Download Epidemiology

Infectious process
Learning objectives:
After completing the course, the students will be able to:
1. identify the six requisites for the perpetuation of a communicable
disease (the cycle of infection)
2. explain mechanisms of disease production
3. define reservoir of infection and classify human reservoirs
4. define carrier and list its types
5. identify the portals of exit of infection from the reservoir, and give an
example for each
6. identify different modes of transmission of communicable diseases,
and give an example for each
7. recognize the various defense mechanisms of the human body, and
give an example for each
8. define herd immunity, and list the factors that play a role in community
protection
9. define primary, secondary and tertiary levels of prevention
10. outline the general principles of prevention and control of
communicable diseases
Learning Activities
Solving exercises and M.C.Q’s on cycle of infection
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Definitions:
-
Infections: It is the entry, development and multiplication of an
infectious agent in the body of man or animal. Infection is not
synonymous with infectious disease; the result of infection may be
inapparent or manifest infectious disease.
-
Infectious disease: A clinically manifest disease of man or animal
resulting from infection.
-
Non- infectious disease: A disease is not due to infectious agent as
diabetes and cardiovascular diseases.
-
Communicable diseases: It is an illness caused by an infectious
agent or its toxic product which can be transmitted directly or
indirectly or through vector from the reservoir to a susceptible host.
-
Non- communicable disease: It is an infectious disease which
cannot be transmitted from the reservoir to a susceptible host.
-
Contagious disease: A disease that is transmitted through contact.
E.g. scabies, trachoma and leprosy.
-
Contamination: The presence of living infectious agents on the
exterior surface of the body or on the clothes or articles of the
person or on any inanimate object in the environment including
water and food.
-
Epidemic: It the occurrence in a community or a region of a group of
illness of similar nature, clearly in excess of its normal expectancy.
-
Endemic: A disease that has established itself permanently in a
certain locality or community all the time, e.g. bilharziasis in Egypt.
-
Pandemic: The appearance of a disease in an epidemic form
affecting countries sequentially (at the same time).
-
Outbreak: A more or less localized epidemic affecting certain large
number of a group, in the community, e.g. outbreak of food
poisoning in an institution.
-
Sporadic: means scattered about. The cases occur irregularly,
haphazardly from time to time and generally infrequently. Cases are
few and separated widely in space and time showing no connection
to each other.
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-
Opportunistic infection: This is infection by an organism that takes
the opportunity provided by a defect in the host defense mechanism
e.g. AIDs, Toxoplasmosis.
-
Iatrogenic disease: It is a physician induced disease.
-
Eradication: Termination of all transmission of infection by
extermination of the infectious agent. ( Termination of infection from
the whole world) e.g. Smallpox
-
Period of communicability: The time during which the infectious
agent could be transmitted directly or indirectly from the reservoir to
a susceptible host.
-
Nosocomial infection: (Hospital acquired infection). They are
infections that acquired by the patients during or associated
with delivery of health care which are not present or
incubating at admission.
The requisites for the perpetuation of communicable
diseases:
There are six requisites for the perpetuation of the communicable
disease:
(The cycle of infection)
1- Presence of microbiological agent.
2- Presence of reservoir.
3- Portal of exit.
4- Mode of transmission.
5- Portal of entry ( inlet )
6- Presence of susceptible host
I. Microbiological agent:
Microorganisms responsible for the causation of communicable
diseases are classified into:
a) Viruses: These are the causative agents for a large number of
diseases such as influenza, mumps, chiken pox, poliomyelitis…etc.
b) Bacteria: Cocci ( streptococci, staphylococci, diplococci ), Bacilli (
diphtheria, salmonella, shigella ) and vibrio (classical vibrio, Eltor vibrio)
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c) Rickettsia: The causative agents of a group of infectious diseases
including the typhus group of fevers.
d) Fungi: Including Candida.
e) Protozoa: One cell animal form such as entameba causing dysentery,
plasmodia causing malaria,.
f) Parasites: Such as schistosoma, ancylostoma ….etc.
Mechanisms of disease production ( pathogenesis ):
a) Invasiveness.
b) Toxicity.
c) Hypersensitivity.
a) Invasiveness:
Is the ability of the organisms to invade the tissues and multiply. Each
pathogen has the ability of invasiveness and toxigenicity (e.g.
treponema pallidum, typhoid and paratyphoid organisms have a high
power of invasiveness but they have low toxicity).
b) Toxicity: Toxins are:
Exotoxin: They are released by living organisms, destroyed rapidly by
heating (above 600 C), highly immunogenic and converted to antigenic,
non toxic toxoid by Formalin, heat and acid. They are diffusible, don't
produce fever e.g. (Neurotoxins of tetanus and botulism, erythrogenic
toxins of scarlet fever).
Endotoxin: They are toxins released after disintegration of microorganisms,
highly stable i.e. withstand heating above 600C, weakly immunogenic, not
converted to toxoid, usually produce path physiologic effects as fever,
leukopenea, hypotension hypoglycemia and shock.
c) Hypersensitivity :
It is an allergic state of the host following exposure to certain antigens
of
microorganisms
(e.g.
mycobacterium
tuberculosis),
whereby
subsequent exposure results in a disease state.
Factors affecting disease production in relation to agents:
1. Pathogenicity and virulence:
Pathogenicity: is the ability of the organisms to produce specific clinical
reaction after infection has occurred, however, it does not refer to severity
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of reaction. Virulence: Is the ability of organisms to produce severe
pathological reaction, it refers to severity of the reaction.
It can be measured by:
a) Ratio of clinical to sub-clinical cases =
Clinical cases
Subclinical cases
b) Case fatality rate =
Number of deaths due to a particular disease in an area and year
Total number of cases of this particular disease in the same area and year
2. Antigenic power of the microorganisms:
The ability to initiate the development of antibodies or antitoxin and
associated immunity. It can be measured by:a) Second attack frequency: In certain diseases second attacks are rarely
recorded as in case of measles, mumps and Chicken pox. In other
diseases re-infection occurs as in case of common cold, upper
respiratory infections, syphilis and gonorrhea.
b) Age specific attack rate: In diseases caused by microorganisms of high
antigenic power as measles there is a drop of the attack rate after
young age.
3. Period and ease of communicability:
It can be measured by the secondary attack rate, which is the number
of secondary cases, occurring within the accepted incubation period
following exposure to a primary case, calculated as a percentage of the
number of exposed susceptible.
4. Dose of infection (inoculums): The higher the dose of infection, the more
liability of having an apparent illness, and the severer will be the
diseases.
5. Tissue selectivity (tropism): Is the inherent capacity of the pathogen to
invade some particular tissues. This is the factor that gives each
disease its characteristic symptoms and signs.
6. Host specificity: Some pathogens infect man only as relapsing fever,
other infect animals only, while some others infect both man and
animal such as zoonotic diseases.
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7. Spore formation: The ability of some bacteria to change to a resistant
form under unsuitable conditions and these spores remain viable for
long periods.
When these spores get the chance of coming into contact with a
susceptible host-under favorable condition, they change to vegetative
forms, they cause the disease as in case of tetanus and anthrax.
8. Viability of the organism (the resistance of the organism): Is the ability to
live outside the body, the longer the duration, the more the chance to
come into contact with a new host transmitting the infection to them.
9. Susceptibility of the pathogen to chemotherapy: The degree of
sensitivity to antibiotics differs from one pathogen to the other and even
from one strain of the same pathogen to another.
II. Reservoir of infection:
Definition:
This is the place or depot where the infective agent survives, grows
and multiplies in such a manner that it can be transmitted to a susceptible
host. Reservoirs may be man, animal, plant or soil (or combination of
these)
Man is the most common reservoir of infections followed by
animals.
Human reservoirs:
Human reservoirs of infection may be in the form of cases or
carriers.
a) Cases:
Whether these are typical cases or missed (inapparent or
subclinical)
Inapparent infection: The presence of infection in a host without
recognizable clinical signs or symptoms. They are identifiable only by
laboratory means or positive skin tests.
Severe cases are not necessarily more communicable than mild
subclinical cases. From the public health point of view, mild and
subclinical cases may be more important for the spread of infection
being unrecognized and overlooked.
b) Carriers:
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A carrier is defined as a person or animal that harbors the infectious
organisms and is apparently free from clinical manifestations of the
disease, and serves as a potential source of infection.
Carriers are dangerous because:
1- They do not show any clinical manifestation.
2- The carrier and his contacts are not aware of their condition.
3- It is difficult to discover them.
4- It is not always possible to deal with them.
5- The long duration of carriage in some communicable diseases
(Typhoid, HIV).
6- The high prevalence of carrier state among the population.
Carriers are classified according to :
1. Place of carriage:
a) Upper respiratory carriers as in case of diphtheria, streptococcal and
meningococcal infections.
b) Fecal carriers: In case of typhoid, paratyphoid, cholera, infectious
hepatitis.
c) Urinary carriers: In case of typhoid and paratyphoid.
d) Skin carriers: As in case of staphylococcal infection.
2. Duration of carriage:
a) Transient carrier: A person who harbors and excretes the infectious
organisms up to weeks.
b) Temporary carrier: A person who harbors and excretes infectious
organisms up to 3 months (one year for enterica)
c) Chronic carrier: A person who harbors and excretes organisms for more
than 3 months (more than one year for enterica)
d) Permanent carrier: A person who harbors and excretes the infecting
organisms for life.
Most of the carriers (about 95%) are of the temporary type.
3. Chronologically (according to the spectrum of infection) carriers are
classified into:
a) Incubatory carrier: A person who harbors and excretes the infecting
organisms during the incubation period. For example; hepatitis A,
mumps and poliomyelitis.
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b) Contact carrier: Contact with an infected person as doctors, nurses,
parents as well as servants. Contact carriers are of transient type,
usually the period of carriage ends as soon as the patient is cured or
the contact is over. Contact carriers are common in cholera and
typhoid.
c) Convalescent carriers: A person who discharges the microorganisms
during convalescent period for example typhoid. Here there will be a
need for carrying out 3 consecutive bacteriological examinations before
release of the cases.
Animal reservoirs:
Animals can act as reservoirs, whether in the form of diseased or
carriers. Zoonosis: Infectious disease transmissible under natural
conditions from vertebrate animals to man. The important animal
reservoirs are cattles in bovine T.B, goats in Brucellosis, dogs in rabies
and rats in plague. Mice, rodents, ducks and cows in salmonella, and
monkeys in yellow fever.
There are different modes of transmission of diseases from animals to
man:
1. Direct contact with animal products and excreta: Brucellosis,
anthrax, Toxoplasmosis, rabies
2. Common vehicle; Consuming animal products
as (bovin TB,
salmonella , Brucellosis ,anthrax , Toxoplasmosis)
3. Vector: Aedes Aegypti mosquitoes (Yellow fever), Anopheles
mosquitoes ( malaria) and Flea ( Bubonic plaque)
4. Direct droplet: Inhalation anthrax
Inanimate reservoir:
Soil as in case of tetanus .
The source of infection :
This term refers to the immediate source of infection, i.e. secretion or
excretion, in which the organism is ready to be transmitted to a susceptible
host. The source of infection may not be a portion of the reservoir. For
example, man is the reservoir of shigella infection; a cook who is a carrier
may infect food which is considered as the source of infection. While soil
acts as both the reservoir and source of infection in tetanus.
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III. Portal of exit:
The modes of exit from reservoirs are one of the followings:
i) Respiratory tract:
Organisms will leave the body via the mouth and nose in coughing,
sneezing, laughing or even talking. This is the portal of exit in measles,
whooping cough, diphtheria, streptococcal sore throat, influenza, common
cold, mumps…etc.
ii) Gastrointestinal tract :
The infecting organisms are liberated either in feces as typhoid,
paratyphoid, cholera, eggs of ascaris, …etc. Or through the vomitus as in
case of cholera. In certain diseases where the primary sites of infection is
the bowel, the organisms will pass through the faeces e.g. poliomyelitis (
affects CNS ) and infectious hepatitis .
iii) Urinary tract :
This occurs in some diseases where infection is general and organisms
are found in blood as in case of typhoid, undulant fever, bilharziasis as
well as, in local infection of genito-urinary tract as in case of gonorrhea.
iv) Skin and mucous membrane:
as in case of erysipelas and impetigo, also, discharges from mucous
membrane as in purulent conjunctivitis, and venereal diseases.
v) Insect bite:
As in case of typhus, plague and malaria.
vi) Syringes and taking blood from donors:
As in case of viral hepatitis.
vii) Utero-transmission ( Trans placental ):
Across the placenta from maternal blood to foetal circulation such as
AIDS, Syphilis and German measles.
IV. Mode of transmission:
Studying the modes of transmission in any communicable disease is of
great importance for prevention and control of diseases.
The modes of transmission could be classified into:
A) Contact transmission:
i) Direct contact.
ii) Indirect contact.
iii) Droplet contact.
iv) Transplacental.
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B) Common vehicle transmission, which can occur through :
i) Ingestion.
ii) Inoculation.
Iii)
Deposition.
C) Vector transmission:
i) Mechanical.
ii) Biological.
D) Air-born transmission:
i) Droplet nuclei.
ii) Dust particles.
A) Contact transmission:
Contact transmission means transmission during an association
between the infected man or animal and the new host. This association
(contact ) may be in the form of :
1. Direct contact:
In this situation, there is an association between the infected man or
animal and the new host, without a third object. Direct contact may occur
through sexual intercourse, kissing and touch. Examples for direct contact
transmission are venereal diseases, scabies, contact of saliva of a rabid
animal with abraded skin.
2. Indirect contact:
In this mode of transmission, the spread of infection is through touching
contaminated objects for instances, toys, handkerchiefs, soiled clothing
towels. Microorganisms will be transmitted from hands to mouth or from
hands
to
abraded
skin
or
mucous
membranes,
examples
are
conjunctivitis, trachoma, skin infections, and diphtheria. Also, this mode of
spread may be of importance in spreading hospital infections particularly in
transmitting infections to surgical wounds.
3. Droplet transmission:
Droplets spray onto the mucous membrane of the nose, mouth during
sneezing coughing, spitting or talking ( usually limited to a distance of one
meter or less). These droplets will pass into the nose or mouth of the host
directly. Examples: Measles, streptococcal infections, influenza, diphtheria
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4. Trans placental transmission:
In this type of contact transmission the organism is carried from
mothers to fetus through the placenta, e.g. Syphilis, AIDS and German
measles.
B) Common vehicle transmission
Vehicle of transmission can be water, milk or biological products as
blood, agents via this vehicle are introduced to the susceptible host
through:
1. Contaminated foods or drinks: ingestion of contaminated food and drink
for instance typhoid, paratyphoid, food poisoning, dysentery and cholera.
2. Infected blood and plasma: This is important in case of inoculation of
plasma, blood, serum or even vaccine, e.g. viral hepatitis and syphilis.
3. Agents of diseases inoculated to the skin or mucous membrane :
This is important for the group of diseases that can be transmitted by
the swimming pools, e.g. Conjunctivitis and sinusitis .
*N.B.: The role played by common vehicles depends on the viability of
infective agents outside the body, environmental influences as dryness,
temperature and sunlight.
C) Vector transmission:
Various insects are known to be vectors for disease transmission. There
role is either mechanical or biological.
1- Mechanical transmission: The vectors carry pathogenic organisms of
different infections on the feet or mouth parts, or may be ingested and
pass in the insect feces or vomited later on. The mechanical transmission
can be:
a) Direct:
If the insect as houseflies become contaminated from discharges of
infected eyes and transmit it to a healthy eyes as in case of purulent
conjunctivitis.
b ) Indirect:
This occurs when insects as houseflies and cockroaches carrying
pathogenic
organisms settle on human food or drink, they will
contaminate such food as occurs with typhoid, dysentery and cholera.
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1- Biological transmission:
The agents have to pass through some biological activity inside the
vector which requires a certain period of time " extrinsic
incubation
period ". After this period, the insect is then able to transmit infection to
the new host. The infectious agent may be passed vertically to
succeeding generations (trans-ovarian transmission ).
The biological vector transmission can be:
1- Propagative biological transmission: Simple multiplication of
the causative organism in the vector. The best example is
yersenia pestis agent of plague in flea and virus of yellow fever
in the Aides Aegypti mosquito.
2-Cyclo-propagative: Organisms here multiply and undergo changes
within the vector as in case of malaria.
3-Cyclo-developmental : Here the agent of disease have a cycle of
developmental changes inside the vector, with no multiplication,
example is filaria in mosquito.
D) Air-born transmission:
The dissemination of suspended particles in the air consisting
partially or wholly of microorganisms. They remain suspended for a
long period of time (the reservoir and the host may not be in the
same room). Some organisms retain their infectivity (as TB and
hemolytic streptococci) and the others do not. Air-born transmission
can be :1. Droplet nuclei: The small particles result from evaporation of the
fluids from the droplets of infected person. They contain pathogens
and remain suspended in air for a long period of time. When they
are inhaled by a susceptible host they cause infection.
2. Dust nuclei:- The large particles fall on the ground to mix with the
dust and become part of the dust of the room. The organisms resist
drying for a long time ( TB and hemolytic streptococci ). The source
of these particles is the discharge of infected persons contaminating
bedding, clothes, floor, soil, …
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V. Portal of entry (Inlet)
With some exception, the portal of entry to a host corresponds the portal of
exit from reservoir. Accordingly, we can classify the portals of entry into:
a. Natural inlets including mouth, nose, rectum, vagina, urethra and
conjunctiva.
b. Unnatural inlets for instance by inoculation through blood sucking
insects or by injection.
Incubation period:
The time interval between exposure to an infectious agents and
appearance of the first sign or symptom of the disease. There is an
average period for every communicable disease. However, there is a
range. The length of incubation period depends on:
1- If the agent is an organism or toxin.
2- The virulence of the organism.
3- The dose of the inoculums.
4- The host resistance.
Long incubation periods are encountered in infectious and serum hepatitis,
tuberculosis and AIDS. Short incubation period is that of staphylocooal
food poisoning, as the agent is the enterotoxin present in the ingested food
rather than the organism.
Knowledge of the incubation period is important to :1- It is used practically for surveillance of contacts of communicable
diseases.
2- To apply preventive measures in certain diseases.
- Measles vaccine if given to the contacts in the first three days
after exposure it will prevent the disease.
- If given in the second three days after exposure it will modify the
attack ( subclinical manifestation and result in solid immunity )
3- Identification of the source of infection, water borne epidemic of
typhoid has longer incubation period than milk borne while shell
fish borne typhoid has a shorter one.
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Extrinsic incubation period:This is the period taken by the infective agent outside the human body
until it becomes infective again to a new individual, e.g. yellow fever takes
(9 – 12) days in Aedes Aegypti to be infective after the blood meal .
VI. The susceptible host
The host is
a person or other living animal, including birds and
arthropods, that affords subsistence or lodgment to an infectious agent
under natural condition.
Resistance:
It is the total body mechanisms which act as barriers to invasion or
multiplication of infectious agents or their damaging effects of their toxins.
Types of resistance:
A) Natural barriers ( inherent resistance, non specific ).
B) Acquired resistance ( immunity, specific ).
A) Natural barriers:
It is non-specific resistance of the body against the invading organisms
which does not depend on the presence of specific antibodies or antitoxin
for protection,
but
depends on
the
anatomical or physiological
characteristics of the host.
b) Acquired resistance:
This depends on antibodies production. Immunity may be :1- Passive immunity.
a) Natural.
b) Artificial.
2- Active immunity.
a) Natural.
b) Artificial.
1. Passive immunity: Passive immunity is the type of resistance in which
ready made antibodies are gained.
a) Natural passive immunity ( infant immunity ): It is the resistance of
infant due to the presence of antibodies passed from the mother to the
foetus via the placenta. The mother should have acquired the infection
and / or vaccine and developed specific antibodies against certain
diseases. These passive acquired antibodies in the foetus are at their
highest level at birth, then start to decline gradually until they are
insignificant usually by the end of six months. Such natural passive
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immunity in the infant could be induced artificially by immunizing the
mother during pregnancy, as in case of immunization by tetanus toxoid
to protect the newly born infant against tetanus neonatorum. Breast
milk,
specially
the
colostrums,
contains
plenty
of
antibodies
(immunoglobulin account for about 95% of the protein in colostrums).
Antibodies are continually secreted in breast milk but in a lower level
than that of colostrums.
b) Artificial passive immunity (passive immunization): It is the immunity
induced by injecting immune serum or immunoglobulin. It is of short
duration, remaining for about three weeks, during which the antibodies
given are gradually eliminated. Examples of materials used for passive
artificial immunity:
1- Sera of artificially immunized animals, these are used either for
prophylaxis or treatment as in case of antitetanic and antidiphtheretic
sera.
2- Immunoglobulin: This is a plasma protein fraction that carries most
of antibodies present in the body, used as prophylaxis, e.g. in measles
and infectious hepatitis.
2. Active immunity: This is the type of immunity in which the person
makes or develops his own antibodies. It may be :
a) Natural actively acquired immunity or post infection immunity: It may
be solid or for a long time as in case of mumps and measles or it may
be of moderate duration (for years) as in case of meningitis or it may
be for a short period as in common cold.
b) Artificial actively acquired immunity: This is produced artificially by
active immunization using an immunizing agent which is a specific
antigen when introduced in the body, provoke the formation of
antibodies.
The ideal immunizing agent should be:
1- Antigenically stable.
2- Give durable immunity.
3- Have minimal side effects.
4- Easy in administration.
5- Of reasonable cost as well as being available.
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6- Keeping quality is good.
Types of immunizing agents ( vaccines ):
I- Live attenuated vaccines:
a) Attenuated vaccine: In these vaccines, microorganisms lose their
pathogenicity but retain their power of multiplication and antigenicity.
Attenuation can be done by repeated subcultures or by cultivation under
unfavorable condition, e.g. Sabin vaccine of
polio, measles, German
measles and mumps vaccines.
b) Variant forms of living organisms vaccine: In these vaccines a milder
species of the organisms closely related Antigenically to the human
disease agents, are used, such as smallpox vaccine using cow pox virus
and BCG vaccine using bovine tubercle bacilli.
II- Non living vaccines:(A) Killed or inactivated:
- Killed bacterial organisms ( using heat or chemicals as ether and
Formalin ) examples typhoid, whooping cough vaccines.
- Inactivated virus as salk vaccine of polio virus.
(B) Products of organisms ( Toxoid ):
- Toxoid is the toxins after loosing their toxicity but retaining their
antigenicity e.g. Diphtheria, and tetanus toxoid.
(C) Part of organisms:
- The subunit of hepatitis B surface antigen ( HBs Ag ) prepared from
plasma of HBs Ag positive carriers or by genetic engineering.
- Part of polysaccharide capsule of Nisseria meningitides used as a
vaccine against meningococcal meningitis.
Herd immunity: It is the state of immunity within the community. Herd
immunity is the factor that decides the epidemiological pattern of any
infectious disease among that community. The incidence of diseases rises
at times when the number of susceptible in the population is highest and
the herd immunity is lowest. The best example is measles epidemic in
Fidjii in 1975 in which the attack rate approached 100% and fatality rate
was excessively high. The spread of the disease among all age groups
equally is characteristic of the absence of immunity. Also, the well known
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rhythm of measles epidemic in urban communities is due to the variation in
community susceptibility. Following an epidemic, immunity is at its highest
level. The level of susceptibility increases as new infants are born, an
epidemic will develop after accumulation of susceptible. The herd
immunity could be produced artificially by immunization, or naturally after
infection.
Several factors play a role in community protection namely:
1- The extent of coverage of the immunization program.
2- The degree of resistance to infection afforded by the vaccine.
3- Duration and degree of infectivity of the organism.
4- Past experience with different infections.
5- Overcrowding and environmental sanitation.
General Principles of Prevention
Successful prevention depends upon knowledge of causation and
dynamics of disease transmission, identification of risk factors and risk
groups and availability of prophylaxis or early detection and treatment.
Levels of prevention:
It has become customary to define prevention in terms of three levels:
1. Primary prevention.
2. Secondary prevention.
3. Tertiary prevention.
1- Primary prevention.
Actions taken prior to the onset of the disease which control the
causation and the risk factors thus limit the incidence or prevent the
possibility of occurrence of a disease. Example of primary prevention:
1- Health education.
2- Environmental sanitation.
3- Nutritional interventions.
4- Specific protection.
-
Immunization.
-
Chemoprophylaxis.
-
Use of specific nutrients.
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-
Protection from carcinogens and allergens.
-
Protection against accidents
-
Protection against occupational hazards.
2- Secondary prevention.
Actions hinder the progress of a disease at its early stage and prevent
complications. The specific interventions are early diagnosis (e.g.
Screening tests and case finding programs) and adequate treatment
before irreversible pathological changes have taken place. It may also,
protect others in the community from acquiring the infection and thus
provide at once secondary prevention for the infected individuals and
primary prevention for their potential contacts. Secondary prevention is
largely the domain of clinical medicine. The health programs initiated by
governments are usually at the level of secondary prevention. Early
diagnosis and treatment though not as effective and economical as
primary prevention, it may be critically important in reducing the high
morbidity and mortality in certain diseases such as essential hypertension,
cancer cervix and breast cancer.
An example of secondary prevention is mass treatment. A mass treatment
is used in the control of certain diseases, e.g. Trachoma, malaria and
bilharziasis. The rationale for a mass treatment program is the existence of
at least four to five cases of latent (incubating) infection for each clinical
case of active disease in the community. Patients with a latent infection
may develop disease at any time. In such cases, mass treatment is a
critical factor in the interruption of disease transmission.
3- Tertiary prevention:
Actions taken when the disease process has advanced beyond its early
stages i.e. intervention in late pathogenesis phase. The aim of tertiary
prevention is :
- To limit impairments and disabilities.
- To decrease suffering.
- To promote the patient's adjustment to rehabilitation.
 Rehabilitation:- It is a measure to train disable individuals to reach the
highest level of functional ability by using combined coordinated
medical, social and educational measures.
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Examples of rehabilitation:1- Special schools for blind pupils.
2- Provision of aids for crippled.
3- Reconstructive surgery for leprotics.
4- Modification of life for tuberculous or cardiac patients.
General Principles of Prevention
And Control of Communicable Diseases
Prevention:Measures applied to prevent the diseases before their occurrence.
Control:Measures applied to avoid the spread of the diseases after their
occurrence i.e. Reduce the incidence and / or the prevalence of these
diseases.
I - Measures applied to the agent: Sterilization and disinfections.
II – Measures applied to the reservoir of infection:
A) Measures applied to cases:
1- Case finding.
2- Reporting.
3- Isolation.
4- Treatment.
B) Measures applied to carriers:
1- Detection.
2- Exclusion from work.
3- Treatment for the carrier state.
C) Measures applied to animal reservoir.
III - Measures applied to the contacts:
1- Enlistment.
2- Surveillance.
3- Isolation.
4- Increase resistance.
5- Health education
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IV - Measures applied to the environment:
- Improving Sanitation.
V - Measures applied to the host:
1- Non-specific.
2- Specific.
I - Measures applied to the agent:
Sterilization:- Killing all forms of living organisms. It is used for medical
and surgical instruments.
Disinfections:- Killing the infectious agents outside the body by direct
exposure to chemical or physical agents:1- Concurrent disinfection:- Is the application of disinfective measures
as soon as possible after the discharge of infectious material from
the body of an infected person or after the soiling of articles with
such discharge.
2- Terminal disinfection:- Application of disinfective measures after the
patients has been removed by death or to a hospital or has been
ceased to be a source of infection (by treatment). Disinfection is
necessary only for diseases spread by indirect contact.
II – Measures applied to the reservoir of infection:
A) Measures applied to cases
1- Surveillance: It is the collection , analysis, interpretation and
dissemination of information about a selected health event. This
information is important
to plan, implement and evaluate a
health program.
2-Case finding and early detection: It is the first step in the
control of a communicable disease.
3-Reporting: Notifying an appropriate authority of the occurrence
of a communicable disease. Diseases in man are reported to the
local health authority, those in animals to sanitary, veterinary or
agriculture authority. Zoonotic diseases are reported to both
authorities. Diseases under international health regulation
should be reported to WHO. The aim of reporting is to provide
information to permit appropriate control measures and to
compare the frequencies of diseases occurrence between
different countries.
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4- Isolation of the patient: Isolation means the separation, for the
period of communicability of the infected person, from other healthy
persons, under conditions that will prevent the direct or indirect
transmission of the infectious agent to the susceptible person. Isolation
is continued for the period in which the patient is dangerous to other.
Since the period of communicability varies in different diseases, the
period of isolation must vary correspondingly. In many instances, it is
possible to determine it by exact laboratory methods.
Isolation has a limited value, and cannot be relied upon to prevent
spread of infection in a community for the following reasons:
a- Many diseases are highly communicable during the early stage.
b-The exact period of communicability is not known in many diseases.
c- Carriers of infective agents may go undetected.
d- Many mild cases of infection spread disease without being
detected.
- Isolation in a hospital or in separate quarter is required for cholera,
plague
- Isolation at homes: If the condition of home is suitable patients with
typhoid, paratyphoid and meningococcal meningitis, whooping
cough, poliomyelitis and infective hepatitis.
5- Treatment: Chemotherapy: The use of chemicals to cure clinically
recognizable disease or to limit its further progress. This is not only
for the sake of the patient, but also, for reducing the period of
communicability, and to limit the transmission to the contacts.
B) Measures applied to carriers:
1- Detection: Detection of carriers is important in diseases in which
carriers are an important reservoir of infection, e.g. Enteric fever.
However, the value of detection for carriers depends on:
a) The proportion of carriers in the community.
b) The occupation of the carriers and its intimacy to contacts as
food handlers and those working in closed community ( e.g. School
or day care centers ).
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2- Exclusion from work: Must be done if his occupation is a food
handler (e.g. Typhoid carrier) or a teacher (e.g. Diphtheria carrier).
3- Treatment for the carrier state.
C) Measures applied to animal reservoir:
Reservoir eradication is only possible in diseases transmitted from
animal to man (zoonotic diseases), e.g. Killing the infected animals( in
rabies), slaughtering ( in bovine tuberculosis), immunization (in
Brucellosis), careful husbandry and sterilization of animal products (in
anthrax).
III – Measures applied to the contacts:
Contacts are persons in close association with infected persons or an
animal or material handled by an infected person. Contacts are
undergoing
special risk of contracting infection. Contacts at large include household
contacts, occupational and school contacts. The following are measures
for contacts:1- Enlistment: Contacts are listed and dealt with according to the
disease.
2- Surveillance of persons :(put under observation) The supervision of
contacts to permit recognition of infection or illness without
restricting their movements. The period of observation is equal to
the longest incubation period of the disease under observation.
Taking the temperature is the best criterion for the development of
the disease.
3-Isolation: It means separation of contacts in a specific place
to
prevent direct or indirect contact between them and unexposed
individuals. Period of isolation is for the longest incubation period counted
from date of last exposure. This measure
is only applied in case of
contacts of pneumonic plague only.
4- Increasing the resistance of the contacts: This is done either by :
a) Active immunization as in case of measles.
b) Passive immunization as in case of diphtheria.
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c) Chemoprophylaxis: is administration of chemicals including
antibiotics, to prevent the development of an infection or the
progression of an infection to active manifest disease.
Examples of Chemoprophylaxis:(1) Isoniazid for contacts of tuberculous patients.
(2) Sulphadiazine or Rifampicin for contacts of meningococcal meningitis.
(3) Erythromycin or penicillin for unimmunized contacts of a diphtheria
case.
(4) Tetracycline for contacts of cholera and pneumonic plague.
(5) Penicillin for contacts of syphilitic patients.
IV - Measures applied to the environment:
This include chlorination of water, sanitary sewage and refuse
disposal, rodent control, vector and food sanitation.
V – Measures applied to the host:
a) Non-specific measures:
1- Health education.
2- Good standard of nutrition.
3- Provision of adequate housing.
4- Provision of adequate recreation facilities.
5- Provision of suitable working condition.
6- Periodic selective examination.
7- Personal hygiene.
8- Protection against accidents.
b) Specific measures:
Use
of
specific
passive
and
active
immunization
and
Chemoprophylaxis.
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