Lecture 13: Disease Reservoirs and Transmission Flashcards
Disease transmission is a result of interaction between
host, agent, and environment
Infectious disease
disease caused by the invasion and multiplication of a living agent in/on a host
Infestation
Invasion, but not multiplication of an organism in/on a host (fleas/ticks, some parasites)
Contagious
disease transmissible from one human/animal to another via direct or airborne routes
Communicable
disease caused by an agent capable of transmission by direct, airborne, or indirect routes from an infected person, animal, plant or a contaminated inanimate reservoir
Zoonotic disease
disease that is transmitted from animals to humans
Latent period
microbe is replicating but not yet enough for the host to become infectious
Incubation period
microbe is replicating but not symptomatic yet.
Does the incubation period always correlate with the latent period?
Nope
Reservoir
- Habitat in which an infectious agent normally lives, grows, and mulitplies
- Maintain pathogens over time, from year to year or generation to generation
How do pathogens make it so that animals become susceptible again and again
They mutate
How do pathogens allow infections to occur after a short time period?
They evade immunity
Balanced pathogenicity
Chronic infections with minimal symptoms
Are all sick animals reservoirs?
Nope
Three questions that you must answer yes to in order for soemthing to be a reservoir
- Is it naturally infected with the pathogen?
- Can the species of animal maintain the pathogen over time?
- Can this source transmit disease to a new, susceptible host?
Two main types of transmission
- Vertical
2. Horizontal
Two types of horizontal transmission
- Direct
- Indirect
Vertical transmission
From a reservoir host to its offspring
Congenital transmission
- Vertical
- Some pathogens can cross the placenta, infect eggs, ect
Perinatal transmission
- Vertical
- During parturition, via colostrum
Horizontal transmission
from reservoir to new host
Direct horizontal transmission
Directly from the reservoir to a susceptible host
Indirect horizontal transmission
Via any sort of intermediary, animate or inanimate
Three types of horizontal/direct transmission
- Direct transmission - contact
- Direct projection - droplet spread
- Airborne - Considered to be a form of direct
Vehicle
Inanimate object which communicates the disease
Vector
Living organism that communicates the disease
Two types of vehicles
- Common - food, water, contaminated IV drugs
2. Fomites - object that can be contaminated and transmit disease on a limited scale (ex. door knob)
Most vectors are
arthropods (mosquitoes, flies, fleas, ants, ticks)
Mechanical vector
Agent does not multiply or undergo part of its life cycle while in/on arthropod
Biological vector
Agent undergoes changes or multiplies while in the vector and these changes are required for transmission
Emerging disease
- Previously unknown disease that suddenly appears
- Known disease that suddenly appears in a new population
Re-emerging disease
-Known disease, previously on the decline, that is becoming more common and will likely continue to do so
Exposure
- Introduction of a new agent into a susceptible population
- Exposure is caused by movement of infectious agents
New pathogens originate from
old pathogens
Transmission
- Adoption, establishment, and dissemination in the susceptible population
- Requires a pathogen that can adapt to, and transmit between, these hosts
7 determinants of emergence
- Type of agent (pathogen)
- Mutation/change (pathogen)
- Phylogenetic distance (reservoir)
- Reservoir size (transmission)
- Pathogen prevalence (transmission)
- Contact frequency (transmission)
- Susceptibility (host)
What percentage of human pathogens are zoonotic
61%
What percentage of emerging diseases are zoonotic
75
How likely are zoonotic diseases to be associated with emerging diseases?
Twice as likely
What do mutations do for pathogens
- Increased antibiotic resistance
- Increased virulence (transmissibility within or between species)
- Evasion of host immunity
Phylogenetic distance between reservoir and new host
- Best transmission is within species
- More likely to be crossed to closely related species
Pathogens that cross very different species often cause
Very different, often more severe, disease
Two reasons for new host susceptibility
- Intensive agriculture
- More populations with weakened immune systems
Three factors that increase transmission
- Increasing abundance of reservoir
- Increasing pathogen prevalence in reservoir
- Increasing contact between the reservoir and new host
