Virology Flashcards
Viruses are
Small, infectious agents
Outside of their host, viruses
Do not show any properties of life (ie. cannot replicate)
Viruses cannot produce
Their own energy or macromolecules
Viruses posses ___ type of nucleic acid
One (either DNA OR RNA, not both)
Viruses consist of a ___ enclosed in a ____
Genome (genetic code); protein shell (capsid)
Nucleocapsid is
The virus’s Capsid and Genome
True or False: Viruses can have both DNA and RNA
False: Viruses can only posses DNA or RNA, never both
Some viruses may contain an external layer surrounding the nucleocapsid, which is called
The envelope
What project from the surface of the virus envelope
Glycoprotein peplomers (spikes)
What may be present between the virus envelope nucleocapsid and the envelope
A matrix (layer of viral protein)
The Capsid is composed of
Capsomeres
Capsomeres are
Repeating units of polypeptides (amino acids) that make up the Capsid
The number of Capsomeres comprising the Capsid (and the way they are arranged) allows for
Classification of virus families
Two types of Capsid shapes
-Icosahedral (cube)
-Helical (spiral)
What capsid strength provides maximum strength and volume and is made of 20 equilateral triangles
Icosahedral (cube)
What capsid shape forms a spiral within the Capsomere?
Helical (spiral)
The envelope is derived from
Host cell membranes (ie. plasma, nucleus, or organelle membranes)
The envelope consists of a
Lipid bilayer, which is added to the virus during the final stage of virus replication and assembly
The envelope contains two types of virus-encoded proteins
-Peplomers (spikes)
-Matrix protein
___ project from the envelope and are used in attachment of the virus to the host cell
Peplomers (spikes)
What is located between the envelope and capsid, and provides rigidity to the virus
Matrix protein
Enveloped vs Non-Enveloped Virus (2 points each)
Enveloped:
-fragile, lose effectivity easily
-bind to host via peplomers
Non-Enveloped:
-tough
-bind to host via capsid proteins
Viral nucleic acid carries
The genetic information required for production of new viral proteins
In most viruses, the nucleic acid is
A single molecule
Two examples of viruses that have segmented RNA
-influenza virus
-reovirus
Nucleic acid of viruses may be (3)
-DNA or RNA
-Double or single stranded
-linear or circular
SS stands for
Single-stranded
dS stands for
Double stranded
What is used to replicate viruses
Double stranded host cell “machinery” (energy, organelles, and reagents)
Viruses usually replicate in either
-cytoplasm (if RNA virus)
-nucleus (if DNA virus)
Replication cycle can range from
6-40 hours
Which types of viruses usually destroy the host cell?
Non-Enveloped
6 main replication steps for viruses:
- Attachment to Host cell
- Entry into cell
- Unloading of DNA/RNA
- Replication of viral nucleic acid & synthesis of viral proteins
- Assembly
- Release from cell
Enveloped viruses bind to host cell receptors via
Peplomers
In non-enveloped viruses, attachment to host cell receptors is via
External capsid proteins
Which type of virus is fragile and loses infectivity more readily?
Enveloped viruses
Virus entry into cells is via
Endocytosis
Some enveloped viruses can also enter the cell via
Fusion
During the uncoating phase,
Viral nucleic acid is released from the capsid
DNA viruses have the same ___ as the host
Genome
ALL viruses produce
mRNA which is then translated into proteins
Positive sense RNA means ___ and give two examples
-RNA virus acts directly as mRNA
-coronavirus and picornaviruses
Negative sense RNA means ___ and give two examples
-RNA virus must make a copy mRNA from their own genome
-hantaviruses and filoviruses
By the end of step 4 (replication of viral nucleic acid and synthesis of viral proteins) there are (2)
-multiple copies of viral DNA/RNA
-all viral proteins (Capsomeres, spikes, matrix, etc) required to form new virus particles have been produced
Polypeptides associate spontaneously to form ___ which then self assemble to form ____
Capsomeres; capsids
Viral nucleic acid is packaged within the
Capsid
Non-enveloped viruses are released from host when
Host cell lyses (dies) due to accumulation of virus particles causing cell membrane to burst
Enveloped viruses are released by
Budding through the host cell membrane (host cell may survive this process)
In each virus replication cycle, the viral genome
May be changed
Viral genetic change can occur via (3)
-mutation (point, deletion/insertion)
-recombination (viral, virus-host)
-reassortment
Substitution of a single nucleotide is called
Point mutation
When nucleotides are added or deleted, it is called
Insertion or Deletion (mutation)
Rate of mutation is higher in
RNA viruses
Replication enzymes in RNA viruses are (2)
-error prone
-lack proof-reading correction
Mutations lead to the
Production of potentially different viral proteins
Two types of Recombination
-viral
-virus-host
Viral recombination
When 2 different viruses simultaneously infect the same cell, exchange of nucleotide sequences may occur (ex. Fowlpox virus)
When the virus incorporates host cell’s nucleotides into viral genome it is called
Virus-host cell recombination (ex. Some retroviruses)
Progeny viruses contain
Different combinations of the “parent” gene segments
If 2 viruses with segmented genomes infect the same cell, segments can be swapped in a process called
Reassortment
Quasispecies (2)
-after replication, the viral population becomes a cluster of viral genetic variants
-still belong to same virus species
Quasispecies may be able to (5)
-replicate / be transmitted faster
-replicate in different cells or hosts
-become more or less virulent
-be able to evade host immune response
-become resistant to antiviral drugs
A change in tropism means
Virus can now replicate in different cells types
A species jump means
Virus can now infect different hosts
An example of a Quasispecies is
Feline infectious peritonitis
3 broad approaches used for diagnosis of viral infections
- Detecting virus in tissue or blood
- Detecting antibodies to the virus
- Detecting characteristic pathology caused by the virus
Timing and Sample type to detect the virus
Timing: when the disease is occurring
Sample Type: wherever the virus is most likely to be (ie. faeces, urine, resp, repro, blood)
Timing and Sample type to detect antibodies
Timing: 1-2 weeks after acute disease
Sample Type: blood
Timing and Sample type used to detect pathology
Timing: when disease is occurring or post-mortem
Sample Type: whichever organ is most likely to be affected
4 methods used in lab to detect the Virus
- Virus Isolation (ie. growing the virus)
- Detection of viral nucleic acid (DNA/RNA)
- Detection of viral antigens (proteins)
- Electron microscopy (ie. viewing the virus)
viruses can only replicate in
living cells
In order to culture a virus we need to provide living cells in the form of (3)
-whole animals
-chicken embryos
-cell cultures (most common)
Using lab animals for virus isolation (3)
-rare (ethical concerns)
-mainly done for research
-examples: some arthropod-borne viruses, encephalitis viruses
Using chicken embryos for virus isolation (4)
-sample is inoculated into embryonated egg
-variety of routes/location in egg are used depending on virus
-evidence of virus growth includes: death of embryo, curling/stunting of embryo, virus antigens in egg fluid
-Examples: influenza virus
Using cell cultures for virus isolation (5)
-cells are artificially grown in the lab
-grown in liquid medium containing glucose, amino acids, vitamins, inorganic ions, and maintained at a neutral pH
-serum is often included for optimal growth
-grow as a monolayer (single layer) OR suspended in the medium (free-floating)
-need sterile working conditions
most common cells used for isolation of virus in cell cultures are
cell lines
changes to host cell due to virus is called the
cytopathic effect (CPE)
some viruses can cause production of _____ which can be seen in stained cell cultures
inclusion bodies
inclusion bodies can consist of (3)
-whole virions
-components of virus particles
-reaction by the cell to the virus
inclusion bodies can be in the ___ or ___ which will stain ____ or ____, respectively
Nucleus –> acidophilic (stains red/pink)
Cytoplasm —> basophilic (stains blue/purple)
an example of an intracytoplasmic inclusion body virus is
rabies
an example of an intranuclear inclusion body virus is
cytomegalovirus (herpes)
detection of viral nucleic acid (DNA/RNA) is performed by the
Polymerase Chain Reaction (PCR)
PCR is a
very sensitive molecular test used for detection of viral nucleic acid
two types of PCR are:
-conventional PCR (amplified product is viewed on a gel)
-real-time PCR (results as a computer ouput)
detection of viral antigens (proteins) is performed by
serological tests
antigens are the
protein components of the virus (ie. capsid or peplomer proteins)
serological tests are based on the
binding of a specific antibody to the antigen
3 examples of serological tests
-ELISA
-agglutination (rotavirus)
-rapid antigen test (RAT) (feline leukemia)
viruses are not visible in a light microscope, so we must use an
electron microscope
electron microscopy is useful for the diagnosis of
poxvirus
which virus detection approach takes a long time and is expensive
virus isolation
which virus detection approach is quick and cheap
serological tests for antigen detection
which virus detection approach is only found in specialized labs
electron microscopy
which virus detection approach has very high sensitivity and a limited range to the virus it is detecting
molecular tests (PCR) for DNA/RNA detection
a positive antibody result could mean (4)
-current infection
-previous infection (months or years ago)
-response to vaccination
-maternal antibodies (young animals)
a negative antibody result could mean (2)
-virus being tested for is NOT the cause of disease
-sample was taken too early
antibody tests must be tested
1-2 weeks AFTER period of disease/clinical signs
some viral infections have characteristic pathology such as (2)
-distinct lesions
-inclusion bodies
viral infections cannot be treated with
antibiotics
compounds (medication) that interfere with viral replication are often
toxic to the host cell
two human pandemics that have provided incentive for antiviral drug development are:
-HIV
-COVID
two examples of antiviral drugs
-Acyclovir (Zovirax) used for treatment of herpesvirus in humans and felines
-Remdesivir/GS-441524 used for treatment of human SARS-CoV-2 and Feline Infectious Peritonitis (FIP)
3 basic ways viruses are transmitted
- Direct Contact
- Indirect Contact
- Arthropod-borne
2 types of Arthropod-Borne Transmission
- Biological (“arboviruses”)
- Mechanical
direct contact transmission is a common route for
fragile viruses (enveloped) that do not survive well in the environment (ex. rabies and herpesvirus)
during indirect contact, the infected animal deposits the virus into (3)
-environment
-air
-fomite (inanimate object)
indirect contact transmission is common for
tough viruses (unenveloped) that can survive in the environment (ex. parvovirus and rotavirus)
the arthropod transmits the virus between the vertebrate host but the virus does NOT replicate in the arthropod is called
Mechanical transmission of arthropod-borne transmission
two examples of arthropod-borne mechanical transmission
-equine infectious anemia virus
-myxoma virus in rabbits
when the virus replicates in BOTH the vertebrate host and arthropod it is called
biological transmission (arboviruses)
two examples of arthropod-borne biological transmission
-culicoides transmits Bluetongue Virus to sheep
-ticks transmit African Swine Fever Virus to pigs
with many arboviruses there is an endemic cycle, this means
there is no disease in either species and the cycle is restricted to a particular geographic area
arbovirus epidemics occur when (4)
-an “unusual” host is infected
-climatic changes (causing an increase in arthropod vectors)
-ecological disturbance (causing a change in distribution of arthropods or an increased number of arthropods)
-transport of the virus to previously virus-free areas
viruses produce two broad effects on the host cell:
- Cytotoxicity (cell death)
- Oncogenesis (development of cancer)
3 main cancer causing viral families
-retroviruses
-herpesviruses
-papillomaviruses
during oncogenesis, the virus disrupts normal host cell growth and division in three ways
-affecting host cell oncogenes (c-onc)
-carrying an oncogene in the viral genome (v-onc–> rare)
-suppressing the action of host cell tumour suppressor genes
4 general patterns of viral infection
- Acute infection
- Chronic infection
- Latent infection
- Slowly progressive infection
patterns of viral infection differ in (3)
-timing of disease
-duration of virus present in host
-period of virus shedding
most common pattern of viral infection is
Acute Infection
during acute infection (4)
- host is infected
- period of disease occurs
- virus sheds
- immune system eliminates virus
an example of acute viral infection is
canine parvovirus
during chronic infection (2)
-the virus continues to be present in the animal and can be shed months or years later
-eventually, shedding/antibodies will decline to undetectable levels
two examples of chronic viral infection are
-canine adenovirus-1 (infectious canine hepatitis)
-feline calcivirus
which virus goes through a pattern of latent infection
herpesviruses
during latent infection (5)
-virus develops latent state (ie. no replication)
-periodic recurrences of diseases/virus shedding
-remains in animal for life
-virus only detectable during periods of reactivation
-antibodies are generally detectable throughout life
4 examples of slowly progressing viral infection are
-feline leukemia (FeLV)
-retroviruses such as HIV
-equine infectious anaemia
-bovine leukemia virus
during slowly progressing viral infection (5)
-virus replication increases over time, eventually leading to death
-no disease signs at initial infection
-host immune response is ineffective
-host is infected for life
-virus and antibodies are generally detectable throughout life
a disease causing agent that consists only of protein (ie. no DNA/RNA) is called a
prion
PrP^c is the
host cell protein
a prion is an
abnormally folded variant/isoform of a host cell protein
PrP^Sc is the
prion agent
the prion differs from the normal host cell protein in that (3)
-has a different structure (Beta sheets instead of alpha helices)
-less soluble
-resistant to proteases (enzymes that break down proteins)
prion protein is (2)
-extremely resistant to environment
-difficult to inactivate
recommended prion inactivation includes (3)
-autoclaving hotter and longer
-hot solutions of sodium hydroxide
-high concentrations of sodium hypochlorite
complete destruction or infectivity of prions
CANNOT be guaranteed
prions cause
neurological disease (spongiform encephalopathy)
prion diseases have common features (5)
-long incubation periods
-progressive, eventually fatal
-brain lesions are similar
-minimal inflammatory response
-no detectable immune response
two prion diseases of humans
-creutzfeldt-jakob disease (CJD)
-Kuru
prion disease of sheep
scrapie
prion disease of cattlw
bovine spongiform encephalopathy (BSE)
two ways prions cause disease
- Externally (outside of body due to ingestion of prion protein)
- Internally (inside body due to mutation of gene)
BSE and scrapie disease are caused
externally –> ingestion of prion protein PrP^Sc
Once the prion protein enter the brain (3)
-induces normal protein change (ie. converts PrP^c into PrP^Sc)
-aggregation of PrP^Sc forms amyloid deposits and fibrils
-eventually leads to disruption of brain function
species barrier
a host species (humans) may be resistant to infection by prions (ex. humans cannot get scrapies) BUT can be infected with prions from another (ex. humans CAN be infected with BSE)
the species barrier is due to
differences between amino acid sequence and shape of the prion proteins between the two species
Prions are NOT classified according to ___. Instead, they are
-family/genus/species
-differentiated by molecular and biological properties
Scrapie is
a progressive, neurological disease of sheep
differentiating prions can be done in three ways
-amino acid sequence of prion protein reflects host species they are derived from
-banding on Western blot (serological assay)
-bioassays in mice (shows incubation period & mortality pattern; and distribution of lesions in brain)
Scrapie incubation period is
1-5 years
Death of scrapie-infected sheep usually occurs
Within 6 months from onset of clinical signs
Method of transmission for scrapie is via
Ingestion (probably during neonatal period via exposure to placental fluids and milk from infected dam)
Scrapie occurs in
Most of the world
Pastures grazed by scrapie infected sheep may
Remain contaminated for year
Control programs for scrapie (3)
-surveillance
-depopulation of infected herds
-breeding programs for more resistant sheep
Clinical signs of scrapie (8)
-behaviour change
-itching
-in coordination
-tremors
-loss of condition
-emaciation
-paralysis
-death
incubation period of scrapie is ____, so affected sheep are usually ____
1-5 years; >2 years old
method of transmission of scrapie is via
ingestion (typically during the neonatal period ie. placental fluids and milk from infected dam)
Bovine Spongiform Encephalopathy (BSE) has similar clinical signs to scrapie except there is no
itching
BSE studies found an association with feeding of
meat-and-bone meal
Which two types of transmission do NOT occur with BSE
-horizontal transmission (cow to cow)
-maternal transmission (cow to calf)
ingestion of BSE-affected meat resulted in
the human disease called Variant CJD
two forms of Creutzfeldt-Jakob Disease (CJD) in humans
-sporadic CJD
-variant CJD
Sporadic CJD is caused by (2)
-mutation of PrPc gene
-spontaneous conversion of PrPc to PrPSc
Sporadic CJD usually affects
people over the age of 50
Variant CJD (vCJD) is caused by
the ingestion of BSE-affected beef
symptoms of sporadic CJD (6)
-sensory disturbances
-sleeping disorders
-dementia
-motor disturbances
-coma
-death
Variant CJD differs from sporadic CJD in these three ways
-symptoms = psychiatric disorders
-pathology = lesions are more florid
-age = 19-45 years old
Kuru (4)
-human prion disease
-occurred in people living in Papua New Guinea
-caused by consumption of human brains during ritualistic practices
-cannibalism was abandoned in the late 50s
