Unit 10 Flashcards
Describe viruses
Found everywhere
Goal: to infect cells
Can exists but cannot replicate outside of cells (can only replicate inside host cells) And their life cycle depends on infection
Can infect bacterial/animal/plant cells
10-1000 nanometers (smaller than bacteria) (second smallest -first is a prion) can only see with an electron microscope
NOT cells, considered particles
Define Bacteriophage
Viruses that infect bacteria
What is a virion
A completely assembled infectious virus particle outside its host cell
What are 4 components of viruses (which 2 do ALL viruses have?)
Every virus has
A protein shell called a Capsid
Genetic material
SOME have protein protruding from the surface (spikes)
SOME have an envelope
A capsid and the enclosed genome is called the
Nucleocapsid
Capsid is made up of protein sub units called
Capsomeres (usually 1-3 different proteins arranged in repeating units)
Viral proteins located on the outside of viruses are
Important for attaching to host cells during infection and are what the immune system identifies viruses by
What are the 3 basic shapes of viruses
Icosahedral (20 sided structure)
Helical (hollow tube)
Complex (doesn’t fit into other categories)
Each can have an envelope or not
Describe enveloped viruses
Viruses surrounded by a phospholipid membrane called the envelope
The envelope is derived from the host cell membrane
More easily damaged (detergents, drying, heat and disinfectants easily damage the phospholipid layer that is needed for structure)
Describe naked viruses compared to enveloped viruses
Naked viruses are more resistant to heat, drying, and many disinfectants (only some disinfectants are labeled against naked viruses)
Describe complex viruses
Includes bacteriophages
Distinct shape with;
1) head: contains genetic info
2) spikes: attach to bacterial surface
3) tails: insert into bacterial cell and acts as a tube that the viral genome enters into the cytoplasm through
Describe the viral genome
Depending on the virus the genome can be made up of:
1) double or single stranded DNA
2) double or single stranded RNA
Viruses are classified by their genome type
The type of genome they have affects how viruses infect cells and how drugs are designed to stop infections
True or false
Viruses have very few genes
True
Most of the genes viruses have encode what?
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, where do they get these enzymes and what does this explain?
Extra enzymes come from the cells they infect
Explains why viruses can only replicate inside cells
Describe the virus life cycle
Virions exist in host of environment
Virus attaches to a host cell (determined by viral attachment proteins that bind to proteins in the surface of the cell)
Either the viral genome alone or within the capsid enter the host cell
It hijacks the DNA replication machinery and the protein synthesis machinery of the cell
Synthesis of copies of the viral genome and viral proteins begins
Capsid proteins self assemble around a copy of the viral genome
The new virus particles are released from the host cell
What is the Lytic Virus Cycle
Only applies to viruses WITHOUT an envelope
Cells that are infected continue to produce virus particles, when the cell releases these virus particles the cell lysis
Host cell dies once the virus particles are released
What is the non lytic virus cycle
Only applies to enveloped viruses
Virus particles self assemble in the cytoplasm, at the same time special virus proteins are inserted into the host cells’ plasma membrane. The virus capsule binds to the proteins in the cell membrane, the 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 longer periods of time and does not lyse on release of the virus
True or false
Many viruses are considered part of normal flora
FALSE
there a very few viruses considered to be part of normal flora
What do the bacteriophages that are considered to be part of normal flora do
Bacteriophages in rumen infect bacteria and cause lysis -this releases nutrients for the animal to use
Intestines also have a few bacteriophages -lyse intestinal bacteria to release nutrients
What are the stages of viral infection
Transmission (same route as bacteria)
Entry into and movement throughout the body
Attachment to The host cell
Entry into the host cell
Replicate within the host cell
Released from the host cell
Spread to other cells and animals
In order for viral infections to occur, what must the virus do
Avoid the immune system
Define tropism
Attraction
What is cell/tissue tropism
Give an example
When a virus can only bind to certain types of cells or tissues
Ex. Papilloma virus can only bind to and infect epithelial cells
What is host tropism
Give an example
When a virus can only bind to cells in a certain species
Ex. Ovine herpes virus can only infect sheep
Transmission and infection of the host much be at
Minimum infectious dose to cause disease
What happens during incubation
Virus is finding its specific cell/tissue, attaching, entering and replicating
What happens during the prodromal period
Fever, lethargy
Usually related to the immune system’s initial response to the virus
What happens during the clinical period
Clinical signs Specific to the tissue/cell infected appear
What stage is after clinical period
resolution
What stage is after resolution
Convalescence
What stage is after convalescence
Recovery
What are alternate ways an infection can result in
Chronic clinical disease
Chronic latent infections
Death
Describe latent infections
Certain viruses can “hide” inside infected cells -very slow replication or complete dormancy
There is NO SHEDDING during a latent infection
Periodically, it will replicate faster -may be able to shed and cause disease again
Stress events trigger replication
Explain how damage occurs in the cell from a viral infection
Viruses can inhibit synthesis of host cell DNA/RNA
Some viruses will insert their genome into the host cell genome which damages essential genes
Some viral proteins can have direct toxic effects on the cell
Immune system attacks the viral proteins that are expressed on the surface of the host cell
The accumulation of virus particles inside the cell that damage normal cell structure
Cell can be lysed in the process of releasing non enveloped virus particles (lytic cycle)
Some viruses cause infected cells to become transformed -the cells become cancerous
A few viruses can cause cancer, explain their mechanism for doing so
Virus inserts it’s 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 begin to divide continuously
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
What are the 3 things viruses have to do in order to achiever their goal; to replicate
1) infect as many cells as possible (determined by virulence and immune status of host) (more virulent = faster spread)
2) try to avoid killing the host cell (lytic viruses kill the host cell they must be able to rapidly spread to continue replicating)
3) avoid the immune response (viruses have very few proteins which are mainly structural, these are recognized by the immune system but they replicate quickly and have a high rate of spontaneous mutation of capsid proteins -the immune system would have to re-train how to identify the virus)
The seasonal fluid vaccine changes every year, explain how this works
The vaccine contains 2 viral antigens that the immune system is taught to recognize (Neuraminidase (N)) and (Hemagglutinin (H)) these are spike proteins found on the influenza envelope Used for attaching and tropism
Amino acid changes due to spontaneous mutation lead to a slightly different N and H protein
Define antigen
Protein recognized by the immune system
What is antigenic DRIFT
Gradual change in these proteins recognized by the immune system
The immune system has time to relearn how to fight the virus
What is Antigenic SHIFT
Rapid, large change in antigens
Occurs when two influenza strains infect the same cell and the genes mix and match during virion assembly
Results in mixing of Hemagglutinin from one strain with Neuraminidase from another strain
When is the best time to destroy the virus
BEFORE it infects the cell
Once viruses enter cells, the immune system must destroy the infected cell in order to
control the infection
What is one major cause of pathology in viral infections
Immune medicated destruction of the body’s own cells
describe how antivirals are used in vet medicine
Very few exist
These target viral enzymes that take part in replication of virus genomes made up of single stranded DNA and double stranded RNA
Describe vaccination in vet medicine
Best defence
Teaching the immune system to find and destroy free virion particles before the infect AND to identify and destroy infected cells
Some vaccines prevent infection, other only prevent clinical signs from occurring and the virus can still spread
What are some other ways veterinary medicine control viral infections besides antivirals and vaccination
Disinfection
Must select appropriate disinfectant
Enveloped viruses are easiest to destroy
Small, non enveloped viruses are more difficult to destroy only certain disinfectants will destroy these (Virox, Accel, Virkon, bleach)
Describe the scale of microbial resistance to chemical disinfectants from most resistant to least resistant
Bacterial spore
Mycobacterium
Hydrophilic viruses (non lipid, non enveloped)
Fungi
Vegetative bacteria
Lipophilic viruses (lipid containing, enveloped)
