Friday, May 31, 2024

Role of Stress to Produce Disease


Physiological stress and physical injury are the primary contributing factors of fish disease and mortality in aquaculture. Stress is defined as physical or chemical factors that cause bodily reactions that may contribute to disease and death. Many potential fish disease pathogens are continually present in the water, soil, air, or fish. In nature fish are often resistant to these pathogens, and they are able to seek the best living conditions available. Food fish reared under commercial aquaculture conditions are confined to the production unit and are weakened by stress conditions including: increased fish density and poor water quality (i.e., low dissolved oxygen, undesirable temperature or pH, increased levels of carbon dioxide, ammonia, nitrite, hydrogen sulfide, organic matter in the water); injury during handling (i.e., capture, sorting, shipping); inadequate nutrition; and poor sanitation.

These conditions can result in decreased resistance by the fish, resulting in the spread of disease and parasite infestation. Stress and injury initially trigger an alarm reaction (fight or flight response), which results in a series of changes within the fish. A blood sugar increase occurs in response to hormone secretion from the adrenal gland as liver glycogen is metabolized. This produces a burst of energy which prepares the animal for an emergency situation. In addition, the inflammatory response, a defense used by fish against invading disease organisms, is suppressed by hormones released from the adrenal gland.

Water balance in the fish (osmoregulation) is disrupted due to changes in the metabolism of minerals. Under these circumstances, freshwater fish absorb excessive amounts of water from the environment (over-hydrate); saltwater fish lose water to the environment (dehydrate), This disruption increases energy requirements for osmoregulation. Respiration increases, blood pressure increases, and reserve red blood cells are released into the blood stream. Fish are able to adapt to stress for a period of time; they may look and act normal.  However, energy reserves are eventually depleted and hormone imbalance occurs, suppressing their immune system and increasing their susceptibility to infectious diseases.

What Causes Stress?

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. Stress is caused by placing a fish in a situation that is beyond its normal level of tolerance. Specific examples of things that can cause stress (stressors) are listed below.

Chemical stressors

  1. Poor Water Quality – low dissolved oxygen, improper pH
  2. Pollution – intentional pollution: chemical treatments -accidental pollution: insect spray, spills
  3. Diet composition – type of protein, amino acids
  4. Nitrogenous and other metabolic wastes – accumulation of ammonia or nitrite

Biological stressors

  1. Population density – crowding
  2. Other species of fish – aggression, territoriality, lateral swimming space requirements
  3. Microorganisms – pathogenic and nonpathogenic
  4. Macroorganisms – internal and external parasites

Physical stressors

  1. Temperature: This is one of the most important influences on the immune system of fish.
  2. Light
  3. Sounds
  4. Dissolved Gases

Procedural stressors

  1. Handling
  2. Shipping
  3. Disease Treatments

Factor responsible for stress

There are several factors or stressors (which may be external or internal) responsible for stress in fish.

External factors: The following physical, chemical and biological factors (as external factors) are associated with stress in fish.

  • Water circulation:  In stagnant water, biological activity takes place very slowly than in circulated water.  Absence of water circulation may cause accumulation of unwanted products causing pollution of ecosystem, production of toxic materials due to anaerobic decomposition. All are responsible for stress.
  • Temperature: Water temperature affects the rate of metabolism, food intake, respiration and reproduction of fish. If water temperature exceeds the upper and lower limit of tolerance, then the fish fail in stress. The gradual decline in temperature directly affect the growth and stress factor for out break of EUS in fish.
  • Turbidity: Due to excessive turbidity, ihc gill of fish is clogged. As a result water circulation for respiration loxygeiialionuieralion) is hampered and finally the fish full in stress. Turbidity limited the productivity due to lack of prope light penetration, limited (he movement of fish and fish become blind. As a result, fish arc physically injured and infected by bacteria or fungus.
  • Salinity: Wild fish live within very specific salinity levels (levels of salt in the water). Their bodies work hard to maintain the osmotic gradient between themselves and their environment. It their environment salinity is not. specific to their needs and is not held at a steady level, they have to work harder to maintain their osmotic gradient, which generates chronic stress.
  • Light: Salmonids are very sensitive to light during the development of the eggs and larvae. The longer and more intensive the light, the higher the damages and losses. The developing fry are insensitive to the action of light.
  • Dissolved oxygen: Oxygen is the key factor to life of all environments. Oxygen deficiency is a more severe factor causing stress. If dissolved oxygen level is decreased from 5.0 ppm to 2.5 ppm then fish will be victim to stress and finally they will die. On the other hand, abundant supply of O2 is also harmful to fish.
  • pH: Both highly acidic and highly alkaline pH is harmful to fish. Highly acidic and alkaline pH causes aeidosis and alkalosis, Large fluctuation of pH of a water body also causes stress in fish.
  • NH3: Excess ammonia is harmful for fish, It is toxic substance especially attach on gill and create respiratory and circulatory problem.
  • Density: Every water body has a constant carrying capacity, if the number of population exceeds than that of the carrying capacity, the organism inhabiting the water body must be in stress due to competition among the individual forfood and space. As a result, fish become diseased.
  • Food and nutrition: Food and nutritional deficiency results in reduced growth and weakened of fish which ultimately make fish viable to stress and pathogen.
  • Predation: It is a severe interference of one individual. In this case, the small fishes arc directly ingested or injured by the predator and no! only causing stress but causing death. The fish also become in stress as they loss their immunity power by losing energy during escaping from the predator.
  • Interference; Interference of one individual undoubtly disturbs the life process of others. Inference by netting, angling or other physical means will cause an excess mucous secretion and injury. Due to less mucous and injury fish become viable to stress and pathogen.

Internal factors: Following internal factors arc associated with stress in fish.

  • Blood: Any change in blood composition cause abnormalities and thus produces stress. For example decrease in RBC level cause anemia, decrease in inflammatory response may result from less WBC and decrease thrombocytcs reduce the capacity of blood clotting.
  • Microbes   and    pathogens:  Microbes   and   other   pathogens   attack   fishes externally and internally, directly or indirectly influence the normal life process. External flora reduces the respiratory surface and internal flora disturbs the digestive process.
  • Metabolic activity: Abnormal metabolic activities of cells undoubtedly produce stress. Stress produce from metabolic disorder may effects cell structure.
  • Hormone: Adrino-cortico-tropic hormone (ACTH) or cortisol is called the stress hormone. Increasing the level of this hormone indicates the stress condition of a fish. The endocrine system is particularly important in physiological response of fish to stress, because this system together with nerves system is first acted upon by stress. It is also very important because a minute change in hormonal system is sufficient to cause a vagarious change in life process of an animal.
  • Heart beat rate: There is a certain range of heart beat rate for every animal. Any change in heart beat rate indicates circulatory disorder and obviously is a factor responsible for stress.
  • Nervous function: Any part of body cannot remain in function without nervous connection and hence nervous disturbance is a serious stress factor. Besides, stress from any sources first effect the central nervous system (CNS).
  • Enzyme: Stress may also generate due to enzymatic disturbance. All biochemical reactions in the body arc mediated and promoted by enzymes. Enzymatic disturbance causes digestive and metabolic disorders.
  • Excretory:  The water, salt balance and other ionic balance in fishes are maintained by excretory system. Stress also may be generated due to excretory and osmoregulatory imbalance.

There are also many factors like (handling, hauling, stocking, feeding methods shipping and disease treatments) which are responsible for stress in fish. All factors are dependent upon one other.

Mechanism of stressors to create disease

The unfavorable factors which cause stress first acts upon the physiological control system through the simulation of central nervous system and endocrine system by way of the hypolhalamus and pituitary gland. There is a rapid elevation of adrenalin and non adrenalin and if the stressful condition continues, the adrenal cortex is stimulated to release increased amount of cortisol. The hormonal changes (primary effects of stress) generate various physiological alterations (secondary effects of stress) which are given below-

  • Increased blood glucose level (hypcrglycemia): The rapid elevation in adrenalin and non-adrenalin metabolizes muscle glycogen into blood sugar which cause depletion of the reserved muscle glycogen and increase in blood glucose level. As a result, rise in blood pressure and ultimately fish loss their immunity.
  • Decreased blood chloride level (Hypochloremia): As the physiological control system is affected by stress, an ionic imbalance takes place through decrease in blood chloride levels.
  • Increased number of circulating tlirombocytes: It is usually accompanied by the contraction of the spleen and decrease in number of blood cell (but enhance strongly the clotting power, of blood by increasing the number of thrombocytes) as a result blood circulation will be hampered and the fish will be infected by the blood parasites like Trypanosoma. Cryptobia and finally fishes will be diseased.
  • Decreased liver glycogen: In balance condition, liver glycogen is very important factor for normal activity of any animal (fish). If it becomes failure, the body will be attack by germ. Decrease in liver glycogen is accomplished by the increase in glucose through mctabolization of glycogen into glucose by adrenaline and non-adrenalin.
  • Decreased inflammatory response: Inflammation is the state of being flame with pain. As a result, the part of the body shows swelling, redness and increasing temperature of the body. If nervous system is failure to the inflammatory response, the fish will be infected and finally become diseased.
  • Decreased serum protein: The increase release of cortical causes metabolization of some body proteins into plasma acid. Protein is used to produce new sugar through gluconeogenesis, when adequate glucoses from external sources are not supplied. If the serum protein is decreased, the hemoglobin will be destroyed. As a result, fish will be infected by pathogen and finally become diseased.
  • Increased plasma 17-hydroxy corticosteroid levels: It is a fat hormone secreted from the adrenal gland. In stress condition, its secretion is excess due co-nervous stimulation. Excess secretion of this hormone may result in disease such as progressive Icucopenia, furanculosis (infection is carried by the combination of steroid infection and heat stress).
  • Increased blood flow in muscle: Due to nutrient dficiency, the blood flow is increased in muscle and this is compensated by a decreased blood flow in the digestive organ and acute digestive disturbance take place.
    • Vitamin A, C and D deficiency are usually synergistic to infection
    • Vitamin-B complex plus mineral imbalance influence the infection to fish.

The above major changes take place as the secondary effect of stress. The normal physiological process of fish is greatly hampered due to the vigorous unfavorable change in fish. The immune system becomes greatly imbalanced due to the synergic effect of the unfavorable changes and the fish fail to exhibit inflammatory response. The fish body becomes immediately exhausted and disease is caused.

Effect of stress on protective barriers

There arc some protective barriers (mucous, scale and skin, antibody, killer cells) in fish like other animals. When fish falls in stress, the following changes take place in those barriers.


Mucus (slime layer) is the first physical barrier that inhibits entry of disease organisms from the environment into the fish. It is also a chemical barrier, containing enzymes and antibodies which can kill invading disease organisms. Mucus also lubricates the fish, aiding their movement through water, and is important for osmoregulation. Injury as a result of handling (i.e., capture, transport, etc.) and certain chemicals in the water (i.e., poor water quality, disease treatments) remove or damage the mucous layer, reducing its effectiveness as a barrier against infection at a time when it is needed most. This damage decreases the chemical protection of the slime layer and also results in excessive uptake of water by freshwater fish and dehydration by saltwater fish. Decreased lubrication causes the fish to expend more energy to swim at a time when its energy reserves are already depleted.

Scales and skin

Scales and skin function as a physical barrier which protects the fish. These are injured most commonly by handling, rough surfaces of tanks or cages, and by fighting caused by overcrowding or reproductive behavior. Parasite infestations can also result in damage to gills, skin, fins, and loss of scales. Damage to scales and skin of the fish can increase the susceptibility to infection. It also causes excessive uptake of water by freshwater fish or loss of water from marine species (osmotic stress). Fish which are heavily parasitized may die from bacterial infections which gained initial entrance to the fish’s body through damaged areas in the skin.


Inflammation is a natural immune response by the cells to a foreign protein, such as bacterium, virus, parasite, fungus, or toxin. Inflammation is characterized by swelling, redness, and loss of function. It is a protective response, an attempt by the body to wall off and destroy the invader. Any stress causes hormonal changes which decrease the effectiveness of the inflammatory response. Temperature stress, particularly cold temperatures, can completely halt the activity of the immune system, eliminating this defense against invading disease organisms. Excessively high temperatures are also extremely detrimental to the fish’s ability to withstand infections. High water temperature may favor rapid population growth of some pathogens. High temperature also reduces the ability of the water to hold oxygen and increases the metabolic rate and resulting oxygen demand of the fish.


Unlike inflammation and other nonspecific forms of protection, antibodies are compounds formed by the body to fight specific foreign proteins or organisms. The first exposure results in the formation of antibodies by the fish which will help protect it from future infection by the same organism. Exposure to sublethal concentrations of pathogens is important for fish to develop a competent immune system. Animals raised in a sterile environment will have little protection from disease. Young animals may not have as effective an immune response as older animals and therefore, may be more susceptible to pathogens in the environment. Stress impairs the production and release of antibodies. Temperature stress, particularly rapid changes in temperature, severely limits the fish’s ability to release antibodies, giving the invader time to reproduce and overwhelm the fish. Prolonged stress reduces the effectiveness of the immune system, increasing the opportunities for disease-causing organisms.

Symptoms and prevention of stress

Symptoms: We should observe our fish often for any of these signs of stress.

  • Gasping at the surface: Fish is gasping his mouth at the surface, this is a sign of stress brought on by poor water conditions, usually a lack of oxygen.
  • Appetite: Fish is stressed, oftentimes he will not eat.
  • Disease: Ich, characterized by white spots on the body of a fish, and other diseases can appear as a result of your stress. If you observe this or any other visible ailments or sores on your fish, you should talk to your veterinarian about possible treatments.
  • Strange swimming: When fish are stressed, they often develop odd swimming patterns. If your fish is swimming frantically without going anywhere, crashing at the bottom of his tank, rubbing himself on gravel or rocks, or locking his fins at his side, he may be experiencing significant stress. Talk to your veterinarian about treatment and look into what may be causing the stress and alleviating it.

Prevention of stress: The key to prevention of stress is good management. This means maintaining good water quality, good nutrition, and sanitation.

  • Good water quality involves preventing accumulation of organic debris and nitrogenous wastes, maintaining appropriate pH and temperature for the species, and maintaining dissolved oxygen levels of at least 5 mg/I. Poor water quality is a common and important stressor of cultured fish and precedes many disease outbreaks.
  • Feed a high quality diet that meets the nutritional requirements of the fish. Each species is unique and the nutritional requirements of different species will vary. Supplementing diets with fresh vegetables and live food is a good way to provide a balanced diet for fish which have poorly understood nutritional requirements. Fish in ponds have an advantage over fish raised indoors, because of the variety of natural foods available.
  • Proper sanitation implies routine removal of debris from fish tanks and disinfection of containers, nets, and other equipment between groups of fish. Organic debris which accumulates on the bottom of tanks or vats is an excellent medium for reproduction of fungal, bacterial, and protozoal agents. Prompt removal of this material from the environment will help decrease the number of agents which exposed to fish. Disinfection of containers and equipment between groups of fish helps minimize transmission of disease from one population to another.


Stress compromises the fish’s natural defenses against invading pathogens. When disease outbreaks occur, the underlying stress factors, as well as the disease organism, should be identified. Correcting stress factors should precede or accompany chemical disease treatments. A disease treatment is only an artificial way of slowing down an infection so that the fish’s immune system has time to respond. Any stress which adversely affects the fish will result in an ongoing disease problem. Prevention of disease outbreaks is more cost-effective than treating dying fish. As a student of M.S in Aquaculture knowledge on role of stress to produce disease is essential.


Floyd, R.F. 1990. Stress Its Role in Fish Disease, pp. 1-4.

Rottmann, R.W., Floyd, F.R. and Durborow, R. 1992. The Role of stress in Fish Disease, pp 1-7.

Islam, M.A. 2004. Fish Phraseology, Mymensing, Bangladesh, pp. 118-125.

Foster and Smith, 1992. Stress and Fish Health, 1 pp.

Kumar, A. 2005. Fish biology, pp. 97-101.

Wilhelm Schaperclaus 1991. Fish disease, Oxonian press, New Delhi, 958 pp.

Hartz, 1997, Stress in Fish: Symptoms and solution. 1 pp.

Related Articles

Latest Articles