Download Communicable disease

EPIDEMIOLOGY AND
PUBLIC HEALTH
Dr. Mohammad Shakeeb ,MD
Specialist in clinical
pathology/microbiology and
immunology
EPIDEMIOLOGY
EPIDEMIOLOGY
 Introduction
• An epidemiologist studies the factors that determine the
frequency, distribution, and determinants of diseases in
human populations.
• With respect to infectious diseases, these factors include:
 the characteristics of various pathogens
 Susceptibility of different human populations
 Locations (reservoirs) where pathogens are lurking
 Various ways in which infectious diseases are transmitted
EPIDEMIOLOGY
• epidemiologists are concerned with:
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Who becomes infected?
What pathogens are causing the infections?
Where do the pathogens come from?
When do certain diseases occur?
Why do some diseases occur in certain places but not in
others?
 How are pathogens transmitted?
 Do some diseases occur only at certain times of the year? If
so, why?
• Epidemiologists also develop ways to prevent, control, or eradicate
diseases in populations.
 Epidemiologic Terminology
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Communicable and Contagious Diseases
Zoonotic Diseases
Incidence and Morbidity Rate
Prevalence
Mortality Rate
Sporadic Diseases
Endemic Diseases
Epidemic Diseases
Pandemic Diseases
Communicable and Contagious Diseases
• Communicable disease: infectious disease is transmissible
from one human to another (i.e., person to person).
• Contagious disease :is defined as a communicable disease
that is easily transmitted from one person to another.
Zoonotic Diseases
• Infectious diseases that humans acquire from animal sources,
Incidence and Morbidity Rate
• The incidence of a particular disease is defined as the number
of new cases of that disease in a defined population during a
specific time period.
• morbidity rate expressed as the number of new cases of a
particular disease that occurred during a specified time period
per a specifically defined population (usually per 1,000,
10,000, or 100,000 population).
Prevalence
• The period prevalence of a particular disease is the number
of cases of the disease existing in a given population during a
specific time period.
• The point prevalence of a particular disease is the number of
cases of the disease existing in a given population at a
particular moment in time.
Mortality Rate
• The mortality rate (also known as the death rate) is the ratio
of the number of people who died of a particular disease
during a specified time period per a specified population
(usually per 1,000, 10,000, or 100,000 population).
Sporadic Diseases
• A sporadic disease is a disease that occurs only occasionally
(sporadically) within the population of a particular geographic
area.
• Example : In the United States, sporadic diseases include
botulism, cholera, gas gangrene, plague, tetanus, and typhoid
fever.
• they are kept under control as a result of immunization
programs and sanitary conditions.
Endemic Diseases
• Endemic diseases are diseases that are always present within
the population of a particular geographic area.
• The number of cases of the disease may fluctuate over time,
but the disease never dies out completely.
• Endemic infectious diseases of the United States include:
tuberculosis (TB), sexually transmitted diseases (STDs) like
gonorrhea and syphilis and viral diseases such as the common
cold, influenza.
Epidemic Diseases
• An epidemic (or outbreak) is defined as a greater than usual
number of cases of a disease in a particular region, usually
occurring within a relatively short period of time.
• Read the following examples about the epidemics that have
occurred in the United States within the past 35 years (you
will be asked about this topic in the exam).
• 1976. An epidemic of a respiratory disease
(Legionnaires’ disease or legionellosis)
• 1992–1993. An epidemic involving Escherichia coli
O157:H7-contaminated hamburger meat occurred in
the Pacific northwest.
• 1993. An epidemic of hantavirus pulmonary
syndrome
• 1993. An epidemic of cryptosporidiosis (a diarrheal
disease) occurred in Milwaukee, Wisconsin.
• 2002. An epidemic of West Nile virus (WNV)
infections occurred throughout the United States.
• Waterborne disease outbreaks.
• Foodborne disease outbreaks.
• Epidemics usually follow a specific pattern, in which the
number of cases of a disease increases to a maximum and
then decreases rapidly.
• Epidemics may occur in communities that have not been
previously exposed to a particular pathogen.
• In communities in which normal sanitation practices are
relaxed, allowing fecal contamination of water supplies and
food, epidemics of typhoid fever, cholera, giardiasis and
dysentery often occur.
• Visitors to these communities should be aware that they are
especially susceptible to these diseases.
• Influenza (“flu”) epidemics occur in many areas during certain
times of the year and involve most of the population because
the immunity developed in prior years is usually temporary.
• Epidemics of influenza cause approximately 20,000 deaths per
year in the United States.
• Ebola virus has caused several epidemics of hemorrhagic
fever in Africa.
• The source of the virus is not yet known.
• In a hospital setting, a relatively small number of infected
patients can constitute an epidemic
• situation must be brought to the attention of the Hospital
Infection Control Committee.
Pandemic Diseases
• A pandemic disease is a disease that is occurring in epidemic
proportions in many countries simultaneously—sometimes
worldwide.
• The 1918 Spanish flu pandemic was the most devastating
pandemic of the 20th century.
• That pandemic killed more than 20 million people worldwide,
including 500,000 in the United States.
• Almost every nation on Earth was affected.
• infectious diseases are responsible for approximately half the
deaths that occur in developing countries.
• approximately half of those are caused by three infectious
diseases:
 human immunodeficiency virus/ acquired immunodeficiency
syndrome (HIV/AIDS).
 Tuberculosis.
 Malaria.
• Read about these pandemics from your textbook
• Collectively, HIV/AIDS, TB, and malaria cause more than 300
million illnesses and more than 5 million deaths per year.
INTERACTIONS AMONG PATHOGENS,
HOSTS,AND ENVIRONMENTS
INTERACTIONS AMONG PATHOGENS,
HOSTS,AND ENVIRONMENTS
• Whether or not an infectious disease occurs depends
on:
1. Pathogen factors.
2. Host (i.e., the person who may become infected)factors.
3. Environmental factors.
• Pathogen factors includes The virulence of the pathogen, a
portal of entry, The number of organisms that enter the body.
• Host factors include :
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person’s health status
person’s nutritional status
Age
lifestyle [behavior],
Socioeconomic level
Occupation
Travel
immune status
• Environmental factors includes:
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geographic location
Climate
Heat ,Cold ,Humidity
season of the year.
Availability of appropriate reservoirs, intermediate hosts, and vectors.
Sanitary and housing conditions.
adequate waste disposal.
adequate healthcare
CHAIN OF INFECTION
CHAIN OF INFECTION
• There are six components in the infectious disease
process (also known as the chain of infection).
• The six components in the chain of infection are :
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A pathogen.
A reservoir of infection.
A portal of exit.
A mode of transmission.
A portal of entry.
A susceptible host.
STRATEGIES FOR BREAKING
THE CHAIN OF INFECTION
STRATEGIES FOR BREAKING
THE CHAIN OF INFECTION
• measures must be taken to break the chain of
infection at some point (link) in the chain:
 Eliminate or contain the reservoirs of pathogens or curtail the
persistence of a pathogen at the source.
 Prevent contact with infectious substances from exit
pathways.
 Eliminate means of transmission.
 Block exposure to entry pathways.
 Reduce or eliminate the susceptibility of potential hosts.
• Some of the specific methods of breaking the chain of
infection are:
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Practice effective hand hygiene procedures
Maintain good nutrition and adequate rest and reduce stress
Obtain immunizations against common pathogens
Practice insect and rodent control measures
Practice proper patient isolation procedures
Ensure proper decontamination of surfaces and medical instruments
Dispose of sharps and infectious waste properly
Use gloves, gowns, masks, respirators, and other personal protective
equipment, whenever appropriate to do so
 Use needle safety devices during blood collection
RESERVOIRS OF INFECTION
RESERVOIRS OF INFECTION
• A reservoir is any site where the
pathogen can multiply or merely survive
until it is transferred to a host.
• Reservoirs may be living hosts or
inanimate objects or materials.
RESERVOIRS OF INFECTION
• Living Reservoirs
Human Carriers
Animals
Arthropods
• Nonliving Reservoirs
RESERVOIRS OF INFECTION
Human Carriers
• A carrier is a person who is colonized with a particular
pathogen, but the pathogen is not currently causing disease in
that person.
• The pathogen can be transmitted from the carrier to others,
who may then become ill.
• Passive carriers carry the pathogen without ever having had
the disease.
• An incubatory carrier is a person who is capable of
transmitting a pathogen during the incubation period of a
particular infectious disease.
• Convalescent carriers harbor and can transmit a particular
pathogen while recovering from an infectious disease (i.e.,
during the convalescence period).
• Active carriers have completely recovered from the disease,
but continue to harbor the pathogen indefinitely.
• Respiratory secretions or feces are usually the vehicles by
which the pathogen is transferred.
• Directly or indirectly (through food or water ).
• Staphylococcal and streptococcal infections as well as in the
spread of hepatitis, diphtheria, dysentery, meningitis, and
STDs.
Animals
• Zoonotic diseases (zoonoses): are infectious diseases that
humans acquire from animal sources.
Arthropods
• Many different types of arthropods serve as reservoirs of
infection, including insects (e.g., mosquitoes, biting flies, lice,
fleas), and arachnids (e.g., mites, ticks).
• When involved in the transmission of infectious diseases,
these arthropods are referred to as vectors.
• The arthropod vector may first take a blood meal from an
infected person or animal and then transfer the pathogen to a
healthy individual.
Examples of Zoonotic Diseases
CATEGORY
DISEASE
PATHOGEN
ANIMAL RESERVOIR(S)
MODE OF TRANSMISSION
Nonliving Reservoirs
• Nonliving or inanimate reservoirs of infection include air, soil,
dust, food, milk, water, and fomites.
• Respiratory viruses----transmitted through the air on droplets
of respiratory tract secretions.
• fungal respiratory diseases (e.g., histoplasmosis) are
frequently transferred by dust containing yeasts or spores.
• Soil contains the spores of the Clostridium species that cause
tetanus, botulism, and gas gangrene.
MODES OF TRANSMISSION
• The five principal modes by which
transmission of pathogens occur are contact
(either direct or indirect contact), droplet,
airborne, vehicular, and vector transmission.