Lecture 9 - Evolution of virulence

What is virulence?

A parasite’s impact on host fitness through increased mortality or decreased fecundity.

What does natural selection act on in parasites?

Parasite fitness, including survivorship and fecundity, not virulence directly.

Why is it incorrect that parasites always evolve to become harmless?

Evolution favors traits that increase parasite fitness; reduced virulence is not always advantageous.

Why can within-host competition increase virulence?

Competing parasites are selected to reproduce more within hosts, which can increase host damage.

What is coinfection, and why does it matter?

A host is infected by multiple parasites simultaneously, allowing within-host competition.

How can increased virulence benefit a parasite?

It may increase transmission, helping the parasite reach more hosts.

How can virulence increase transmission in trophic transmission?

An infected intermediate host becomes easier for its predator, the definitive host, to catch.

How might lethargy benefit a vector-transmitted parasite?

A less active host may allow vectors, such as mosquitoes, to feed more easily.

How does rabies illustrate a transmission benefit of virulence?

Rabies in saliva can cause aggressive biting, helping transmit the virus.

What is the cost of killing a host too quickly?

The parasite may lose opportunities to transmit to susceptible hosts.

What is the trade-off model of virulence?

Virulence provides transmission benefits but also imposes costs; selection can favor an intermediate level.

In R₀ = βN/(α + d + ν), what does β represent?

Transmission rate.

In the R₀ equation, what do α, d, and ν represent?

Disease-induced mortality, natural death, and recovery rates, respectively.

If β increases while everything else stays constant, what happens to R₀?

R₀ increases, potentially favoring the parasite.

If disease-induced mortality α increases while everything else stays constant, what happens to R₀?

R₀ decreases because hosts die sooner, shortening the opportunity for transmission.

Why can both very low and very high virulence be disadvantageous?

Low virulence may mean insufficient transmission; high virulence may kill hosts too quickly.

Why are vertically transmitted parasites predicted to be less virulent?

Their transmission depends on host reproductive success, so host damage directly reduces parasite transmission.

What does a single-foundress fig-wasp brood indicate?

One foundress supplies the nematodes and produces the wasp offspring, indicating vertical transmission.

In the fig-wasp study, what does a larger proportion of single-foundress broods suggest?

Greater opportunity for vertical transmission.

In the fig-wasp study, how was virulence estimated?

As 1 − V, where V is infected wasps’ lifetime reproductive success relative to uninfected wasps.

What relationship did the fig-wasp nematode study support?

Greater vertical transmission was associated with lower virulence.

Why did myxoma virus cause different disease severity in different rabbit species?

It caused mild tumors in South American rabbits but was highly lethal in European rabbits.

How was myxoma virus transmitted among rabbits in Australia?

Mechanically by fleas, mosquitoes, and other insects carrying virus on their mouthparts.

Why was myxoma virus introduced into Australia in 1950?

As biological control for introduced rabbits that had become major agricultural pests.

What happened after myxoma virus was introduced into Australia?

It initially caused extremely high rabbit mortality, but lower-virulence strains evolved.

How can host evolution affect parasite virulence?

Host resistance can change selection on parasites and may contribute to selection for greater virulence.

What is the key prediction of the trade-off model?

The balance between transmission benefits and virulence costs can favor an intermediate optimum.

What is one limitation of the trade-off model noted in the lecture?

Few examples clearly demonstrate the specific trade-offs the model requires.

A parasite reproduces faster inside its host, but the host dies sooner. What must you evaluate?

Whether increased transmission outweighs the loss of transmission opportunities caused by early host death.

A parasite becomes less virulent but also transmits less effectively. Must it be favored?

No. Selection depends on overall parasite fitness, not virulence alone.

What are the two definitions for evolutionary fitness?

1. an organism’s ability to survive at a high level of capacity over time

2. an organism’s ability to reproduce (completely or in part) and evolve over successive generations