Journal of Biology, Agriculture and Healthcare www.iiste.org ISSN 2224-3208 (Paper) ISSN 2225-093X (Online) Vol.4, No.27, 2014 116 Effect of Stress on Animal Health: A Review Aleme Asres 1* Negassie Amha 2 1.Department of Animal Sciences, Adigrat University, Ethiopia 2.School of Animal and Range Sciences, Haramaya University, Ethiopia *Corresponding author E-mail: almasres06@gmail.com Abstract Stress, as it relates to bodily functions, has been defined as the sum of all biologic reactions to physical, emotional, or mental stimuli that disturb an individual’s homeostasis. Stressor can be defined as any internal or external stimuli or threat that disrupts homeostasis of the body, and elicits a coordinated physiological response within the body in an attempt to reestablish homeostasis. The different stressors that enhance stress it can be chemical stressors: poor water quality low dissolved oxygen, improper pH, Pollution intentional pollution: chemical treatments, accidental pollution: insect spray, Diet composition - type of protein, amino acids, Nitrogenous and other metabolic wastes accumulation of ammonia or nitrite. Biological stressors: population density crowding, mixing of different species of animal aggression, territoriality, space requirements, Microorganisms pathogenic and nonpathogenic, Macroorganisms internal and external parasites. Physical stressors: Light, sounds, dissolved gases and temperature. The stressor can be any internal or external stimuli or threat that disrupts homeostasis of the body, and elicits a coordinated physiological response within the body in an attempt to reestablish homeostasis. The different stressors that enhance stress and disturb the normal physiological functions of the different organs of animal health in different ways. Finally stress increased incidence of diseases and suffering animals as well as if the stress level is chronic it leads to death. Keywords: Health, Stressor and stress 1. Introduction Stress is a broad term, generally used in negative connotation and is described as the cumulative detrimental effect of a variety of factors on the health and performance of animals. Stress is a condition in which an animal is unable to maintain a normal physiologic state because of various factors adversely affecting its well being. The term “stress” has been widely used in biology to describe a set of physiological and behavioural changes elicited by aversive stimuli (Agarwal and Prabhakaran, 2005). Cannon (1929) described stress as the sympatho adreno medullary (SAM) system’s attempt to regulate homeostasis when threatened by a variety of aversive stimuli or stressors. Stress, as it relates to bodily functions, has been defined as the sum of all biologic reactions to physical, emotional, or mental stimuli that disturb an individual’s homeostasis. Therefore, a stressor can be defined as any internal or external stimuli or threat that disrupts homeostasis of the body, and elicits a coordinated physiological response within the body in an attempt to reestablish homeostasis. A stressor is a chemical or biological agent, environmental condition, external stimulus or an event that causes stress to an organism and an event that triggers the stress response. The different stressors that enhance stress it can be chemical stressors: poor water quality - low dissolved oxygen, improper pH, Pollution - intentional pollution: chemical treatments, accidental pollution: insect spray, Diet composition - type of protein, amino acids, Nitrogenous and other metabolic wastes accumulation of ammonia or nitrite. Biological stressors: population density - crowding, mixing of different species of animal- aggression, territoriality, space requirements, Microorganisms - pathogenic and nonpathogenic, Macroorganisms - internal and external parasites. Physical stressors: Light, sounds, dissolved gases and temperature. Temperature is one of the most important influences on the immune system of animals and Procedural stressors are handling, shipping and disease treatments (Altan et al., 2003). Later on, Selye (1936) conducted some of his classic studies on the response of the hypothalamic- pituitary-adrenal (HPA) axis to noxious stimuli and suggested that the organism reacted in a non-specific manner to a wide variety of aversive stimuli, mainly with an increase in the HPA axis activity. Stresses broadly categorize Environmental (thermal), Physiologic (nutritional deficiency) Psychological/psychosocial i.e. overcrowding. Stressors can be conveniently divided into physical stressors, social stressors resulting from the interactions with individuals of the same species and stressors related to handling by humans. Stressors have additive effects. This means that when several stressors impinge upon the animal at the same time, the resulting stress response will be much higher than if the animal was exposed to one stressor only. Therefore, circumstances such as weaning and transport can be particularly difficult for the animals (Nienaber and Hahn, 2007). The stress response includes several changes that may have negative effects on the performance of farm animals. These effects include changes in the immune function and increased susceptibility to disease, decreased feed intake and rumination, inhibition of oxytocin release, and reduced fertility, among others (Nardone et al., 2010).