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Week 3 - Module 3 (Lec 8) (Boundaries of Epidemiology (Epidemiology…
Week 3 - Module 3 (Lec 8)
Boundaries of Epidemiology
Epidemiology provides a profile of the type of individual who is likely to contract a disease after being exposed to an agent
time, place, demographics
Identifies agents that are associated with an increased risk of disease in groups of individuals
Quantifies the amount of excess disease that i associated with the agent
Disease Ecological Equilibriums
Host = population density, immune (herd) status, sub-populations: age, gender
Environment = food availability, terrain, climate
Pathogen = prevalence, virulence, transmissibility, environmental survival
Agent and host classification
Agent
Pathogenicity
severity of disease of the same
agent strain in populations of other host
types (Heterogeneities of HOST RANGE)
Virulence
describes severity of disease
produced by different strains of an infectious
agent in the same host type. Virulence is a
characteristic of the organism.
Host
Reservoir Host
Reservoir hosts are those that serve as a habitat for
the infectious agent and are a common source of infection to other
species.
Definitive Host
a species in which a parasite undergoes the sexual
phase of reproduction.
Intermediate Host
one in which the infectious agent undergoes
development, and possibly replication, without sexual reproduction.
Paratenic Host
is one in which the agent is transferred mechanically, ie
without further development. A paratenic host is not essential for the
long term survival of the infectious agent species.
SEIR model for an immunizing infection
S – Susceptible
E – Exposed (infected)
I – Infectious
R –Recovered (Immune)
S-E interaction:
Routes and Mechanisms of Transmission
E-I interaction:
Latent and Incubation period
Incubation period (A): from exposure to
onset of clinical signs
• Latent period (B): from exposure to onset
of agent shedding
Zoonoses
Diseases of animals transmitted to humans
Direct Transmission
"true" zoonoses = animal reservoirs e.g. rabies
"reverse" zoonoses = human reservoir e.g. cysticercosis
Indirect Transmission
Infection passed from animal to human via a third party
Contagiousness
Contagious
Infectious agent
transferred from one
animal to another by
direct contact or via
fomites
Non-contagious
Infectious agent not
transferred from one
animal to another by
direct contact or via
fomites
Force of infection
Probability for a susceptible host to acquire the infection
Effective Contact Rate
= m x (I / N) x t
m : “mixing” rate (contact pattern)
– I / N : proportion of contacts with infectious hosts
– t : probability of transmission of the infection once a
contact is made between an infectious host and a
susceptible host
Disease
Clinical disease
Percute
Acute
Sub acute
Chronic
Other
Sub clinical
Factors influencing infectiousness
Transmission and entry
Infectivity
Probability of contact
Contact rate
Duration of shedding