Aquaculture Viruses

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Aquaculture Viruses
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What a Virus Isnt

• Not a bacterium
• not an independently-living organism
• cannot survive in absence of a living cell within
  which to replicate
• antibiotics do no harm to a virus, unless
  indirectly
• treatment of a flu virus with antibiotics is only
  the treatment of its symptoms
• you dont kill the organism that causes the flu

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What are Viruses? an Introduction

• Infectious agents composed mainly of nucleic acid
  with a protein coat (capsid)
• can only be seen with an electron microscope
• range in size from 10 to 200 nanometers (nM)
• carry on normal cell-like function unless free,
  then infectious
• in infectious form, they neither grow nor respire
• can enter living plant, animal or bacterial cell

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What do Viruses Look Like?

• Most viruses have a capsid, core and genetic
  material (DNA/RNA)
• capsid outer shell of the virus which encloses
  genetic material (link chemical structure of
  capsid helps determine immune response to virus)
• capsid is made of many identical individual
  proteins, precisely assembled
• protein core under capsid protecting genetic
  material
• sometimes an additional covering (lipid bilayer
  w/embedded proteins) on outside known as an
  envelope
• resembles a baseball
• various forms rods, filaments, spheres, cubes,
  crystals

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Virus Appearance capsid
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capsomere unit/molecule associated with capsid
structure
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Typical Virus Shapes
SPHERES
RODS
CUBES
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More Virus Shapes
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Composition of T-Even Bacteriophage

• Capsid brains of virus, tightly-wound protein
  protecting nucleic acids
• Body attached to capsid head, rod-like
  structure w/retractible sheath, hollow core
• Tail at end of core is a spiked plate carrying
  6 slender tail fibers, anchor virus to its host

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What are Viruses? an Introduction

• Viruses make use of the host cells chemical
  energy, protein and nucleic acid synthesizing
  ability to replicate themselves
• bacteriophage infect the host through a
  tail-like adaptation
• each virus attacks a specific type of cell
• cold viruses attack cells of the lung
• the AIDS virus attacks T4 cells of the immune
  system

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Bacteriophage Attack
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What are Viruses? an Introduction

• Viral nucleic acids are single- or double-
  stranded and may be DNA and RNA
• after viral components are made by the infected
  host cell, viral particles are released
• often, the virus alters the intracellular
  environment enough to damage or kill the cell
• if enough cells are destroyed, disease results
• some viruses do not kill cells, but transform
  them into a cancerous state, remaining latent for
  a long time

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Role of RNA/DNA

• Supplies the codes for building the protein coat
  (capsid) and for producing enzymes needed to
  replicate more viruses
• codes also provide enzymes that allow the
  newly-built viruses to lyse cells (e.g.,
  bacteriophage)
• cell is ruptured and destroyed

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What do Viruses Actually Do?

• All viruses only exist to make more viruses
• all, with the exception of some bacterial
  viruses, appear to be harmful
• their replication leads to the death of the cell
  which the virus has entered
• virus enters the cell by first attaching a
  specific structure on the cells surface
• depending on the virus, either the entire virus
  enters the cell or only the genetic material is
  injected

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The Virus Invasion

• Phase 1 spike and fibers attach themselves to
  the walls of the cell or bacteria
• Phase 2 the sheath contracts and drives the
  core through the cell wall (injection)
• Phase 3 the nucleic acid passes through the
  core, from the capsid head, into the host cell
• Phase 4 nucleic acid disappears, afterwards
  (10m) hundreds of virions appear causing the cell
  to rupture, releasing hundreds of small viral
  replicates
• this is how it can replicate so quickly

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The Virus Invasion
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What Things Can Become Infected by a Virus?

• All living things have some susceptibility to a
  particular virus
• virus is specific for the organism
• within a species, there may be a 100 or more
  different viruses which can affect that species
  alone
• specific for example, a virus that only affects
  one organism (humans and smallpox)
• influenza can infect humans and two animals

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Different Types of Viruses

• Major classification animal, plant, bacterial
• Sub-classified by arrangement and type of nucleic
  acid
• animal virus group double-stranded DNA,
  single-stranded DNA, double-stranded RNA,
  single-stranded RNA, retrovirus
• influenza SS-RNA
• for all viruses, regardless of the kind of
  arrangement of genetic material, the virus is
  capable of replicating within a living cell and
  can produce progeny

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Do Viruses ever Change?

• Sometimes during viral replication, mutations
  occur
• if the mutation is harmful, the new virus
  particle might no longer be functional
  (infectious)
• however, because a given virus can generate many,
  many copies, a small number of non-functional
  viruses is not important
• mutation is not necessarily damaging to the virus
  -- it can lead to a functional but new strain of
  virus

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Defense Against Viruses

• First Line skin and mucous membrane, which also
  lines the gastrointestinal and respiratory
  passageways
• skin is tough and stomach acidity acts as a
  disinfectant
• Second Line after the virus enters the blood
  and other tissues, white blood cells and related
  cells (phagocytes) consume them
• accumulation of phagocytes in area of infection
  is known as puss

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Defense Against Viruses
Antibodies attacking chickenpox virus
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Defense Against Viruses

• Antibodies are the best defense against viruses
• unfortunately, they are specific in their action
• chickenpox antibody will only attack a chickenpox
  virus
• a particular virus stimulates the production of a
  particular antibody

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Defense Against Viral Infection

• Humans are protected in a couple of ways
• 1) intracellular if a particular virus attacks
  cells, our bodies produce interferons
• interferons (alpha, beta or gamma) are proteins
  which interact with adjacent cells and cause them
  to become more resistant to infection by the
  virus
• if the resistance is not quite good enough, we
  become sick

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Defense Against Viral Infection

• 2) immune system (extracellular) kills the
  virus outside the cell
• also kills the infected cells
• virus cannot spread
• eventually the virus is completely removed and we
  get better
• exception HIV because it infects cells of the
  immune system, itself
• chemicals/drugs acyclovir, AZT, HIV protease
  inhibitor

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Major Viral Infections in Fish

• Infectious pancreatic necrosis (IPN)
• Viral hemorrhagic septicemia (VHS)
• Infectious hematopoetic necrosis (IHN)
• Channel catfish virus disease (CCVD)

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(1) Infectious Pancreatic Necrosis (IPN)

• Acute, viral infection of salmonids, especially
  trout and char
• causes high mortality in fry, sometimes
  fingerlings, rare in larger fish
• isolated in Pacific NW in 1960s, wiped out brook
  trout in Oregon in 1971-73
• classified as a reo-like virus
• only 65 nM in diam, smallest of fish viruses

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IPN general notes

• Single capsid shell, icosohedral symmetry, no
  envelope
• contains two segments of DS-RNA
• fairly stable and resistant to chemicals (acid,
  ether, etc.), variable resistance to freezing
• remains infectious for 3 months in water
• causes general viremia, but targets pancreas and
  hematopoietic tissues of kidney/spleen

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IPN epizootiology

• Host/geographic range all salmonids, brook
  trout most susceptible, report from various
  marine fish (flounder) and some inverts
• Reservoirs carriers, once a carrier always a
  carrier, virus particles shed in feces/urine
• Transmission horizontal, by waters via carriers
  or infected fry vertical from adults to progeny
  experimentally by feeding infected material, IP
  injection
• Pathogenesis entry via gills, digestive tract
• Environmental factors mortality reduced at
  lower temps (carrier not reduced)

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IPN pathology

• Gross external sudden mortality in fry, largest
  affected first, whirling, rotating, lethargy,
  exophthalmia, hemorhrhages at base of fins
• Gross internal petechae of pyloric cecae (small
  spot on surface of membrane, caused by localized
  hemorraging), muscle, viscera pale liver,
  spleen, no food in digestive tract
• Histopathology necrosis of pancreatic cells,
  mild necrosis of kidney tissue, intestinal lining

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IPN detection, diagnosis and control

• Isolation whole fry, kidney, spleen, pyloric
  cecae, sex fluids are all good sources
• Presumptive tests epizootiological evidence
  and/or typical PCE in infected cells
• Definitive tests serology (FAT, serum
  neutralization)
• Control avoid virus in water, virus-free stock,
  destruction of infected stock, some vaccination
  possible in future

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(2) Viral Hemorrhagic Septicemia (VHS)

• Viral disease of European salmonids
• recognized in Denmark in 1949
• isolated from Pacific Coast in 1989
• rhabdovirus, bullet-shaped (one rounded end), 185
  x 65 nM, lipoprotein envelope
• non-segments SS-RNA
• sensitive to ether and chloroform, heat, acid
• resistant to freeze-drying

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Viral Hemorrhagic Septicemia

• Produces a general viremia, tissue and organ
  damage, liver necrosis, spleen, kidney
• epizootiology cultured rainbow trout, also
  brown trout, steelhead, chinook, coho
• mainly found in Washington state
• reservoirs survivors are life-long carriers,
  usually rainbow trout, brown in Europe
• transmission horizontal through water, virus
  can occur on eggs spawned by carriers, IP
  injection, birds, hatchery equip

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Viral Hemorrhagic Septicemia (VHS)

• Pathogenesis infection results in viremia,
  disrupts many organ systems, 200-300g fish most
  affected
• Environmental factors low temp (lt 8oC)
• External pathology lethargis, hanging downward
  in water, swimming in circles, exopthalmia, dark
  discoloration, hemorrhages in roof of mouth, pale
  gills w/focal hemorrhages

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Viral Hemorrhagic Septicemia (VHS)

• Internal pathology gut devoid of food, liver
  pale, hemorrhages in connective tissue, kidney
  gray and swollen (chronic), red and thin (acute)
• Histopathology necrosis of liver, kidney
  nephrons, spleen, pancreas, melanin in kidneys
  and spleen
• Isolation/tests isolated from kidney/spleen,
  epizootiological evidence, CPE isolation,
  definitive test is serum neutralization or
  fluorescent antibody

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Viral Hemorrhagic Septicemia (VHS)
External hemorrhages
Internal hemorrhages
Liver red in acute stage
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Viral Hemorrhagic Septicemia

• Prevention clean broodstock, water fish,
  avoid infected broodstock, test and slaughter
• can spread very quickly from farm to farm avoid
  close proximity to other farms
• vaccines are under development

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Infectious Hematopoetic Necrosis (IHN)

• Salmon and trout, 100 million mortalities between
  1970-1980, 70 mortality
• agent bullet shaped rhabdovirus, non- segmented
  SS-RNA, sensitive to heat and pH, glycoprotein is
  spiked on surface of virus
• host/range sockeye, chinook, rainbows cohos
  resistant mortality in young fish spread by
  shipment

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Infectious Hematopoetic Necrosis (IHN)

• Reservoirs survivors become life-long carriers,
  adults shed virus at spawning
• transmission horizontal, primary mode is
  vertical via ovarian fluid (virus hitches ride on
  sperm into egg) feeding and inoculation have
  worked experimentally
• pathogenesis gills suspected incubation period
  depends on temp, route, dose, age fry most
  susceptible extensive hemorrhaging, necrosis of
  many tissues death usually due to kidney failure

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Infectious Hematopoetic Necrosis (IHN)

• Environmental factors temp very important,
  slows below 10 C, holding in tanks/handling
  increase severity
• External pathology lethargy, whirling, dropsy,
  exopthalmia, anemia, hemorrhaging of
  musculature/fins, scoliosis
• Internal pathology liver, kidney, spleen pale
  stomach/intestines filled with milky fluid
  petechial hemorrhaging
• Histopathology extensive necrosis of
  hematopoetic tissue of kidney/spleen

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Infectious Hematopoetic Necrosis (IHN)

• Definitive diagnosis serum neutralization, FAT,
  ELISA
• Prevention avoidance, quarantine, clean water
  with UV, ozone, virus-free stock test,
  slaughter, disinfect disinfect eggs vaccines
  under development elevated water temp

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Channel Catfish Virus Disease (CCVD)

• Contagious herpes virus affecting only channel
  catfish less than four months old
• occurs in SE United States, California, Honduras
• acute hemorrhagia, high mortality, first
  discovered in 1968
• agent enveloped capsid, icosohedral
  nucleocapsid with 162 capsomeres
• physio/chemical properties easy to kill,
  sensitive to freeze-thaw, acid, ether, etc.

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Channel Catfish Virus Disease (CCVD)

• Environmental factors optimal temperature 28-30
  C, common during warmer months, cooler water
  big difference
• epizootiology horizontal, vertical suspected
• external pathology spiral swimming float with
  head at surface hemorrhagic fins, abdomen
  ascites pale or hemorrhagic gills exophthalmia

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Channel Catfish Virus Disease (CCVD)

• Internal pathology hemorrhages of liver,
  kidney, spleen, gut, musculature congestion of
  mesenteries and adipose
• Histopathology necrosis of kidney, other
  organs macrophages in sinusoids of liver, etc.
  degeneration of brain
• Presumptive diagnosis clinical signs,
  epizootiological evidence, CPE
• Definitive diagnosis serum neutralization,
  fluorescent antibody

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Channel Catfish Virus Disease (CCVD)

• Prevention avoid potential carriers (survivors)
  or infected fry, keep temperature below 27oC
  (will still produce carriers), attenuated vaccine
  shows some promise
• Therapy none available

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Channel Catfish Virus Disease
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Channel Catfish Virus Disease