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Lecture 3: Aquaculture Viruses

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Aquaculture Viruses
What a Virus Isn’t
• Not a bacterium...
• Not independent...
• Cannot survive in absence of a living cell
within which to replicate...
• Antibiotics generally don’t work on them...
What Viruses Are...
• Infectious agents composed mainly of nucleic acid
with a protein coat (capsid)
• Visible with electron microscope (10-200 nM)
• Carry on normal cell-like function (unless free, then
• In infectious form: no growth; no respiration???
• Can enter living plant, animal or bacterial cell
Virus Appearence?
• 1. Capsid
• 2. 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
• protein core under capsid protecting genetic material
• sometimes an additional covering (lipid bilayer w/embedded
proteins) on outside known as an envelope ( like a baseball)
• various forms: rods, filaments, spheres, cubes, crystals
capsomere: unit/molecule associated with capsid structure
Typical Virus Shapes
More Virus Shapes
Composition of T-Even
• 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
How do viruses work?
• Viruses make use of the host cell’s chemical energy,
protein and nucleic acid synthesizing ability to replicate
• 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
– fish viruses are just as specific
Bacteriophage Attack
Virusal Mechanism
• Viruses contain single- or double- stranded DNA
or RNA
• Often, the virus alters the intracellular
environment enough to damage or kill the cell
• If enough cells are destroyed, disease results!
Role of RNA/DNA
• Supplies the codes for building the protein coat
(capsid) and for producing enzymes needed to
replicate more viruses
• Information given so newly-built viruses can lyse cells
(e.g., bacteriophage)
• Result: cell destroyed.
Bottom Line...
• All viruses only exist to make more viruses
• Most are harmful
• Replication = host cell death.
The Virus Invasion: Step by step
• Phase 1: Attachment of virus to bacteria, etc.
• Phase 2: injection of DNA or RNA
• Phase 3: DNA (RNA) enters the host cell
• Phase 4:DNA incorporated (10 minutes) hundreds of
virions appear causing the cell to rupture, releasing
hundreds of small viral replicates
• This is how it can replicate so quickly!
The Virus Invasion
What’s 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
• Specific: for example, a virus that only affects
one organism (humans and smallpox)
• Influenza can infect humans and two animals
Different Types of Viruses
• Major classifications: animal, plant, bacterial
• Sub-classified by arrangement and type of nucleic
• Animal virus group: double-stranded DNA, singlestranded DNA, double-stranded RNA, singlestranded RNA, retrovirus
• Influenza: SS-RNA
Do Viruses ever Change?
• Mutations do 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
Defense Against Viruses
• First Line: skin and mucous membrane, which
also lines the gastrointestinal and respiratory
• skin is tough and stomach acidity acts as a
• 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”
Defense Against Viruses
Antibodies attacking chickenpox virus
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
Defense Against Viral Infection
• Animals are protected in several 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
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, several fish vaccines available.
Major Fish Viruses
Major Viral Infections in Fish
• Infectious pancreatic necrosis (IPN)
• Viral hemorrhagic septicemia (VHS)
• Infectious hematopoetic necrosis (IHN)
• Channel catfish virus disease (CCVD)
(1) Infectious Pancreatic Necrosis
• What?: viral infection of salmonids (trout and char)
• Time: Acute
• Result: high mortality (fry and fingerlings)
• Rare in larger fish (good thing!)
• History: Isolated in Pacific NW in 1960’s, wiped out brook
trout in Oregon in 1971-73
• Size: Only 65 nM in diam., smallest of fish viruses
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 (uh oh!)
• Targets pancreas and hematopoietic tissues of kidney and spleen
IPN: epizootiology (disease process)
• Who?: All salmonids, brook trout most susceptible,
marine fish (flounder?)
• Reservoirs (where)?: carriers, once a carrier always a
carrier, virus particles shed in feces/urine
• Transmission (how?): 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 (why?); however, carriers not reduced
IPN: pathology (what do we see?)
IPN: detection, diagnosis and control
• Isolation: whole fry, kidney, spleen, pyloric cecae, sex
fluids are all good sources, .i.e. check these!!!
• Presumptive tests: epizootiological evidence and/or typical
PCR in infected cells
• Definitive tests: serology
(fluorescent antibody test (FAT))
• Control: avoid virus in water, virus-free
stock, destruction of infected stock,
vaccine exists now!
How Bad Can It Be??
Fish severely affected by IPNV:
• Atlantic salmon* (Salmo salar)
brook trout* (Salvelinus fontinalis)
brown trout* (Salmo trutta)
danio zebrafish* (Brachydanio rerio)
rainbow trout* (Oncorhynchus mykiss)
yellowtail* (Seriola lalandi)
Other species known to be
• amago salmon (Oncorhynchus rhodurus)
Arctic char (Salvelinus alpinus)
Atlantic menhadden (Brevoortia tyrannus)
carangids (Carangidae)
chinook salmon (Oncorhynchus tshawytscha)
chum salmon (Oncorhynchus keta)
cichlids (Cichlidae)
coho salmon (Oncorhynchus kisutch)
common scallop (Pecten maximus)
cutthroat trout (Salmo clarki)
cyprinids (Cyprinidae)
Danube salmon (Salmo hucho)
drums/croakers (Sciaenidae)
eels (Anguilla spp)
grayling (Thymallus thymallus)
halibut (Hippoglossus stenolepis)
Jap. amberjack (Seriola quinqueradiata)
lampreys (Petromyzontyidae)
loach (Misgurnus anguillicaudatus)
masou salmon (Oncorhynchus masou)
perches (Percidae)
silversides (Atherinidae)
soles (Soleidae)
striped snakehead (Channa striatus)
summer flounder (Paralichthys dentatus)
white seabass (Moronidae)
carp (Cyprinus carpio)
redfin perch (Perca fluviatilis)
yellowfin bream (Acanthopagrus australis)
herrings/sardines (Clupidae)
lake trout (Salvelinus namaycush)
left-eye flounders (Bothidae)
loaches (Cobitidae)
Pacific salmon (Oncorhynchus spp)
pikes (Esocidae)
sockeye salmon (Oncorhynchus nerka)
Southwest European nase (C. toxostoma)
suckers (Cotostomidae)
turbot (Psetta maxima)
whitefish (Coregonidae)
goldfish (Carassius auratus)
southern flounder (P. lethostigma)
Asymptomatic carriers...
• coalfish (Pollachius virens)
common carp (Cyprinus carpio)
discus fish (Symphysodon discus)
goldfish (Carrasius auratus)
heron (Ardea cinerea)
loach (Cobitidae)
minnow (Phoxinus phoxinus)
noble crayfish (Astacus astacus)
Infectious pancreatic necrosis in Atlantic salmon.
pike (Esox lucius)
Note swollen stomach and 'pop eye'
river lamprey (Lampetra fluviatalis) Source: Australian Animal Health Laboratory
shore crab (Carcinus maenas)
Spanish barbel (Barbus graellsi)
white suckers (Catostomas commersoni)
...what now???
(2) Viral Hemorrhagic Septicemia (VHS)
(2) Viral Hemorrhagic Septicemia (VHS)
• What?: Viral disease of European salmonids
• When?: Recognized in Denmark in 1949, isolated
from Pacific Coast in 1989
• Size: rhabdovirus, bullet-shaped (one rounded
end), 185 x 65 nM, lipoprotein envelope
• non-segmented SS-RNA
• Constitution: sensitive to ether and chloroform,
heat, acid, resistant to freeze-drying
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 (most cases in WA state)
• Reservoirs: again...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
Viral Hemorrhagic Septicemia (VHS)
• Pathogenesis: infection results in viremia, disrupts many
organ systems, 200-300g fish most affected
• Environmental factors: low temp (< 8oC, 46oF)
• External pathology: lethargy, hanging downward in water
(dropsy), swimming in circles, exopthalmia, dark
discoloration, hemorrhages in roof of mouth, pale gills
w/focal hemorrhages
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 (OUCH!)
• Isolation/tests: isolated from kidney/spleen,
epizootiological evidence, definitive test is serum
neutralization, or FAT.
Viral Hemorrhagic Septicemia (VHS)
External hemorrhages
Liver red in acute stage
Viral haemorrhagic septicaemia in rainbow trout.
Note pale color of stomach region, pinpoint
haemorrhages in fatty tissue, and pale gills
Source: T HГҐstein
Viral haemorrhagic septicaemia in rainbow trout.
Note swollen stomach and “pop eye”
Prevalent host type and location
Farmed rainbow trout and a few other freshwater fish in continental
Marine fish of the Baltic Sea, Skagerrak, Kattegat, North Sea,
Farmed rainbow trout Denmark
Farmed rainbow trout in Norway, Finland, Gulf of Bothnia
Rainbow trout in Georgia, farmed and wild turbot in the Black Sea[11]
Marine fish of the Baltic Sea
Marine fish of the British Isles and northern France, farmed turbot in
the UK and Ireland, and Greenland halibut (Reinhardtius
hippoglossoides) in Greenland[12]
Marine fish of the Northwest Pacific (North America), North
American north Atlantic coast,[13] Japan, and Korea[1][14]
Freshwater fish in North American Great Lakes region[14]
Virus presence spread through much of the Great Lakes from 2003-2007.
Viral Hemorrhagic Septicemia
• Prevention: clean broodstock and water = clean 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.
• One EPA-approved disinfectant: Virkon® AQUATIC
(made by Dupont). Bleach kills the VHS virus.
(3 ) Infectious Hematopoietic Necrosis
• Who: sockeye, chinook, rainbows; cohos resistant
• When?: 1950’s in Oregon hatcheries. 100 million
mortalities between 1970-1980, if infected, 70%
mortality likely, esp. in young fish (fry: 90-95%
mort. possible)
• What?: bullet shaped rhabdovirus, non- segmented
SS-RNA, sensitive to heat and pH, glycoprotein is
spiked on surface of virus
Infectious Hematopoietic Necrosis (IHN)
• Reservoirs: survivors life-long carriers, adults shed virus
at spawning
• Transmission: horizontal, primary mode is vertical via
ovarian fluid (virus hitches ride on sperm into egg);
however, feces, urine, and external mucus possible. Also,
feeding and inoculation have worked experimentally
• Pathogenesis: gills suspected; incubation period depends
on temp, route, dose, age; extensive hemorrhaging,
necrosis of many tissues; death usually due to kidney
Infectious Hematopoietic Necrosis (IHN)
• Environmental factors: temp. very important,
slows below 10в—¦C, holding in tanks/handling
increase severity (doesn’t occur naturally >15 ◦C)
• 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
Infectious Hematopoietic 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
• No vaccines as of June 2007.
(4) 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, discovered in 1968
• Agent: enveloped capsid, icosohedral nucleocapsid with
162 capsomeres
• Physio/chemical properties: easy to kill, sensitive to
freeze-thaw, acid, ether, etc.
Channel Catfish Virus Disease (CCVD)
• Environmental factors: optimal temperature 28-30◦C,
common during warmer months, cooler water = big
• epizootiology: horizontal, vertical suspected
• external pathology: spiral swimming; float with head at
surface; hemorrhagic fins, abdomen; ascites; pale or
hemorrhagic gills; exophthalmia
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
• Definitive diagnosis: SN or FAT.
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...
Channel Catfish Virus Disease
Channel Catfish Virus Disease
However, you can always take precautions!
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