Download Infectious Disease Summary

Infectious Disease Summary
N. Birkett, Back-to-Basics, 2008
 Terminology
 Nature
of diseases
 Outbreaks/epidemics


Identification
Methods of control
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Terminology (1)
 Infectivity


The ability of an agent to invade and multiply
in a host (an infection).
Dose of organism required to establish
infection in 50% of animals.
 Pathogenicity

The ability of an agent to produce clinically
apparent illness.
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Terminology (2)

Virulence


The proportion of clinical cases which produce severe
disease and/or permanent sequelae.
Immunogenicity



The ability of an agent to produce specific immunity
against the agent
Can be produced in general body or within specific
sites such as the GI tract.
Determines the ability of an agent to re-infect a host
• Measles vs. gonorrhea
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Terminology (3)

RESERVOIR


FOMITES (Vehicle)


Living organisms or inanimate matter in which
infectious agent normally lives and multiplies
Inanimate objects contaminated with infectious agent
(not the reservoir). Example would be toys in a
daycare centre.
VECTOR

An animate source of an infectious agent. The vector
may be infected with the organism (e.g. mosquitoes
and malaria) or just be a mechanical carrier (e.g.
flies). There is disagreement about whether vectors
are restricted to insects or can also include small
mammals.
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Terminology (4)

ZOONOSES


CARRIERS


Diseases transmitted to humans from animals (e.g.
anthrax)
An infected person without apparent clinical disease
who remains infectious (e.g. Typhoid Mary)
INDEX CASE

The person (case) who brings the infection to the
attention of the medical community or the public
agency. Sometimes used to refer to the person who
brings the infection into a community. This will often
(but not always) be the same person.
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Terminology (5)

ATTACK RATE

The probability that people will get ill from the
disease. Usually applied in an outbreak situation.


# new cases in group
# people in group
SECONDARY ATTACK RATE

Probability of infection in a closed group who are at
risk but excluding the index case(s). Formula is:

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# new cases in group - index case(s)
# people in group - index case(s)
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Terminology (6)
 CASE

FATALITY RATE
The probability of death in people with an
infection.
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Pathogenic Mechanisms (1)
 Direct
Tissue Invasion
 Production of Toxins
 Allergic Host Reaction
 Resistant/latent infection (carriers)
 Enhancement of host susceptibility to
drugs (e.g. Reye’s syndrome and ASA).
 Immune Suppression
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Reservoirs (examples of
transmission patterns)
 Human
Human
Human
 Animal
Animal
Human
Animal
Animal
Vector
Vector
Human
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Epidemics (1)

Epidemic (now often called ‘outbreak’)


Endemic


the occurrence of a disease/condition at a relatively
constant level in a given setting
Pandemic


the occurrence in a community or region of cases of a
disease/condition/behaviour clearly in excess of
normal expectancy
an epidemic covering a very wide area and affecting a
large proportion of the population
Pathogen

Infectious and non-infectious substance capable of
producing tissue damage or initiating a process which
can lead to a disease.
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Epidemics (2)
 Common
conditions increasing likelihood
of an epidemic



The introduction of a new pathogen or an
increased amount of, or a change in the
virulence of, a pathogen.
An adequate number of exposed and
susceptible persons.
An effective means of transmission between
the source of the pathogen and the
susceptible person.
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Epidemics (3)

Incubation period and causal agent
Time frame
Examples
Hours
Food toxins
Heavy metals
Days
Bacterial infections
Salmonella / cholera
Weeks
Measles / mumps / Hep A
Months
Hep B / Rabies
Years
Kuru / cancer
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Epidemics (4)

Types of epidemics

Common source
• Point source
• Ongoing exposure
• Need not be geographically localized


Propagated/progressive
Mixed

Epidemic curve
 Spot maps
 Note that epidemics can arise from behaviour as
well as from traditional infectious sources.
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Epidemic Curves: point source (1)
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Epidemic Curves: propagated (2)
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Factors Influencing Spread of
Disease in a Population (1)

Period of infectivity in relationship to symptoms

Includes consideration of carrier states

Herd immunity
 Type of spread





Person to person
Common vehicle
Vector-borne
Zoonoses
Transmission mechanics

Consider sexual vs. droplet spread
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Herd Immunity
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Epidemics (6)
 Two

strategies to control disease spread
Isolation
• Separation of infected persons or animals from
other susceptible people/animals during the period
of communicability.

Quarantine
• Restrictions on the activities of well people who
have been exposed to a communicable disease,
during its period of communicability.
 Immunization
 Chemoprophylaxis
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