The mechanisms of Disease Spread
and Population Growth
Causes of Infectious Disease
• Caused by infective agents:
– Bacteria: These one-cell organisms are responsible for
such illnesses as strep throat, urinary tract infections
and tuberculosis
– Viruses: Even smaller than bacteria, viruses are the
cause of a multitude of diseases — ranging from the
common cold to AIDS
– Fungi: Many skin diseases, such as ringworm or
athlete's foot, are caused by fungi. Other types of
fungi can infect your lungs or nervous system
– Parasites.:Malaria is caused by a tiny parasite that is
transmitted by a mosquito bite. Other parasites may
be transmitted to humans from animal feces.
Disease Spread by Direct Contact
• Coming in contact with someone who is ill or
infected
– Person to person: The most common way for
infectious diseases to spread is through the direct
transfer of bacteria, viruses or other germs from
one person to another
• Individual with the bacterium or virus touches, coughs
on or kisses someone who isn't infected.
• Exchange of body fluids from sexual contact or a blood
transfusion
Disease Spread by Direct Contact
– Animal to person: Pets can carry infectious
agents
• Being bitten or scratched by an infected animal
• Handling animal waste -- toxoplasmosis infection by
scooping your cat's litter box
– Mother to unborn child: A pregnant woman may
pass infectious diseases to her unborn baby
• Some infectious agents can pass through the placenta
• Agents in the vagina can be transmitted to the baby
during the birthing process
Disease Spread by Indirect Contact
• Many infectious agents can linger on an
inanimate object, such as a tabletop, doorknob or
faucet handle
• Environmental transfer-- When you touch the
same doorknob grasped by someone ill with the
flu or a cold, for example, you can pick up the
viruses he or she left behind. If you then touch
your eyes, mouth or nose before washing your
hands, you may become infected.
Disease Spread by Insect transfer
(Insect bites)
• Some infectious agents rely on insect carriers
— such as mosquitoes, fleas, lice or ticks — to
move from host to host
• These carriers are known as vectors
• Mosquitoes can be vectors for malaria
parasite or West Nile virus, and deer ticks may
be vectors for the bacterium that causes Lyme
disease
Disease Spread by Food Contamination
• Contaminated food and water
• Infectious agents spread to many people
through a single source
• E. coli is a bacterium present in or on certain
foods — such as undercooked hamburger or
unwashed fruits or vegetables
Infection vs. Disease
• Infection
– Often the first step,
occurs when bacteria,
viruses or other
microbes enter your
body and begin to
multiply
• Disease
– Disease occurs when the
cells in your body are
damaged — as a result
of the infection — and
signs and symptoms of
an illness appear.
In response to infection, your immune system springs into action: White
blood cells, antibodies and other mechanisms goes to work to rid your
body of whatever is causing the infection
For instance, in fighting off the common cold, your body might react with
fever, coughing and sneezing
Spread of Disease in Ecological
Systems
• Splashing rain, water currents, air currents
• Animal to animal transfer
– Migration
• Animal contamination (fecal deposition)
– Environmental transfer
• Plant to animal
• Insect transfer
Spread of a disease
• The rate of spread also has a lot to do with the nature of the
disease
– Length of being contagious
– Transmission (air, water, food, diarrhea)
– Transmission rate (i.e. the chance that any particular encounter will
transmit the disease)
– Death rate due to the disease
• Epidemic occurs if contagion or transmission rate it exceeds the
norm
– Less lethal diseases will have higher contagion rates without a sense of
emergency (such as the common cold or the common flu)
– Small increases above the norm in diseases such as tuberculosis, HIV,
Ebola, or other such highly lethal viruses, results in a state of
emergency
Spread of a disease
– Major differences between bacterial and viral
illnesses.
• Antibiotics work for bacterial disease, and sometimes
vaccines can be developed for viral disease.
• There isn't always a quick fix to an illness, however,
since both bacteria and viruses mutate and alter their
genetic makeup, making previous treatments noneffective.
Disease Simulation
• Most diseases begin with what is called "the
virgin field"—a scenario in which subjects
(humans or other animals) have no natural or
in case of humans, man-made immunity to
the disease
• http://www.learner.org/courses/envsci/intera
ctives/disease/disease_help.php or
http://goo.gl/VYj0y
zero
Running a simulation
• In this first run-through:
http://goo.gl/VYj0y
– population does not move around the field; they
interact with their neighbors, but do not travel
long distances.
– run the simulator to 100 days (click on Run
button) three times and answer the following:
• Do you get the exact same results each time? How do
the results compare to each other and to your
prediction? What factors might contribute to
susceptibility to the disease?
Running a simulation
• Population density can impact the rate at
which a disease moves through a population
• Set parameters for low, medium, and high
population density and run the simulation
three times each
– What could be done to prevent the spread of
disease in a low population density? What kinds
of challenges would high population density
present to these precautions?
Running a simulation
• Population mixing in a contagious area is
analogous to increasing population density.
Both increased density and increased
movement of people bring more contagious
people into contact with susceptible people,
thus increasing the spread of disease
• Set parameters to low, medium, and then high
mixings and run simulation 3 times in each
mode
Classroom simulation
• Follow directions in handout
Population growth
Births and immigration add individuals to
a population.
Births
Immigration
Population size =
arrivals departures
“Arrivals” are referred
to as NATALITY:
Hatching, Born,
Germinate
PopuIation
size
Emigration
Deaths
Deaths and emigration remove
individuals from a population.
“Departures” are
measured as
MORTALITY: can be
measured as the
number of deaths
Population Growth
• In a population without immigration and
emigration:
Growth = births - deaths
r=b-d
This is referred to as biotic potential
Determination of biotic potential is tedious
and time consuming
Organism
Biotic potential (yearly)
Large mammals:
elephants, rhinoceroses &
humans
Birds
0.02 to 0.5
Small mammals: squirrel,
rabbits
Insects
0.3 to 8.0
Bacteria
3,000 to 20,000
0.05 to 1.5
4.0 to 50
Logistic and exponential growth
Exponential Growth
• Growth without limits
• Characteristic J-shaped curve
• Growing at biotic potential [r]
Simulation of Exponential Growth
• Website: http://goo.gl/ZheoL
• Click “run applet” button
• Follow directions on handout
Exponential Growth: Effect of Birth
Rate
Exponential Growth: Change During
Growth
Logistic Growth
• Carrying capacity (K)- maximum population
• Environmental resistance- slows growth
• Characteristic S-shaped curve
• How well does the logistic model fit the
growth of real populations?
– The growth of laboratory populations of
some animals fits the S-shaped curves
fairly well.
– Some of the assumptions built into the
logistic model do not apply to all
populations.
• It is a model which provides a basis from
which we can compare real populations.
Simulation of Logistic Growth
• Website: http://goo.gl/nedgA
• Click “run applet” button
• Follow directions on handout
Logistic: Effect of Carrying Capacity
Logistic: Effect of Birth Rate
Logistic: Oscillation Around K
Birth rate = 3.0
Logistic: When N far Exceeds K
Snowshoe hare
160
120
Lynx
9
80
6
40
3
0
0
1850
1875
1900
Year
1925
Lynx population size
(thousands)
• Some
populations
show regular
boom and bust
cycles
Hare population size
(thousands)
Population Cycles
Extreme fluctuations in population size are often more
common in invertebrate populations
Commercial catch (kg) of
male crabs (log scale)
730,000
100,000
10,000
1950
1960
1970
Year
1980
1990
Stability and Fluctuation
FIELD STUDY
Researchers regularly surveyed the population of
moose on Isle Royale, Michigan, from 1960 to 2003. During that time, the
lake never froze over, and so the moose population was isolated from the
effects of immigration and emigration.
RESULTS
Over 43 years, this population experienced
two significant increases and collapses, as well as several less severe
fluctuations in size.
2,500
Moose population size
• Long-term population
studies have
challenged the
hypothesis that
populations of large
mammals are
relatively stable over
time
Steady decline probably
caused largely by wolf
predation
2,000
1,500
1,000
Dramatic collapse caused by severe
winter weather and food shortage,
leading to starvation of more than 75%
of the population
500
0
1960
CONCLUSION
1970
1980
Year
1990
2000
The pattern of population dynamics observed
in this isolated population indicates that various biotic and abiotic factors
can result in dramatic fluctuations over time in a moose population.
Stability and Fluctuation
FIELD STUDY
Researchers regularly surveyed the population of
moose on Isle Royale, Michigan, from 1960 to 2003. During that time, the
lake never froze over, and so the moose population was isolated from the
effects of immigration and emigration.
RESULTS
Over 43 years, this population experienced
two significant increases and collapses, as well as several less severe
fluctuations in size.
2,500
Moose population size
• Long-term population
studies have
challenged the
hypothesis that
populations of large
mammals are
relatively stable over
time
Steady decline probably
caused largely by wolf
predation
2,000
1,500
1,000
Dramatic collapse caused by severe
winter weather and food shortage,
leading to starvation of more than 75%
of the population
500
0
1960
CONCLUSION
1970
1980
Year
1990
2000
The pattern of population dynamics observed
in this isolated population indicates that various biotic and abiotic factors
can result in dramatic fluctuations over time in a moose population.
Stability and Fluctuation
FIELD STUDY
Researchers regularly surveyed the population of
moose on Isle Royale, Michigan, from 1960 to 2003. During that time, the
lake never froze over, and so the moose population was isolated from the
effects of immigration and emigration.
RESULTS
Over 43 years, this population experienced
two significant increases and collapses, as well as several less severe
fluctuations in size.
2,500
Moose population size
• Long-term population
studies have
challenged the
hypothesis that
populations of large
mammals are
relatively stable over
time
Steady decline probably
caused largely by wolf
predation
2,000
1,500
1,000
Dramatic collapse caused by severe
winter weather and food shortage,
leading to starvation of more than 75%
of the population
500
0
1960
CONCLUSION
1970
1980
Year
1990
2000
The pattern of population dynamics observed
in this isolated population indicates that various biotic and abiotic factors
can result in dramatic fluctuations over time in a moose population.
Age structure of populations
• The ratio of the various age classes to
each other in a population
• Age pyramids portray the agestructure of a population
• Ratio affects rates of population
growth
– Pre-reproductive (v. important)
– Reproductive
– Post-reproductive
• Mortality and natality rates differ for
different age groups
Population Growth Simulation
• Start by running the simulator to 2050 for all seven
countries (click on Run button). Record their population
growth rates at the end of the simulated period
– How do you suppose living conditions differ between the
country furthest along in the demographic transition compared
to the country earliest in the transition? How would living
conditions in these two countries affect both birth and death
rates?
– Think of social factors that contribute to lower birth rates in the
countries farther along. How might these social conditions be
encouraged to emerge in less developed countries?
– In general, how do the concepts of "early, middle, and late
demographic transition" map to the concepts of "first, second,
and third world countries"?
• Demographic transition
http://goo.gl/RvGWQ
Sources:
• http://www.mayoclinic.com/health/infectious
-diseases/DS01145/DSECTION=causes
• http://www.learner.org/courses/envsci/intera
ctives/disease/disease_help.php
• http://www.learner.org/courses/envsci/intera
ctives/demographics/demo_transition_1.php