Associate Professor

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Dr. Salwa Tayel
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Infectious Disease Epidemiology
By
Dr. Salwa Tayel
Associate Professor Family and Community
Medicine Department King Saud University
Dr. Salwa Tayel
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Learning Objectives

• At the end of this lecture you (will) be able to
• Explain common definitions and basic concepts
  used in epidemiology.
• Describe natural history and spectrum of
  infectious diseases and their implications for
  public health.

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• Infection
• It is the entry, development and multiplication
  of an infectious agent in the body of man or
  animal.
• Outcome of infection varies.
• Infectious disease
• A clinically manifest disease of man or animal
  resulting from infection.
• In-apparent infection
• The infection does not become manifest at any
  stage.

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Communicable Disease

• An illness due to a specific infectious agent or
  its toxic products that arises through
  transmission of that agent from reservoir to
  susceptible host.
• Communicable Disease (typhoid, influenza,..
• Non- Communicable Disease (DM, cancer,..

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Natural history

• Natural history of disease refers to the progress
  of a disease process in an individual over time,
  in the absence of intervention.
• The natural history of a disease describes the
  course of the disease in an individual starting
  from the moment of exposure to the causal agents
  till one of the possible outcomes occurs.

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Spectrum of disease

• The idea that an exposure can lead to varying
  signs, symptoms and severity of the same disease
  in the population is the spectrum of disease.
• Why do we have varying degrees of severity?
  Prognosis?
• The outcome will depend on the interactions of
  host, agent and environmental factors.

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Classification of diseases according to clinical
severity (spectrum)
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Class A Inapparent infection
Examples Tuberculosis, Polio, Hepatitis A,
Meningitis, AIDS
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Class B Classic cases
Examples Measles, Chickenpox
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Class C Severe or Fatal infections
Examples Rabies, Hemorrhagic fevers caused by
Ebola and Murberg viruses.
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Implications for public health
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The relation of severity of illness to disease
statistics.
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The pyramid and iceberg of disease
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Iceberg phenomenon

• Cases of illness correctly diagnosed by
  clinicians in the community often represent only
  the tip of the iceberg.
• Many additional cases may be too early to
  diagnose or may remain asymptomatic.
• Examples Tuberculosis, meningitis, polio,
  hepatitis A, AIDS.
• The risk is that persons with in-apparent or
  undiagnosed infections may be able to transmit
  infection to others.

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Applications of the concepts of natural history
and spectrum of disease

• Persons with in-apparent or undiagnosed
  infections can transmit infections to others.
• Control measures must be directed toward all
  infections capable of being transmitted to
  others
• both clinically apparent cases and
• those with in-apparent or undiagnosed infections.

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Chain of infection
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Chain of infection A process that begins when an
agent leaves its reservoir or host through a
portal of exit, and is conveyed by some mode of
transmission, then enters through an appropriate
portal of entry to infect a susceptible host.
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Cycle of infection
Agent
Susceptible Host
Reservoir
IP
PC
Portal of Inlet
Portal of Exit
Mode of transmission
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• The requisites (essentials) for the perpetuation
  of communicable diseases
• The elements of the cycle of infection
• Presence of microbiological agent.
• Presence of reservoir.
• Portal of exit.
• Mode of transmission.
• Portal of entry (inlet).
• Presence of susceptible host.

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1. Agent

• Microorganisms are responsible for disease
  production (viruses, bacteria, protozoa,
  parasites, fungi,..

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• Mechanisms of disease production (pathogenesis)
• Invasiveness (Pneumococcosis, measles).
• Toxicity (Tetanus, Botulism).
• Hypersensitivity (Tuberculosis).
• Others (Immune suppression AIDS).

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Agent factors that affect disease transmission

• Infectivity,
• Pathogenicity,
• Virulence,
• Antigenicity,

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Infectivity The ability of an agent to invade and
multiply (produce infection) in a susceptible
host. How to measure (Infectivity) ease spread
of infection? Secondary Attack Rate The
proportion of exposed susceptible persons who
become infected. Examples High infectivity
Measles, Chickenpox Low infectivity
Leprosy
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• Pathogenicity
• Is the ability of the organisms to produce
  specific clinical reaction after infection
• It refers to the proportion of infected persons
  who develop clinical disease.
• Examples
• High pathogenicity Measles, Chickenpox (Class B)
• Low pathogenicity Polio, Tuberculosis, Hepatitis
  A, Meningitis, AIDS (Class A)
• It can be measured by

Ratio of clinical to sub-clinical case
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• Virulence
• It refers the ability of organisms to produce
  severe pathological reaction.
• It is the proportion of persons with clinical
  disease who become severely ill or die.
• Examples Rabies, Hemorrhagic fevers caused by
  Ebola and Murberg viruses. (Class C)

It can be measured by Case fatality rate
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Antigenicity (Immunogenicity) The ability of the
organism to produce specific immunity
(antibodies or antitoxin). It can be measured
by Second attack frequency Second attacks are
rare in measles, mumps and chickenpox.
Re-infection occurs as in case of common cold,
syphilis and gonorrhea.
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2. Reservoir of infection
The reservoir of an agent is the habitat in which
an infectious agent normally lives, grows, and
multiplies. Types of reservoirs Humans,
animals, and the environment.
Human reservoirs
Two types of human reservoir exist Cases
persons with symptomatic illness Carriers
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Pathogen Reservoirs
Humans are the most important reservoir of human
infectious disease.
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Carrier

• A person or animal without apparent disease who
  harbors a specific infectious agent and is
  capable of transmitting the agent to others.

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• Carriers are dangerous because
• They do not show any clinical manifestation so
  they carry normal life.
• The carrier and his contacts are not aware of
  their condition so, they take no precautions.
• It is difficult to discover them.
• It is not always possible to deal with them.
• Chronic carriers can remain infectious for a long
  time leading to repeated introduction of the
  disease to contacts.

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• Types of Carriers
• Asymptomatic (In-apparent) carrier
• The carrier state that may occur in an
  individual with an infection that is in-apparent
  throughout its course
• Examples Polio virus, meningococcus, hepatitis
  A virus
• Incubatory, Convalescent, Post-Convalescent
  carriers
• The carrier state may occur during the incubation
  period, convalescence, and post convalescence of
  an individual with a clinically recognizable
  disease.
• Examples of Incubatory carrier Measles,
  chickenpox

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Convalescent carriers Examples Diphtheria,
hepatitis B viruses and Salmonella
species According to duration of carriage The
carrier state may be (transient carrier or
chronic carrier). Chronic carriers They
continue to harbour an agent for an extended time
(months or years) following the initial
infection. Examples Hepatitis B virus and
Salmonella typhi
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Animal reservoirs Zoonoses Infectious diseases
that are transmissible under normal conditions
from vertebrate animals to humans. (with humans
as incidental hosts)

• Zoonotic diseases include
• brucellosis (cows and pigs),
• anthrax (sheep),
• plague (rodents),
• rabies (bats, dogs, and other mammals).

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Zoonoses
Zoonoses are Human Diseases with Animal
Reservoirs.
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Zoonoses
Toxoplasmosis
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• Environmental reservoirs
• Soil, and water
• Soil Agents live and multiply in the soil.
• Examples
• -Tetanus spores and
• - Fungal agents (those causing histoplasmosis)
• Pools of water are the primary reservoir of
• Legionnaires bacillus.

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Bibliotheca Alexandrina
Thank you
Website http//faculty.ksu.edu.sa/73234/default.as
px salwatayel_at_hotmail.com
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