2025-2026 Ecology, epidemiology and control Joachim Mariën
ECOLOGY, EPIDEMIOLOGY AND
CONTROL OF INFECTIOUS DISEASES
INTRODUCTION
• How do infections circulate in a population?
• How does population dynamics affect infections (and vice versa)?
o For human populations this typically stays consistent
o Especially important for animal studies
• How do environmental conditions affect infections?
o Where and when do infection occur?
o How/when do zoonotic infections emerge?
o How are infections linked to biodiversity?
o What is the effect of climate?
POPULATION ECOLOGY
WHAT IS A POPULATION?
• A group of individuals of one species, living at a certain place in space and time
o If ‘one species’ was not there it would be a community instead of a population
• A group of individuals of one species under investigation
WHAT ARE THE LIMITS OF A POPULATION?
• Who to include depends on your hypothesis
o E.g. voting: this is easy because we can see who’s in the Belgian registry and those
people form the population
o E.g. the spread of infectious diseases: this is less easy because Belgium is not an
isolated place where we can use the borders as limits. The Belgian people at the border
will come in close contact with people in France, Germany…
• The population must be ‘counted’
o E.g. a field of cows is easy
o E.g. the population of birds in a bush is harder because this changes constantly
DEFINITIONS
PARASITE POPULATIONS
• Infra population = the specific collection of parasite individuals of a single species residing
within a single host organism at a given time
• Component population = all individuals of a specific parasite species belonging to a single,
defined life history stage (like mature adults) at a given time and place
1
,2025-2026 Ecology, epidemiology and control Joachim Mariën
• Supra population = the sum of all individual populations of a given parasite species across a
specific geographic area
o Blue = host 1 infected with stage 1 of the parasite
o Yellow = host 2 infected with stage 1 of the
parasite
o Orange = host 3 infected with stage 2 of the
parasite
POPULATION SIZE
• Abundance = the number of organisms in a population,
combining ‘inensity’ (density/number of organisms within inhabited area) and ‘prevalence’
(number and size of inhabited areas)
• Density = number of individuals per area
o E.g. # parasites per host
o This is an abstraction!! (but maybe more relevant to the individual experiencing it)
§ Think about 20 students in a big classroom or 20 students in a small classroom
• How do we determine the population size? Counting individuals
o People and sheep are easy to count
o Rodents are harder à there are special techniques to count them
• BUT how do we know which individuals should be counted?
o If we want to study the frog population in a pond
§ Do we only count the adults?
§ Or do we also count the eggs?
o This depends on your research question
§ E.g. What will the population if frogs of the swamp in the future be? Only
counting the adults will not be enough now, the eggs need to be considered
POPULATION DYNAMICS
• The population is influenced by natality, immigration, mortality and emigration
o Nnow = Nthen + B -D + I – E
NATALITY
• Mainly dependent on females (in most sexually reproducing species)
o Number of females
o Age
o Reproductive capacity (for humans this would be 1 child every 9
months from age 12-50 for example)
o Realised reproductive capacity = what happens in reality (a human
female could have 30 children, but she won’t)
§ This depends on the culture e.g. Belgium vs Congo
§ The population in Congo is increasing because of new births, the population in
Belgium is also increasing but that is due to immigration
2
,2025-2026 Ecology, epidemiology and control Joachim Mariën
MORTALITY
• Many factors play a role in mortality
• Effects depending on which part of the population dies
• Maximum lifespan is only rarely relevant!
• The average lifespan depends on different things
• There are 3 types of survival (survivorship curves)
o I – populations where most individuals get old,
after a certain age they start to die all at once
(steep decline in survival)
o II – not talked about
o III – populations where many individuals die in
the beginning, after a certain age they will live long
§ E.g. frogs
• For humans this is a little more complex
o In the 17th century many children died (due to a lack of hygiene, food…), but those who
survived lived long (type III)
o In Britain people become old, from the age of 70 we see a decline in survival (elders start
dying) (type I)
o Niger has a more constant decrease in survival throughout the years (type II)
• This shows us that even within populations there are different death curves = age plays an
important role in the studies of infectious diseases
• This shows us that different populations have different characteristics
POPULATION COMPOSITION
• This plays an important role in population growth
• Age distribution:
o Old people won’t reproduce anymore
§ A population with a lot of old people will have less natality
§ A population with a lot of young people will have many new births
o Age distribution differs within and between populations
§ E.g. human age
pyramids
• Sex distribution
o A population of 10 which
consists of 1 female will
contribute less to the population than a population of 10 which has 8 women
IMMIGRATION AND EMIGRATION
• Very difficult to study in animal populations
• Usually assumed that immigration = emigration à this is wrong!!
o if many fish “emigrate” from Bowl A but few successfully arrive
or survive to live in Bowl B, Bowl A declines while Bowl B doesn’t
gain as much — so I≠EI =E. In nature, different patch sizes, movement mortality, and
directed preferences make that common.
• This is utmost important for parasite ecology
o Transmission between hosts
o Invasion of infection in new areas
3
, 2025-2026 Ecology, epidemiology and control Joachim Mariën
POPULATION GROWTH
• R = reproductive rate = difference in population over a certain time
!"
o !#
=𝐵−𝐷
"#$%
o 𝑅= "#
à no immigration or emigration within this formula
• Net reproductive rate R (=population growth rate l)
o Nt+1 = RNt
• Basic reproductive rate R0
o NT = RTN0 = R0N0
o With T = generation time
o R0 = Rnaught = the factor, on average, which which an individual replaces itself over the
course of a generation
§ R0 > 1 = population increases
§ R0 < 1 = population decreases
o E.g. the generation time for humans is around 20 years
o E.g. in parasitology R0 measures the average number of secondary cases (new
infections) that one infected individual produces in a completely susceptible population
à it combines how easily the infection spreads, how long people interact, and long
people are infectious
• Expressed with birth and death rate
&"
o = (𝑏 − 𝑑)𝑁
&#
• Intrinsic rate of natural increase r (r = b-d)
&"
o = 𝑟𝑁
&#
CARRYING CAPACITY
• Resources are limited; therefore, we have a carrying capacity (K)
o = The maximum population size of a species that an
environment can sustain indefinitely, given the available
resources (food, water, shelter, space) and other environmental
limits (like predation, disease, or competition)
o The more animals, the more difficult survival for individual
animals = density dependent mortality
&" '("
o = 𝑟𝑁( )
&# '
§ N = population size
§ Time
§ K = carrying capacity (the maximum N that the environment can sustain)
o Explanation of the graph
§ Beginning = exponential growth of the population because there is an abundant
number of resources and limited competition
§ Middle = steep upward growth (growth is fastest here)
§ End = growth slows because the resources are being used up à death rate =
birth rate à N stabilizes around K because the environment cannot sustain
more individuals long term
o Red circle = when N = K à birth and death are equal (b=d)
4
ECOLOGY, EPIDEMIOLOGY AND
CONTROL OF INFECTIOUS DISEASES
INTRODUCTION
• How do infections circulate in a population?
• How does population dynamics affect infections (and vice versa)?
o For human populations this typically stays consistent
o Especially important for animal studies
• How do environmental conditions affect infections?
o Where and when do infection occur?
o How/when do zoonotic infections emerge?
o How are infections linked to biodiversity?
o What is the effect of climate?
POPULATION ECOLOGY
WHAT IS A POPULATION?
• A group of individuals of one species, living at a certain place in space and time
o If ‘one species’ was not there it would be a community instead of a population
• A group of individuals of one species under investigation
WHAT ARE THE LIMITS OF A POPULATION?
• Who to include depends on your hypothesis
o E.g. voting: this is easy because we can see who’s in the Belgian registry and those
people form the population
o E.g. the spread of infectious diseases: this is less easy because Belgium is not an
isolated place where we can use the borders as limits. The Belgian people at the border
will come in close contact with people in France, Germany…
• The population must be ‘counted’
o E.g. a field of cows is easy
o E.g. the population of birds in a bush is harder because this changes constantly
DEFINITIONS
PARASITE POPULATIONS
• Infra population = the specific collection of parasite individuals of a single species residing
within a single host organism at a given time
• Component population = all individuals of a specific parasite species belonging to a single,
defined life history stage (like mature adults) at a given time and place
1
,2025-2026 Ecology, epidemiology and control Joachim Mariën
• Supra population = the sum of all individual populations of a given parasite species across a
specific geographic area
o Blue = host 1 infected with stage 1 of the parasite
o Yellow = host 2 infected with stage 1 of the
parasite
o Orange = host 3 infected with stage 2 of the
parasite
POPULATION SIZE
• Abundance = the number of organisms in a population,
combining ‘inensity’ (density/number of organisms within inhabited area) and ‘prevalence’
(number and size of inhabited areas)
• Density = number of individuals per area
o E.g. # parasites per host
o This is an abstraction!! (but maybe more relevant to the individual experiencing it)
§ Think about 20 students in a big classroom or 20 students in a small classroom
• How do we determine the population size? Counting individuals
o People and sheep are easy to count
o Rodents are harder à there are special techniques to count them
• BUT how do we know which individuals should be counted?
o If we want to study the frog population in a pond
§ Do we only count the adults?
§ Or do we also count the eggs?
o This depends on your research question
§ E.g. What will the population if frogs of the swamp in the future be? Only
counting the adults will not be enough now, the eggs need to be considered
POPULATION DYNAMICS
• The population is influenced by natality, immigration, mortality and emigration
o Nnow = Nthen + B -D + I – E
NATALITY
• Mainly dependent on females (in most sexually reproducing species)
o Number of females
o Age
o Reproductive capacity (for humans this would be 1 child every 9
months from age 12-50 for example)
o Realised reproductive capacity = what happens in reality (a human
female could have 30 children, but she won’t)
§ This depends on the culture e.g. Belgium vs Congo
§ The population in Congo is increasing because of new births, the population in
Belgium is also increasing but that is due to immigration
2
,2025-2026 Ecology, epidemiology and control Joachim Mariën
MORTALITY
• Many factors play a role in mortality
• Effects depending on which part of the population dies
• Maximum lifespan is only rarely relevant!
• The average lifespan depends on different things
• There are 3 types of survival (survivorship curves)
o I – populations where most individuals get old,
after a certain age they start to die all at once
(steep decline in survival)
o II – not talked about
o III – populations where many individuals die in
the beginning, after a certain age they will live long
§ E.g. frogs
• For humans this is a little more complex
o In the 17th century many children died (due to a lack of hygiene, food…), but those who
survived lived long (type III)
o In Britain people become old, from the age of 70 we see a decline in survival (elders start
dying) (type I)
o Niger has a more constant decrease in survival throughout the years (type II)
• This shows us that even within populations there are different death curves = age plays an
important role in the studies of infectious diseases
• This shows us that different populations have different characteristics
POPULATION COMPOSITION
• This plays an important role in population growth
• Age distribution:
o Old people won’t reproduce anymore
§ A population with a lot of old people will have less natality
§ A population with a lot of young people will have many new births
o Age distribution differs within and between populations
§ E.g. human age
pyramids
• Sex distribution
o A population of 10 which
consists of 1 female will
contribute less to the population than a population of 10 which has 8 women
IMMIGRATION AND EMIGRATION
• Very difficult to study in animal populations
• Usually assumed that immigration = emigration à this is wrong!!
o if many fish “emigrate” from Bowl A but few successfully arrive
or survive to live in Bowl B, Bowl A declines while Bowl B doesn’t
gain as much — so I≠EI =E. In nature, different patch sizes, movement mortality, and
directed preferences make that common.
• This is utmost important for parasite ecology
o Transmission between hosts
o Invasion of infection in new areas
3
, 2025-2026 Ecology, epidemiology and control Joachim Mariën
POPULATION GROWTH
• R = reproductive rate = difference in population over a certain time
!"
o !#
=𝐵−𝐷
"#$%
o 𝑅= "#
à no immigration or emigration within this formula
• Net reproductive rate R (=population growth rate l)
o Nt+1 = RNt
• Basic reproductive rate R0
o NT = RTN0 = R0N0
o With T = generation time
o R0 = Rnaught = the factor, on average, which which an individual replaces itself over the
course of a generation
§ R0 > 1 = population increases
§ R0 < 1 = population decreases
o E.g. the generation time for humans is around 20 years
o E.g. in parasitology R0 measures the average number of secondary cases (new
infections) that one infected individual produces in a completely susceptible population
à it combines how easily the infection spreads, how long people interact, and long
people are infectious
• Expressed with birth and death rate
&"
o = (𝑏 − 𝑑)𝑁
&#
• Intrinsic rate of natural increase r (r = b-d)
&"
o = 𝑟𝑁
&#
CARRYING CAPACITY
• Resources are limited; therefore, we have a carrying capacity (K)
o = The maximum population size of a species that an
environment can sustain indefinitely, given the available
resources (food, water, shelter, space) and other environmental
limits (like predation, disease, or competition)
o The more animals, the more difficult survival for individual
animals = density dependent mortality
&" '("
o = 𝑟𝑁( )
&# '
§ N = population size
§ Time
§ K = carrying capacity (the maximum N that the environment can sustain)
o Explanation of the graph
§ Beginning = exponential growth of the population because there is an abundant
number of resources and limited competition
§ Middle = steep upward growth (growth is fastest here)
§ End = growth slows because the resources are being used up à death rate =
birth rate à N stabilizes around K because the environment cannot sustain
more individuals long term
o Red circle = when N = K à birth and death are equal (b=d)
4
