Lecture 6 - Vector and Sexual Transmission

What is vector transmission?

Transmission in which a pathogen moves between hosts using another organism, the vector.

What are the two major transmission events involving a vector?

Acquisition of the pathogen by the vector and inoculation of the pathogen into another host.

What is acquisition?

The vector obtains the pathogen from an infected host.

What is inoculation?

The vector transmits the pathogen to a susceptible host.

Why are both acquisition and inoculation necessary?

The vector must first become infected and then successfully transmit the pathogen to another host.

How does feeding duration affect vector transmission?

Longer feeding can increase the probability that a vector acquires and inoculates a pathogen.

What is vector persistence?

How long a vector retains the ability to transmit a pathogen.

What is nonpersistent transmission?

The vector retains the pathogen for seconds to minutes.

What is semipersistent transmission?

The vector retains the pathogen for hours to days and may lose it during molting.

What is persistent transmission?

The vector retains the ability to transmit for days to weeks and may retain the pathogen through molting.

What is circulative transmission?

The pathogen moves between tissues within the vector.

What is propagative transmission?

The pathogen replicates within the vector.

What is a vector latent period?

The time between acquisition of a pathogen and the point when the vector becomes capable of transmitting it.

Why is vector lifespan important?

If vectors die before completing the latent period, they cannot transmit the pathogen.

What happens if the vector latent period is nearly as long as vector lifespan?

A large fraction of vectors may die before becoming infectious.

Why could killing vectors after they reproduce but before they transmit be useful evolutionarily?

It could reduce transmission while potentially placing less selection for resistance on the vector population than killing vectors earlier.

What factors are included in the malaria R₀ equation?

Biting rate, host-to-vector transmission probability, vector-to-host transmission probability, vector density, vector mortality, host recovery, host mortality, and host density.

What does b represent in the malaria R₀ equation?

The vector biting rate.

What does Tvh represent?

The probability of transmission from an infected vector to a susceptible host.

What does Thv represent?

The probability of transmission from an infected host to a susceptible vector.

What does Nv represent?

Vector density.

What does μv represent?

Vector mortality rate.

What does γh represent?

Host recovery rate.

What does μh represent?

Host mortality rate.

Why is there no simple threshold host density for vector transmission in the malaria model?

The key threshold depends on the ratio of vectors to hosts rather than host density alone.

What happens if vector density decreases while host density stays constant?

R₀ decreases because fewer vectors are available to transmit infection.

What happens if vector mortality increases?

R₀ decreases because vectors have less time to survive and transmit.

What happens if biting rate increases?

R₀ increases because vectors make more transmission opportunities.

What happens if vector-to-host transmission probability increases?

R₀ increases.

Why can insecticide resistance make vector control less effective?

Repeated insecticide use can select for vectors that are resistant, reducing the effectiveness of the intervention.

What is one way to reduce vector transmission without directly treating infected hosts?

Reduce vector abundance or vector-host contact through vector control.

Why can bed nets reduce malaria transmission?

They reduce contact between vectors and humans.

Why might vector control be especially effective when the vector latent period is long relative to vector lifespan?

Many vectors may die before becoming capable of transmitting the pathogen.