What is vector-borne disease?

  • A micro- or macroparasite transmitted among vertebrate hosts by an arthropod vector

  • Vector-borne diseases represent some of the deadliest, most impoverishing diseases of mankind and wildlife

Vectors

Vectors

Importance of vector-borne disease

  • Vectors are flying syringes

  • Host preference determines encounter

  • Huge economic cost

  • Some rough diseases

Vectors are flying syringes

  • Vectors take blood or host tissue (including pathogen) and transmit to same (or different) species

  • Means reservoir hosts are really important

Host preference determines encounter

  • Vectors prefer certain hosts, and host preference determines the resulting disease burden

Eigenbrode & Gomulkiewicz 2022 J Economic Entomology

Huge economic cost

  • $12 billion per year (Chilakam et al. 2023 doi:10.2196/50985)

  • $100 billion economic costs per year (USDA-ARS report)

  • More important is the cost in terms of impacts to human and wildlife

Should we ‘extinct’ some species?

“Keep their (Anopheles gambiae) DNA for future research and let them go” -EO Wilson

Fang 2010 Nature; and associated 4+ replies

Some rough diseases

Mosquitos

  • Malaria, Dengue, West Nile, Chikv, Zika, yellow fever,

Ticks:

  • Lyme, E equine encephalitis, Ehrlichiosis, Rocky Mtn spotted fever, Tularemia

Other:

  • Plague (fleas), Chagas (Tiatome bugs), Schisto (aquatic snails), Sleeping sickness (Tsetse flies)

Plague

  • Yersinia pestis, a bacterial pathogen

  • Vectored by fleas

  • Killed 25% of Europe’s pop in 1300’s

  • Exists in enzootic cycles with transmission between rodents and fleas

Lyme disease

  • Borrelia burgdorferi, a bacterial pathogen

  • Vectored by ticks (Ixodes scapularis as main vector)

  • Ancient disease (plagued humans prior to European Colonization)

  • Disappeared around 1850’s due to deforestation

Lyme disease

Mosquito-borne disease is different

  • Dengue (33%+ of world pop at risk)

  • Chikungunya (45 countries affected)

  • Zika (61 countries affected, 47 in Americas)

  • Malaria (kills ~1 million people per year)

Mosquito-borne disease is different

Flores & O’Neill 2018 Nature Reviews Microbiology

And we should be worried

Ainsworth 2023 Nature

How do we model vector-borne disease?

  • Recall the SIR model

Ross-MacDonald model

Ross-MacDonald model

Ross-MacDonald model

  • Host and vector terms are both in there

  • We can look across probable parameter ranges to see the effect of each component

  • Is it more important to reduce vector infection probability ( \(b_v\) ) or host duration of infection ( \(1/r\) )

Ross-MacDonald model

Ross-MacDonald model

  • Controlling mosquitoes has largest effect

Let’s kill the mosquitoes

  • \(R_0\) is most sensitive to changes in mosquito survival

Let’s not kill the mosquitoes (or at least find better ways)

Hancock et al. 2018 PNAS

What are better ways to kill mosquitoes?

  • Brainstorm in class and write on board

Or maybe we don’t need to kill them

  • Maybe we just need to figure out how to reduce biting rates

Thongsripong et al. 2021 Ann Entomol Soc Am

Adding the vector also makes for some confusion about density/frequency dependent transmission

  • Mosquitoes won’t just keep feeding and feeding and feeding, right?

  • Even with 1000 hosts per meter, there’s a limit on how many people a mosquito can bite

  • This results in a non-linear relationship between feeding rate and host density that looks a bit frequency dependent

Thongsripong et al. 2021 Ann Entomol Soc Am

Adding the vector also makes for some confusion about density/frequency dependent transmission

Thongsripong et al. 2021 Ann Entomol Soc Am

What about the other terms in \(R_0\)?

What about the other terms in \(R_0\)?

How variable is \(R_0\)?

Liu et al. 2020 Environmental Research

Is this variability related to transmission mode?

\(R_0\) estimates for COVID; Dhungel et al. 2022 Int J Environ Res Public Health

Do you think that estimates of \(R_0\) would be more or less variable for vector-borne disease relative to others. Why?

(5 minutes of small group discussion)

Look forward to some more fun disease ecology

  • What happens when we start to consider the role of the environment in modifying some of those Ross-MacDonald parameters?

  • How are host and vector distributions shifting with climate change, and what does this mean for the future of vector-borne disease?

  • and other questions!




End of lecture 1

What have we learned?

  • Vector-borne disease is pretty rough

  • We have a decent model of vector-borne disease

  • It can help identify suitable mitigation strategies

Let’s revisit Ross-MacDonald

Let’s revisit Ross-MacDonald

Let’s code up the Ross-MacDonald \(R_0\)

R0 <- function(Nv, Nh, a, bH, bV, p, r){
  (Nv*(a**2)*bH*bV*(p**Nv)) / (Nh*r *(-log(p)))
}

R0(Nv=10, Nh=10, a=0.5, bH=0.5, bV=0.5, p=0.95, r=0.1)
## [1] 7.295507

Influence of host recovery rate

Influence of mosquito survival

Let’s revisit host recovery rate

  • But look across a small range of mosquito survival rates

What did we learn from this exercise?

  • \(R_0\) is pretty sensitive to small changes
  • Computer code is not scary
  • Probably something else

The role of environment on vector-borne disease

Those small changes that have big impacts on \(R_0\) could be driven by the environment

The role of environment on vector-borne disease

The role of environment on vector-borne disease

Does this mean climate change is ‘good’?

  • Mixed, but probably not

  • How could it be good?

  • Why is it especially not good for us?

Climate change could cause disease shifts

or vector distribution shifts

or vector distribution shifts

Anopheles gambiae is the biggest malaria vector, currently limited in distribution

Li et al. 2021 J Biosafety & Biosecurity

or vector distribution shifts

Projected distribution is a bit scarier

Li et al. 2021 J Biosafety & Biosecurity

but the vectors are already here in the USA

Evans et al. 2017 eLife

the bigger question is…


if the vector species are already here, why is the pathogen not found as often?

This points to the importance of the reservoir host community , environmental control of pathogen presence , and/or mitigation efforts

But probably not mitigation efforts. Does Columbia spray for mosquitos? No. Other counties do.

This is not to say that vector-borne disease is not increasing in the US

  • Some diseases we were already struggling with (e.g., Lyme; 20-30k cases per year)

  • Some we are now starting to grapple with (e.g., Zika; local transmission back in 2016-2017, but not since)

  • Some are right around the corner (e.g, Dengue; local transmission in Florida, Texas and Arizona)

  • ~2000 malaria cases per year (some from travel though, so not necessarily local transmission)

The role of vector preference on resulting disease dynamics

The role of vector preference on resulting disease dynamics

The role of vector preference on resulting disease dynamics

Important because:

  • Vector feeding preferences are particularly important for multi-host systems

  • Can introduce variation in exposure across different groups of individuals within a host population

  • Vector preference can inform intervention strategies based on vector and reservoir communities

In terms of the Ross-MacDonald model, what do vector feeding preferences influence?




How would we go about incorporating vector biting preferences and the underlying host community in our estimate of \(R_0\)?

  • I’d probably argue that it largely influences vector infection probability (\(b_v\)) and use some weighted estimate of the susceptibility of the host community to infection as a modifier of \(b_v\).

And considering the reservoir community through the lens of force of infection to humans makes sense …

but let’s not be silly about it

but let’s not be silly about it

  • Response to this (and other bat-vectored disease) is to try to kill bats

  • Often killed the wrong bats

  • Some efforts potentially increased disease in humans

The role of pathogen infection on vector behavior

The role of pathogen infection on vector behavior

The role of pathogen infection on vector behavior

The role of pathogen infection on vector behavior

Parasite manipulation of host behavior

  1. Plasmodium can direct uninfected mosquito blood-seeking and feeding behavior via alteration of vertebrate-host odor profiles and production of phagostimulants

  2. Plasmodium also manipulates its vector during the sporogony cycle to increase transmission (infected mosquitos bite more and take longer bloodmeals)

So the malaria pathogen can shift host preferences and increase transmission rates through manipulation of host behavior

Emami et al. 2019 Pathog Glob Health

The role of vector behavior in response to interventions

The role of vector behavior in response to interventions

The role of vector behavior in response to interventions

Questions to think about


  • What determines how widespread a vector-borne pathogen can be?

  • Why are vector-borne diseases so tough to control?

  • What parameters in \(R_0\) are likely to be most sensitive to environmental change?

  • Do we need to model those parameters directly as functions of environmental variables?

    • Mordecai et al. 2019 Ecology Letters argues this