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“Host Longevity Matters” investigates how the rema “Host Longevity Matters” investigates how the remaining lifespan of a host influences the basic reproduction number (R₀) of infectious diseases. Unlike traditional epidemiological models—which often assume infinite infectious duration or ignore host lifespan—the authors show that R₀ is deeply shaped by host longevity, especially for long-lasting infections.
The study combines two powerful components:
A within-host model capturing pathogen replication, mutation, immune response, and resource dynamics.
A between-host transmission model capturing contact structure, secondary infections, and network effects.
By integrating both layers, the paper explores how pathogen evolution depends on two internal parameters:
Replication rate (ρ)
Successful mutation probability (δ)
and one external ecological parameter:
Host contact rate (α)
The goal is to determine which pathogen strategy maximizes R₀ under different host lifespans.
🔍 Core Insight
Pathogens evolve toward one of two fundamental strategies:
1. Killer-like Strategy
Fast replication
Intermediate mutation rates
High pathogen load
Short, intense infections
Favors rapid spread when:
Host lifespan is short, OR
Host contact rates are low
2. Milker-like Strategy
Slow replication
High mutation rates
Low, sustained pathogen load
Long infection duration
Favors persistence when:
Host lifespan is long, AND/OR
Contact rates are high
The study demonstrates a sharp transition between these strategies depending on the combination of:
Host longevity (Dmax)
Contact rate (α)
This yields a bifurcation line separating killer-like from milker-like evolutionary optima.
📈 Key Findings
1. Host Longevity Strongly Shapes R₀
For short-lived hosts (e.g., insects), R₀ increases roughly linearly with contact rate.
For long-lived hosts (e.g., humans), R₀ rapidly reaches a plateau even with moderate contact.
The impact of longevity is large enough to change evolutionary conclusions from previous models.
2. Strategy Switch Depends on Contact Rate
There exists a critical contact rate αₙ, where pathogens switch from:
Killer strategy (fast replication)
to Milker strategy (slow replication)
The value of αₙ shifts strongly with host lifespan.
3. Above a Certain Longevity Threshold, Only Milker Strategy Is Optimal
For very long-lived hosts:
Killer-like strategies disappear entirely.
Pathogens evolve toward mild, persistent infections.
This explains why many long-standing human diseases show long-duration, low-virulence dynamics.
4. Zoonotic Diseases Are Exceptions
Because they originate from short-lived animals, zoonoses (e.g., avian influenza, Ebola) are often:
Highly virulent
Fast-replicating
Short-lasting (killer-like)
This aligns with the model’s predictions.
🧠 Implications
For Evolutionary Epidemiology
Host longevity must be included when predicting pathogen evolution.
Long-lived species tend to select for milder, persistent pathogens.
For Public Health
Models ignoring host lifespan may misestimate epidemic thresholds.
When evaluating disease control strategies, lifespan restriction (e.g., culling, selective breeding) can alter pathogen evolution.
For Theory
This model is among the first to show that R₀ is not purely a pathogen trait, but emerges from interaction between:
Host immune dynamics
Lifespan constraints
Contact structures
Pathogen mutation and replication
🧭 In Summary
“Host Longevity Matters” shows that the lifespan of a host is a critical, previously overlooked determinant of pathogen fitness and evolution.
Long-lived hosts push pathogens toward slow, stealthy, “milker-like” behavior.
Short-lived hosts favor fast, damaging “killer-like” pathogens.
This work demonstrates that R₀, infection strategy, and pathogen evolution are inseparable from host longevity.... |