extremes of temperature and noise, and many more. All through their rearing life, scavenging chickens are exposed to most, if not all, of these stressors.
Heat stress is one of the most important stressors particularly in the hot regions of the world (Altan et al., 2003) and to that effect, the influence of hot and cold regimes on birds have been investigated. Injections with various pharmacological preparations e.g. norepinephrine, social groupings and alterations in population density, feeding various nutritional formulations have also been tested (Thaxton and Puvadolpirod, 2000a). While elevated plasma corticosterone and increased circulating H/L ratio (Gross and Siegel, 1983; Quintero-Filho et al., 2010; Altan et al, 2010; Puvadolpirod and Thaxton, 2000b; Gross and Chickering, 1987; Cravener et al., 1992) are the two most widely accepted indicators of the stress condition in birds, other measures such as tonic immobility (TI) have also been applied.
2.13.1 Duration of tonic immobility
Also known as animal hypnosis, tonic immobility (TI) is a phenomenon found in many different species which is induced by a brief period of physical restraint, typically administered by holding
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an animal down on a flat surface (Gallup, 1974). It is a behavioral state characterized by lack of movements and an apparent lifeless position (Gallup and Rager, 1996; Miyatake et al., 2009;
Edelaar et al., 2012). Tonic immobility is a fear-potentiated response induced by physical restraint.
The technique has been applied in various species including beetles, crabs, lizards, pheasants, chickens and sharks. Fear is an important component of stress and can potentially reduce welfare in birds and duration of TI is widely accepted and applied as a measure of fearfulness in birds (Altan et al., 2003; Hrabcakova et al., 2012). The more fearful birds show longer immobility reactions when tonic immobility is induced (Altan et al., 2003). The TI test is based on a natural defensive reaction used by birds living in the wild when escaping a predator (Hrabcakova et al., 2012). The principle behind this defense strategy is to remain completely motionless and appear virtually lifeless in order to dissuade the predator from attacking.
Although heat stress influences fearfulness, it is as yet unclear how slow-growing free-range chicken behaviour, particularly physiological responses, is influenced by environmental factors such as high temperatures. In addition, interspecific differences in response to various environmental stimuli have not been evaluated under outdoor rearing conditions. Outdoor conditions are becoming increasingly popular partly because housing of chickens in conventional cages with high stocking densities is associated with increased fearfulness (Keeling and Gonyou, 2001). This triggers welfare concerns. Animal well-being can be improved by housing animals in species-specific, natural or near-to-natural environments (Hrabcakova et al., 2012) but there is need to further investigate salient complications associated with the same.
36 2.13.2 Heterophil to lymphocyte (H/L) ratio
Physiological variables can be used to measure the condition of thermal comfort of animals. Any variation in them is indicative of attempts to maintain thermal equilibrium, and consequently, if an animal is or not under stress (Nascimento et al., 2012). The main haematological response is a change in the heterophil/ lymphocyte (H/L) ratio. This is premised on the fact that the numbers of heterophils and lymphocytes per unit of blood increase and decrease, respectively, in birds under stress, but their ratio is less variable. The H/L ratio is thus a better indicator than individual cell numbers (Gross and Siegel, 1983).
Physiological and physical stressors such as fasting, water deprivation, frustration, crowding, stocking density, high temperature and housing increase the H/L ratio (Cravener et al., 1992).
Extensive research has been done on the effects of stress on H/L ratio in various livestock species including chickens (Askar and IsmaeI, 2012; Guémené et al., 2010; Altan et al., 2003; Thaxton and Puvadolpirod, 2000a), hawks and Peregrine falcons (Parga et al., 2001) and various other species. For birds, a normal ratio is about 0.42 but this can rise to 8 under severe stress. Changes in H/L have been observed in response to thermal stress and treatment with corticosterone (EC, 2000). Exposure of birds to heat stress results in an increase in the H/L ratio (McFarlane and Curtis, 1989; al-Murani et al., 1997; Altan et al., 2000a). Fear is an important component of stress and prolonged or intense fear can markedly reduce welfare and performance. The influence of high temperature on H/L ratio in slow-growing chickens has not been evaluated, let alone effect of cyclic exposure to such temperatures.
37 2.13.3 Other indicators of stress
Total white blood cell counts have been used as indicators of stress in birds. Other factors that have been applied in the study of stress responses in chickens include; mediation of the adrenal glands directly by exogenous administration of adrenocorticotropin (ACTH) and exogenous administration of steroid moieties, including corticosterone (Gross and Siegel, 1983), cortisone, cortisol, deoxycorticosterone, and dexamethasone (Thaxton and Puvadolpirod, 2000b). Plasma enzymes have also been widely used as indicators of stress. Common examples include creatine kinase (CK), aspartate transaminase (AST), lactate dehydrogenase (LDH) and alkaline phosphatase (ALP) (EC, 2000).