Lecture 22

Question 1

Ebola virus

Answer 1

• Causes ebola hemorrhagic fever
• Spread by contact with bodily fluids
• High fever, headache, stomach, and chest pain, vomiting, and severe internal bleeding

Question 2

Virulence

Answer 2

• Disease severity as assessed by reductions in host fitness due to infection
• Interpreted as the additional mortality rate that a pathogen imposes on an infected individual

Question 3

Conventional Wisdom

Answer 3

• Claim: Pathogens that harm their hosts thereby harm themselves
• Predictions: Over time, coevolution of pathogens and their host will lead to a mutualistic association. Instances of highly virulent pathogens are cases where the host-pathogen relationship is recent

Question 4

H5N1 Bird Flu

Answer 4

• Nearly every case involves close contact with infected birds

Question 5

Challenges to conventional wisdom

Answer 5

Tuberculosis
- Around since ancient Egyptian times, maybe 3000-4000 BCE
- Still quite virulent

Myxoma Virus
The virus causes disease called myxomatosis. Early after release, disease was highly virulent

Question 6

What is fitness for a parasite

Answer 6

• R0: The # of new infections generated by a single infected individual
• Depends on things like replication or growth within a host
• Opportunities for transmission between hosts

Question 7

Tradeoff hypothesis

Answer 7

• Level of virulence is a consequence of pathogen replication in host -> cost of replication
• Pathogen replication is required for transmission between hosts -> benefit of replication
• Replication rate of pathogens evolves to maximize overall transmission and spread

Question 8

Simple model for virulence evolution

Answer 8

• Pathogens maximize # of new infections R0
• R0 = T(r) x D(r)
• Tradeoff between transmission and virulence
• Increased replication rate (r) increases transmission rate, but decreases duration of infection (increases virulence)

Question 9

Optimizing virulence

Answer 9

• The most virulent strains of a pathogen will have high transmission rates, but kill host quickly
• The least virulent strains of a pathogen may lead to long infections, but transmit poorly from host
• Prediction: strains of intermediate virulence will increase in frequency

Question 10

Modes of disease transmission

Answer 10

• Direct host to host transmission: Hosts must be mobile and functioning relatively well in order to transmit disease
  
  • Prediction: Low replication rate and therefore low virulence
• Vector transmission: Hosts do not need to be mobie in order to transmit disease
  
  • Prediction: Higher replication rate and therefore higher virulence

Question 11

Cholera

Answer 11

• Caused by bacterium Vibrio cholerae
• Infection results in severe diarrhea and dehydration
• Outbreaks still occurring in some regions
• Disease is transmitted through contaminated water

Question 12

Can we influence evolution of virulence

Answer 12

• Expect pathogens will evolve to be highly virulent when transmission between hosts is easy
• Make transmission harder: Provide uncontaminated drinking water, increase basic cleanliness, remove vectors, isolate patients

Question 13

Graphical argument for virulence evolution

Answer 13

• Virulence may be unavoidable consequence of parasite transmission
• Parasites need to exploit their host in order to get transmitted to new hosts
• An increase in transmission comes with the cost of an increase in virulence

Question 14

Predicting optimal virulence

Answer 14

• Benefit - Cost is maximized at intermediate levels of virulence
• At low replication rates, MB > MC
• At high replication rates, MB < MC
• At intermediate replication rate, MB = MC

Question 15

Relative transmission is what matters

Answer 15

• A host can be infected by several parasite strains because of multiple infections or rapid within host evolution
• If strains compete for same resource, there is tragedy of the commons
• Strains that exploit host more will gain a bigger share of host resources, outcompeting other strains and transmitting at higher rate
• Even though overall transmission from host may be lower, the more virulent strain has higher relative transmission
• Theory predicts multiple infections select for increasinf virulence

Question 16

Multiple infections select for higher virulence

Answer 16

• More virulent (Host is more sick, more anemic)
• Less virulent (Host is less sick, less anemic)
• The more virulent strains do better in competition (multiple infections)