Host-Pathogen Heterogeneity

80/20 Rule

20% of hosts contribute to 80% of transmission potential

Superspreaders

the tail end, 20% that contribute the most to transmission

Can be broken into super-shedders, super-receivers, etc

make outbreaks rarer, but more explosive

80/20 Rule Treatment Implication

identifying and treating the core 20% is probably more effective than trying to treat the whole population

How does animal behavior affect disease transmission heterogeneity?

o Variability in the rates and numbers of contacts with others

o Social organization

o Individual animal’s personality

Contact heterogeneity

Tasmanian Devil Facial Tumor Disease example

Tasmanian Devil Facial TUmor Disease

Transmissible cancer contracted via biting, mating interactions

death from starvation, secondary infections, organ failure

Contact network

Social Organization as Heterogeneity

FIV in Pumas example

FIV in pumas

Feline immunodeficiency virus, affects most felids but species-specific, chronic lifelong infection

directly transmitted via biting, fighting

Puma Social Organization

males: larger home range, compete with other males for mating opportunities

females: smaller home range, compete with other females for resources to raise offspring

prevalence of FIV varies by sex, age, and location

Individual Animal Personality

Bold: less risk averse, more exploratory behavior, may affect contact rate

ex. bold feral cats had higher reproductive success but higher probability of FIV infection

Resistance

reduce pathogen’s growth, geneally function of immune function

Tolerance

reduces damage by pathogen, but does not reduce pathogen growth or reproduction

Resistance vs Tolerance

resistance: limit pathogen growth

tolerance: limit pathogen damage

all else equal, a resistant host is less infectious than a tolerant host

both can be observed for the same pathogen

Body condition

can increase infectiousness if body lacks resources for strong immune response

can increase infectiousness if body has sufficient resources for pathogen to increase pathogen load/shedding

coinfection

multiple infectious diseases interacting within an individual can increase or decrease infection rates, infectious period, and duration of infection

attractiveness to reservoirs

example: mosquitoes attracted to high density, low diversity of bacteria

differences in attraction could alter likelihood of disease transmission rates

reproduction implications

change in hormones stimulate/inhibit immunity in both males and females

behavior changes associated with reproduction and mating

sickness behavior

can be caused by infection, generally including reduced food intake, social activity, energy expenditure, etc

can vary by context (ex. zebra finches do not exhibit when around others) and pathogen (ex. rabies induces aggression favorable for transmission via biting)

pathogen genotype and transmission heterogeneity

affects disease severity, host susceptibility, pathogen load shed into environment, infectious duration, host range, etc

ex. COVID variants

pathogen reproduction and transmission heterogeneity

pathogens may reproduce and/or shed at variable rates within hosts

Scales in Heterogeneity

Individual

Classes of individuals (age class, sex)

Species

Individual Scale

Personality

Stress

Immune function

Coinfection

Dominance/social position

vector preferences

body condition

Classes of Individuals Scale

sex differences in territoriality, behavior, space use

age differences in physiology

body size

mating behavior

Species Scale

Multi-host Pathogens

when a pathogen is transmitted between two hosts, there can be variability in how successful it is in different host species

some host species may not be able to maintain a pathogen in a population on their own

Maintenance Population

Population (scale) that can maintain a pathogen without transmission from other populations r0>1

Non-maintenance population

population that cannot maintain a pathogen without transmission from other populations r0<1

Maintenance Community

two or more non-maintenance populations that together can maintain a pathogen

Reservoir

population that maintains a pathogen (maintenance population) AND transmits it to the target population

often not clinically affected by the pathogen

Bighorn Sheep Pneumonia Example

Target: bighorn sheep

Reservoir: domestic sheep/goats

Maintenance population: Both bighorn and domestic sheep/goats

Spillover Pathogen

Target host cannot sustain transmission alone (non-maintenance)

Transmission from reservoir to target species is low

result: pathogen doesn’t appear endemic in target

Apparent Multi-host Pathogen

Target host cannot sustain transmission alone (non-maintenance)

Transmission from reservoir to target species is high

result: infection is persistent in target species (appears endemic)

True Multi-host Pathogen

Transmission within AND between reservoir and target species is high

Result: both species considered maintenance hosts

Potentially Emerging Pathogen

Pathogen can persist within target species

Rate of transmission between reservoir and host is low

Result: target host is rarely exposed to the disease

Multi-Host Pathogen Types

Spillover Pathogen

Apparent Multi-host Pathogen

True Multi-host Pathogen

Potentially Emerging Multi-host Pathogen

Bovine Tuberculosis (bTB) Example

mass slaughter of cattle revealed anther source of transmission, badgers found to transmit

Target population: cattle

Reservoir: badgers?? and cattle