Lecture 10

Question 1

Viruses are and there goal

Answer 1

Everywhere
Goal: to infect cells
Can exist, but cannot replicate outside of cells
Can only replicate inside host cells
Life cycle requires infection
Can infect bacteria, animal cells, and plant cells
Viruses that infect bacteria are called bacteriophage

Question 2

Major diseases caused by viruses

Answer 2

Influenza
AIDs (HIV virus)
Ebola
Measles
Smallpox
Common cold (adenovirus)
Herpes
Canine parvovirus
Rabies virus
West nile virus
Papilloma virus

Question 3

Virus sizes

Answer 3

10-1000 nanometers
Considered “particles” - they are not cells
Second smallest infectious agent
Smallest is a prion which is a single protein
Can only be seen with an electron microscope

Question 4

Virion is

Answer 4

completely assembled infectious virus particle outside its host cell

Question 5

Virus components

Answer 5

A protein shell - VIRAL CAPSID
Genetic material
Some have proteins protruding from the surface (spikes)
Some have an envelope

Question 6

Structure of the virus

Answer 6

Capsid and enclosed genome is called the nucleocapsid
Capsid made up of protein subunits called capsomeres
Usually 1-3 different proteins that are arranged in repeating units
Viral proteins located on the outside are:
Important for attaching to host cells during infection
What the immune system sees

Question 7

Enveloped viruses are

Answer 7

Enveloped viruses are SURROUNDED by a phospholipid membrane
This phospholipid membrane is called the ENVELOPE
Envelope derived from host cell membrane

Question 8

How to get rid of enveloped viruses

Answer 8

Enveloped viruses are more easily damaged
Detergents, drying, heat, disinfectants easily damage the phospholipid layer

Question 9

How to get rid of naked viruses

Answer 9

Naked viruses are more resistant to heat, drying, many types of disinfectants
For veterinary medicine, only some disinfectants are labelled for naked viruses

Question 10

Complex viruses are

Answer 10

Viruses that do not fit into the other categories
Includes bacteriophages
Distinct shape with head, tail and spikes

Question 11

Shape of complex viruses

Answer 11

Head contains genetic information
Spikes attach onto the bacterial surface
Tail inserts into the bacterial cell and acts as a tube through which the viral genome enters the cytoplasm

Question 12

The genome can be made up of

Answer 12

Double stranded DNA
Single stranded DNA
Double stranded RNA
Single stranded RNA

Question 13

The viral genome types and can be classified by

Answer 13

Viruses can be classified by their genome type
Type of genome affects how viruses infect cells
Type of genome determines how drugs are designed to try and stop viral infections

Question 14

Relationship b/w genome and life cycle

Answer 14

Viruses have very few genes
Most of the genes encode:
Capsid proteins
Proteins that allow the virus to attach to host cells
Special enzymes involved in replication of genomes made up of RNA and single stranded DNA
In order to replicate, viruses need more enzymes than they have genes for
Extra enzymes come from cells that they infect
Viruses can only replicate inside cells

Question 15

Virus life cycle

Answer 15

Virions exist in the environment or in a host
Virus attaches to a host cell
Certain viruses only attach to certain types of host cells
Determined by viral attachment proteins that bind to proteins on the surface of the cell
Either the viral genome alone, or within the capsid, enters the host cell
“Hijacks” the DNA replication machinery and the protein synthesis machinery of the cell
Synthesis of copies of the viral genome and viral proteins occurs
Capsid proteins self-assemble around a copy of the viral genome
The new virus particles are released from the host cell

Question 16

Lytic virus cycle

Answer 16

Only applies to viruses without an envelope
Cells that are infected continue to produce virus particles
Eventually the virus particles are released
Causes cell lysis
Host cell dies once it has produced enough virus

Question 17

Non-lytic virus cycle

Answer 17

Applies to enveloped viruses
1a. Virus particles self-assemble in the cytoplasm
1b. At the same time, special virus proteins are inserted into the plasma membrane
2. The virus capsule binds to the proteins in the cell membrane
3. Virus wraps itself in a phospholipid bilayer in the process of BUDDING
Virus are continually being produced and released from the cell
The cell remains infected for a longer time and is not lysed during the process of viral release

Question 18

Normal flora viruses

Answer 18

Viruses must infect cells to survive
Cause damage to a cell
There are very few viruses that can be considered normal flora of animals
Bacteriophage in rumen infect bacteria and cause lysis releases nutrients for animal to use
Intestines also have a few bacteriophages
Lyse intestinal bacteria to release nutrients

Question 19

Stages of viral infection

Answer 19

Transmission (same as routes for bacteria)
Entry into and movement within the body
Replication to the host cell
Entry into the host cells
Replication within the host cell
Release from the host cell
Spread to other cells and other animals
Virus must avoid the immune system
Virus will cause varying types of damage to the host

Question 20

Tropism is

Answer 20

Attraction

Question 21

Cell tropism is

Answer 21

CELL TROPISM or TISSUE TROPISM means a virus can only bind to certain types of cells or tissues
Eg. Papilloma virus can only bind to and infect epithelial cells

Question 22

Host tropism is

Answer 22

a virus can only bind to cells in a certain species
Eg. Ovine herpes virus can only infect sheep

Question 23

Stages of viral disease

Answer 23

Transmission and infection of the host
Must be a minimum infectious dose to cause disease
Incubation
Virus is finding its specific cell/tissue, attaching, entering and replicating
Prodromal period
Fever, lethargy
Usually related to the immune system’s initial response to the virus
Clinical period
Specific to the tissue/cell infected
Resolution
Convalescence
Recovered
Alternately, infection can result in:
Chronic clinical disease
Chronic latent infections
Death

Question 24

Chronic latent infection in virus is

Answer 24

Certain viruses can “hide” inside the infected cell
Very slow replication or complete dormancy
There is NO SHEDDING (transmitting of virus to others) during a latent infection
Periodically, it will replicate faster
May be able to shed or cause disease again

Question 25

How damage occurs from viruses

Answer 25

Virus can inhibit synthesis of host cell DNA and RNA
Some viruses will insert their genome into the host cell genome → damages essential genes
Some viral proteins can have a direct toxic effect on the cell
Immune system attacks the viral proteins that are expressed on the surface of the host cell
The accumulation of virus particle inside the cell can damage normal cell structure
Cells can be lysed in the process of releasing non- enveloped virus particles
Some viruses cause infected cells to become transformed - cells become cancerous

Question 26

Mechanism of virus causing cancers

Answer 26

Virus inserts its genome directly into the host cell genome as part of the replication process
If the virus inserts into and disrupts a gene that stops cell growth → cell will divide
If the virus inserts right before a gene that encodes a growth promoter → excess production of the growth promoter along with synthesis of viral protein

Question 27

Viruses that cause cancer

Answer 27

Epstein Barr virus
Rous Sarcoma virus
Human papillomavirus
Hepatitis B (specific form of liver carcinoma in people)
Feline leukaemia virus
Bovine leukaemia virus
Canine/bovine papilloma viruses

Question 28

What is a viruses virulence dependent on

Answer 28

Determined by the virulence of the virus and the immune status of the host animal
More virulent → faster spread

Question 29

Why do viruses have to replicate fast

Answer 29

Try to avoid killing the host cell if possible
Because lytic viruses kill the host cell they must be able to rapidly spread to continue replicating

Question 30

Why does the virus try and avoid the immune response

Answer 30

Viruses have very few proteins
Proteins are recognized by the immune system when it fights a virus infection
However, viruses replicate very quickly and have a high rate of spontaneous mutation of capsid proteins
If a structural protein mutates, the immune system has to “re-train” how to identify the virus

Question 31

Influenza virus antigen drift

Answer 31

Every year, there is a new strain of influenza virus
To protect humans against the flu, there is a seasonal flu vaccine that changes every year
The seasonal flu VACCINE contains 2 viral antigens that the immune system is taught to recognize
Neuraminidase (N)
Hemagglutinin (H)
Neuraminidase and Hemagglutinin are spike proteins found on the surface of the influenza envelope
Used for attaching to cells and tropism
These are the proteins that the immune system recognizes
Amino acid changes due to spontaneous mutation lead to a slightly different N and H protein
Proteins recognized by the immune system = ANTIGENS
Gradual change in these proteins recognized the immune system = ANTIGENIC DRIFT
Immune system has to relearn how to fight virus

Question 32

Influenza virus antigen shift

Answer 32

ANTIGENIC SHIFT = rapid large change in antigens
Occurs when two influenza strains infect the same cell and genes mix and match during virion assembly
Results in mixing of hemaglutinin from one strain with neuraminidase from another strain

Question 33

immune system response to infection

Answer 33

The best time to destroy the virus is BEFORE it infects the cell
Once viruses enter cells, the immune system must destroy the infected cell in order to control the infection
Immune-mediated destruction of the body’s own cells is a major cause of pathology in virus infections

Question 34

Anti viral drugs to control viral infection

Answer 34

Very few exist
These target viral-enzymes that take part in replication of virus genomes made up of ssDNA and dsRNA

Question 35

Vaccination for viral infections

Answer 35

Best defense
Teaching the immune system to find and destroy free virion particles before they infect
Teaching the immune system to identify and destroy infected cells
Some vaccines prevent infection, othe

Question 36

Disinfectant for viral infections

Answer 36

Must select appropriate disinfectant
Enveloped viruses are easiest to destroy
Damaging the envelope damages the virus
Small, non-enveloped viruses are difficult to destroy
ONLY certain disinfectants will destroy these
Virox, Accel, Virkon, bleach