Lecture Twenty-One - Host-Parasite Dynamics and Disease Flashcards
Describe microparasites.
- Small
- Usually intracellular
- Exist in vast numbers within hosts
What are the smallest parasites called?
Microparasites
Give three kinds of micro parasite.
- Measles (virus)
- Typhoid (bacterium)
- Plasmodium (protozoa)
What is prevalence?
The number of hosts parasitised at a particular time.
Describe macroparasites.
- Grow on or in hosts
- Create infective stages which transmit within hosts
Give four macroparasites.
- Tapeworms
- Ticks
- Common dodder (in plants)
- Mistletoe
What are some differences between the habitat of a free-living organism and a parasite’s habitat?
- Biology of the host can influence its susceptibility to the habitat, e.g. immune reactions, evolution.
- Host has its own population dynamics.
How do parasite-host models differ from predator-prey models?
- Contact between hosts results in transition between parasites; parasites depend on movement patterns of hosts themselves, rather than direct contact between predator and prey.
Give a species that requires a vector.
Plasmodium falciparum, causing malaria (requires mosquitos)
Give the three types of host in the host-parasite model.
- Susceptible hosts
- Infected hosts
- Resistant hosts
What are susceptible hosts?
Hosts that can pick up the infection.
What are resistant hosts?
Following infection, hosts become resistant and are no longer susceptible.
Give the equation for the change in the number of susceptible hosts over time.
dS/dt = b(S + I + R) - dS - BSI
B = parameter for rate of transition.
What is Beta in the parasite-host model?
Probability that contact between susceptible and infected host results in infection.
Give the equation for the change of the number of infected individuals over time.
dI/dt = BSI - dI - AI
In the parasite-host model, what is Alpha?
Declines via natural and parasite-induced mortality.
Give the equation for host-parasite model that takes into account the number of resistant individuals.
dI/dt = BSI - dI - AI - GI
In the host-parasite model, what is gamma?
The number of infected hosts becoming resistant.
Give the equation for the change in resistant individuals over time.
dR/dt = -dR + GI
What is the basic reproductive rate of a parasite?
The number of new infections arising from one infection.
What is R0 representative of?
The basic reproductive rate.
What is the basic reproductive rate determined by?
1) The number of susceptible individuals.
2) The transmission parameter.
3) The length of time that each infected individual remains contagious.
What is the value of R0 for
a) Measles?
b) Rubella?
c) Smallpox?
d) HIV/AIDS?
e) Ebola?
a) Measles = 12-18
b) Rubella = 5-7
c) Smallpox = 5-7
d) HIV/AIDS = 2-5
e) Ebola = 1.5-2.5
Which disease has the
a) Larges R0?
b) Smallest R0?
a) Largest R0 = Measles
b) Smallest R0 = Ebola
When does disease spread?
If R0 is greater than 1.
What is meant by low transmission threshold?
Does not require many individuals to be able to spread.
If a parasite has a large Beta, what does this signify?
That it is highly infectious
If a parasite has a large L, what does this signify?
It has a long period of infectiousness
What do species with a high transmission threshold require to persist?
A large host population
Give a disease with a high transmission threshold.
Measles (Morbillivirus)
What can be said about the epidemic curve for a species with high R0?
- Faster growth
- Faster removal of Susceptibles
What population number does Measles require to clear the number of susceptibles?
300,000
What is the control of disease outbreak dependent on?
Movement of infected individuals.
Give the equation for the minimum proportion of population to immunise.
pc = 1 - (1 / R0)
What is pc?
The minimum proportion of population to immunise.
For measles, approximately what percentage of the population needs to be immunised to eradicate the disease?
Approx 93%
Why might parasites or pathogens have an R0 of less than 1?
- Not adapted to the host
- Due to control measures, e.g. vaccination
- Environmental conditions of host dynamics do not allow spread.
What allowed HIV to spread so readily in Africa?
Changes to partner acquisition.
Give a species with cyclic population dynamics, and the parasite likely to cause this cyclic pattern.
Red grouse, caused by Nematode worms.
