Viruses and Viral pathogens of humans

Question 1

Define a pathogen

Answer 1

An organism that can cause disease (damage) in a host

Question 2

Define pathogenicity

Answer 2

The ability to cause disease (Damage)

Question 3

Define virulence

Answer 3

A relative measure of disease causing potential

Question 4

Define Virulence factor

Answer 4

A pathogen derived substance or structure that contributes to virulence/damage

Question 5

Why are Viruses not cells?

Answer 5

–They do not grow and divide

–They do not have cell walls, a nucleus or any other organelles seen in cells

–They are incapable of cellular metabolic activity

•They must hijack a host cells metabolic and synthetic machinery to make more virus

–Therefore, they cannot survive without a host and so are obligate parasites

Question 6

Describe the general structure of a virus

Answer 6

–Genome (nucleic acid core)

•Contains the genetic information as ds/ss - RNA or DNA

– Capsid
•Protein shell for genome

• Gives protection during transmission
• Target for host immune response
• Made up of protein sub-units called CAPSOMERS

–Envelope (viral membrane)

•Not present in all viruses

•Contains host membrane components
–CSM or nuclear membrane

•Contains viral proteins
–Viral spikes proteins carrying receptors

•May have other proteins between envelope and capsid
–Tegument
–Matrix proteins

Question 7

What is a completed virus?

Answer 7

This is known as Virion

Question 8

Explain the known shapes of viruses

Answer 8

–Virus size: Typically 10 - 400nm

–Majority of viruses rod or sphere shaped

–Rods are said to have helical symmetry and spheres have icosohedral symmetry

–Can be a combination of both and have complex symmetry.

Question 9

Explain the known structure of the Helical capsids

Answer 9

–These are assembled from rings of protomeres

–This type of symmetry enables very stable rods to be assembled

-Straight and flexible rods formed

Question 10

What are complex viruses?

Answer 10

–Helical or icosohedral or neither

–May possess additional structures

•Pox viruses
–Small pox
–White pox

Question 11

Explain the role of viral membranes in viruses

Answer 11

•May combat immune system

–May contain host or mimic host proteins
–Proteins for avoidance – protein A

•In enveloped viruses receptors are often on viral proteins = spikes

–Enable interactions with host cells
–Specific interactions = Tropism

•Allow entry of capsid and genome release

–Fusion with cell membrane
–Fusion with membrane of endosomes

Question 12

What are the main aims for virus?

Answer 12

–To infect host cells

–Make more virus

–Infect more cell

–Be released

Question 13

What must a virus do to infect the body?

Answer 13

the body virus must first reach a suitable entry site or portal of entry

Question 14

List the different portals of entry for a virus

Answer 14

–Mucous membranes
Respiratory tract
Gastrointestinal tract(GI)
Urinogenital tract
Conjunctiva (Eye)

-Skin
Abrasions
Vectors
Iatrogenic infection.

Question 15

How do viruses bind to permissive cells?

Answer 15

Using receptors

Question 16

What is a permissive cell?

Answer 16

Allows replication when the virus binds to its receptors

Question 17

Define a primary pathogen

Answer 17

They make you ill despite of your immune system

Question 18

How do viruses enter the cell?

Answer 18

• Direct penetration – naked (non-enveloped) viruses
• Fusion with the cell membrane – enveloped viruses
• Entry via ENDOCYTOSIS or engulfment– enveloped viruses

Question 19

Penetration and uncoating is essential in viral infection of cells as it allows the ?

Answer 19

release of the viral genome and starts the replication cycle

Question 20

What is the main difference between the Lytic cycle and Lysogenic cycle?

Answer 20

The lytic cycle includes the virus using the cell to duplicate hundreds of times before bursting

While the Lysogenic cycle includes an integration of the virus DNA into the cells DNA and will stay this way until the environment is suitable.

Question 21

Explain the two different ways the virus can exit the cell

Answer 21

–Direct lysis (bursting) of the infected cell

•Often naked viruses
•Most enteroviruses do this - Polio v
–Exit via enzymatic disruption inner cytoplasmic membrane

–Budding through/from the cell membrane or nuclear membrane
•Enveloped viruses
•HIV, HSV, influenza
–Both part of lytic life cycle

Question 22

Explain infection and replication for viruses

Answer 22

–Replication in susceptible (permissive) cells at site of entry

–Overcome local innate defences
•Modulate immunity, colonise immune cells

–Spread from site of inoculation – viraemia

–Undergo further replication in target organ

–Escape from host in large enough numbers to infect new host
•Fecal, respiratory secretions, urine, semen
•Exit site not always linked to entry

Question 23

What are the consequences of infection?

Answer 23

–Symptoms of viral infection are not caused by a secreted product - no metabolites!

Symptoms of disease often caused by
•Deregulation of cellular function
•Direct damage to cells from viral replication
•Host immune response to infection
–Stimulants of immune responses produced – cytokines
–Interferon - γ (gamma)

–A change in cellular characteristics may occur - CPE

Question 24

Explain Recovery from viral infections

Answer 24

–Destruction of virus infected cells
•Innate and specific immune responses

–Immune response directed towards capsid/binding proteins – important viral antigens

–Immune response can prevent re-infection

• Often targets for vaccine production – influenza vaccine- formulated from strain specific H & N antigens
• If receptors blocked cant bind and infect cells–Neutralising antibodies

Question 25

What causes Polio virus?

Answer 25

Poliomyelitis from the family Picornaviridae

Genus: Enterovirus
Containing RNA and naked

It is transmitted from human to human through faecal oral route

It takes 3 to 21 days to incubate

Question 26

What does the Polio virus cause?

Answer 26

Polio multiplies in tonsins or Peyer’s patches then local lymph nodes

–Passes into blood

–Attacks CNS if no antibodies

–All enteroviruses are lytic, destroy host cells

–Can result in limb wasting, paralysis and death

Question 27

What causes Yellow fever?

Answer 27

–Family: Alphavirus

–Genus: Faviviridae

–Is an example of an ARBOVIRUS

–RNA & spherical capsid

–Transmission: Human to human via mosquito vector.

–Incubation: 3 – 6 days

Question 28

What does Yellow fever cause?

Answer 28

–Spreads from the lymph nodes and damages liver to cause jaundice (hence name).

–Also infects spleen kidney and heart

.–Symptoms include, fever chills, neuralgia, back pain and vomiting.

–Can cause in haemorrhaging in infected organs.

–Infection may lead to death