epidemiology of communicable disease

7/27/2019 Epidemiology of Communicable Disease

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EPIDEMIOLOGY OF

COMMUNICABLE DISEASE

Dr. Nelson E. Hora


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Chain of Infection

6 links

I. Etiologic agent

II. Reservoir

III. Portal of Exit

IV. Mode of Transmission

V. Portal of Entry

VI. Susceptible Host


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I. Etiologic Agents

seven categories of biological agentsthat can cause infectious diseases:

1. Metazoa

2. Protozoa3. Fungi

4. Bacteria

5. Rickettsia6. Viruses

7. Prions


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I. Etiologic Agent

Host-Parasite (Infectious Agent) Interactions

Infectivity

Ability of an agent to invade and multiply (produce infection)

in a host

Pathogenicity

Ability to produce clinically apparent illness

Virulence

Proportion of clinical cases resulting in severe clinical

manifestations (including sequelae)

Immunogenicity

Infections ability to produce specific immunity


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I. Etiologic Agent

Pathogenetic Mechanisms

1. direct tissue invasion

2. production of toxin

3. immunologic enhancement or allergic

reaction leading to damage to the host

4. persistent or latent infection

5. enhancement of host susceptibility to drugsof otherwise minimal toxicity

6. immune suppression


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II. Reservoir

Human

Animals

Environment


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II. Reservoir

Human Reservoir

Levels1. Colonization

2. Inapparent infection (covert or subclinical infection)3. Infectious disease

All infected persons, including those with

colonization only, are potential sources ofinfection to others


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II. Reservoir

Human reservoir

Carrier an infected person who does nothave apparent clinical disease but is,

nevertheless, a potential source of infection toothers

Type of Carrier Inapparent throughout

Incubatory carrier Convalescent carrier

Chronic carrier


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II. Reservoir

Animal Reservoirs

Zoonoses - infections transmissible under

natural conditions form vertebrate animals to

man

Environmental Reservoirs

Plant, soil and water


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III. Portal of Exit

The route by which the disease agent

may escape from the human or animal

reservoir

1. Respiratory

2. Genitourinary

3. Alimentary

4. Skin

5. Transplacental


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Necessary to bridge the gap between the

portal of exit form the reservoir and the

portal of entry into the host

Two Basic Modes

1. Direct

2. Indirect


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. Direct transmission

Consists of essentially immediate transfer of

an infectious agent form an infected host or

reservoir to an appropriate portal of entry

Include spray by droplets


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. Direct transmission

Person-to-person spread of disease

Three important aspects

Generation time

Herd immunity

Secondary Attach Rate


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Secondary attack

rate =

number of new cases in

group minus initialcase(s)

during specified time

periodnumber of susceptible

persons in group minus

initial case(s)*


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Indirect transmission

Vehicle borne

Vector borne

Air borne

2 types of particles implicated

Dust

Droplet nuclei


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V. Portal of Entry

Usually the same as the portal of exit form

the reservoir


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VI. Susceptible Host

Susceptibility is affected by:

Genetic factors

General resistance factors

Specific acquired factors


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THE INFECTIOUS DISEASE

SPECTRUM

Some do not become infected at all

Some become infected but develop no

symptoms

Some become infected and develop mild

or moderate symptoms

Some become infected and developsevere symptoms

Some die as a result of their infection


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SPECTRUM

Part of this variation is due to the capacity

of the agent to produce disease and to

differing levels of resistance of the hosts.


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SPECTRUM

The existence of the infectious disease spectrum

can make it challenging to find out the extent of

transmission in a particular population.

Most cases with inapparent or mild symptomswill never be discovered or reported, since these

people will not seek health care.

So when moderate or severe cases are reported

they may represent the tip of the iceberg


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SPECTRUM

Another challenge is posed by the fact that

many diseases look alike.

A variety of agents may produce

essentially similar clinical syndromes.

That is why laboratory identification of the

specific disease agent is so important in

any epidemiological investigation.

epidemiology of communicable disease

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