Semester 5- week 1
COMMUNITY MEDICINE
INTRODUCTION OF MODULE EPIDEMIOLOGY RELATED TO INFECTIOUS
DISEASES
LEARNING OBJECTIVES
•
•
•
At the end of the lecture, students should be able to:
Know about the classification of diseases.
Describe the:
– modes of communicable disease transmission
– prevention of communicable diseases
– tools available to the “state” for the control of communicable disease
INTRODUCTION
• Disease of infectious origin, been a courage for most of man’s history.
• Progress in the understanding of these diseases, started with the discovery
of the microscope and development of the science of bacteriology, in the
late 19th century.
• The first sanitary revolution along with development in the science of
epidemilogy, in the western countries, helped in restricting the spread of
these diseases.
• However specific treatment and prevention had to wait till the emergence
of the antibiotics era.
CLASSIFYING DISEASES
• Acute Diseases
– Acute diseases are those conditions in which the peak severity of symptoms occurs
within three months (usually sooner), and recovery in those who survive is usually
complete
• Chronic Diseases
– Chronic diseases or conditions are those in which symptoms continue longer than
three months and in some cases for the remainder of the person’s life. Recovery is
slow and sometimes incomplete.
• Communicable (Infectious) Diseases
– Diseases for which biological agents or their products are the cause and which are transmissible
from one individual to another
– The disease process begins when the causative agent is able to lodge and grow or reproduce within
the body
– The process of lodgment and growth of a microorganism or virus in the host is termed infection.
• Non-communicable (Noninfectious) Diseases/Illnessses
– Those diseases or illnesses that cannot be transmitted from an infected person to a susceptible,
healthy one
– Several, or even many, factors may contribute to the development of a given non-communicable
health condition
– The contributing factors may be genetic, environmental, or behavioral in nature.
Classification of Diseases
Types of Diseases_____Examples______________
• Acute Diseases
– Communicable
Common cold, pneumonia, mumps,
measles, pertussis, typhoid fever, flu
– Non-communicable
Appendicitis, poisoning, trauma (e.g., (incl. trauma)
automobile accidence, fires,
etc.)
due to
• Chronic Diseases
– Communicable
Lyme disease, tuberculosis, AIDS,
syphilis, rheumatic fever following
herpes
– Non-communicable
Diabetes, coronary heart disease,
osteoarthritis, cirrhosis of the liver dur
hyptertension
streptococcal infections,
to alcoholism,
COURSE OF INFECTIOUS DISEASE
• Exposure/Invasion of Host
• Incubation -- period of time between exposure and onset of symptoms -- e.g., interval
between HIV infection and development of AIDS can be as long as 10-15 years
• Host reaction
• Disease runs course -- treatment, recovery/death (most people don’t die from
infectious diseases).
INCUBATION PERIOD
• Varies by disease
• Salmonella -- 12-72 hours after infection; symptoms usually resolve in 5-7 days,
unless infected person is in a very weakened health status
• Measles (rubeolla) -- approx. 10-12 days (prodromal -- i.e., interval between the
earliest symptoms and the appearance of the rash or fever -- rash onset, on average, 14
days
• HIV -- 6 weeks upward to months; interval between HIV infection and development of
AIDS can be as long as 10-15 years
• 2-6 weeks after infection in many, but not all, diseases, most people develop
antibodies against re-infection.
MODES OF COMMUNICABLE DISEASE TRANSMISSION
•
Direct Transmission
•
Indirect Transmission
DIRECT TRANSMISSION
• Immediate transfer of the disease agent by direct contact between the infected and the
susceptible individuals
• Occurs through such acts as touching, biting, kissing, sexual intercourse, or by direct
projection (droplet spread) by coughing or sneezing within a distance of one meter
• Examples of diseases for which transmission is usually direct are AIDS, syphilis,
gonorrhea, and the common cold.
INDIRECT TRANSMISSION
• May be one of three types: air-borne, vehicle-borne, or vector-borne
• Air-borne transmission -- transmission of microbial aerosols to a suitable port of
entry, usually the respiratory tract
– Microbial aerosols are suspensions of dust or droplet nuclei made up wholly or in
part by microorganisms -- may be suspended and infective for long periods of time
– Examples of air-borne diseases include tuberculosis, influenza, histoplasmosis, and
legionellosis
•
Vehicle-borne transmission -- contaminated materials or objects (fomites) serve as
vehicles, nonliving objects by which communicable agents are transferred to a
susceptible host
– The agent may or may not have multiplied or developed on the vehicle
–
Examples of vehicles include toys, handkerchiefs, soiled clothes, bedding, food
service utensils, and surgical instruments
– Also considered vehicles are water, milk, food (e.g., common vehicles), or
biological products such as blood, serum, plasma, organs and tissues
– Almost any disease can be transmitted by vehicles, including those for which the
primary mode of transmission is direct, such as dysentery and hepatitis
•
Vector-borne transmission -- disease transfer by a living organism, such as a
mosquito, fly, or tick
– Transmission may be mechanical, via the contaminated mouth parts or feet of the
vector, or biological, involving multiplication or developmental changes of the
agent in the vector before transmission occurs
– In mechanical transmission, multiplication and development of the disease do not
usually occur -- e.g., organisms that cause dysentery, polio, cholera, and typhoid
fever have been isolated from such insects as cockroaches and house flies and
could presumably be deposited on food prepared for human consumption.
– In biological transmission, multiplication and/or developmental changes of the
disease agent occur in the vector before transmission occurs
– Biological transmission is much more important than mechanical transmission in
terms of its impact on public/community health
– Examples of biological vectors include mosquitoes, fleas, ticks, lice, flies and other
insects.
– Mosquitoes are extremely important vectors of human diseases -- e.g., they transmit
the viruses that cause yellow fever and dengué fever as well as 200 other viruses -they also transmit malaria, which infects 100 million people in the world each year
(most in tropical areas), killing at least 1 million of them each year
– Ticks are another important biological vector, transmitting Rocky Mountain spotted
fever, relapsing fever, and Lyme disease
DISEASE AND INJURY PREVENTION
AND CONTROL
•
Prevention
– Includes individual, clinical, or personal health services such as immunizations,
screening for high blood pressure and follow-up services, or the use of Pap smears
to detect the precursors to cancer of the cervix
•
Protection
– Includes the activities of organizations, both public and private, to reduce exposure
to hazards such as polluted water, contaminated food, traffic accidents, mosquitoes,
or use of electric saws without safety devices
PREVENTION OF COMMUNICABLE DISEASES
• Primary Prevention
– In the chain of infection model, primary prevention strategies are evident at each
link of the chain
– Successful application of each strategy can be seen as weakening the link -- with
the ultimate goal of interrupting the disease transmission cycle.
Community measures -- e.g., chlorination of the water supply, inspection of restaurants,
immunization programs that reach all citizens, maintenance of a well-functioning sewer
system, proper disposal of solid waste, and control of vectors and rodents.
– Personal/Individual actions -- hand washing, proper cooking of foods, adequate
clothing and housing, use of condoms, obtaining all of the available immunizations
against specific diseases.
• Secondary Prevention
– Community effort includes measures taken to control or limit the extend of a
disease outbreak/epidemic -- e.g., maintaining records of cases and compliance
with regulations requiring the reporting of notifiable diseases, investigating cases
and contacts, those who may have become infected through contact with cases.
–
Individual effort includes either (1) self-diagnosis and self-treatment with
nonprescription medications or home remedies, or (2) diagnosis and treatment with
an antibiotic or other physician-prescribed medicine
–
– Occasionally, secondary disease control measures may include isolation or
quarantime
– Isolation = separation, for the period of communicability, of infected persons or
animals from others so as to prevent the direct or indirect transmission of the
communicable agent to a susceptible person/host
– Quarantine = limitation of the freedom of movement of well persons or animals
that have been exposed to a communicable disease until the incubation period has
passed
– Further measures may include disinfection -- the killing of communicable agents
outside the the host, and mass treatment with antibiotics
– Public health education and health promotion should also be used as both primary
and secondary preventive measures.
• Tertiary Prevention
– Convalescence from infection, recovery to full or partial health, and return to
normal activity
– In some cases, such as paralytic polio, return to normal activity may not be
possible, even with extensive physical therapy
– At the community level, proper removal of infected items such as clothing,
disinfection, and burial of the dead, for example
– Tertiary prevention may also involve the reapplication of primary and secondary
measures to prevent further cases -- e.g., in Japan and South Korea, people with
colds or flu wear gauze masks in public to reduce the spread of the disease
TOOLS AVAILABLE TO THE “STATE” FOR THE CONTROL
OF COMMUNICABLE DISEASE
• REPORTING -- Physicians and other health professionals must report specified
diseases to a designated authority, usually to local or state health authority
• LABORATORY REPORTING -- In many states, licensed laboratories must report
positive results for certain diseases to the health department, even though a diagnosis
may not have been established
• SURVEILLANCE -- The systematic measurement of health status and risk factors
• MONITORING -- Involves the ongoing assessment of a condition after intervention
has been initiated
• LABORATORY ANALYSIS -- Involves public health laboratories with authority for
the study and detection of infectious diseases
• CONTACT INVESTIGATION -- Once a case of a particular disease has been
diagnosed, personnel from the health department are authorized to interview the victim
to establish a list of possible contact. (A practice of considerable debate recently
because of AIDS.)
•
•
•
•
•
•
TREATMENT -- Public health agencies are required to provide treatment services for
specified infectious diseases. (In many cities, special hospitals were built for this
purpose.)
Local health departments are also required to provide services for people infected with
sexually transmitted diseases or tuberculosis.
These services are not regarded as welfare services but rather as tools to prevent the
spread of dangerous communicable diseases. It is important to note, however, that the
state cannot require treatment.
The state can only force treatment if it can prove that the victim is
(1) gravely disabled, and
(2) a danger to self or others because of the disability
• ISOLATION -- Means separation of infected people from non-infected people during
the period of communicability. Follows the “least restrictive” principle.
• IMMUNIZATION -- All states have requirements for the immunization of children
against certain infectious diseases -- most common are diphtheria, pertussis (whooping
cough), tetanus, rubella (German measles), and polio.
•
The controlling agency is usually the school system, which is required to prevent
entry of any child who has not been properly immunized
• INVESTIGATION -- In addition to the specific authorities noted above, most health
departments are required to investigate unusual occurrences of disease or injury.
• This includes the authority to review medical records, to perform laboratory
investigations, to examine patients, and to interview both patients and others who may
have been exposed to the disease or injury.
•
(The public health authorities may require court authorization to undertake these
investigations.)
-------------------------------------------------------[----------------------------------------------------