Introduction to Traits

VectorByte Methods Training

The VectorByte Team (Cat Lippi, PhD, Virginia Tech)

Introduction to Traits - Objectives


• Define a “trait”

• Understand the role of traits (and their variation) in VBDs

• Discuss how traits can be incorporated into VBD predictions

What is a vector?

Any agent which carries and transmits an infectious agent between hosts

What kinds of animals are vectors?

Many! Although arthropods are most commonly studied

Overview of VBD Transmission

Mechanical Transmission

Overview of VBD Transmission

Biological Transmission

Pathogens undergo obligate develoment in the mosquito

VBD Transmission is the Intersection of Two Cycles

Intrinsic and Extrinsic Incubation Periods

What elements do we need to include?

What is a Trait?

Any measurable feature of an individual organism


Functional traits

\(\Rightarrow\) feeding rate, size, metabolic rate, eggs per day


Behavioral traits

\(\Rightarrow\) questing, host/food preference, thermal regulation, site selection

Variability in Traits


SIR Model

We can get more complex/realistic models…

H=host, V=vector, \(_S\)=susceptible, \(_I\)=infected, \(_R\)=recovered

But with every parameter added, more data are needed!

H=host, V=vector, \(_S\)=susceptible, \(_I\)=infected, \(_R\)=recovered a=biting rate, b=vector to host transmission success (proportion of bites), d=recovery

H=host, V=vector, \(_S\)=susceptible, \(_I\)=infected, \(_R\)=recovered a=biting rate, b=vector to host transmission success (proportion of bites), d=recovery, c=host to vector transmission success (proportion of bites), P=extrinsic incubation period, \(\mu\)=adult vector mortality rate

\(R_0\): Directly-Transmitted Pathogen

\(R_0\): Vector-Borne Pathogen

Thermal Performance Curves (TPCs)


  • Vectors of interest are typically ectothermic

  • Biological reactions typically go faster as temperature increases

But there’s a limit!

  • Nearly every aspect of life history is influenced by temperature

  • We want to include parameters in relation to temperature

  • Biologically meaningful models

  • Old assumptions were linear

  • Unimodal curves are more realistic

Different traits have different curves

Thermal limits are useful for predictions and mapping

Mapping current and future thermal limits to suitability for malaria transmission by the invasive mosquito Anopheles stephensi. Ryan et al. 2023. Malaria Journal.

Estimating TPCs

TPCs are derived using empirical data from laboratory experiments.

Need to Collect Data Across a Thermal Gradient


  • Ideally want to capture data across a wide thermal gradient
  • Need enough temperatures to estimate minimum, maximum, and optimum values

Fitting TPCs

TPCs can have many possible shapes:

Model Validation

Data from the field or other experiments can help validate estimates.