UNIT 3 DAY 3 - EVOLUTION OF VIRULENCE

Question 1

traditional argument

Answer 1

pathogens should evolve to be benign (since the longer the host lives, the longer the pathogen can) –> only sometimes: benignity should evolve when the pathogen requires a mobile host in order for the pathogen to spread

Question 2

traditional vs current view of pathogen/parasite evolution

Answer 2

1. selection always maximises selection (dispersal to new host)
2. evolution can be on short time scale
3. selection occurs among parasites/ pathogens within a host

Question 3

virulence

Answer 3

damage done to host by parasite or the host’s parasite-induced loss of fitness

Question 4

within-host virulence vs between-host virulence

Answer 4

within-host selection favours increased virulence, while between-host virulence favours decrease virulence

Question 5

between and within-host selection

Answer 5

• number of strains that infect a host / compete for its resources and the pathogens mode of transmission influence evolution of virulence
  –> when number of strains increase and rapid transmission, increase in 1 strain in virulence is favoured

Question 6

why does within-host selection tend to favour increased virulence?

Answer 6

• host dies as soon as the more virulent strains reach a critical mass
• more virulent strain also suffers because its host dies because it exists in many more copies than its competitor
• less virulent loses out because it exists in fewer copies, it is less likely to reach a new host, dies anyways because the competing strain fails to show the same self-restrain
• more virulent strain of pathogen will become more common because more copies of it are transmitted to the next generation, such that the population evolves increased virulence
• less virulent strain would be favoured only if transmission to new hosts is so difficult and uncommon such that host survival is very important and at least some hosts are not are multiple infected, so that the less virulent strategy has a chance

Question 7

why does between-host selection tend to favour decreased virulence?

Answer 7

• host #1, infected by a less virulent strain of pathogen, lives a long time as does the pathogen
• less virulent pathogen has fewer numbers at any one time, but much more time exists for it to reach another host
• host #2 infected by a more virulent strain of pathogen dies quickly
• this more virulent pathogen has greater numbers at its peak because they have only a short time to reach another host
• less virulent strain of pathogen lives much longer, giving it more time to infect new hosts
• overtime, it becomes more common, such that the population evolves reduced virulence
• more virulent strain would be favoured only if transmission to new host is so easy and frequent such that host survival is not that important

Question 8

2 factors that influence the evolution of virulence

Answer 8

1. the number of strains a pathogen that infects a host and compete for the reason of the host
2. the mode of transmission, which determines whether it is easy or difficult to infect new hosts
   –> increased virulence is favoured by multiple strains competing within a host and ready for rapid transmission among hosts
   –> decreased virulence is favoured when a host is infected by just one strain at a time or when transmission to new hosts is infrequent and difficult

Question 9

example of within-host transmission

Answer 9

malaria –> as it is transferred by mosquitos

Question 10

example of between-host transmission

Answer 10

common cold –> individuals are still well enough to do normal activities so it can spread faster

Question 11

Why are hospitals selected for increased virulence in pathogens?

Answer 11

• large number of patients may be admitted with infectious diseases normally transmitted by personal contact
• people who are severely ill do not move around much, but hospital personnel and equipment move rapidly from such people to others not yet infected
• clean hands, thermometers or eating utensils can be quite effective cultural vectors and transmit diseases may rapidly become more virulent

Question 12

HIV example

Answer 12

if HIV transmission is decreased, becomes less virulent so that hosts can live longer and spread more pathogens

Question 13

COVID example

Answer 13

covid is easily transmissible, not had to sacrifice virulence in order to survive

Question 14

chimera

Answer 14

organisms containing combination of genetically different tissues

Question 15

human microbiome

Answer 15

all microbes that live in/on human tissues

Question 16

human virome

Answer 16

all viruses that live in/on human tissue

Question 17

metagenome

Answer 17

sequencing of genetic material from all environmental samples

Question 18

commensalism

Answer 18

relationship between 2 organisms in which one organism benefits and the other is unaffected

Question 19

mutalism

Answer 19

both species benefit

Question 20

Ledenburg

Answer 20

microbial presence within human body tends to less of an us vs them mentally, many are good/helpful

Question 21

benefits that humans derive from microbes

Answer 21

1. increased metabolism, bacteria help the body harvest more energy from digested food
2. increased immune function, can have anti-inflammatory properties

Question 22

why would microbes be helpful?

Answer 22

• selection for microbes would be helpful so that microbes can be passed down during childbirth

Question 23

transmission by vector selects for high virulence

Answer 23

• no advantage to being nice to host and multiple advantages to being nasty
• hosts survival becomes less important than viruses survival

Question 24

transmission by mobile host selects for low virulence, how does COVID escape this limitation?

Answer 24

• host must remain sufficiently healthy to be mobile
• highly virulent virus with abundant opportunities to reach new hosts before the original host died

Question 25

transmission by host reproduction

Answer 25

• resident gut microbes that are transmitted from mother to child
• selects for zero virulence (commensalism) or even benefitting the host (mutalism)
• microbe has the same interest as host: a host that successfully reproduces
• a gene in a resident gut microbe is perpetuated by the same means as a gene in a liver or kidney cell, by successful host (human body) reproduction
• thus, just as selection favours gene expressed in liver cells that help the body survive and reproduce, selection will also favour genes in the gut microbe that help the host survive and reproduce

Question 26

stone age peoples views

Answer 26

• they were exposed to natural toxins

Question 27

why are plant toxins there

Answer 27

plants can’t protect themselves so toxins keep herbivores from eating plants

Question 28

what is a human adaptation for dealing with plant toxins

Answer 28

• swallowed toxins –> denatured by stomach acid and digestive enzymes, stomach lining protects stomach

Question 29

why do herbivores have a diverse diet

Answer 29

• to avoid overloading any kind of detoxification machinery

Question 30

why do humans eat less greens than deer

Answer 30

• as the human body can’t handle detoxifying as many enzymes

Question 31

selective breeding and toxins

Answer 31

• since agricultural innovation, selectively breeding plants to overcome evolved defences creates new species that are naturally resistant to diseases

Question 32

novel toxins

Answer 32

some extremely different chemically from those which are adapted to cope –> no enzymatic machinery for complex toxins

Question 33

mutagen

Answer 33

chemicals that cause mutations

Question 34

teratogen

Answer 34

chemicals that interfere with normal tissue development and causes birth defects

Question 35

evolutionary hypothesis

Answer 35

kids hate veggies as they contain a lot of plant toxins

Question 36

why is “means of transmission” crucial in determining whether a pathogen evolves greater or lesser virulence?

Answer 36

must take into consideration in a given host of new infections:
- the extent to which these competing pathogens differ in virulence
- the rate of origin of new strains by mutations within a host
- the extent to which these new strains differ in virulence

Question 37

what climate would select for greater or lesser virulence in Malaria

Answer 37

• hot, humid and rainy climates = lesser
• cold,dry = greater