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Influence of Stress on Immune System of Fish

Biraj Bikash Sharma* and Gadadhar Dash

Department of Aquatic Animal Health

Faculty of Fishery Sciences, West Bengal University of Animal and Fishery Sciences

Kolkata - 700094, West Bengal, India

*Corresponding author:


Over the last twenty years commercial aquaculture has experienced extravagant expansion in terms of production and resource utilization. Fish and shrimp protein meet the protein requirements as a valuable source of energy at present. The diversified cultural strategy leads to the shifting of small-scale farming to large-scale global production. As the intensification and the managerial practices have increased it results in the occurrence of the different types of stress. The stress brings the physiological changes to the health it is related to and the immunological aspects of the fish.


The term stress was first employed in a biological context by the endocrinologist Hans Selye in the 1930s. Stress can also be called as the lives of all living beings. Low level stress can be harmless and even beneficial. High level stress for longer periods creates health problems whereas most fishes can tolerate environmental conditions. Stress is induced by an environmental change and that renders the individual more vulnerable to further environmental change. It plays a very important role in susceptibility to disease and the outcome of the disease process. Though stress has been defined in many different ways, the basic components are universally same.


Stressors are the means by which animals become stressed. Many stressors have been identified that impact aquaculture operations. Some of these can be easily and cost effectively controlled and others cannot at any cost.

External and Internal Stressors:

  • External stressors are the results from adverse physical conditions (such as pain or hot or cold temperatures) or stressful psychological environments (such as poor working conditions or abusive relationships).

  • Internal stressors can also be physical (infections and other illnesses, inflammation) or psychological (such as intense worry about a harmful event that may or may not occur).

Acute and chronic stressors:

  • Acute stress (fights or flight response) is the reaction to an immediate threat. These threats can be any condition that is recognized, remains even hidden or falsely. The common management practices lead to the development of this stress such as handling, netting, photo electric shock, transport, aggression, noise, vibration etc. However, in most of the cases after the acute phase has passed, the stress hormone level returns to normal which is termed as the relaxation response.

  • Chronic Stress has long term consequences. Common chronic stressors include low dissolved oxygen, inadequate temperature, pH imbalance, improper hardness, salinity fluctuation, heavy metals, pathogens, high dissolved organics, high suspended solids, turbidity crowding photo stress, toxins etc.

Aquaculture Stressors can be divided into four categories:

  1. Human interference (i.e. Procedural)

  2. Extreme changes in the physical environment (i.e. Physical)

  3. Water pollution (i.e. Chemical)

  4. Animal interaction (i.e. Biological)

Defining what levels of stressors are normal and acceptable is not easy. A level of a stressor that is problematic under one set of environmental conditions might not be under another. Stressors that are present in combinations might be benign by themselves but pose a far greater threat because they are present together. In healthy animals this may not pose a problem, but it can create a serious problem for those animals carrying other pathogens. There are many observations as to what levels of specific water chemistry parameters are problematic. Snieszko was the first to theorize that host-pathogen and environment interrelationships were applicable to fish (and by extension to shrimp) disease.


Fish undergo a series of biochemical and physiological changes in an attempt to compensate for the challenge imposed upon it. The sum of all above changes are referred to as Stress Response. Acute response is an adaptive response that provides the fish with the means to cope with the stressor in order to maintain its normal or homeostatic state. Overly severe or long lasting stress impacts can be seen on growth, disease resistance, reproductive capacity and quality of the final marketable product.

Physiology of Stress Response:

The response initiated by the Central Nervous System (CNS) begins with release of Corticotrophin Releasing Hormone (CRH) or Corticotrophin Releasing Factor (CRF) from the hypothalamus in the brain. The CRF stimulates the corticotrophic cells to secrete adenocorticotrophin (ACTH). The Circulating ACTH, in turn stimulates the interrenal tissue (adrenal cortex homologue) located in the kidney to synthesis and release corticosteroids, mainly Cortisol, into circulation for distribution to target tissues.

Types of Stress Responses:

In fishes the response is broadly classified into three categories:

  1. Primary Response [activation of neuroendocrine system and production of stress hormone, cortisol (Alarm reaction)]:

This response includes the rapid release of stress hormones such as catecholamines and cortisol, catecholamines releases from the chromaffin tissue situated in the head kidney of teleost and also from the endings of adrenergic nerves. The cortisol hormone released from interrenal tissue is located in the pronephros in response to several pituitary hormones. However the predominant hormone is Adrenocorticotrophic hormone (ACTH).

  1. Secondary Response [effects of cortisol on physiological systems (Resistance)]:

This response comprises the biochemical and physiological effect mediated by the stress hormones. It leads to the alteration of blood chemistry and hematology. Blood glucose levels increase to assist the need of energy by brain, gills and muscle to cope with the increased energy demand for which gluconeogenesis and glycogenolysis occur in the liver. Adrenalin and cortisol increases the glucose production but adrenalin cleared from the blood very rapidly, however blood glucose level remains elevated.

  1. Tertiary Response [consequence of physiological changes (Exhaustion)]:

This response represents a whole animal and population level change associated with stress. If the animal fails to cope, this leads up to decreased reproductive capacity as well as results in decreased growth.


Chronic stress leads to lower the ability of the immune system to respond effectively. This stress increases the susceptibility of the fishes to diseases by decreasing their defense ability (both first line, second line and third line).

Affects on the first line of defense:

  1. Mucus:

It is a physical barrier that inhibits entry of pathogens. It is also a chemical barrier that contains enzymes (lysozymes) and antibodies (immunoglobulin). Mucus also lubricates the fish and aids in movement and is important for osmoregulation.

How stress affects mucus (1st line of defense)?

  • Stress causes chemical changes in the mucus :

    • Upsets the normal electrolyte (sodium, potassium, chloride) balance

    • Which results in excessive uptake of water by freshwater fish and dehydration in saltwater fish

  • Handling stress physically removes mucus resulting in decreased chemical protection, osmoregulatory function and lubrication, hence, causing the fish to use more energy. This facilitates pathogens to invade.

  • Chemical Stress (i.e. Disease treatment) causes the same as handling stress.

  1. Scales and Skin:

It is the physical barrier which protects against injury. On the other hand damaged scales and skin provide a window for invaders.

How stress affects scales and skin (1st line of defense)?

  • Handling stress: Most common factor for scale and skin damage, leads to pathogen invasion.

  • Trauma caused by fighting: Mainly by reproductive and behavioral stress

  • Parasite infestation: Can result in damage to gills, fins, and loss of scales. Many times fishes heavily infested with parasites die due to bacterial infections.

Affects on second line of defense:

  1. Inflammation:

It is a non specific complex cellular response of vascular tissue to an invading agent such as bacteria, virus, parasite, toxin etc. the significant sign of the reaction is pain, heat, swelling, redness and loss of function. It is also an attempt by the body to ward off and destroy the invader.

How stress affects inflammation (2nd line of defense)?

  • The hormonal changes resulting from the stress decreases the effectiveness of an inflammatory response.

  • Temperature stress: Includes cold stress and heat stress. Both the stress can completely halt the activity of the "Killer cells."

Affects on the third line of defense:

Antibody production:

Antibodies are a specific cellular response to fight invading proteins or organisms and are formed when the fish is exposed to an invader for the first time. These antibodies will protect the fishes from future infection by the same organism. Young animals do not have a well developed immune response.

How stress affects Antibodies (3rd line of defense)?

  • Particularly temperature stress severely impairs the ability of the fish quickly to release antibodies against an invading organism.

  • Long term stress ruthlessly limits the effectiveness of the immune system, thereby increasing the opportunities for an invader to cause disease.


  • Maintaining optimum water quality, proper nutrition and sanitation of the target.

  • Adhering to recommended loading densities.

  • Following establish methods of disease prevention.

  • Selective breeding is to aim at producing stress-tolerant strains for aquaculture.

  • Fish transportation methods should incorporate approaches designed to stress.


Many factors are associated with fish stress. Minimizing and eliminating sources of stress increases the chances of keeping fishes healthy. The exact amount of stress an individual fish can take depends greatly on what species it is, its age and size, etc. Overall a stressed fish is a weakened fish.


  • Brett, J. R. (1958). Implications and assessment of environmental stress: In the Investigation of Fish-Power Problems (ed. P. A. Larkin), Vancouver: University of BC, Institute of Fisheries. pp. 69-93.

  • Barton, B. A. and Iwama, G. K. (1991). Physiological changes in fish from stress in aquaculture, with emphasis on the responses and effect of corticosteroids, Ann. Rev. Fish Dis, 1: 3-26.

  • Tort, L. (2007). Functional, genomics and stress responses in aquacultured fish. Dpt. Cell Biology, Physiology and Immunology. Research Group in Stress and Immunophysiology in Aquaculture Universitat Autonoma de Barcelona, pp 20-26

  • Roberts, R. J. (2003). Fish Pathology, 3rd ed., Elsevier Science, pp 133-143.



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