Diseases (Chapter 10) Why study disease? One of most serious

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Diseases (Chapter 10)

• Why study disease?
• One of most serious limiting factors in
  aquaculture. Costs of disease outbreaks can be
  high (3 billion loss of cultured shrimp in 1994)
• Practically defined as any process that limits
  the productivity of a system
• There are infectious diseases (caused by germs)
  and non-infections diseases (caused by management
  practices e.g., environment, nutrition,
  genetics)
• Topics
• Major pathogens
• General principles
• Philosophy of disease control
• Disease management techniques
• Major diseases

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Diseases

• General principles - interactions
• Sneizko three-ring model of interactions between
  host (cultured species), pathogen, and the
  environment (Fig. 10.1)
• Obligate pathogens are the most threatening group
  of pathogens

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Diseases

• General principles density and disease
• Spread of pathogens is density-dependent
  pathogens are shared more easily when multiple
  hosts are in close proximity
• Immobile pathogens follow diffusion laws
  (concentration gradient), and there is also
  natural attrition of pathogens in the
  environment,
• If a pathogen does not reach a host in a defined
  period of time, chance of establishing new
  infection is reduced
• Higher host densities allow the genetic selection
  of mutant pathogens that are virulent

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Diseases

• Philosophy of disease control
• Absence of pathogens is desired state
• However, this is not practical or cost effective
  in most situations
• Number of factors are considered when deciding on
  control measures
• Cost of control measure
• Some pathogens must be completely eradicated
  since their presence makes culture uneconomical
  thus control measure must be implemented
  regardless of cost
• Other pathogens may be self-limiting and only
  affect production by a certain percentage learn
  to live with pathogen
• Likelihood of reinfection
• Ideally, there should be no chance for pathogen
  to be reacquired following application of control
  measure
• Not often the case thus, control measure coupled
  with development of immunity will hopefully
  suffice
• Sometimes, surrounding environment mat be
  supersaturated with pathogen in such cases,
  giving up may be an alternative
• Availability of adequate assays for pathogen
• Assay to detect pathogen is needed in order to be
  able to decide on control measure (if you dont
  know who are you fighting against, you dont know
  how to fight it)

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Diseases

• Disease management techniques
• Most important factor for movement and
  introduction of pathogens is movement of animals
  (mainly broodstock, and also larval fish for
  stocking)
• Unrestrained movement of contaminated animals
  accounts for an estimated 99 of introduction of
  pathogens to pristine system
• Completely pathogen-free broodstock are most
  likely unavailable goal is to minimize chances
  of infection

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Diseases

• Disease management techniques (continued)
• Available procedures to limit pathogens within
  culture system
• Batch culture
• In continuous culture, early batches serve as
  pathogen factories (magnifying effect)
• Thus, all in, all out strategy
• Drying out and sterilization of culture systems
  and associated items stops magnification of
  pathogen numbers
• This strategy is widely used in hatcheries, but
  not so common in grow out systems except for
  salmonid and shrimp culture
• Incoming water treatment
• Essential in recirculating systems
• More practical in hatcheries than in grow out
  systems (for reasons of volume of water)
• Means of water treatment include
• Chemical sterilization (chlorine, iodophores,
  ozone)
• Physical sterilization (UV light)
• Use of settlement ponds prior to treatment helps
  (reduces particulate matter which provides
  substrate, shelter and nutrition for bacteria)
• Chorine should not be used in organically
  polluted waters (such as aquaculture effluent)
  because of formation of chloramines (highly
  reactive and toxic to cultured organisms)
• Ozone particularly useful especially in
  recirculating systems

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Diseases

• Disease management techniques (continued)
• Available procedures to limit pathogens within
  culture system (continued)
• Lower stocking density
• Probability of pathogen transmission is reduced
• Social stress and competition for space and food
  also diminishes
• Single spawning stockings
• Differential growth is good indicator of poor
  health in captive population
• Thus, use of single spawning stocks will help
  monitor general health by allowing for detection
  of runts (the weak members of the population that
  are likely to carry pathogens and express
  disease)
• Not practiced often enough in fish culture
  (mostly in invertebrate culture)
• Specific pathogen-free broodstock
• If available, this helps
• Vaccination
• In theory, not all fish need to be vaccinated. If
  half are vaccinated, this is equivalent to
  halving the stocking density
• In most cases, practical method of delivery is by
  immersion at the nursery stage (Table 10.3)
• Several factors affect the success of vaccination
  (Table 10.4)

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Diseases

• Major diseases
• Most literature regarding diseases in aquaculture
  concerns shrimp and salmonids
• Shrimp
• White spot syndrome virus (WSSV origin China)
• Stocks have become partially resistant to WSSV
  carriers are now common
• Taura syndrome virus (origin South America)
• Initially this syndrome thought to be caused by
  accumulation in shrimp ponds of fungicides used
  in banana plantations. More recent work showed a
  virus to be responsible.
• Salmonids
• Bacteria are the major pathogens in salmonid
  aquaculture
• Vibrio use of antibiotics has resulted in the
  development of antibiotic-resistant strains.
  Effective vaccines have now been developed for
  Vibrio species.
• Aeromonas salmonicida is probably the most
  important fish pathogen in fish mariculture
• Over 20 viral diseases have been also recognized
  in salmonids