Disease Ecology Flashcards
What is considered a disease?
an “atypical” condition in a living organism that causes some sort of a physiological impairment
what are non-infectious diseases caused by?
environmental toxins, genetics, etc.
what are infectious diseases caused by?
pathogens - include viruses, bacteria, worms, fungi, protists
How are pathogens transmitted?
- bodily fluids/feces
- soil/water
- vector boune
Zoonotic diseases
diseases transmitted from animals to humans
What is disease ecology?
the ecological study of host-pathogen interactions within the context of their environment and evolution
Examples of Zoonotic diseases?
- 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)
Two goals of disease ecology
- to understand pathogen transmission and spread over time and space
- the impacts on host populations
Virulence
a pathogens ability to damage the host
Define Direct Transmission
direct contact between hosts, via air, water, soil or other surfaces
Define Indirect Transmission
indirect transfer of the infectious agent, e.g., by airborne, formites, animal vectors
Horizontal Transmission
among individuals of the same generation
Vertical Transmission
from mother (parent) to child (offspring) before or during birth
Replication of Viruses
- virus binds to and enters cells
- the virus delivers its genome to a site where it can produce new copies of viral proteins and RNA
- viral proteins and RNA assemble into new viral particles, and exit the host cell; via the cell wall bursting (aka ‘lysing’)
Is COVID-19 vertically or horizontally transmitted?
mostly horizontally transmitted
- the virus enters the respiratory system via the human mouth or nose and then spreads from there
- vertical transmission is rare
Does Pathogen transmission vary with Host Density?
- 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
Density-dependent Transmission
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
Frequency-dependent Transmission
- 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
Endemic
constant presence but relatively low spread
- e.g., malaria in African Regions/ SE Asia
Epidemic
sudden increase (high spread) in certain regions
- e.g., Lyme disease
Pandemic
global epidemic
- e.g., swine flu (2009). COVID-19
R0 - the basic reproduction rate of a pathogen
- describes the initial growth of a pathogen in a previously unexposed host population
How is Pathogen success measured?
in terms of their host behaviour and pathogen traits
R0 interpretation of a disease and under what assumptions?
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)
How does R0 vary?
- spatially (from region to region)
- temporally (over time)
- based on model used
How do we slow a disease?
- behavioural change
- herd immunity
Isolation vs. Quarantine
- 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
What is Herd Immunity?
Herd immunity threshold (H): minimum percent of population that needs to be immunized to reach herd immunity
Relationship between R0 and herd immunity thresholds ?
highly contagious pathogens have higher herd immunity thresholds because they spread fast
When does Herd Immunity Occur?
when immunity rates are high enough that the population growth rate (r) of the pathogen is negative and may go extinct
Host-pathogen evolution
parasites and hosts frequently evolve in response to one another and to their changing environments
Exceptions for Vaccinations?
- babies cannot be vaccinated until a certain age
- some individuals are allergic to components of certain vaccines, have an abnormal autoimmune response
Why do parasites harm their host, given that they depend on their hosts for their own transmission?
- 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)
What is a possible explanation for Virulence?
- 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
Tolerance vs. Resistance
- 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)
Reasons why hosts aren’t more resistant to pathogens?
- 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
Co-evolutionary dynamics of host-parasite interactions?
- increase of the genetic diversity of both hosts and pathogens via co-speciation events and genetic arms race
How are the deaths of individuals in the host population broken down:
- deaths that are unrelated tot he disease and occurs in unaffected and immune individuals
- deaths that occur in infected individuals
