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1

3 stages of virulence evolution

1. Accidental infection
2. Virulence evolution soon after invasion
3. Evolution of optimal virulence

2

Accidental infection

Pathogens cross species boundaries
Mostly low virulent.
This rarely happens.

3

How might pathogens infect novel hosts?

- Infect one, but secondary infections won't happen
- Short-lived, high virulence infection that dies quickly

4

Why might accidental infections be very virulent?

Cross species, so new host as no immune defence against novel pathogen

5

Secondary infections

Infection from host to host

6

What does successful invasion require?

Chain of host to host transmissions

7

Epidemic

Rapid increase in number of infected hosts

8

What happens during stage two of virulence evolution

Epidemic happens
Rapid evolution of pathogen and virulence
Many hosts infected

9

What happens during stage 3 of virulence evolution

Reach trade-off boundary between high & low virulence.
Evolution of pathogen slows, adaptation occurs.
Transmission frequency is optimized.

10

Problem with confirming death due to flu

Death is usually doe to associated illnesses, so difficult to confirm if flu was actually the cause.

11

Once you get one strain of the flu...

You are immune to that strain for life

12

HA

Hemagglutinin protein
Surface protein that allow virus to bind to target cell

13

NA

Surface protein that allow virus to escape from host cell and infect other cells

14

Seasonal flu caused by

Influenza A

15

Pandemics are caused by...

Influenza A

16

Influenza A evolves...

very fast

17

Rapid evolution of the flu occurs at what sites on the virus?

HA and NA sites (recognized by the immune system)

18

Descendent of recent flu strains?

From single ancestor

19

What happens at antigenic sites on HA?

Continuous nucleotide substitutions

20

Where does mutations occur on HA and NA?

Antigenic sites

21

___ plays a strong role in determining influenza evolution

Human immune system

22

What do we use to make vaccines?

The most successful strain of the current year (most number of mutations)

23

How are vaccines tested?

When new strains of the virus appear, they are tested against current vaccines. If it does not work new vaccines must be made.

24

How long does it take to make a vaccine?

6 months

25

How are vaccines made?

Inactivated viruses cultured in eggs

26

How are viruses used for vaccines inactivated?

Damage nucleic acid by chemicals or radiation

27

What type of vaccine is used?

Trivalent vaccine:
- 2 influenza A strains - H1N1, H3N2
- 1 influenza B strain

28

Main point of getting a vaccine

Stop transmission to OTHER people

29

3 types of the flu

1. Seasonal flu
2. Pandemic flu
3. Avian flu

30

Seasonal flu

respiratory illness transmitted host-to-host. Immunity and vaccine available