Disease Ecology

Question 1

What is considered a disease?

Answer 1

an “atypical” condition in a living organism that causes some sort of a physiological impairment

Question 2

what are non-infectious diseases caused by?

Answer 2

environmental toxins, genetics, etc.

Question 3

what are infectious diseases caused by?

Answer 3

pathogens - include viruses, bacteria, worms, fungi, protists

Question 4

How are pathogens transmitted?

Answer 4

• bodily fluids/feces
• soil/water
• vector boune

Question 5

Zoonotic diseases

Answer 5

diseases transmitted from animals to humans

Question 6

What is disease ecology?

Answer 6

the ecological study of host-pathogen interactions within the context of their environment and evolution

Question 7

Examples of Zoonotic diseases?

Answer 7

• Nipah Virus (small fruit bats), a RNA virus
• Lyme disease (a bacterial infection, transmitted via deer ticks)
• malaria (caused by single-celled protists, transmitted to humans via mosquito)

Question 8

Two goals of disease ecology

Answer 8

1. to understand pathogen transmission and spread over time and space
2. the impacts on host populations

Question 9

Virulence

Answer 9

a pathogens ability to damage the host

Question 10

Define Direct Transmission

Answer 10

direct contact between hosts, via air, water, soil or other surfaces

Question 11

Define Indirect Transmission

Answer 11

indirect transfer of the infectious agent, e.g., by airborne, formites, animal vectors

Question 12

Horizontal Transmission

Answer 12

among individuals of the same generation

Question 13

Vertical Transmission

Answer 13

from mother (parent) to child (offspring) before or during birth

Question 14

Replication of Viruses

Answer 14

1. virus binds to and enters cells
2. the virus delivers its genome to a site where it can produce new copies of viral proteins and RNA
3. viral proteins and RNA assemble into new viral particles, and exit the host cell; via the cell wall bursting (aka ‘lysing’)

Question 15

Is COVID-19 vertically or horizontally transmitted?

Answer 15

mostly horizontally transmitted
- the virus enters the respiratory system via the human mouth or nose and then spreads from there
- vertical transmission is rare

Question 16

Does Pathogen transmission vary with Host Density?

Answer 16

• transmission of a pathogen increases (linearly or non-linearly) with host density
• the chance that an individual because infected by a pathogen is proportional to the number of possible hosts
• as a consequence, there exists a threshold density of hosts below which the pathogen cannot persist
• threshold host density (Nt) - minimum density of hosts required to sustain a disease within a population

Question 17

Density-dependent Transmission

Answer 17

can regulate host populations (i.e., reduce and hold them to a lower density than without the pathogen present) in the absence of any other density-dependent factors influencing host abundance

Question 18

Frequency-dependent Transmission

Answer 18

• transmission that is relatively unaffected by host density
• the per capita rate at which a susceptible individual becomes infected increases with the fraction of the host population that is infectious but does not increase with host density
• as a result, there is no threshold density for pathogens

Question 19

Endemic

Answer 19

constant presence but relatively low spread
- e.g., malaria in African Regions/ SE Asia

Question 20

Epidemic

Answer 20

sudden increase (high spread) in certain regions
- e.g., Lyme disease

Question 21

Pandemic

Answer 21

global epidemic
- e.g., swine flu (2009). COVID-19

Question 22

R0 - the basic reproduction rate of a pathogen

Answer 22

• describes the initial growth of a pathogen in a previously unexposed host population

Question 23

How is Pathogen success measured?

Answer 23

in terms of their host behaviour and pathogen traits

Question 24

R0 interpretation of a disease and under what assumptions?

Answer 24

average number of people that will be infected by a person with a disease under the following assumptions:
- in a naive population
- everyone is susceptible in that population
- w/o transmission interventions (e.g., vaccinations)

Question 25

How does R0 vary?

Answer 25

- spatially (from region to region) - temporally (over time) - based on model used

Question 26

How do we slow a disease?

Answer 26

1. behavioural change 2. herd immunity

Question 27

Isolation vs. Quarantine

Answer 27

- isolation is separating sick individuals from healthy individuals - quarantine is separating individuals who have been exposed to the illness, even is they are not presenting any symptoms of the disease

Question 28

What is Herd Immunity?

Answer 28

Herd immunity threshold (H): minimum percent of population that needs to be immunized to reach herd immunity

Question 29

Relationship between R0 and herd immunity thresholds ?

Answer 29

highly contagious pathogens have higher herd immunity thresholds because they spread fast

Question 29

When does Herd Immunity Occur?

Answer 29

when immunity rates are high enough that the population growth rate (r) of the pathogen is negative and may go extinct

Question 29

Host-pathogen evolution

Answer 29

parasites and hosts frequently evolve in response to one another and to their changing environments

Question 29

Exceptions for Vaccinations?

Answer 29

- babies cannot be vaccinated until a certain age - some individuals are allergic to components of certain vaccines, have an abnormal autoimmune response

Question 30

Why do parasites harm their host, given that they depend on their hosts for their own transmission?

Answer 30

- parasites should evolve to become benign and prolong the lives of their hosts - yet, many parasites cause substantial harm (e.g., parasite replication, damages host tissues and consumes host resources)

Question 31

What is a possible explanation for Virulence?

Answer 31

- parasites that replicate too slowly will not produce sufficient transmission stages - parasites that replicate too quickly will kill their host before they can transmit - selection may favour pathogens with intermediate levels of within-host replication that balance transmission benefits of higher replication with the costs of faster host deaths

Question 32

Tolerance vs. Resistance

Answer 32

- tolerance: the ability of a host to tolerate infection with a pathogen by minimizing the damage done but without impeding replication or transmission of the pathogen - resistance: reducing the probability that a host is infected, reduce pathogen replication within the host, and/or increase the speed of pathogen clearance (recovery)

Question 33

Reasons why hosts aren't more resistant to pathogens?

Answer 33

- trade-offs between resistance traits and other fitness-related traits - pathogen evolution to evade/counter host resistance - trade-offs among defenses aimed at different parasite types of strains

Question 34

Co-evolutionary dynamics of host-parasite interactions?

Answer 34

- increase of the genetic diversity of both hosts and pathogens via co-speciation events and genetic arms race

Question 35

How are the deaths of individuals in the host population broken down:

Answer 35

1. deaths that are unrelated tot he disease and occurs in unaffected and immune individuals 2. deaths that occur in infected individuals