Transcript chap11-eks-3ed.ppt

Epidemiology Kept
Simple
Chapter 3
The Infectious Disease Process
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1
Reasons to Study Infx D. Epi
• Infectious disease
control, including
emerging agents and
bioterrorism
• To illustrate general
principals of
epidemiology
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2
What is Infection?
• Infection ≡ biologic agent is
living and replicating within a
host
• Contamination ≡ agent living
on exterior surface of host
• Silent Infection ≡ infection
without disease (“commensal”)
• Infectious disease ≡ infection
accompanied by pathology
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Pathos
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Components of the Infx
Disease Process
1.
2.
3.
4.
5.
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Agent
Reservoir
Portals of entry and exit
Mode of transmission
Immunity
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1. Agents (from large to small)
• Helminths (parasitic worms)
• Fungi & yeast (parasitic plants, lack
chlorophyll)
• Protozoa (eucaryotes; complex life
cycles)
• Bacteria (independent reproduction)
• Rickettsia (intracellular agents;
require Ixodes tick carrier)
• Viruses (submicroscopic; incapable
of multiplication outside of host)
• Prions (infectious proteins)
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2. Reservoirs
• Reservoir ≡ where the
agent multiplies and
perpetuates
• Types of reservoirs
–
–
–
–
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Cases [symptomatic]
Carriers
Animals & insects
Inanimate objects
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Carrier Reservoirs
• Carrier ≡ contagious
without discernable
signs
• Types of carriers
– Inapparent [throughout]
– Incubatory
– Convalescent
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Animals Reservoirs (Zoonoses)
• Zoonosis = an animal disease
transmissible to humans
Types of zoonoses
• Direct zoonosis:
vertebrate animal  human
(e.g., rabies)
• Cyclozoonoses:
 species #1 species #2
 human  species #1
(e.g., Echinococcus tapeworm,
right)
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Source: www.cdc.gov
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Types of Zoonoses (cont.)
• Metazoonoses
 vertebrate animal 
invertebrate
 human
e.g., malaria (right)
• Saprozoonoses
 vertebrate animal 
inanimate object  human
e.g. (coccidiomycosis) Valley
fever
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Inanimate Reservoirs
•
•
•
•
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Water
Food
Soil
Other
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Portals of Entry & Exit
•
•
•
•
•
•
•
Respiratory
Conjunctiva
Urogenital
Gastrointestinal
Skin
Placenta
Etc.
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Transmission by Contact
• Direct (host  host)
• Indirect (host  secretion
 host)
• Droplet (airborne, short
distance)
• Nuclei (airborne,
suspended)
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Transmission via Intermediaries
Vector (living)
Vehicles (inanimate)
Vectors types: mechanical, developmental, propagative, cyclopropagative.
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Transmission Dynamics
Common source
Serial transmission
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Transmission: Cycle
in Nature
Each agent has its unique cycle in nature
Example: the blood worm (Shistosoma sp.).
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Immunity
• Immunity ≡ all factors that alter likelihood and
severity of infection after host is exposed
• Types of immunity (figure)
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Innate Immunity
• Physical barriers:
skin, cilia, mucosal,
sheaths
• Chemical barriers:
acidity, enzymatic,
etc.
• Non-specific cellular
& physiologic
responses: phages,
polymorphs,
inflammation
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Acquired Immunity
Cellular (immunocytes)
Lymphocytes
Granulocytes
Non-cellular (humoral)
Antibodies
Cytokines
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Interaction of Innate & Acquired
Immunity
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Immunization
• Immunization ≡ the act of acquiring immunity
• Active immunization ≡ host response to exposure
(Natural exposure or artificial vaccination)
• Passive immunization ≡ receipt of immunity products
from others (Therapeutic e.g., anti-serums or maternal
(transplacental, colostrum)
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Types of Vaccines
• Killed vaccine: killed
agent, not capable of
self-replication
• Modified live
vaccine: attenuated
version of agent
capable of replication
• Toxoid: denatured
toxin (no agent)
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Herd Immunity
Non-susceptible individuals represented by dark circles.
You do not need to vaccinate the
entire herd to achieve infection
control if the agent meets a lot of
dead ends
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Herd Immunity Animation
• http://www.immunisation.nhs.uk/About_Im
munisation/Science/Herd_immunity__animation
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