Chapter two cdc course

Addis Ababa Medical University College
Hargeisa Campus
Course: Communicable Disease Control
For Health Officers
Instructor: Ahmed Ismail Batun
(Mph, PhD Candidate)
Cell Phone : 063 4420289
E-mail shaafi88@hotmail.com

CHAPTER TWO
TRANSMISSION, PREVENTION AND CONTROL OF
COMMUNICABLE DISEASES
Learning Objectives
At the end of this chapter the student will be able to:
•Define communicable disease.
•Describe the factors involved in the chain of
communicable disease transmission.
•Identify the different levels of disease prevention.
•Apply the different control methods of
communicable diseases.

Communicable Diseases
These are illnesses due to specific infectious agents
or its toxic products, which arise through
transmission of that agent, or its toxic products
from an infected person, animal or inanimate
reservoir to a susceptible host, either directly or
indirectly, through an intermediate plant or animal
host, vector or inanimate environment.

Chain of Disease Transmission
This refers to a logical sequence of factors or links
of a chain that are essential to the development
of the infectious agent and propagation of
disease. The six factors involved in the chain of
disease transmission are:
a. Infectious agent
b. Reservoir
c. Portal of exit
d. Mode of transmission
e. Portal of entry
f. Susceptible host

a.Infectious agent:
Any microorganism that can cause a disease such as
a bacterium, virus, parasite, or fungus. Reasons that
the organism will cause an infection are virulence
(ability to multiply and grow), invasiveness (ability
to enter tissue), and pathogenicity (ability to cause
disease).
a.Reservoir of infection:
Any person, animal, arthropod, plant, soil or
substance (or combination of these) in which an
infectious agent normally lives and multiplies, on
which it depends primarily for survival and where it
reproduces itself in such a manner that it can be
transmitted to a susceptible host

Types of Reservoirs
1. Man: There are a number of important pathogens that
are specifically adapted to man, such as: measles,
smallpox, typhoid, meningococcal meningitis, gonorrhea
and syphilis. The cycle of transmission is from human to
human.
2. Animals: Some infective agents that affect man have
their reservoir in animals. The term “zoonosis” is applied
to disease transmission from animals to man under
natural conditions. For example:
Bovine tuberculosis - Cow to man
Brucellosis - Cows, pigs and goats to man
Anthrax - Cattle, sheep, goats, horses to man
Rabies - Dogs, foxes and other wild animals to man
Man is not an essential part (usual reservoir) of the
lifecycle of the agent.

3. Non-living things as reservoir: Many of the agents
are basically saprophytes living in soil and fully
adapted to live freely in nature. Biologically, they are
usually equipped to withstand marked
environmental changes in temperature and
humidity. For Example
Clostridium botulinum etiologic agent of Botulism
Clostridium tetani etiologic agent of Tetanus
Clostridium welchi etiologic agent of gas gangrene

c. Portal of exit :
This is the site through which the agent
escapes from the reservoir. Examples include:
GIT: typhoid fever, bacillary dysentery,
amoebic dysentery, cholera, ascariasis, etc.
Respiratory: tuberculosis, common cold, etc.
Skin and mucus membranes: Syphilis

d. Mode of transmission:
Refers to the mechanisms by which an infectious
agent is transferred from one person to another or from a
reservoir to a new host. Transmission may be direct or
indirect.
1. Direct transmission: Consists of essentially immediate
transfer of infectious agents from an infected host or
reservoir to an appropriate portal of entry. This could
be:
a. Direct Vertical: Such as transplacental transmission of
syphilis, HIV, etc.
b. Direct horizontal: Direct touching, biting, kissing, sexual
intercourse, droplet spread onto the conjunctiva or onto
mucus membrane of eye, nose or mouth during sneezing
coughing, spitting or talking; Usually limited to a distance of
about one meter or less.

2. Indirect transmission
a. Vehicle-borne transmission: Indirect contact
through contaminated inanimate objects
(fomites) like:
 Bedding, toys, handkerchiefs, soiled clothes,
cooking or eating utensils, surgical instruments.
 Contaminated food and water
 Biological products like blood, serum, plasma or
IV- fluids or any substance serving as intermediate
means by which an infectious agent is transported
and introduced into a susceptible host through a
suitable portal of entry. The agent may or may not
multiply or develop in the vehicle before it is
introduced into man.

b. Vector-borne transmission: Occurs when the infectious agent is
conveyed by an arthropod (insect) to a susceptible host.
1. Mechanical transmission: The arthropod transports the
agent by soiling its feet or proboscis, in which case multiplication of
the agent in the vector does not occur. (e.g. common house fly.)
2. Biological transmission: This is when the agent multiplies in the
arthropod before it is transmitted, such as the transmission of
malaria by mosquito.
C. Air-borne transmission: Dissemination of microbial agent by air to
a suitable portal of entry, usually the respiratory tract. Two types of
particles are implicated in this kind of spread: dusts and droplet
nuclei.
Dust: Small infectious particles of widely varying size that may arise
from soil, clothes, bedding or contaminated floors and be
resuspended by air currents.
Droplet nuclei : Small residues resulting from evaporation of fluid
(droplets emitted by an infected host). They usually remain
suspended in the air for long periods of time.

Portal of entry:
Successful transmission of the infectious agent requires it to
enter the host through a specific part of the body before it can
cause disease. The routes of entry are:
The respiratory tract: some infectious agents enter the body
in air breathed into the lungs. E.g Mycobacterium tuberculosis.
The gastrointestinal tract: some infectious agents enter
through the mouth. Example: the infectious agents causing
diarrhoeal diseases enter through the mouth in contaminated
food, water or on unclean.
The skin provides a natural barrier against entry of many
infectious agents, but some can enter through breaks in the
skin. Example: malaria parasites (Plasmodium species) get into
the body when an infected mosquito bites through the skin to
suck blood.

f. Susceptible host: The person who is at risk
for developing an infection from the disease.
Several factors make a person more susceptible
to disease including age (young people and
elderly people generally are more at risk),
underlying chronic diseases such as diabetes or
asthma, conditions that weaken the immune
system like HIV, certain types of medications,
invasive devices like feeding tubes, and
malnutrition
.

Carrier and Its Type
A carrier is an infected person or animal who does not have
apparent clinical disease but is a potential source of infection to
others. There are four types of carriers.
a. Healthy or asymptomatic carriers: These are persons whose
infection remains unapparent. For example, in poliovirus, hepatitis
virus infections, and meningococcus, there is a high carrier rate.
b. Incubatory carriers: These are individuals or persons who excrete
the pathogen during the incubation period (i.e. before the onset of
symptoms or before the characteristic features of the disease are
manifested). E.g. Measles, mumps, chickenpox and hepatitis.
c. Convalescent Carriers: These are those who continue to harbor
the infective agent after recovering from the illness. E.g. Diphtheria,
Hepatitis B virus infection.
d. Chronic Carriers: The carrier state persists for a long period of
time. E.g. Typhoid fever, Hepatitis B virus infection

Time Course of Infectious Diseases
Incubation period: It is the interval of time between
infection of the host and the first appearance of
symptoms and signs of the disease.
 Prodormal period: It is the interval between the
onset of symptoms of an infectious disease and the
appearance of characteristic manifestations. E.g., in
a measles patient, fever and coryza occur in the first
three days and Koplick spots in the buccal mucosa and
characteristics skin lesions appear on the fourth day.
Period of communicability: The period during which
that particular communicable disease (infectious agent)
is transmitted from the infected person to the
susceptible host

ASSIGNMENT 1 FOR GROUP 1
Disease Prevention
Introduction
The concept of prevention
The levels of prevention
• Aims of prevention
• Phase of disease
• Target
• Intervention Examples
Prevention and public health
Benefits of prevention

Communicable Disease Control
This refers to the reduction of the incidence and
prevalence of communicable disease to a level
where it cannot be a major public health problem.
Methods of Communicable Disease Control
There are three main methods of controlling
communicable diseases:
1. Elimination of the Reservoir
2. Interruption of transmission
3. Protection of susceptible host

1. Elimination of the Reservoir
a. Man as reservoir: When man is the reservoir, eradication of
an infected host is not a viable option. Instead, the following
options are considered:
Detection and adequate treatment of cases: arrests the
communicability of the disease (e.g. Treatment of active
pulmonary tuberculosis).
 Isolation: separation of infected persons for a period of
communicability of the disease. Isolation is indicated for
infectious disease with the following features:
- High morbidity and mortality
- High infectivity
 Quarantine: limitation of the movement of apparently well
person or animal who has been exposed to the infectious
disease for a duration of the maximum incubation period of
the disease.

b. Animals as reservoir: Action will be determined by the
usefulness of the animals, how intimately they are
associated to man and the feasibility of protecting
susceptible animals. For example:
Plague: The rat is regarded as a pest and the objective
would be to destroy the rat and exclude it from human
habitation.
Rabies: Pet dogs can be protected by vaccination but
stray dogs are destroyed.
Infected animals used for food are examined and destroyed.
c. Reservoir in non-living things: Possible to limit man’s
exposure to the affected area (e.g. Soil, water, forest, etc.).

2. Interruption of transmission
This involves the control of the modes of transmission from
the reservoir to the potential new host through:
Improvement of environmental sanitation and personal
hygiene
Control of vectors
Disinfections and sterilization
3. Protection of susceptible host: This can be achieved
through:
Immunization: Active or Passive
Chemo-prophylaxis- (e.g. Malaria, meningococcal
meningitis, etc.)
Better nutrition
Personal protection. (e.g. wearing of shoes, use of
mosquito bed net, insect repellents, etc.)

Chapter two cdc course

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Chapter two cdc course