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Disease Epidemiology > Virulence > Flashcards

Virulence Flashcards

1
Q

Virulence

A

The pathogen-caused reduction in host fitness (reduction in host survival/reproduction)

  • Pathogens differ greatly in their morbidity and mortality. Scale where pathogens are ranked from the least to most dangerous.
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2
Q

High virulence and Low virulence example

A
  • High virulence will be associated with more dangerous pathogens Ex. Ebola
  • Low virulence example: rhinovirus (common cold)
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3
Q

Malaria virulence

A
  • Most important vector-borne disease of humans
  • Virulence of 8
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4
Q

Is there a grand theory of virulence? Why or why not?

A

There is no grand theory of virulence because there is so much variation in pathogens.

Pathogens have varying virulence and all differ in evolutionary history in human hosts and mode of transmission

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5
Q

Variation in virulence among strains of the same pathogen

A

Same pathogen species often consist of multiple strains, and these strains can differ in their virulence.

Ex. Influenza strains- some more deadlier than others

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6
Q

Virulence theory

A
  • Explains differences in virulence among strains rather than differences in virulence among pathogen species
    Ex. H1N1 vs. H5N1 strains of influenza
  • Uses epidemiological models to determine the optimal level of virulence for a pathogen based on trade-offs! Trade-offs include transmission, recovery, and virulence. Not looking at genes
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7
Q

List some pathogen trade-offs

A
  • transmission
  • host recovery from pathogen
  • virulence
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8
Q

What is virulence a trait of?

A

A trait of the pathogen, but measured in the host

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9
Q

SIR models including virulence

A

Need to include virulence’s impact on mortality rate (mu + alpha).

Alpha= pathogen-induced mortality from virulence

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10
Q

Avirulence theory

A

The host is the environment of the parasite and a good parasite should not kills its host. In new host-parasite interactions, virulence is high and over time the parasite and host will adapt so that virulence decreases

However, many endemic infectious diseases have not evolved to become less deadly

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11
Q

What trade-offs are included when looking at modern virulence theory?

A
  • Transmission
  • Recovery/clearance
  • Virulence
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12
Q

What does a pathogen need to do to maximize R0?

A
  • Maximize transmission (beta)
  • Minimize virulence (alpha)
  • Minimize rate of recovery (v)
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13
Q

Virulence-transmission trade off

A

Reduction in host survival (high virulence) is an unavoidable consequence of pathogen replication within the host. Therefore a negative association between transmission and host survival (evolutionary trade-off)

  • Pathogens with low abundance in host tissues have long duration of infection but low transmission whereas pathogens with high abundance will have high transmission but host will have short lifetime
  • Pathogen needs to balance virulence and abundance in host so that transmission success is maximized over lifetime of infection
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14
Q

Max R0 and virulence

A
  • Max R0 occurs at an intermediate value of virulence
  • There will be no intermediate optimum with accelerating or linear relationships
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15
Q

HIV and virulence-transmission trade off

A

Infectious stage lasts from 2-15 yrs,but allows for high levels of transmission because infected are healthy enough to still pass pathogen on to new hosts.

  • Variation in infectious stage length is due to variation in the set-point viral load where a higher number means a higher transmission but it is more virulent and host will die sooner

-Highest lifetime transmission occurs when intermediate set-point viral load (SPVL)

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16
Q

Set-point viral load (SPVL)

A

Virus persists at stable abundance in the host
- Affects transmission and duration of infectious stage

17
Q

Lifetime transmission potential (LTP)

A

The number of people one infected person could infect over the duration of infectious period

Transmission rate x duration of infectious period= LTP

18
Q

Set-point viral load (SPVL) and lifetime transmission potential (LTP) (using HIV example)

A

An intermediate SPVL will result in the highest LTP

  • Low SPVL has lower LTP because transmission rate per sex act is low
  • High SPVL has lower LTP because duration of infectious period is too short
19
Q

Virulence-clearance trade off

A

Trade off between the virulence of pathogen and the clearance rate (or the recovery rate)
- Need virulence for transmission, but not too much that it results in death
- Don’t want too little virulence where clearance (recovery) occurs very quickly

Mediated by pathogen abundance in host tissues

20
Q

Clearance rate

A
  • The rate at which host immune system clears infection
  • Avirulent strain will have high clearance rate and low transmission
21
Q

Virulence-clearance trade-off in MYXV strains

A
  • 6 strains of MYXV in rabbits

Negative relationship between virulence and clearance rate (showing trade-off present)
- Need to have intermediate virulence that will result in highest fitness for parasite where host will be virulent enough to transmit pathogen but not too virulent that the host dies

22
Q

MYXV & a decrease in virulence

A

Introduction of MYXV in rabbits in Australia, resulted in huge population decrease which lead to a change in optimal virulence and clearance of pathogen

Due to a lower population density meaning it was more difficult for infected to transmit the pathogen which lead to the selection of MYXV strains that were less virulent with a longer infectious period and more chance to run into susceptibles.

At lower host density, early death due to virulence was a bigger problem than early death due to clearance so less virulent strains were selected for.

23
Q

Why did virulence of MYXV increase during study of pathogen introduction in Australia?

A

MYXV caused strong selection in rabbit population. The rabbit population selected for MYXV resistance, causing rabbit immune system to increase clearance of the pathogen and the virulence of the pathogen to decrease in the population.

Then MYXV needed to evolve higher virulence to avoid this clearance

Therefore host resistance selects for higher pathogen virulence

24
Q

MYXV & avirulence theory

A

Contradicts theory because although during a period of time the pathogen showed a decrease in virulence, there was also periods of increased virulence depending on host clearance ability

Disease Epidemiology (9 decks)

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