Download What are Viruses?

Aquaculture Viruses
What a Virus Isn’t
• 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 don’t kill the organism that causes the flu
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
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
Virus Appearance: capsid
capsomere: unit/molecule associated with capsid structure
Typical Virus Shapes
SPHERES
RODS
CUBES
More Virus Shapes
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
What are Viruses?: an
Introduction
• Viruses make use of the host cell’s 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
Bacteriophage Attack
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
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
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 cell’s surface
• depending on the virus, either the entire virus
enters the cell or only the genetic material is
injected
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
The Virus Invasion
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
Different Types of Viruses
• Major classification: animal, plant, bacterial
• Sub-classified by arrangement and type of nucleic
acid
• animal virus group: double-stranded DNA, singlestranded DNA, double-stranded RNA, singlestranded 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
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
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”
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 antibody
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
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
Major Viral Infections in Fish
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Infectious pancreatic necrosis (IPN)
Viral hemorrhagic septicemia (VHS)
Infectious hematopoetic necrosis (IHN)
Channel catfish virus disease (CCVD)
(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 1960’s, wiped out
brook trout in Oregon in 1971-73
• classified as a reo-like virus
• 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
• causes general viremia, but targets pancreas
and hematopoietic tissues of kidney/spleen
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)
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
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
(2) Viral Hemorrhagic
Septicemia (VHS)
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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
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
Viral Hemorrhagic Septicemia
(VHS)
• Pathogenesis: infection results in viremia,
disrupts many organ systems, 200-300g fish most
affected
• Environmental factors: low temp (< 8oC)
• External pathology: lethargis, hanging downward
in water, 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
• Isolation/tests: isolated from kidney/spleen,
epizootiological evidence, CPE isolation,
definitive test is serum neutralization or
fluorescent antibody
Viral Hemorrhagic Septicemia
(VHS)
External hemorrhages
Internal hemorrhages
Liver red in acute stage
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
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
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
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
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
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.
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
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
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