- Its Role in Fish Disease
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
Stress is caused by placing a fish in a situation which is beyond its
normal level of tolerance. Specific examples of things which can cause
stress (stressors) are listed below.
Biological stressors :
- Poor Water Quality - low dissolved oxygen, improper pH
- Pollution - intentional pollution: chemical treatments - accidental
pollution: insect spray, spills
- Diet composition - type of protein, amino acids
- Nitrogenous and other metabolic wastes - accumulation of ammonia or
- Population density - crowding
- Other species of fish - aggression, territoriality, lateral swimming
- Microorganisms - pathogenic and nonpathogenic
- Macroorganisms - internal and external parasites
- Temperature: This is one of the most important influences on the
immune system of fish.
- Dissolved Gases
Alarm reaction ("fight or flight" response) See Figure 1
- Disease Treatments
|Figure 1 .
- Increases in blood sugar are caused by a secretion of hormones from
the adrenal gland. Stored sugars, such as glycogen in the liver are
metabolized. This creates an energy reserve which prepares the animal
for an emergency action.
- Osmoregulation is disrupted because of changes in mineral
metabolism. Under these circumstances, a freshwater fish tends to
absorb excess water from the environment (over-hydrate). A saltwater
fish will tend to lose too much water to the environment (dehydrate).
This disruption requires that extra energy be used to maintain
- Respiration increases, blood pressure increases, and reserve red
blood cells are released into the circulation.
- The inflammatory response is suppressed by hormones released from
the adrenal gland.
See Figure 1 .
An animal is able to adapt to stress for a finite period of time.
During this period the animal may look and act normal, but is depleting
energy reserves because of the extra requirements placed upon it.
See Figure 1 .
The animal's reserves have been depleted and adaptation fails because
the stress was too severe or lasted too long.
What Is Disease?
Disease is an abnormal condition characterized by a gradual degeneration
of a fish's ability to maintain normal physiologic functions. The fish is
not "in balance" with itself or its environment.
All fish do not get sick and die each time a disease outbreak occurs.
There are many factors which affect how an individual responds to a
potential pathogen. The pathogen (bacteria, parasite, or virus) must be
capable of causing disease. The host (fish) must be in a susceptible
state, and certain environmental conditions must be present for a disease
outbreak to occur ( Figure 2 ).
|Figure 2 .
Defense Against Disease
Protective barriers against infection
Effect of stress on protective barriers
- Mucus (slime coat) is a physical barrier which inhibits entry of
disease organisms from the environment into the fish. It is also a
chemical barrier because it contains enzymes (lysozymes) and
antibodies (immunoglobulins) which can kill invading organisms. Mucus
also lubricates the fish which aids movement through the water, and it
is also important for osmoregulation.
- Scales and skin function as a physical barrier which protects the
fish against injury. When these are damaged, a window is opened for
bacteria and other organisms to start an infection.
- Inflammation (nonspecific cellular response) is a cellular response
to an invading protein. An invading protein can be a bacteria, a
virus, a parasite, fungus, or a toxin. Inflammation is characterized
by pain, swelling, redness, heat, and loss of function. It is a
protective response and is an attempt by the body to wall off and
destroy the invader.
- Antibodies (specific cellular response) are molecules specifically
formed to fight invading proteins or organisms. The first time the
fish is exposed to an invader, antibodies are formed which will
protect the fish from future infection by the same organism. Exposure
to sublethal concentrations of pathogens is extremely important for a
fish to develop a competent immune system. An animal raised in a
sterile environment will have little protection from disease. Young
animals do not have an immune response which works as efficiently as
the immune response in older animals and therefore, may be susceptible
- 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
- 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.
- 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
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 the fish is exposed to. Disinfection of
containers and equipment between groups of fish helps minimize
transmission of disease from one population to another.
Prevention of Disease
Fish farm management should be designed to minimize stress on fish in
order to decrease the occurrence of disease outbreaks. When disease
outbreaks occur the underlying cause of mortality should be identified, as
well as underlying stress factors which may be compromising the natural
survival mechanisms of the fish. Correction of stressors (ie. poor water
quality, excessive crowding, etc.) should precede or accompany disease
Stress compromises the fish's natural defenses so that it cannot
effectively protect itself from invading pathogens. A disease treatment is
an artificial way of slowing down the invading pathogen so that the fish
has time to defend itself with an immune response. Any stress which
adversely effects the ability of the fish to protect itself will result in
an ongoing disease problem; as soon as the treatment wears off, the
pathogen can build up its numbers and attack again. Rarely would a
treatment result in total annihilation of an invading organism. Disease
control is dependent upon the ability of the fish to overcome infection as
well as the efficacy of the chemical or antibiotic used.
The keys to minimize disease outbreaks on your fish farm are maintenance
of good water quality, proper nutrition and sanitation. Prevention of
disease outbreaks is more rewarding and cost-effective than treatment of
dying fish. Disease treatments should never be applied in a haphazard
fashion. When needed, chemical or antibiotic treatment should be targeted
at a specific problem. Any management deficiencies in water quality
management, nutritional management, or sanitation should be corrected.
Fish which do not respond to a correctly administered treatment should be
reevaluated by a fish health professional.
Ruth Francis-Floyd, extension
veterinarian, Department of Large Animal Clinical Science, Cooperative
Extension Service, Institute of Food and Agricultural Sciences, University
of Florida, Gainesville, 32611.
This document is copyrighted by the University of Florida, Institute
of Food and Agricultural Sciences (UF/IFAS). Published: December
1990. Reviewed: May 1997.