1.An introduction to communicable disease control

Communicable Disease Control
by: Aschalew M.

Course Chapter Contents
1. INTRODUCTION
2. ORAL-FECAL TRANSMITTED DISEASES
3. AIR-BORNE DISEASES
4. VECTOR-BORNE DISEASES
5. SEXUALLY TRANSMITTED DISEASES
6. ZOONOTIC DISEASES
7. FOOD-BORNE DISEASES
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Course outlines
• Introduction
• Scope of communicable Disease
• Why CDC
• Natural hx of disease
• Chain of Disease Transmission
• Principles of CDC
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• What is Disease?
• What do you think ‘Communicable Disease’ is?
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INTRODUCTION
• Diseases can be classified can be classified based on the time course
and cause.
• Based on time course
1.Acute (characterized by a rapid onset and a short duration),
2.Chronic disease (characterized by prolonged duration).
• Based on cause
1. infectious, (i.e. caused by living parasitic organisms such as viruses,
bacteria, parasitic worms, insects, etc.),
2.noninfectious (which are caused by something other than a living parasitic
organism).
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INTRODUCTION…
• However, most of the common diseases in Africa are environmental
diseases (infectious).
• These infectious diseases are called communicable diseases, because
they spread from person to person, or sometimes from animals to people.
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Why are we concerned about Communicable diseases
• Many of them are very common
• Some of them are serious and cause death and disability
• Some of them cause widespread out breaks of disease or epidemics
• Most of them are preventable by fairly simple means.
• Poor socio-economic status of the individuals makes them vulnerable
to a variety of diseases
• Low educational status
• Lack of access to modern health care service
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Scope of Communicable Diseases in Ethiopia.
• In the past 7 decades, there has been a dramatic fall in the incidence of
infectious diseases, due:
Immunization
Anti-microbial chemotherapy
Improved nutrition
Better sanitation and housing
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Scope of CDC…
• Ethiopia, as part of the developing world, has two big health problems.
These are:
• Infectious diseases (communicable diseases) 80% of these can be prevented
by simple sanitary measures.
• Nutritional problems
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Table 1.1 Top 10 leading causes of outpatient visits in most regions of Ethiopia,
September 2008–August 2009. (From: Federal Ministry of Health (2010) Health
and Health Related Indicators: 2008/9, Addis Ababa, Ethiopia)
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The top leading causes of deaths
• Tuberculosis of the respiratory system (10.1%)
• Pneumonia (7.3%) –
• All types of malaria (4.6%)
• Bacillary dysentery (2.2%)
• Meningitis (1.5%)
• Gastroenteritis and colitis (1.1%)
• AIDS (0.8%) –
• Leishmaniasis (0.5%
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Term Definitions
• Epidemiology- the study of the frequency, distribution and determinants
of disease and other health related conditions in human populations, and the
application of this study to the promotion of health and to the prevention
and control of health problems.
• "Frequency" shows that “epidemiology” is a quantitative science (e.g.
occurrence of illness is measured using morbidity rates).
• Distribution" refers to the occurrence of disease by place, person and
time.
• "Determinants” These are factors that determine whether or not a
person will get a disease.
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Definitions of Terms…
• Epidemics - the occurrence of any health related condition in a given
population in excess of the usual frequency in that population.
• Endemic - a disease that is usually present in a population or in an area at a
more or less stable level.
• Sporadic - a disease that does not occur in that population, except at
occasional and irregular intervals.
• Pandemic - an epidemic disease which occurs worldwide.
• Infestation – presence of living infectious agent on the exterior surface of
the body
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Natural History of Disease
• Natural History of Disease: is the course of disease over time
without/unaffected by treatment or intervention
They have four phases
1. Stage of susceptibility: Period of exposure
2. Stage of subclinical disease (Pre-symptomatic stage)
3. Stage of clinical disease
4. Stage of outcome
NB: recovery will be at any stage in the course of
infection
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Course of Disease over time
• It begins with an exposure of a susceptible host to a causative
agent.
• Each disease has its own natural history
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Recovery is possible
Pathological Onset of Symptoms Usual time of Diagnosis
Changes
Exposure
Stage of Stage of Stage of Clinical Stage of outcome: Time
Susceptibility Subclinical Disease Disease Disease or
(Pre-symptomatic disease) (Symptomatic Stage)| Recovery or
Death
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Course of disease over time…
1.Stage of susceptibility
• Disease has not yet developed but risk factors are present
Ex. Unprotected sex practitioner is susceptible to HIV.
2. Stage of pre-symptomatic disease (Sub-clinical stage)
• The disease process has already begun but not manifested i.e. no signs and symptoms
of disease are detectable
• The disease can only be detected through special tests.
Example:
• Detection of antibodies against HIV in an apparently healthy person
• Ova of intestinal parasite in the stool of apparently healthy children
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Course of disease over time …
3. The Clinical Stage
 Signs and symptoms of the disease are manifested in this stage
 Clinical stage of different diseases differs in duration, severity, disability and
death.
4. Stage of Outcome
 In this stage the clinical disease results in one of the following:
• Impairment: symptoms at organ level ,e.g. broken leg
• Disability: objective alteration of behavior or performance at individual
level e.g. can’t work
• Handicap: changed interaction with others at social or environmental
level.
• Death
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Chain model of infectious diseases
Susceptible
host
Causative
agent
Reservoir
Portal of
entry Mode of
transmission
Portal of
exit
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Chain of Transmission…
1.Infectious agent: An organism that is capable of producing infection
or infectious disease.
• Etiological agents are generally classified into:
• Metazoan (multicellular organisms). (e.g. Helminths).
• Protozoa (Unicellular organisms) (e.g. Ameobae)
• Bacteria (e.g. Treponema pallidum, Mycobacterium tuberculosis, etc.)
• Fungus (e.g. Candida albicans)
• Virus (e.g. Chickenpox, polio, etc.)
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2.Reservoir of infection: Any person, animal, arthropod, plant, soil or
substance (or combination of these) in which an infectious agent
normally lives and multiplies.
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Types of reservoirs
a. Man:
• There are a number of important pathogens that are specifically adapted to man
like sexually transmitted diseases, measles, mumps, etc.
b. Animals:
Some infective agents that affect man have their reservoir in animals. The term
“zoonosis” is applied to disease transmission from animals to man.
e.g
• 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
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Reservoirs…
c. Non-living things as reservoir:
• Many of the agents are basically saprophytes living in soil and fully
adapted to live freely in nature.
E.g. Clostridium botulinum etiologic agent of Botulism
Clostridium tetani etiologic agent of Tetanus
Clostridium welchi etiologic agent of gas gangrene
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3. 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
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4. Mode of transmission: Refers to the mechanisms by which an
infectious agent is transferred from one.
Transmission may be direct or indirect.
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1. Direct transmission
• Consists of essentially immediate transfer of infectious agents from an
infected host or reservoir to an appropriate portal of entry.
Through:- touching, biting, kissing, sexual intercourse, mother to child.
a. Direct Vertical
e.g Transplacental transmission of syphilis, HIV, etc. b. Direct
horizontal.
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
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2.Indirect transmission
a. Vehicle-borne transmission: Indirect contact through contaminated
inanimate objects (fomites) like:
- cooking or eating utensils, surgical instruments.
- Contaminated food and water
- Biological products like blood, serum, plasma or IV-fluids
b. Vector-borne transmission: Occurs when the infectious agent is
conveyed by an arthropod (insect) to a susceptible host.
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Types of Vector-borne transmission…
i. 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.)
ii. Biological transmission: This is when the agent multiplies in the
arthropod before it is transmitted, such as the transmission of
malaria by mosquito.
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Indirect transmission…
C. Air-borne transmission
• Dissemination of microbial agent by air to a suitable portal of entry,
usually the respiratory tract.
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5.Portal of entry:The site in which the infectious agent enters to the
susceptible host. For example:
• Mucus membrane
• Skin
• Respiratory
• GIT
• Blood tract
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6. Susceptible host (host factors):
• A person or animal lacking sufficient resistance to a particular
pathogenic agent to prevent disease if or when exposed.
• Occurrence of infection and its outcome are in part determined by
host factors.
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Carrier
• Carrier: is an infected person or animal that does not have a
clinical sign /symptom of the disease but serves as a potential
source of infection to other.
• Unaware of their condition known by bacteriological/lab.
Four different types carrier
• Healthy/Asymptomatic carriers: Infection remain unapparent.
no sign symptom suggestive of clinical disease. ex. polio virus,
hepatitis
• Incubatory/precocious carriers: Excrete the pathogen during
incubation periods, before onset of symptom or sign
• Convalescent carrier: Continue to harbor pathogen after
recovering from the illness. Ex. diphtheria from time of recovery
to when shedding stops.
• Chronic carriers: Carriers state persists for long period of time.
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Time Course of Infectious Diseases
Different periods are encountered in the course of development of infectious
disease. Those periods are described below:
1. Pre-patent Period
• Is the time interval between infection and the point at which the infection
can first be detected
• Measured by the first shedding of the infectious agent by the host.
• Sometimes infection cannot be detected when the agent is first shed, only
later when antibodies to the agent appear, as in HIV
2. Incubation Period
• The time interval between infection and the first clinical manifestation of
disease.
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Time course…
3. Prodromal period
• The time interval between the onset of symptoms of an
infectious disease and the appearance of characteristic
manifestations.
Ex. In measles from the onset of fever and coryza to the
development of characteristic signs like koplick spots and
characteristic skin lesions.
4. Communicable Period
• The period during which an infected host can transmit the
infection to others
• Measured by the length of the time in which the agent is shed
by the host
• Degree of transmissibility does not remain constant throughout
the period of communicability, as the amount of infectious
agent that is shed by the infected host is variable at different
points time
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Time course…
5. Generation time
• Is the period between the onset of infection in a host and the
maximal communicability of that host
• Maximal communicability may be during or after the
incubation period.
6. Latent Period
• The time interval between recovery and the occurrence of a
relapse or recrudescence of infectious disease
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Principles of Communicable Disease Control
• The actions may be effected through:
A. Attacking the source
B. Interrupting the mode of transmission and
C. Protecting the host (susceptibility)
A. Attacking the source
Domestic animals as reservoirs
• Immunization ex. Brucellosis
• Destruction of infected animals ex. Rabies
Humans as reservoirs
• Isolation of infected persons & separation of infected
persons from other for the period of communicability.
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Principles of Communicable Disease Control…
Treatment of cases (clinical) and carriers
• Mass treatment – where large proportion are known to have a
disease it is sometimes advisable to treat everybody, without
checking whether individuals have disease or not.
Ex. Cholera epidemic
Quarantine – the limitation of freedom of movement of
apparently healthy persons or animals who have been exposed to
a case or infections disease.
• Cholera, plaque, and yellow fever are the 3 internationally
quarantinable diseases.
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B. Interrupting transmission
For Transmission by in gestation
Purification of water
Pasteurization of milk
Inspection procedures designed to ensure safe food supply
Improve housing conditions
For Transmission by in halation
Chemical disinfections of air
Improving ventilation
For transmission by vector or intermediate hosts
• Vector control measures
• Environmental manipulation
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C. Measures that protect host
Immunization
• Active immunization – when either the altered organism
or its products is given to a person to induce production of
antibodies. Ex. BCG
• Passive immunization – provision of ready – made
antibodies. Ex. TAT
Chemoprophylaxis:
• Use of antibiotics for known contacts to a case
Ex. Ciprofloxacin for contacts to a case of M. meningitis
Better Nutrition
• Malnourished children get infections more easily & suffer
more severe complications
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NB:
Effective control of disease is most likely when a
combination of methods
attacking the source,
interrupting transmission, and
protecting the host
is used at the same.
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1.An introduction to communicable disease control

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