Disease Ecology

Frauke Ecke 2025-09-16
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Reservoir - hosts and reservoir hosts

Reservoir is where a disease remains in constant concentration. Usually an organism. The reservoir can be soil, or in a lake or stream for a bacteria or parasite. Viruses usually cannot live outside a host
Host species takes the pathogen
Some can be both
- Beavers and a certain bacteria - can be infected and die, but also can have the disease endemic to them
Example:
- Bats can be reservoirs for SARS-CoV2, but are not the host (can be infected but don't develop the disease)

Vectors

Carry an infectious agent through purely mechanical means
Can support growth or changes in the agent
- Classic example is mosquito or tick
- However something like a mouse can also be a vector if the disease attaches to its fur for example
Difference between a vector and a reservoir:
- Pathogens don't usually replicate in a reservoir:
  - e.g. Lyme disease causing bacteria in ticks do not replicate inside the tick, it's just carried within the tick between animals
Is a deer also a vector when it carries the ticks? Good question (I think no)

Virulence

The cost of infection on host fitness

Host mortality rate is the classic example
Decrease in a number of offspring
Severity of the disease (Immune response)
Parasite replication rate

Disease Risk

Hazard x Exposure x Vulnerability = Risk

These are all components of disease risk

Transmission modes

Close contact

(rodents example) - they come in contact for competition, mating, social behaviour
Aerosols from bodily fluids

Horizontally and vertically transmitted pathogens

Vertical - via offspring
Horizontal - peer-to-peer

Indirect transmission

Example: Pathogen shed into water and transported to households
Eat contaminated food or pick berries that were contaminated by another animal

Intermediate transmission

Disease that includes a complex lifestyle:
- Intermediate component or reservoir in another species
- Tropical diseases or parasites which reproduce in snakes orslugs

Vector-borne transmission

Mosquito, tick, borne

Infection Dynamics

Typical Example

Newborn animals are protected from infection from antibodies that they inherit, such as hantavirus in mice. After being infected once, you can't receive this disease again.

Typical dynamic is Susceptible, exposed, infected, recovered, and then susceptible again.

Infect-shed-spill-spread cascade

Diseases that can be spread from humans to animals and back again
Mouse is infected with mouse disease, sheds the disease into the environment, it is spilled over to humans, then spread within humans
Examples:
- Hantavirus is not an example here since it cannot be spread among humans (only to humans from mice)
- COVID-19 is a whole cascade, since it can be spread among humans
Shedding the virus involves contact among reservoir and humans

Human-rodent contact

Rodents entering human spaces, or humans entering wild spaces (outdoor recreation, work, agriculture)

Pathogen transmission / Virulence Trade-off

As virulence increases, transmission increases to a maximum then decreases.

If virulence is too high, the negative effects on the host reduce the capacity for transmission

Susceptible-Infected-Removed (SIR) model

only for a completely susceptible population

R_{0} \propto (\frac{i n f e c t i o n}{c o n t a c t}) \times (\frac{c o n t a c t}{t i m e}) \times (\frac{t i m e}{i n f e c t i o n})

R_{0} \propto τ \times \bar{c} \times d

Examples: Ebola and SARS

Ebola (in this example) starts by infecting two from patient zero, therefore $R_{0} = 2$ .
SARS patient zero infects four ( $R_{0} = 4$ )

Biodiversity is good for our health

Dilution effect

Pathogen diversity increases with biodiversity - this is good since they compete with each other and reduce the overall infection rate
This concept was first examined in plant diseases but recently has been applied to animal diseases as well (reverse of behavioural ecology)

Example - rewilding and restoration reducing disease risk

Adding a new species (Increasing species richness) can result in encounter reduction within the population if the new species is competitive with the original species
Susceptible host regulation - Introducing a non-host species to the disease gives the disease less opportunities
Infected host mortality - a predator is more likely to predate on infected individuals if they are weaker, decreasing overall disease risk

"old friends hypothesis"

Our gut microbiome and other microorganisms result in greater overall health

Rodents

Rodents are very species-rich - we have a lot of reservoirs, but many more non-reservoir species (and many are reservoirs of zoonotic diseases in humans, but also other epizootic diseases within the rodents)
Voles, mice and squirrels are the most important reservoir species
Specialist species are less likely to be reservoirs, generalists more likely

Why are rodents often reservoirs?

High reproduction rate
- can happen all year round in some species like brown rats
- some species like lemmings reproduce in the winter under the snow and reach sexual maturity in up to 14 days, but only live one year.
Sexual maturity age
High litter size
Very social

Why do rodents move indoors?

Mus i hus - citizen science app where people report mice inside - data can be used to determine correlation between indoor mouse reports and disease incidences based on time of year
ROdents which overwinter indoors are more likely to survive the winter

Drivers of rodent population cycles:

Food availability
Predators
Disease

Wood lemmings

have a cryptic population cycle - their genes result in lots of females sometimes
They only eat moss
They sometimes die suddenly and surprisingly
Why?
- Not predators.
- Is it food or disease? Currently being studied.

Tick-borne illnesses

Sheep are important in spreading tick-borne encepholitus (TBE)

Lyme Disease

Tick larvae (six legs) - predate on rodents, this is where infection occurs
Nymphs predate onsmall animals and humans and can spread infection
Adults predate on large ungulates, however usually after predation they are "cleared" of infection. Nevertheless, this also allows them to produce eggs and start the cycle again.
Wild boars spread zoonotic diseases like swine fever, however they have no role in spreading tick-borne pathogens

Tularemia

Reservoir in wetlands, can spread from mice to beavers and hares through ticks and mosquitoes (and people) - it is often fatal in larger animals
Recently it has increased in Scandanavia - this could be due to increased precipitation,increasing wetness, wetlands are reservoirs for the disease, also more mosquitoes
Beaver increase in Sweden also contributes to more wetness, and also beavers themselves as hosts
Bank vole population cycle correlates with Tularemia increases
Tengmalm's Owl is a vole specialist and important predator
There is likely a relationship between Tengmalm's owl populations and Tularemia, especially since the owls selectively prey on infected bank voles

Othohantavirus puumalaense (PUUV)

Only infects bank vole, which typically outcompetes grey-sided vole, but this virus has resulted in the otherspecies increasing
Common shrews which prey on bank voles will take out infected individuals (susceptible host mortality)
Higher community complexity (biodiversity) means lower hazard from virus
Similar to in Tularemia, the Tengmalm's owl selectively preys on infected bank voles
Bank voles infected with the virus move and urinate more, making them easier targets for the owls.
Weather conditions also play an important role, therefore climate change has contributed to increased transmission risk
- Wetter autumns (i.e. warmer) leads to :
  - Increased environmental persistence
  - decreases the health of rodents
  - Reduces the space available for voles and therefore increases transmission
Overall a very complicated ecology of this disease