Fish Immunology (1+1)

Unit 12 - Stress and immune response

12.3. Stress and immune system
Stress affects the immune system of fish as it affects in higher animals, the cause of stress is referred as stressor. The stress response in fish is controlled by HPI axis. This stands for Hypothalamus-Pituitary- inter-renal axis. That means that the brain, pituitary, and inter-renal tissue.
Stress stimulates a part of the brain called the hypothalamus. The hypothalamus releases a hormone called corticotrophin releasing factor, or CRF and this hormone stimulate the pituitary gland to release Adrenocorticotropic hormone, or ACTH. ACTH intern stimulates the inter-renal tissue to release Two important hormones.

• Epinephrine
• Cortisol

Epinephrine
Epinephrine, is responsible for what is called the "fight or flight" reaction. This hormone produces a number of physiological changes that prepare the animal to stand and face the problem or fly away from the area.
When Epinephrine is release numerous reactions occur, such as

• Increase in heart rate, that result in increased blood pressure and respiration.
• Increase of glucose content in blood to provide a quick energy source.
• Increase of blood flow to the brain

Cortisol
Cortisol is the other hormone released by the inter-renal tissue in response to stress. This hormone prepare the animal to overcome stress. The level of cortisol increase rapidly when fish are crowded together or handled badly. Cortisol can directly interfere with the normal functioning of the immune system, specifically it interfere with the phagocytosis. If cortisol stays for a long time in body it can exhaust the body and cortisol itself become stressors and this exhaustion in the animal leads to the diseases associated with stress,
Physiological changes produced by the cortisol are

• Protein breakdown
• Elevated thyroid hormones
• Biochemical exhaustion

In fish, physiological stress and physical injury are the major factors that cause disease and mortality. In aquaculture fish are confined to the production unit and are weakened by stressors like:

• Increased fish density
• Poor water quality
• Injury during handling
• Inadequate nutrition
• Accumulation of metabolic waste.

These conditions can result 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.