Aquaculture Disease Processes

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Transcript Aquaculture Disease Processes

Aquaculture Disease
Processes
Dr. Craig Kasper
FAS 2253/2253L
Lecture 1: Introduction to Disease
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What is disease?
Types of diseases
Dynamics of infectious disease
Epizootiology of infectious diseases
What you have to do to be a disease agent
Disease reservoirs
Transmission
The host
Stages in an epizootic
What is Disease?
• Definition: any alteration of the body
or one of its organs so as to disturb
normal physiological function
• opposite of health = unhealthy or
dysfunctional
Why are diseases of such
concern in aquaculture?
– 1990: WSSV, a virus, devastates shrimp
culture in China, $600 million lost
– 1971: Flexibacter columnaris, a bacterium,
kills 14 million wild fish in Klamath Lake
– the Idaho trout industry loses 10 cents on
every dollar made to disease (death,
weight loss)
– future of finfish and shrimp culture may
hinge on our ability to control vibriosis*
*more on “vibrio” in a later lecture!
Types of Diseases
1) infectious: diseases due to the action of
microorganisms (animal or plant):
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viruses: CCV, WSSV, TSV, YHV
bacteria: Vibrio sp.
protozoans
metazoans
fungi: Saprolegnia sp.
crustaceans: O. Isopoda
Types of Diseases
2) non-infectious: diseases due to non-living
causes (environmental, other)
– even a moderately adverse environment can lead
to stress, stress leads to epizootics (a disease that
appears as new cases in a given animal
population, during a given period, at a rate that
substantially exceeds what is "expected" based on
recent experience)
– a very adverse environment can cause disease
and mortalities directly (e.g., nitrogen gas bubble
disease, brown blood disease)
– the “other” category refers to nutritional, genetic
and developmental diseases
Types of Diseases
3) treatable vs. non-treatable
– non-treatable diseases are some of the worst
– include pathogens such as viruses, drug-resistant
bacteria, myxozoans
– white spot syndrome virus (shrimp) has no known
treatment
– Vibrio sp.: because of rampant over-use of
antibiotics in Central America, South America,
new, more virulent strains are developing
Dynamics of Infectious
Diseases
• First mode of infection demonstrated by Robert
Koch (1876) and his work with Bacillus anthracis
(anthrax)
• reached epidemic proportions in cattle, sheep
and other domesticated animals
• also can occur in man (as we are well aware!)
• Koch showed that a bacterium caused the
disease by using the following method:
Koch’s Method (Postulates)
• 1) find the organism common to all
infected animals, demonstrate its absence
in healthy ones
• 2) isolate the organism in pure culture
• 3) reproduce the disease in suitable
experimental animals
• 4) reisolate the same organism from
experimentally infected animals
Dynamics of Disease: Germ
Theory
• Koch’s work lead to what is known as the
germ theory: germs cause disease
• if you have germs you are diseased
• Renes Dubos (1955) refined the concept in
the following statement:
“There are many situations in which the microbe is a constant
and ubiquitous component of the environment but causes
disease only when some weakening of the patient by
another factor allows infection to proceed unrestrained, at
least for a while. Theories of disease must account for the
surprising fact that, in any community, a large percentage of
healthy and normal individuals continually harbor potentially
pathogenic microbes without suffering any symptoms or
lesions.”
Dynamics of Disease: stress
• Definition: any stimulus (physical, chemical or
environmental) which tends to disrupt homeostasis in an
animal.
• The animal must then expend more energy to maintain
homeostasis = less energy to combat disease!
• Aquatic organisms are fundamentally different from
terrestrials = they are immersed in their environment,
can’t go somewhere else as easily.
• Some disease agents are almost always present in the
water (ubiquitous)
• examples: Aeromonas sp., Pseudomonas sp., Vibrio
sp.
REM: Disease?
• Definition: any alteration of the body
or one of its organs so as to disturb
normal physiological function
• opposite of health = unhealthy or
dysfunctional
How Disease Occurs
Three-step model:
1. susceptible host
2. pathogenic agent
3. environment unfavorable to host/favorable to
agent
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exceptions??: extreme numbers or
extreme virulence of agent
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stress is the monkeywrench
Dynamics of Infectious
Diseases
• infection  parasitism  disease (infection can result
from parasitism, but neither necessarily results in
disease
• symbiosis: any association between 2 species
involving an exchange of matter and energy
• commensalism: symbiosis where one gains, the
other is neutral
• parasitism: symbiosis where one (parasite) is
metabolically dependent the other (host); some
harm intuitive, but not necessary
Epizootiology of Infectious
Diseases: terminology
• epidemiology: branch of medicine describing
occurrence, distribution and types of diseases in
populations of animals at distinct periods of time
and at particular places
• epizootiology: same as above (non-human)
• epidemiology is the study of the who, what,
when, where, how and why of disease
outbreaks
Epizootiology of Disease:
outbreak terminology
• enzootic (affects animals) vs. epizootic (disease
epidemic for animals)
• incidence: a measure of the risk of developing some
new condition within a specified period of time.
• incidence rate: the number of new cases per
population in a given time period.
• prevalence: the total number of cases in the population,
divided by the number of individuals in the population,
• proportion: number affected/population
• mortality: number of deaths over a time interval in the
total population (%, frequency)
How to be a parasite:
1.
2.
3.
4.
5.
6.
Find a proper host
Get inside
Find a home
Grow and multiply
Get out once done or developed
Be transmitted to a new host
Host:Parasite Specificity
• Specificity is required for steps 1 and 3, above (find a
proper host, find a home inside)
• host specificity example: Shasta rainbow trout are highly
susceptible to Ceratomyxa shasta (myxosporean
parasite) while Crystal Lake individuals are completely
resistant
• reason: physiological specificity (the host must meet all
of the metabolic requirements of the agent without
destroying it immunologically)
Host:Parasite Specificity
• Another example: Why are bluegill and bass
infected with black spot metacercariae while
walleyes aren’t?
• Answer: ecological specificity -- the host and
agent must overlap in time and space
• Another type of specificity: tissue specificity
What Contributes to Infection?
1.
2.
3.
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6.
number of organisms (overwhelming)
infectivity (ability to get in)
virulence (ability to produce disease)
susceptibility of the host
agent’s ability to overcome host’s defenses
level of stress (REM!)
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Probablility of disease (Theobald Smith Model)
= (# agents x virulence of agents)÷(resistance
of host)
Possible Fates of an Agent
within its Host
1. host dies: agent proliferates, overwhelms
host, good parasites don’t do this, $$$$$
2. host lives: largely dependent on stress
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host gets sick, but recovers (defense worked)
host doesn’t get sick (agent not virulent, wrong host)
survivors:
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agent either eliminated or
carrier state established (host infected, but no obvious
disease, big problem)
– latent (not easily observed)
– patent (ongoing/observable)
Mortality Curves: bell shaped
25
Mortality Rate (fish/wk)
• Infectious agent or toxic
substance moves into the
population and then, after
time, no longer affects
events in population.
• Transmission is horizontal
with width of curve
proportional to incubation
time and period of
communicability.
20
15
10
5
0
1 2 3 4 5 6 7 8 9
Week
Agent??: typically bacterial
Mortality Curves: sigmoidal
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Mortality Rate (fish/wk)
• Slight deviation from
bell-shaped curve
due to lag period in
course of disease (lag
phase of growth)
• Also, periods in which
the disease is not
communicable.
20
15
10
5
lag
0
1 2 3 4 5 6 7 8 9
Week
Agent??: typically bacterial
Mortality Curves: point
source
25
Mortality Rate (fish/wk)
• Population at risk was
exposed to agent at a
single point in time.
• All susceptible
members affected.
• Highly virulent
infectious type
disease of toxic agent
• Exposure to toxin.
20
15
10
5
0
1 2 3 4 5 6 7 8 9
Week
Agent??: chemical, viral
• Indicates exposure over
a long period of time
• slow incubation
• slow transmission
Agent??: possibly
nutritional
Mortality Rate (fish/wk)
Mortality Curves: plateaushaped
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14
12
10
8
6
4
2
0
1 2 3 4 5 6 7 8 9
Week
Mortality Curves: multiple
spiked
14
Mortality Rate (fish/wk)
• Due to frequent but
intermittent exposure
to disease agent
12
10
8
6
4
2
0
1 2 3 4 5 6 7 8 9
Week
Agent??: physical parameter (e.g., low D.O.)
Degree of Infection
• Acute: high degree of mortality in short period
of time, external signs might be completely
lacking (e.g., CCV, IHNV, TSV, WSSV)
• Chronic: gradual mortality, difficult to detect a
peak (Aeromonas septicemia, furunculosis)
• Latent: disease agent present, but host shows
no outward sign, little or no mortality, sometimes
associated with secondary pathogen/infection
(CCV and Edwardsiella ictaluri)
The Reservoir Concept
• reservoir: the sum of all sources of the agent, the
natural habitat of the agent, where the agent comes
from
– The size of the reservoir is proportional to the chance of
spread of a pathogen
• transient reservoir: situation in which the epizootic
displays a seasonal pattern of either cases or carriers
• permanent reservoir: usually associated with
disease in which chronic carriers are shown
– good example: water supply, itself
Transmission
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Definition: mode of transfer of disease to
a new host
Method 1) direct transmission: from one
host to another, either a) vertically or b)
horizontally
a) vertical transmission: from parent to offspring
 via male (Girodactylus, trematode in pipefish)
 via female (IHN)
b) horizontal transmission: from one member of a
population to another, one offspring to another
• contact: typically water borne (e.g., fish to fish)
• ingestion of agent or of infected aquatic
Transmission
• Method 2) indirect transmission: infection via
an inanimate vehicle, vector or intermediate
host
– vehicle: an inanimate object such as handling
equipment (nets, waders, etc.) or feed (e.g.,
aflatoxin)
– vector or intermediate host: animate object
• mechanical: vector is not essential to life cycle of
agent
• biological: agent spends some part of life cycle
in vector (e.g., water boatman and WSSV)
Disease Transmission:
getting in the door
Portals of entry, not as easy as they sound:
1. ingestion: e.g., Ceratomyxa shasta, BKD,
Myxobolus cerebralis
2. gill lamellae: e.g., Schizamoeba salmonis,
Ichthyobodo necatur
3. lesions: bacteria (Vibrio sp.), fungi
(Saprolegnia sp.)
4. active penetration: some metazoans,
dinoflagellates
The Host
• The ability of a host to acquire a disease
agent and demonstrate disease symptoms
can be expressed both qualitatively and
quantitatively
• qualitatively: resistance (ability of a host to
withstand the effects of an agent; e.g.,
Litopenaeus stylirostris to TSV)
• quantitatively: susceptibility (a measure of
the host’s ability to tolerate an agent)
Resistance: Primary
Factors
Physical barriers, inflammation, natural immunity,
acquired immunity
1. physical barriers: refers to innate
characteristic of animal body to penetration
(e.g., mucous slime layer, intact skin, mucous
membranes, exoskeleton)
• for fish, the mucous slime layer itself displays
an immune response (phagocytic properties,
antibodies)
Resistance: Primary
Factors
2. inflammation: basic response to any wound,
designed to seal off the area and reduce further
infection/damage
• manifestations (humans) include swelling, reddening,
loss of function, heat, pain
• manifestations (fish) possibly include heat and pain
• histological changes: local edema (swelling);
infiltration of neutrophils (type of white blood cell
produced in bone marrow) , lymphocytes (lymph
proteins), macrophages; fibroplasia (formation of
fibrous tissue in wounds)
Resistance: Primary Factors
3) Immune Response
1. natural immunity: inherited (discussed in detail
later)
2. acquired immunity: either active or passive
a) active: obtains antibody via contact with antigen
b) passive: antibody obtained via donor (vaccination)
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discussed in following lecture
Resistance: secondary
factors
• Secondary factors associated with disease
resistance are either environmental in nature or
somatic (associated with host, itself)
• environmental factors: mainly stress resulting
from deviation in temperature, dissolved oxygen,
ammonia; inadequate nutrition; mechanical, etc.
• somatic factors: age, sex, species (e.g., IPN
affects only largest fry, potential for exposure,
immune experience via exposure, black
spermataphore, TSV)
Stages in Epizootic
• REM: epizootic is an outbreak of disease
1. incubatory: agent has penetrated host barrier,
found home and multiplying
2. clinical or subclinical: host adversely affected
(manifestations)
– depression (reduced activity)
– color change
– interrupted feeding behavior
– body contortions
– respiratory change
– mortality
Stages in Epizootic
3. terminal: host either dies or recovers
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exception: in some very acute, highly pathogenic
diseases (e.g., MBV) death may occur so fast that
obvious signs don’t develop