Download Purposes and uses of epidemiology The ultimate purpose of

Purposes and uses of epidemiology
The ultimate purpose of epidemiology is prevention of diseases and
promotion of health.
Purposes of epidemiology
1.
To identify disease / health problem
2.
To link with the cause / risk factors
3.
To establish causal relation- ship
4.
To design an intervention for controlling problem
5.
To evaluate the effectiveness of intervention
Uses of epidemiology:
1- Study the trend of the disease.
2- Community diagnosis.
3- Planning and evaluation.
4- Risk assessment.
5- Syndrome identification.
6- Searching for causes / risk factors for establishing causal relationship
Epidemiological Triad
EPIDEMIOLOGICAL TRIANGLE
AGENT
HOST
ENVIRONMENT
Changes in one of the elements of the triangle can influence the
occurrence of disease by increasing or decreasing a person’s risk for
disease.
Changes in characteristics of Agent, Host or Environment
 Disease in The Host
Homeostatic Balance
Agent
Entity necessary to cause disease in a susceptible host
Agent factors:
- Biological (bacteria, virus, parasites, fungi etc)
- Physical (heat, cold, sound, radiation etc))
- Chemical (Endogenous e.g. urea, insulin, etc)
(Exogenous e.g. insecticide, gases, drugs, etc)
- Nutrients (proteins,fats,carbohydrates,vitamines
excess or deficiency may lead to disorders)
and minerals. Any
- Mechanical agents:accedents,burns,earthquakes……………etc.
- Psycho-social agents:stresses,addiction of alcohol, drugs……….etc.
Agent factors in disease causation:- Infectivity – ability of the infectious agents to invade and multiply in a
host (produce infection).
- Pathogenicity – ability to induce clinically apparent illness (cause
disease).
- Virulence – ability to cause severe clinical manifestation and death.
- Immunogenicity-Ability of an organism to produce an immune
response that provides protection against reinfection with the same or
similar agent
 Can be lifelong or for limited periods
 Important information for development of vaccines
Host:
Definition: person that is susceptible to effect of agent
Characteristics
Demographic characteristics:- Age- e.g. infants & elderly
- Sex
- Ethnicity- e.g. sickle cell anemia
- Occupation- e.g. pneumoconiosis
- Social class
- Social and economic characteristics
- Genetic-inherited diseases
- Immunity-natural or acquired
Environment
COMPONENTS
Three components:
OF
ENVIRONMENT:
PHYSICAL: air, water, soil, housing, climate, geography, heat, light,
noise, debris, radiation, etc.
BIOLOGICAL: man, animals, plants, viruses, microbial agents, insects,
rodents, …. etc.
PSYCHOSOCIAL: cultural values, customs, beliefs, habits, attitudes,
morals, religion, education, lifestyles, community life, health services.
ENVIRONMENT
Example:
Environmental conditions  major influence on health status of
individuals in the population
The mission of the epidemiologist is to break one of the legs of the
triangle, which disrupts the connection between environment, host, and
agent, stopping the continuation of disease occurrence.
The goals of public health are the control and prevention of disease.
By breaking one of the legs of the triangle, this public health intervention,
can stop epidemics
An epidemic can be stopped when one of the elements of the triangle is
interfered with, altered, changed or removed from existence.
 Epidemiology is the study of the distribution and determinants of
health states and the use of this information in the control of
disease.
 Disease control can be achieved by:
Destroying or removing the agent at its source
Altering the environment to reduce transmission of the agent
Improving the host’s resistance to the agent:- Altering the host’s behaviors
- Improved nutrition
- Immunization
- Exercise
- Health education
Immune System
Immunity
All mechanisms used by the body to protect itself against foreign things.
Types of Immunity
There are two types of immunity:
1- Innate immunity (Natural or Non-specific immunity)
2- Adaptive immunity (Acquired or Specific immunity)
Innate immunity (Natural):
Includes all elements with which we are born.
Components of innate immunity
- Skin and mucosal membranes.
-
Phagocytes in lymph nodes
-
White blood cells and humoral components (complement,
cytokines etc.)
-
Immune cells in liver, spleen, bone marrow and other organs.
Functions of innate immunity
- Killing invading microbes
- Activating the acquired immunity
Phagocytosis
Phagocytosis, is a process or a mechanism by which certain living cells
called phagocytes ingest or engulf other cells or particles such as
(bacteria, dead tissue cells, protozoa, various dust particles, and other
minute foreign bodies).
Acquired immunity:
Acquired immunity is the resistance that individual acquires during life.
Types of acquired immunity:

Active acquired immunity.

Passive acquired immunity.
Active acquired immunity:
It is a resistance developed as a result of antigenic stimulus.It is divided
into:
 Naturally active acquired immunity (developed after recovery
from an infection).
 Artificially active acquired
administration of a vaccine).
immunity
(developed
by
Passive acquired immunity:
It is a resistance given in a ready made form. It is divided into:
 Naturally passive acquired immunity (ready antibodies that child
acquires from his/her mother).
 Artificially passive acquired
antibodies e.g. antitetanus sera)
Main types of Vaccines
immunity
(administration
of
 Live vaccines (not used currently due to infection risk).
 Live, attenuated vaccines
 Killed or inactivated vaccines
 Toxoid vaccines
Others
 Conjugate vaccines
 DNA vaccines
 Recombinant vector vaccines
Live, attenuated vaccines:
Live, attenuated vaccines contain a version of the living microbe
that has been weakened in the lab so it can’t cause disease. (e.g. vaccines
against measles, mumps, and chickenpox).
Killed or inactivated vaccines:
Inactivated vaccines are produced by killing the disease-causing
microbe with chemicals, heat, or radiation. Inactivated vaccines can be
composed of either whole viruses or bacteria, or fractions of either. Such
vaccines are safer than live vaccines (e.g. vaccines against Cholera,
Typhoid).
Toxoid Vaccines:
These are Produced from toxins that secreted by certain bacteria. These
toxins are inactivated by treating them with formalin (e.g. Diphtheria
vaccine).
Vaccination
1- Basic Vaccination (routine vaccination):
The main vaccinations identified in the Vaccination Certificate,
from birth to entering the first primary grade, aims to protect children in
the community from childhood diseases.
MMR:
The MMR vaccine is given against measles, mumps, and rubella (German
measles). It is a mixture of live attenuated viruses of the three diseases,
administered via injection (dose: 0.5 ml).
Polio vaccines:
1Oral polio vaccine (OPV) “Sabin vaccine” is an attenuated vaccine.
It is safe, effective, and induces long-lasting immunity. It is given orally
(by mouth). (Dose: 2-3 drops).
2Inactivated polio vaccine (IPV) “Salk vaccine”: It consists of
inactivated (killed) poliovirus strains of all three poliovirus types. IPV is
given by intramuscular injection.
BCG:
It is live attenuated bovine tuberculosis bacillus, (Mycobacterium bovis).
Except in neonates, a tuberculin skin test should always be done before
administering BCG. BCG is given as a single intradermal injection
Tetanus vaccine (Tetanus toxoid):
Tetanus vaccine is a vaccine composed of deactivated tetanus toxins
(dose: 0.5 ml).
2- Seasonal vaccination:

Cholera vaccine

Typhoid vaccine

Hepatitis vaccine

Meningitis vaccine

Yellow fever vaccine

Rabies vaccine

Others
Process of infection
Chain of infection includes the following items:
1- Infectious agent
2- Reservoir
3- Portal of exit
4- Means of transmission
5- Portal of entry
6- Susceptible host
1- Infectious agent
Bacteria, viruses, parasites, fungi ....etc
2- Reservoir
The reservoir of an infectious agent is the habitat in which the agent
normally lives, grows, and multiplies.
Reservoirs include:
- Humans,
- Animals, and
- The environment.
• Types of reservoirs
Human reservoirs: Many common infectious diseases have human
reservoirs. Diseases that are transmitted from person to person without
intermediate host include the sexually transmitted diseases, measles,
mumps, streptococcal infection, and many respiratory pathogens.
Animal reservoirs: Humans are also subject to diseases that have
animal reservoirs. Many of these diseases are transmitted from animal to
animal, with humans as incidental hosts. The term zoonosis refers to an
infectious disease that is transmissible under natural conditions from
vertebrate animals to humans. Long recognized zoonotic diseases include
brucellosis (cows and pigs), anthrax (sheep), plague (rodents), and rabies
(bats, dogs, and other mammals).
Environmental reservoirs: Plants, soil, and water in the
environment are also reservoirs for some infectious agents. Tetanus is
caused by Clostridium tetani, the bacteria live usually in soil.
3- Portal of exit
Portal of exit is the path by which a pathogen leaves its host. The
portal of exit usually corresponds to the site where the pathogen is
localized. For example schistosomes through urine, cholera vibrios in
feces. Some bloodborne agents can exit by crossing the placenta from
mother to fetus (toxoplasmosis), while others exit through cuts or needles
in the skin (hepatitis B) or blood-sucking arthropods (malaria).
4- Modes of transmission
An infectious agent may be transmitted from its natural reservoir to
a susceptible host in different ways.
There are different classifications for modes of transmission. Here
is one classification:
• Direct
– Direct contact
– Droplet spread
– Placenta
• Indirect
– Vehicle-borne (3-5 micron)
– Vector-borne (mechanical or biologic)
– Air-borne
Direct transmission:
Direct transmission occurs when someone exposed directly to the
infectious lesion or infected body fluids (e.g. Blood, saliva, semen,
vaginal secretions)
- Direct contact:
Getting causative agent through touching, kissing .... etc (e.g.
Scabies, Leprosy).
- Droplet spread:
Inhaling causative agent that discharged from infected person
while coughing, sneezing .... etc (e.g. Whooping cough, influenza).
- Placental (mother-to-child transmission, vertical transmission):
Causative agent comes from mother to her child through placenta
(e.g. AIDS, Toxoplasmosis).
Indirect Transmission:
1- Vehicle-borne
- Water
- Food
- Milk
- Blood and plasma
- Organs
2- Vector-borne:
An insect or any living carrier that transports an infectious agent
from an infected individual or its wastes to a susceptible individual or its
food or immediate surroundings.
Vector-borne
Vectors transmit causative agent by mechanically or biologically.
Mechanical transmission
In which the transmission process takes place without any change
in form and number of the causative agent.
e.g. House fly transmits Salmonella typhi mechanically
Biological transmission
- cyclo-propogative (change in form and number)
e.g. Plague bacilli in Rats
- cyclo-developmental (change in form)
e.g. Malaria parasite in mosquito
- Propogative (change in number)
e.g. Micoflaria in mosquito
3- Air-borne:
Occurs when microbial particles or dust particles containing
pathogens remain suspended in the air for a prolonged period, and then
are spread widely by air currents and inhaled by susceptible person (e.g.
Tuberculosis, Measles).
5- Portal of entry
The way the causative agent can enter the host from e.g. mouth, nose,
injuries…etc.
6- Susceptible host
The final component in the chain of disease transmission is the
susceptible host who can become infected with the causative agent
People who are more susceptible:
- The very young because their immune system does not fully
develop until about age 6 months
- The very old because the age is associated with declining immune
system function.
- Suppressed immune system
- Malnourished people.
- Hospitalized persons.
- Pregnant women.