Patterns of viral infection

Question 1

Define tropism.

Answer 1

The predilection of viruses to infect some tissues and not others

Question 2

What three factors are tropisms based on?

Answer 2

Susceptibility – receptor interactions

Permissivity – ability to use the host cell to complete replication

Accessibility – ability of the virus to reach the tissue

Question 3

Describe the type of tropism exhibited by HIV.

Answer 3

GP120 on HIV interacts with primary receptor CD4 on the T cell

It attaches to co-receptors CCR5 and CXCR4

Question 4

Describe the type of tropism exhibited by Measles.

Answer 4

• SLAM and nectin 4 are used
• entry to new host - slam on the immune cells
• exit from infected host - nectin 4 on the airway epithelia
• when the virus is infecting someone for the first time measles enters via the resp tract
• Virus binds to SLAM on immune cells
• they hijack a ride to the lymph nodes
• here the virus sees more cells expressing SLAM which it then infects
• When measles wants to leave the host and move on to another host, it uses NECTIN 4
• Virus is carried from the lymph nodes to the lungs
• on the nectin 4 it can travel from the blood to the airspace
• once the virus has entered the airspace it replicates via a resp route

Question 5

How can influenza tropism be changed?

Answer 5

• Influenza is enveloped in haemagglutinin (HA)
• HA helps influenza latch onto the surface of cells
• HA binds to sialic acid which is everywhere
• so technically influenza can attack everywhere but it tends to go for resp because of accessibility
• Once bound onto sialic acid, it enters the cell via the endosome
• Low endosomal pH allows a massive conformational change
• the protein of the virus needs to be cut into two at a particular point
• the correct proteases for the snipping are only present in the lungs
• viruses move around until they come into contact with the necessary proteases which can chop the HA into two pieces.
• then the virus fuses with the membrane and uncoats

Question 6

At what point does Varicella-Zoster infection become symptomatic?

Answer 6

After the secondary viraemia

Question 7

What is the classic triad of Rubella?

Answer 7

Eye Abnormalities (e.g. cataracts)
Deafness
Congenital Heart Disease

Question 8

Describe the latency of Herpes simplex virus.

Answer 8

Herpes simplex virus can remain latent on nerve cells and then become activated in an attack of shingles

The virus then moves end of the neurones and replicates through the skin (forming a cold sore)

Question 9

Give four examples of oncogenic viruses and the types of cancer that they cause.

Answer 9

HHV8 – Kaposi Sarcoma
HTLV-1 – Leukaemia
Merkel Cell Polyoma Virus – Merkel Cell Polyoma
HPV – Cervical Cancer

Question 10

What type of cancer do Hepatitis B and C cause?

Answer 10

Hepatocellular Carcinoma

Question 11

What types of cancer can Epstein-Barr Virus cause?

Answer 11

95% of us are infected with

Epstein-Barr Virus
Burkitt’s Lymphoma
Hodgkin’s Lymphoma
Nasopharyngeal Carcinoma

Question 12

Give an example of viral load affecting the outcome of an infection.

Answer 12

Second child in a household infected with chicken pox will have a worse infection because they’ll get a large dose from the first child

Question 13

Give an example of co-infections affecting the outcome of an infection.

Answer 13

• HHV8 can cause Kaposi Sarcoma in people who are infected with HIV and hence are immunocompromised
• Hepatitis Delta Virus only infects people with Hepatitis B

Question 14

Give an example of genetic resistance to a viral disease.

Answer 14

Delta 32 mutation causes the formation of a different CCR5 receptor and so these people are naturally resistant to HIV

Question 15

who might be immune to HIV?

Answer 15

• mutation in CCR5 receptor
  which means they don’t make a proper
  CCR5 receptor and hence are resistant to HIV

-produce large amounts of chemokine which means the co- receptors are blocked

Question 16

what is the influenza tropism determined by?

how have mutations influences the HA cleavage site?

Answer 16

• determined by the availability of host proteases

- some viruses have mutated so other proteases in other parts of the body can now do the snipping

Question 17

what does the outcome of the virus infection depend on?

Answer 17

• virulence and host response

Question 18

what does 
iatrogenic - 
Nosocomial - 
vertical - 
horizontal - 
germ line -

Answer 18

• iatrogenic - due to medical care
• Nosocomial - acquired in hospital
• vertical - from parent to offspring
• horizontal - all other forms
• germ line - part of the host genome

Question 19

how does the chicken pox virus work?

varicella zoster virus

Answer 19

• virus comes in through resp
• Varicella Zoster Virus can infect many cell types including peripheral blood mononuclear cells and skin cells
• can infect sensory neurones where it remains latent
• Virus can be reactivated when cellular immunity is
  impaired causing a painful rash at nerve endings -
  SHINGLES

Question 20

example of acute infection and clearance ?

example of acute infection and death?

Answer 20

• colds and influenza
• Adaptive immune response provides immunity
• Smallpox and Dengue

Question 21

how might a virus cause cancer?

Answer 21

• may code for an oncogene
• ## interferes with the host cell cycle to enhance their own replication

Question 22

how might strains of a virus differ?

Answer 22

• there are two strains of polio
• one mutation might make it into a live attenuated virus
• another mutation might invade motor neurones and cause complete paralysis