pathogen virulence: evolutionary ecology outline: 29 jan 15 functionally dependent life-history...
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Pathogen Virulence: Evolutionary ecology
Outline: 29 Jan 15
• Functionally Dependent Life-History Traits:
Virulence Important Example
• Pathogen Traits Evolve via Strain Competition
• Spatially Structured Transmission
Dispersal Limitation Reduces Virulence
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Virulence
Property of Host-Parasite Interaction
Parasite Generation Time Much Shorter
Virulence: Parasite’s “Strategy” for Exploiting Host
Virulence Evolution Affects Correlated Demographic Traits
Functional Dependence = Pleiotropic Interaction
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Increased Parasite Virulence
Faster Consumption of Host Resources
(1) Pathogen Reproductive Rate Increases
(2a) Host’s Mortality Rate Increases
or
(2b) Rate of Clearance by Immune System Increases
or
(2c) Host Reproduction Decreases
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Virulence Trade-Off
Antagonistic Pleiotropy
Pathogen Increases Propagule Production (Hence, Infection Transmission) Rate
Duration of Infectious Period Decreases
Evidence Reviewed
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How Does Virulence Evolve?
Pathogen-Stain Competition
2 Phenotypes Differ in Virulence (Resident, Mutant)
Compete Between (and) Within Hosts
3 Modes of Strain Competition
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Pathogen-Strain Competition
1. Cross-Reactive Immunity
Competition Strictly Between-Host Scale
Flu strains
2. Coinfection: Two Strains Exploit Same Host Individual
Compete Both Within & Between-Host
3. Superinfection: More Virulent Strain Excludes Other
Compete Both Within & Between-Host
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Strain Competition
Important Ecological Generality
Cross-Reactive Immunity
Example of
Pre-emptive Competition
Two Species (Strains),
Same Niche
(Allstadt et al. 2009)
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Strain Competition
Important Ecological Generality
Coinfection: Example of
Scramble Competition =
Exploitative Competition
Two Species Interact Indirectly Through Exploitation of Same Limiting Resource
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From quizlet.com
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Strain Competition
Important Ecological Generality
Superinfection: Example of
Interference Competition
Two Species Interact Directly Aggressive Exploitation ofSame Limiting Resource
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quizlet.com
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Strain Competition:Adaptive Dynamics
Host-Pathogen Dynamics
Exert Selection Pressure on Competing Strains
Mutant-Resident Competition
Competitive Exclusion; Alter Parameters of Dynamics
Evolutionarily Stable Strategy (ESS) Resists Invasion
Adaptive Dynamics: Interplay of Ecology, Evolution
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Strain Competition:Adaptive Dynamics
“Solve” Strain Competition for a Preemptive Case
General: ESS Virulence Graphically
Virulence Evolution in a Second Preemptive Case
Pathogen with Free-Living Stage
e.g., Bacteriophage
Superinfection, Vary Pathogen Dispersal Distance
Impact on ESS Virulence
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Cross-reactive Immunity
One Strain per Infected Host Individual
Strain Competition: Between-Host Scale Only
Ecology: Preemptive Competition
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Host Preemption
Assume Homogeneous Mixing Host Population
“Optimally Virulent” Strain, Max R0
Equivalently
Minimizes Equilibrium Density Susceptible Hosts
No Strain Coexistence (Pure ESS)
Recall: Same Niche13
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Host Preemption
Homogeneous Mixing, No Recovery
Transmission-Infectious Period Trade-off
() Transmission Efficiency, Direct Contact
() Virulence, Extra Infected-Host Mortality
Host Exploitation Strategy: d/d > 014
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Natural Selection: Optimize
Invasion Dynamics (Conceptual Core)
Can Rare Mutant Invade Resident * at ecological (dynamic) equilibrium?
This case: ESS does Max R0( )
: Background Host Mortality
S: Susceptible Density15
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Natural Selection: Optimize
SI Transmission
Plus Host Birth, Death
Resident Pathogen’s Dynamics Sets Resource Availability (Susceptible Density) for Mutant Strain of Pathogen
Can Mutant find enough hosts to grow when rare?
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Natural Selection: Optimize
b Per-capitum Birth
b Transmission Rate (Mass Action)
Non-Disease Mortality (All)
( + ) Infective Mortality
: Virulence > 0
No Recovery from Infection
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Dynamics of Epidemic
=
𝑑 𝐼 𝑡𝑑𝑡
=𝛽𝑆𝑡 𝐼 𝑡− (𝜇+𝛼 ) 𝐼 𝑡
Birth, Infection Transmission, Death
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Analysis
: New Cases/Case When Invading Pathogen Rare
Epidemiology: Invade All-Susceptible Population
Evolutionary Ecology: Invade Host-Resident Strain at Endemic Equilibrium
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Natural Selection: Optimize
Transmission Rate: Infections/Time =
Transmission Duration: Time =
Transmission Ends at Host Death
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𝑅0(𝑀𝑢𝑡𝑎𝑛𝑡 ,𝑅𝑒𝑠𝑖𝑑𝑒𝑛𝑡)=𝛽𝑚𝑆𝑟𝑒𝑠
(𝜇+𝛼𝑚)
Mutant Invades:
Recall: among strains
Note: ; Background Mortality & Virulence
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Natural Selection: Optimize
1,;
1,
*0
*
0
RifAdvancesInvaderRare
RmEquilibriuEndemic
SR
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Natural Selection: Optimize
InvaderforDensityeSusceptiblSets
SR
*
**0 ,
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Natural Selection: Optimize von Baalen & Sabelis (1995, Am Nat)
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Natural Selection: Optimize
1. ESS Virulence Maximizes R0 (for any Susceptible Density)
2. ESS Virulence Minimizes Susceptible Density
Too Few Susceptible Hosts for Mutant Invasion
3. Greater Background Mortality Greater Virulence
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Natural Selection: Optimize
4. ESS May Exhibit Intermediate Virulence
Under Host Preemption; Natural Diversity
5. No Strain-Coexistence Possible
Under Well-mixed, Preemptive Competition
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Preemptive Host Competition
Pathogen with Free-Living Stage
Life History:
Alternates Intra-Host Environment, External Environment
Bacteria/Viruses, Including Bacteriophage
“Curse of the pharaoh”
Persistent free-living stage costly; Requires conversion of large amount of host resources; Pathogens with persistent free-living stage likely virulent
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Host-Pathogen Dynamics
S(t) Susceptible Density
I(t) Infectious Density
P(t) Free-living Stage
(Virions, Spores)r Host Reproduction
c Host Self-Regulationa Transmission (Adsorption)q Mortality; Includes Virulence
g FLP Shed Rateb FLP Burst Size
FLP Decay Rate: Focus
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Host-Pathogen Dynamics
Equilibria
Endemic Equilibrium
(Extinction Unstable)
Disease Free: ()
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Local Stability: FLP Persistence =
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ESS Virulence: Pathogen Strain Competition
Preemptive Competition: ESS Minimizes S*
Positive Equilibrium Density of Susceptibles
Traits: Functionally DependentAltering Virulence: Antagonistic Pleiotropy
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ESS Virulence: Pathogen Strain Competition
Curse of the Pharaoh: Increased persistence of FLP
(reduced ) demands more host resources,
and virulence () increases.
Equivalently:
Functional Constraint: ;
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Functional Constraint: Virulence(Decay Rate)
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Minimize S*
Suppose Shed Rate = 0; Burst Size > 0
Lytic Virus, Bacteriophage
Then = Decay/(Adsorption x Burst size)
ESS Reduces and Increases Virulence
Virulent and Persistent
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Minimize S*
Suppose Shed Rate > 0; Burst Size = 0
Animal Virus; Bacterial, Fungal Infection
Then
For : Strain Competition Reduces Decay Rate
Virulent and Persistent
For : Competition Favors Intermediate Virulence
; Curse Broken
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Shed Rate > 0 and Burst Size > 0
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Preemptive Host Competition
Strain Minimizing Equilibrium Density of Susceptibles Should be ESS
No Coexistence of Different Levels of Virulence
(Not True for Coinfection and Superinfection)
Curse of the Pharaoh Oversimplifies
Strain Competition
Caraco annd Wang (2008) J Theor Biol 250:569-579
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Homogeneous Mixing Host Population
Assumed in Dynamics
Full Mixing: Hosts Highly Mobile over Timescale of Expected Lifespan
Might Preclude Terrestrial Plants, Territorial Animals, etc.: “Viscous Populations”
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Contact Structure, Van Baalen (2000)
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Pathogen: Dispersal Limitation
Contact Structures: Constrain Opportunities for Pathogen to Generate New Infections
Ecology: Dispersal Limitation, Neighborhood Interactions
Ecological Implications:
Epidemic Invasion, Endemic Infection Levels
Evolutionary Implications:
(Including) Virulence
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Pathogen: Dispersal Limitation
Contact Structure: (L x L) Lattice
Each Site: One of 4 Elementary States
Local Neighborhood: All Ecological Interactions
• Opportunities for Host Reproduction (Open Sites)• Sources of Infection
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SPATIAL SUPERINFECTION
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SPATIAL SUPERINFECTION
Virulent Can Displace “Avirulent” Strain
Interference Competition
Discrete-Time Dynamics
Transmission (Virulence); No Recovery
Key: Superinfection (Virulence Difference)
Within & Between-Host Competition
Neighborhood Size: 8, 48
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Develop Concepts
1. Mean-Field Analysis: Homogeneous Mixing
2. Pair Approximation: Local Correlation
3. Simulate Full Stochastic Spatial Model:
Large-Scale Correlated Fluctuations,
Strong Clustering Possible
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Develop Theory: Deduce Predictions
Pairwise Invasion Analyses: Adaptive Dynamics
Resident Strain at Ecological Equilibrium
Can Invading Strain (Mutant) Advance?
Assumed Time Scales
Convergence Stability; Evolutionary Stability
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SPATIAL SUPERINFECTION
Dynamics: Local Transition Probabilities
Stochastic Spatial Model
How do local interactions produce ensemble effects (population, community scales)?
Model/Theory: Caraco et al. (2006)
Theoretical Population Biology 69:367-384
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Mean-Field Results
Pairwise InvasionHomogeneous Mixing
Evolution to
Criticality
Coexistence:
Niche Difference
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Mean-Field Results
Pairwise InvasionHomogeneous Mixing
Coexistence:
Niche Difference
Competition-Colonization
Trade-Off
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Spatial Model Results
Increased Virulence
Decreased Infection
Increased Clustering
Pair Correlation Model OK
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Adaptive dynamics spatial process
Pair Approximation
Convergent Stable
Evolutionarily Stable
(Local ESS)
Virulence Constrained
By Contact Structure
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Adaptive dynamics spatial process
Simulation
Max Virulence Lower
Local ESS Reduced
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Adaptive dynamics spatial process
Weaker Competitive Asymmetry Via
Superinfection
Reduce ESS
Reduce Coexistence
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predict
1. Spatial Structure Constrains Maximal Virulence
Capable of Dynamic Persistence, Through
Extinction of Highly Virulent Strains
2. Spatial Structure Reduces Evolutionarily Stable
Level of Virulence
3. Larger Neighborhood Relaxes Constraint,
Dynamic Penalty of Clustering Attenuated
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predict
4. Spatial Structure Promotes Coexistence:
Extended Transmission/Low Virulence,
Poor Interference Competitor/Good Colonizer
and
Attenuated Transmission/High Virulence,
Advantage of Superinfection/Poor Colonizer
5. Coexistence Increases with Neighborhood Size
6. Comp. Asymmetry Increases Coexistence
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Contemporary Questions
Virulence in Pathogens with Both Contact and Environmental Transmission
Avian Flu: Contacts; Virus Persists In Drinking Water
Hyperparasites & Hypovirulence
Vertical Transmission
Sterilizing vs Killing Pathogens
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Contemporary Questions
Vector-Borne More Virulent Than Direct Contact (?)
FLP: “Curse of the Pharaoh”
Conditions for More Virulence
Infective Dose: Remarkable Variation
Ecological Consequences
Strain Competition?
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Contemporary Questions
Within-Host DynamicsParasite, Specific Immune Cell DensitiesAffects Between-Host TransmissionPopulation Dynamics
Host-Pathogen CoevolutionTransmission Resistance, ToleranceVirulence, Optimal Immune Response
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