1
Epidemiology
of infectious diseases
Kolářová M., EPI Autumn 2015
EPIDEMIOLOGY
is the study (scientific, systematic, data-driven)
of the distribution (frequency, pattern)
and determinants (causes, risk factors)
of health-related states or events (not just diseases)
in specified populations (patient is community,
individuals viewed collectively),
and the application (since epidemiology is a discipline
within public health)of this study to the control of health
problems.
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In tracking a disease outbreak, epidemiologists may
use any or all of three types of investigation:
a) descriptive epidemiology
is the collection of all data describing the occurrence of the disease
b) analytical epidemiology
attempts to determine the cause of an outbreak
c) experimental epidemiology
tests a hypothesis about a disease or disease treatment in a group
of people.
 Incidence - the incidence rate refers to the number of new
cases of a disease in a given population over a period of time.
 Prevalence - the prevalence rate refers to the number of total
cases of a disease in a given population at a specific time.
 Sporadic disease Disease that occurs
occasionally in a population.
 Endemic disease Disease constantly present in a
population.
 Epidemic disease Disease acquired by many
hosts in a given area in a short
time.
 Pandemic disease Worldwide epidemic.
Occurrence of Disease
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Epidemiologic investigations are
largely mathematical
descriptions of persons in groups,
rather than individuals.
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The basic quantitative
measurement in epidemiology is
a count of the number of persons
in the group being studied who
have a particular disease.
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Epidemiologists arrange their data in various ways,
depending on what aspect of the information they want
to emphasize.
One of the most powerful tools an
epidemiologist can use is case reporting:
reporting specific diseases to
* local,
* state and
* national health authorities,
who accumulate the data
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Modern infectious disease
epidemiology
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According to clasic definition,
epidemiology of infectious
disease
in its theoretical part
studies the chain of infections
(epidemic process)
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1. the presence of rezervoir (source) of infection
2. the way of transmission
3. the susceptibility of the population or its individual member
to the organism concerned
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THE CAUSATIVE AGENT OF INFECTION (bacteria, viruses, fungi, prions, protozoa)
1. the presence of rezervoir (source) of infection
man, animal at the ende of incubation period
acute stage
cariers
2. the way of transmission
3. the susceptibility of the population or its individual member
to the organism concerned
THE INFECTION
= 1. source of infection
. . . . . .
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THE CAUSATIVE AGENT OF INFECTION (bacteria, viruses, fungi, prions, protozoa)
1. the presence of rezervoir (source) of infection
man, animal at the ende of incubation period
acute stage
cariers
2. the way of transmission A/ direct contact
touching, kissing or sexual intercourse (Staphylococcus spp., Gonococcus spp.,HIV …),
- vertical transmission – from mother to fetus (VHB, VHC, HIV, listeria, rubella, cytomegalovirus…)
B/ indirect contact
- inhalation of droplets containing the infectious agents (TBC, measles, influenza…)
- ingestion of food or water that is contaminated (salmonella, giardia, Norwalk virus, VHA….)
- biological transmission by insects (malaria, borellia….)
3. the susceptibility of the population or its individual member
to the organism concerned
THE INFECTION
= 1. source of infection
. . . . . .
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THE CAUSATIVE AGENT OF INFECTION (bacteria, viruses, fungi, prions, protozoa)
1. the presence of rezervoir (source) of infection
man, animal at the ende of incubation period
acute stage
cariers
2. the way of transmission A/ direct contact
touching, kissing or sexual intercourse (Staphylococcus spp., Gonococcus spp.,HIV …),
- vertical transmission – from mother to fetus (VHB, VHC, HIV, listeria, rubella, cytomegalovirus…)
B/ indirect contact
- inhalation of droplets containing the infectious agents (TBC, measles, influenza…)
- ingestion of food or water that is contaminated (salmonella, giardia, Norwalk virus, VHA….)
- biological transmission by insects (malaria, borellia….)
3. the susceptibility of the population or its individual members to the organism
concerned Host factors : a g e , n u t r i t i o n, g e n e t i c s
i m m u n i t y – natural (nonspecific),
- acquired
THE INFECTION
= 1. source of infection
……….
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THE CAUSATIVE AGENT OF INFECTION (bacteria, viruses, fungi, prions, protozoa)
1. the presence of rezervoir (source) of infection
man, animal at the ende of incubation period
acute stage
cariers
2. the way of transmission A/ direct contact
touching, kissing or sexual intercourse (Staphylococcus spp., Gonococcus spp.,HIV …),
- vertical transmission – from mother to fetus (VHB, VHC, HIV, listeria, rubella, cytomegalovirus…)
B/ indirect contact
- inhalation of droplets containing the infectious agents (TBC, measles, influenza…)
- ingestion of food or water that is contaminated (salmonella, giardia, Norwalk virus, VHA….)
- biological transmission by insects (malaria, borellia….)
3. the susceptibility of the population or its individual members to the organism
concerned Host factors : a g e , n u t r i t i o n, g e n e t i c s
i m m u n i t y – natural (nonspecific),
- acquired
THE INFECTION
= 1. source of infection
……….
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the agent of infection (e.g., particular
bacterium or virus),
Organism characteristic:
infectivity – capacity to multiply in host
pathogenicity – capacity to cause disease in host
virulence - pathogenicity in a specific host
immunogenicity – capacity to induce specific and
lasting immunity in host
antigenic stability – can induce long-life immunity
resistance - in environment
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Organisms vary in their capacity to survive in the
free state and to withstand adverse
environmental conditions, for example:
* heat, cold, dryness.
Sporo-forming organisms, such as tetanus bacilli
which can survive for years in a dormant state,
have a major advantage over an organisms like
the gonococcus which survive for only a very
short time outside the human host.
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1. the presence of source of
infection
is the site or sites in which a disease agent normally lives and
reproduces.
May be classified as:
- human - at the ende incubation period,
if is ill,
reconvalescent,
carriers – healthy,
chronic diseases
- animals - at the ende incubation period,
if is ill,
carriers – healthy,
reconvalescent, chronic
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2. the metod of transmission
A/ direct contact
touching, kissing or sexual intercourse (Staphylococcus spp.,
Gonococcus spp.,HIV …),
- vertical transmission – from mother to fetus (VHB, VHC, HIV,
listeria, rubella, cytomegalovirus…)
B/ indirect contact
- inhalation of droplets containing the infectious agents (TBC,
measles, influenza…)
- ingestion of food or water that is contaminated (salmonella,
giardia, Norwalk virus, VHA….)
- biological transmission by insects (malaria, borellia….)
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Routes of transmission
 Air
 Food, Drink or Water
 Direct or indirect contact
* Transplacental
 Insects (Artropods)
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Among the important environmental factors that affect
an epidemic of infectious diseases are:
poverty, overcrowding, lack of sanitation,
and such uncontrollable factors:
as the season and climate.
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3. the susceptibility of the population or its
individual member to the organism
concerned, and the characteristic of the
organism itself.
Host factors :
a g e - the very young and the very elderly are more susceptible to infetious
diseases than are older children and younger adults
n u t r i t i o n
g e n e t i c s
i m m u n i t y – natural, acquired and population
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Contamination of humans by micro-organisms.
Many parts of the body are colonized by normal flora,
which can be the source of endogenous infection.
Large numbers of micro-organisms are found in moist
areas of the skin (e.g. the groin, between the toes),
the upper respiratory tract, the digestive tract (e.g. the
mouth, the nasopharynx), the ileum and large
intestine, the anterior parts of the urethra and the
vagina.
Other routes are interhuman transmission of infections
and exposure to exogenous contamination.
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Main portals of entry
 Respiratory tract
 Gastro-intestinal tract
 Genito-urinary tract
 Direct break through skin
* surgical and wounds
 Direct into blood via needles/catheters
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Slime-producing coagulase-negative staphylococci. Scanning electron micrograph of the
surface of an intravascular catheter incubated in vitro with (a) slime-producing and (b)
nonslime-producing strains of Staphylococcus epidermidis. With permission from
Christensen.9
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Staphylococcus aureus
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Staphylococcal nasal carriage. This patient had a small staphylococcal abscess beneath the
mucosa of the nose, illustrating how Staphylococcus aureus, which colonizes the nares, can
infect skin and submucosa. Intact mucosa is highly resistant to infection; such infections
usually occur as a result of defects in the mucosal membranes or via hair follicles inside the
nose.
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 Impetigo in a child.
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Septic pulmonary emboli. Multiple nodular pulmonary infiltrates secondary to a
dialysis catheter-associated infection. The patient presented with high fevers, cough
and pleuritic chest pain. Staphylococcus aureus was isolated from multiple blood
specimens.
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 b-Hemolytic streptococci group A on a blood agar plate.
Note the clear b-hemolytic zone.
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 Electron microscopy of group A streptococcus. The
fuzzy M protein layer can be seen protruding from
the cell wall..
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Erysipelas. Note the sharp demarcation of the
affected skin.
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Scarlatina (scarlet fever)
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 Necrotizing fasciitis caused by group A strepococci.
There is only moderate erythema but at surgery there
was extensive soft tissue damage.
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Ziehl-Neelsen stain of 'cords' of Mycobacterium
tuberculosis isolated from a broth culture. Tubercle bacilli
aggregate end to end and side to side to form serpentine
cords, especially in broth cultures.
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Mixed culture of two morphotypes of Enterobacteriaceae
on blood agar plate (Escherichia coli and Salmonella spp.).
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Pseudomonas aeruginosa monotrichous polar flagellum
seen on electron microscopy.
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Burned leg that has been superinfected with Pseudomonas
aeruginosa.
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Cultured Helicobacter pylori in coccoid and bacilli forms,
bound to immunomagnetic beads.
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Obtained after an outbreak, this micrograph depicts Gram-positive
Clostridium difficile bacteria.
Source: CDC
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Under a moderately-high magnification of 8000X, this colorized scanning
electron micrograph (SEM) revealed the presence of a small grouping of Gramnegative
Salmonella typhimurium bacteria that had been isolated from a pure
culture. See PHIL 10986 for a black and white version of this image.
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Rotavirus
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HEPATITIS A VIRUS
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Primoinfection HIV
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Helical structure of Treponema pallidum with the
periplasmic flagella.
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Secondary syphilis with typical skin rash.
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Gonococcal urethritis.
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Colonies of Nocardia asteroides showing smooth chalkywhite
appearance.
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Primary cutaneous nocardial infection is characteristically painless, localized
and slowly progressive. (a) There is marked swelling and erythema in this
child's finger. (b) However, because the finger was painless the child was not
brought to medical attention until the infection had progressed to involve the
entire finger.
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Typical rash of meningococcal septicemia. Fine
erythematous macules and petechiae are present in some
areas.
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Varicella (chickenpox)
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Varicella (chickenpox). Lesions at various stages, including
vesicles, can be seen.
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Morbilli (Measles). A disseminated erythematous rash can be
seen over the trunk and arms.
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Rubella. A pink macular rash can be seen on the forearm.
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Rubella
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Parotitis epidemica ( mumps)
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Sarcoptes scabiei
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Scabies
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Crusted or Norwegian scabies in a patient who has AIDS.
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Giemsa stain of blood with Borellia burgdorferi.
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Tick - Ixodes ricinus
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Lyme boreliosis (LB)
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LB - Typical erythema migrans rash.
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A blood-engorged female Aedes albopictus mosquito feeding on a
human host.
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Francisella tularensis
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Tularemia
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Tularemia
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Hlístice Trichinella spiralis
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Pathogenesis of rabies.
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 The distribution of the
smallpox rash is
usually similar to that
shown here. It is
most dense on the
face, arms and hands,
legs and feet. The
trunk has fewer pocks
than the extremities.
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Smallpox is a disfiguring disease. Three out of ten cases may die. It is caused by variola
virus. The disease is spread by secretions from the patient's mouth and nose, and by material
from pocks or scabs. It is transmitted directly from one person to the next. Close contact with
patients, or their clothing or bedding, is thus required for infection. A patient who has
developed the distinctive symptoms of smallpox will have been exposed to the virus about two
weeks previously.
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If the epidemiology is know, we can
interfere with transmission:
„BREAKING THE CHAIN OF
INFECTION“
Different infections have different
epidemiologies and thus require different
methods of control
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In the practical part it is preoccupied with
preventive measures
repressive measures
related to infectious diseases
The 14 steps of an epidemic investigation
1. Confirm the existence of an epidemic.
2. Verify the diagnosis.
3. Develop a case definition.
4. Develop a case report form.
5. Count the cases (i.e., an approximate analysis).
6. Orient the data (i.e., time, place, and person).
7. Analyze the data (e.g., agent, transmission, and host).
8. Develop a hypothesis.
9. Test the hypothesis.
10.Plan and implement control and prevention measures.
11.Evaluate the implemented measures.
12.Establish or improve the public health surveillance.
13.Write a report.
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