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Effect of stress on protective barriers
- Mucus:
Any stress causes chemical changes in mucus which decrease its effectiveness as a chemical barrier against invading organisms. Stress upsets the normal electrolyte (sodium, potassium, and chloride) balance which results in excessive uptake of water by fresh water fish and dehydration in salt water fish. The need for effective osmoregulatory support from mucus components is increased.
Handling stress physically removes mucus from the fish. This results in decreased chemical protection, decreased osmoregulatory function (at a time when it is most needed), decreased lubrication thereby causing the fish to use more energy to swim (at a time when its energy reserves are already being used up metabolically), and disruption of the physical barrier against invading organisms.
Chemical stress (ie. disease treatment) often damages mucus resulting in loss of protective chemical barrier, loss of osmoregulatory function, loss of lubrication, and damage to the physical barrier created by mucus.
- Scales and Skin:
Scales and skin are most commonly damaged by Handling Stress. Any break in the skin, or removed scale, creates an opening for invasion by pathogenic organisms.
Trauma caused by fighting (Reproductive Stress or Behavioral Stress) could result in breaks in the skin or scale loss.
Parasite infestations can result in damage to gills, skin, fins, and loss of scales which could create breaks in the skin for bacteria to enter. Many times, fish which are heavily parasitized actually die from bacterial infections; but the parasite problem, associated physical damage, and stress response create a situation which allow the bacteria in the water to invade the fish, causing a lethal disease.
- Inflammation:
Any stress causes hormonal changes which decrease the effectiveness of the inflammatory response.
Temperature stress, particularly cold temperatures, can completely halt the activity of "killer cells" of the immune system, thus, eliminating an important first defense against invading organisms. Excessively hot temperatures are also very detrimental to fish, although the precise impact of sudden increases in temperature on the immune system is not known.
- Antibody Production:
Temperature stress, particularly a sharp decrease in temperature, severely impairs the fish's ability to quickly release antibodies against an invading organism. The time lapse required to mount an antibody response gives the invader time to reproduce and build up its numbers, therefore giving it an advantage which may allow it to overwhelm the fish.
Prolonged stress severely limits the effectiveness of the immune system, thereby increasing the opportunities for an invader to cause disease.
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