Aquaculture Disease Processes

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Aquaculture Disease Processes

• Dr. Craig Kasper
• FAS 2253/2253L

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Lecture 1 Introduction to Disease

• 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

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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

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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!

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Types of Diseases

• infectious diseases due to the action of
  microorganisms (animal or plant)
• viruses CCV, WSSV, TSV, YHV
• bacteria Vibrio sp.
• protozoans
• metazoans
• fungi Saprolegnia sp.
• crustaceans O. Isopoda

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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

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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

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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

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Kochs 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

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Dynamics of Disease Germ Theory

• Kochs 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.

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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, cant go somewhere else as easily.
• Some disease agents are almost always present in
  the water (ubiquitous)
• examples Aeromonas sp., Pseudomonas sp., Vibrio
  sp.

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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

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How Disease Occurs

• Three-step model
• susceptible host
• pathogenic agent
• environment unfavorable to host/favorable to
  agent
• exceptions?? extreme numbers or extreme
  virulence of agent
• stress is the monkeywrench

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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

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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

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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)

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How to be a parasite

1. Find a proper host
2. Get inside
3. Find a home
4. Grow and multiply
5. Get out once done or developed
6. Be transmitted to a new host

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HostParasite 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)

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HostParasite Specificity

• Another example Why are bluegill and bass
  infected with black spot metacercariae while
  walleyes arent?
• Answer ecological specificity -- the host and
  agent must overlap in time and space
• Another type of specificity tissue specificity

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What Contributes to Infection?

• number of organisms (overwhelming)
• infectivity (ability to get in)
• virulence (ability to produce disease)
• susceptibility of the host
• agents ability to overcome hosts defenses
• level of stress (REM!)
• Probablility of disease (Theobald Smith Model)
  ( agents x virulence of agents)(resistance of
  host)

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Possible Fates of an Agent within its Host

• host dies agent proliferates, overwhelms host,
  good parasites dont do this,
• 2. host lives largely dependent on stress
• host gets sick, but recovers (defense worked)
• host doesnt get sick (agent not virulent, wrong
  host)
• survivors
• agent either eliminated or
• carrier state established (host infected, but no
  obvious disease, big problem)
• latent (not easily observed)
• patent (ongoing/observable)

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Mortality Curves bell shaped

• 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.

Agent?? typically bacterial
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Mortality Curves sigmoidal

• 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.

lag
Agent?? typically bacterial
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Mortality Curves point source

• 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.

Agent?? chemical, viral
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Mortality Curves plateau- shaped

• Indicates exposure over a long period of time
• slow incubation
• slow transmission

Agent?? possibly nutritional
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Mortality Curves multiple spiked

• Due to frequent but intermittent exposure to
  disease agent

Agent?? physical parameter (e.g., low D.O.)
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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)

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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

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Transmission

• 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
• vertical transmission from parent to offspring
• via male (Girodactylus, trematode in pipefish)
• via female (IHN)
• 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

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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)

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Disease Transmission getting in the door

• Portals of entry, not as easy as they sound
• ingestion e.g., Ceratomyxa shasta, BKD,
  Myxobolus cerebralis
• gill lamellae e.g., Schizamoeba salmonis,
  Ichthyobodo necatur
• lesions bacteria (Vibrio sp.), fungi
  (Saprolegnia sp.)
• active penetration some metazoans,
  dinoflagellates

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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
  hosts ability to tolerate an agent)

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Resistance Primary Factors

• Physical barriers, inflammation, natural
  immunity, acquired immunity
• 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)

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Resistance Primary Factors

• 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)

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Resistance Primary Factors

• 3) Immune Response
• natural immunity inherited (discussed in detail
  later)
• acquired immunity either active or passive
• active obtains antibody via contact with
  antigen
• passive antibody obtained via donor
  (vaccination)
• discussed in following lecture

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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)

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Stages in Epizootic

• REM epizootic is an outbreak of disease
• incubatory agent has penetrated host barrier,
  found home and multiplying
• clinical or subclinical host adversely affected
  (manifestations)
• depression (reduced activity)
• color change
• interrupted feeding behavior
• body contortions
• respiratory change
• mortality

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Stages in Epizootic

• terminal host either dies or recovers
• exception in some very acute, highly pathogenic
  diseases (e.g., MBV) death may occur so fast that
  obvious signs dont develop