Disease Transmission Flashcards
Factors in Disease Dynamics
changing hosts, pathogens, vectors, environments; plus new emerging diseases
How has disease changed in the US since the 1900s?
we have moved from more infectious causes of death (flu, pneumonia, TB) to more chronic disease (heart disease, cancer)
5 Drivers of Infectious Disease Emergence
- Climate Change
- Global trade and travel
- Migration, social unrest
- Human Population Growth
- Urbanization
3 Types of Prevention
- Primary
- Secondary
- Tertiary
Primary Prevention
protect the uninfected to prevent development of disease (ex: vaccination, biosecurity measures, closed herds)
Secondary Prevention
detect the preclinical cases to reduce severity/complications
Tertiary Prevention
reduce the impact of clinical disease with treatments, therapies, and rehab
John Snow and Cholera in London
in 1855+, was able to determine an unknown organism was transmitting through the water supply via an unknown mechanism by mapping out neighborhood pumps and deaths
When did germ theory come about?
1890s
DO you have to know the causative agent to do anything about disease spread?
No - outbreak can be stopped by understanding disease patterns without the knowledge of the causative agent
Smith, Kilbourne, and Curtis
determined that cattle dying from Texas fever and the natural range of cattle ticks overlapped geographically, so hypothesized ticks caused disease - this was the first determination of an infectious arthropod vector; also established life cycle of the tick
Babesia bigemina
agent transmitted by ticks that causes Texas Fever
endemic
usual (habitual) occurrence of a disease within a given area (what are normal levels?)
sporadic
separate/scatted disease incidents occurring at a low frequency
epidemic/outbreak
more disease than expected for a given time and place
pandemic
epidemic affecting several continents
reservoir
any animal/arthropod/plant/soil/inanimate matter in which an infectious agent normally lives and multiplies
nidus
localized reservoir that persists over a long time period
vehicle
object/substance/non-receptive living being that serves as an intermediate in transmitting pathogen from host to host
vector
a LIVING creature which acquires a pathogen from one living host and transmits it to another
fomite
object/substance that by itself is not harmful but on which pathogens may be conveyed (a type of vehicle, technically)
carrier
individual which harbors the organism and can infect others, but is not clinically ill
incubationary carrier
preclinical, many organisms shed
convalescent carrier
harbors and sheds organisms for a period of time following recovery
transient carrier
subclinical but organisms are shed for variable periods of time
chronic carrier
postrecovery animals who shed organism for a long period of time
incubation period
time between exposure to pathogen and the onset of clinical signs
latent infection
inapparent infection that has potential to develop signs of disease
source of infection
animal/person/object from which an infectious agent passes immediately to the host
Use
extrinsic host factor for animals synonymous with occupation in humans
Husbandry
extrinsic host factor for animals synonymous with socio-economic status in humans (ex: nutrition, crowding, open or closed herds, ventilation, vax status, handling, etc)
pathogenicity
ability to produce disease
virulence
degree of pathogenicity, or the severity of a disease
Case-Fatality Rate (CFR)
number of deaths among cases; way to evaluate agent virulence
host range
range of hosts in which an agent can survive
viability
ability of an infectious agent to survive in the “free” state, aka environment
infectivity
ability of agent to lodge and multiply within the host; minimum number of infectious particles required to establish an infection
infectiousness
ease with which the agent is transmitted from one host to another
2 Ways to Measure Infectiousness
- Secondary Attack Rate
- Basic Reproduction Number (Ro)
The Basic/Effective Reproduction Number (Ro)
measure of infectiousness of an agent in a totally susceptible population; the number of new cases a single case will generate during its infectious period
What must the Ro be for a disease to die out/be removed from a population?
Ro less than 1
Ro of Foot and Mouth Disease?
infinity (one of the most infectious animal diseases)
Non-Pharmaceutical Interventions (NPI)
think COVID, when they made everyone stay home, no events, schools were closed, and everything was friggin lame
Herd Immunity
resistance of an “at risk” population to an attack by a disease to which a large proportion of the members of the group are immune
Most critical factor in herd immunity?
frequency of contact
Can herd immunity still be achieved if new susceptible animals occurs?
no
Horizontal Transmission
among “peers”
Vertical Transmission
through generations
Examples of Indirect Modes of Transmission (2)
common vehicle, vector
Fragile infectious agents are best/only transmitted via what mode?
direct transmission
Expiratory Droplets
produced by cough/sneeze, short airborne life, evaporate before hitting the ground; needs direct transmission
Droplet Nuclei
evaporated remains of expiratory droplets/other fluids, they are truly airborne, small; via indirect tranmission
Which is more infectious - expiratory droplets or droplet nuclei?
droplet nuclei (by about ten times)
6 Requirements for Disease Spread
- Causative agent capable of infecting host
- Reservoir/source of agent
- Mode of escape from reservoir or intermediate source
- Mode of transmission to new host
- Mode of entry to new host
- Susceptible host
disease
abnormality of structure and/or function that interferes with the well-being of an individual animal
preclinical disease
not yet clinically apparent but destined to be
subclinical disease
not clinically apparent and not destined to be
