Group Title: Circular
Title: Stress, its role in fish diseases
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 Material Information
Title: Stress, its role in fish diseases
Series Title: Circular
Physical Description: 4 p. : ill. ; 28 cm.
Language: English
Creator: Francis-Floyd, Ruth
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville Fla
Publication Date: 1992?
Subject: Fishes -- Effect of stress on   ( lcsh )
Fishes -- Diseases   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
Statement of Responsibility: Ruth Francis-Floyd.
General Note: Title from caption.
 Record Information
Bibliographic ID: UF00014503
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA6943
ltuf - AJG5680
oclc - 26977802
alephbibnum - 001752723
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Stress-Its role in fish diseases

Ruth Francis-Floyd*

Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, dean

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 which is beyond its
normal level of tolerance. Specific
examples of things which 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
Biological stressors:
(1) Population density
(2) Other species of fish
aggression, territoriality,
lateral swimming space

(3) Microorganisms
pathogenic and non-
(4) Macroorganisms internal
and external parasites
Physical stressors:
(1) Temperature This is one
of the most important
influences on the immun~n,),
system of fish. L. -
(2) Light
(3) Sounds 8
(4) Dissolved Gass?.i.'-,..

Procedural stressors:
(1) Handling
(2) Shipping
(3) Disease Treatments

Alarm reaction (fight or
flight" response)
(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.
(2) Osmoregulation is disrupted
because of changes in mineral
l f A

Alarm Reaction





Figure 1. Stress triggers a chain of events
which result in an "alarm
reaction" (fight or flight response)
by the fish which then triggers a
series of hormonal changes. As
the fish tries to adjust to the insult
it uses up energy reserves, but
during this time it is able to resist
or compensate for the insult. If
the insult is not removed, its
energy reserves become depleted
and the fish becomes"exhausted".
At this phase its ability to resist
disease organisms, with which it
is in constant contact, is severely
compromised and the fish may
become sick or die.

*IFAS, extension veterinarian, Department of Large Animal Clinical Sciences and Department of Fisheries and Aquaculture, Institute of Food
and Agriculture Sciences, University of Florida, Gainesville, FL 32611

Circular 919

metabolism. Under these circum-
stances, 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 re-
quires that extra energy be used to
maintain osmoregulation.
(3) Respiration increases, blood
pressure increases, and reserve red
blood cells are released into the
(4) The inflammatory response is
suppressed by hormones released
from the adrenal gland.
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 deplet-
ing energy reserves because of the
extra requirements placed upon it.
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 degen-
eration of a fish's ability to main-
tain normal physiologic functions.
The fish is not "in balance" with
itself or its environment.

Disease resistance
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. Disease rarely results from simple
contact between the host (fish)
and potential pathogen. Mitigating
circumstances, such as poor
water quality, excessive crowding,
or similar stressor, are usually
present before fish become sick.
Identification and correction of
these problems is essential for
successful control of disease

Defense against disease
Protective barriers against
(1) 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 osmoregula-
(2) 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.
(3) Inflammation (non-specific
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 character-
ized 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.

(4) 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 suble-
thal 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 to disease.
Effect of stress on protective
(1) 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 osmoregula-
tory support from mucus compo-
nents is increased.
(2) Handling stress physically
removes mucus from the fish. This
results in decreased chemical
protection, decreased osmoregula-
tory 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 disrup-
tion of the physical barrier against
invading organisms.
(3) Chemical stress (ie. disease
treatment) often damages mucus*
resulting in loss of protective
chemical barrier, loss of osmoregu-
latory function, loss of lubrication,

1 -,, zb.

and damage to the physical barrier
created by mucus.
Scales and skin
(1) 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 organ-
(2) Trauma caused by fighting
(Reproductive Stress or Behavioral
Stress) could result in breaks in the
skin or scale loss.
(3) 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 physi-
cal damage, and stress response
create a situation which allow the
bacteria in the water to invade the
fish, causing a lethal disease.
(1) Any stress causes hormonal
changes which decrease the
effectiveness of the inflammatory
(2) Temperature stress, particu-
larly cold temperatures, can com-
pletely halt the activity of "killer
cells" of the immune system, thus,
eliminating an important first
defense against invading organ-
isms. 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
(1) Temperature stress, particu-
larly a sharp decrease in tempera-
ture, severely impairs the fishes
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.
(2) Prolonged stress severely
limits the effectiveness of the
immune system, thereby increasing
the opportunities for an invader to
cause disease.

Prevention of stress
The key to prevention of stress is
means maintaining good water
quality, good nutrition, and sanita-
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/1. 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. Supple-
menting 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 ex-
posed to. Disinfection of containers
and equipment between groups of

fish helps minimize transmission of
disease from one population to

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 identi-
fied, 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 treatments.
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 patho-
gen solthat the fish has time to
defend itself with an immune
response. Any stress which ad-
versely effects the ability of the fish
to protect itself will result in an on-
going disease problem; as soon as
the treatment wears off, the patho-
gen can build up its numbers and
attack again. Rarely would a
treatment result in total annihila-
tion 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 manage-
ment, or sanitation should be
corrected. Fish which do not
respond to a correctly administered
treatment should be reevaluated
by a fish health professional.

director, in cooperation with the United States Department of Agriculture, publishes this information to further the purpose of the May 8 and June 30,
1914 Acts of Congress; and is authorized to provide research, educational information and other services only to individuals and institutions that
function without regard to race, color, sex, handicap or national origin. Single copies of extension publications (excluding 4-H and youth publications)
are available free to Florida residents from county extension offices. Information on bulk rates or copies for out-of-state purchasers is available from
C.M. Hinton, Publications Distribution Center, IFAS Building 664, University of Florida, Gainesville, Florida 32611. Before publicizing this publication,
editors should contact this address to determine availability.

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