Infectious diseaseppt

1. BASIC CONCEPTS IN INFECTIOUS
DISEASE EPIDEMIOLOGY
YIMER SEID (BSC, MPH epi and Bio)
Lecturer, Epidemiology Unit.
AUGUST, 2018
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2. Outline
♦ Definition of terms
♦ Features of infectious disease epidemiolog
♦ Infectious disease process
♦ Chain of Disease Transmission
♦ Estimating the Transmission
Probability
♦ Time lines of infection
♦ Natural History of Disease
♦ Levels of disease occurrence
♦ Levels of disease prevention
♦ Methods of controlling communicable diseases
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3. Definition of terms
♦ Communicable disease: disease
transmitted from one host to another
♦ Contagious disease :A contagious
disease is the one that is transmitted
through contact. Examples include
scabies, trachoma, STD and leprosy
♦ Infection: is the entry and development
or multiplication of an infectious agent in
the body of man or animals. An infection
does not always cause illness.
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4. Defn
♦ Contamination: The presence of an
infectious agent on a body surface, on or in
clothes, beddings, toys, surgical instruments
or dressings, or other articles or substances
including water and food
♦ Infestation: It is the lodgment, development
and reproduction of arthropods on the surface
of the body or in the clothing, e.g. lice, itch
mite. This term could be also used to describe
the invasion of the gut by parasitic worms,
e.g. ascariasis
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5. Defn…
♦ Infectivity – ability to invade a host
(# infected / # susceptible) X 100
♦ Pathogenicity – ability to cause disease
(# with clinical disease / # of infected) X 100
♦ Virulence – ability to cause death
(# of deaths / # with disease (cases)) X 100
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6. Defn…
♦ Infectious disease: An illness due to a
specific infectious agent or its toxic products
that arises through transmission from an
infected person, animal or reservoir to a
susceptible host, either directly or indirectly
through an intermediate plant or animal host,
vector or inanimate environment.
Last JM, Dictionary of Epidemiology, 1988
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7.  Two or more populations
– Humans
– Infectious agents
– Vector
– Animals
 A case is a risk factor …
Infection in one person can be transmitted to
others
 The cause often know
 An infectious agent is a necessary cause
Features of infectious disease
epidemiology?
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8. What is infectious disease
epidemiology then used for?
 Identification of causes of new, emerging
infections, e.g. EBOLA, SARS
 Surveillence of infectious disease
 Identification of source of outbreaks
 Studies of routes of transmission and natural
history of infectious disease
 Identification of new interventions
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9. Infectious disease process
(Pathogenesis of host damage)
♦ Pathogens cause damage in a number of
different ways
– directly cause tissue damage by the elaboration of
proteolytic enzymes that destroy or damage tissue
– Production of Toxins
– induce an excessive immune response resulting in
damage e.g., endotoxin induction of cytokines
– cause a hypersensitivity reaction as may be seen in
endocarditis with immune complex
glomerulonephritis
– may cause malignant transformation of host cells
as is seen in hepatitis B or with helicobacter.
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10. Chain of Disease Transmission
 The six factors involved in the chain of
disease transmission include:
– Infectious agent
– Reservoir
– Portal of Exit
– Mode of Transmission
– Portal of entry
– Susceptible host
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11. Infectious Agent
♦ is an organism that is capable of causing
infectious disease.
•Metazoa (multi cellular) – E.g.
Helminthes
•Protozoa (unicellular) – E.g. Amoeba
•Bacteria E.g. M.tuberclusis
•Fungus – E.g. C.albicans
•Virus- E.g. HIV
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12. Reservoir
♦ Is any living or non living things, it could be a
person, animal, plant, soil or substance in
which an infectious agent normally lives &
multiplies
Types of reservoir
– Man
– Animals
– Non-living things eg soil
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13. Portal of Exit
– Is the site through which the agent
escapes from the reservoir.
E.g.
♦ GIT – Typhoid, Shigellosis, cholera,
Ascariasis
♦ RT – TB, common cold
♦ Skin – Scabies
♦ GUT – STIs
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14. Mode of Transmission
a. Direct transmission
I. direct contact
II. Direct projection
III. Transplacental
b. Indirect transmission
I. Air born disease
II. Vehicle born
III. Vector born
1. mechanical
2. biological
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15. Portal of Entry
– Is the site in which the infectious
agent enters to the susceptible host.
Example:
oMucus membrane and skin
oRespiratory tract
oBlood
oGIT
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16. Susceptible host (Host factors)
♦ A person or other living animal susceptible
for infection
♦ Refers to resistance (immunity) of the
host
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17. Estimating the Transmission
Probability
♦ The probability that successful transfer of
the agent will occur so that the susceptible
host becomes infected.
♦ Two common ways of estimating
transmission probability are:
– Secondary attack rate
– Binomial Model
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18. Secondary attack rate
♦ A secondary attack rate is a measure of
the frequency of new cases of a disease
among the contacts of known cases. The
formula is as follows:
• To calculate the total number of contacts,
we usually subtract the number of primary
cases from the total number of people.
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19. Example: Secondary Attack Rate
♦ Seven cases of hepatitis A occurred among
70 children attending a child care center.
Each infected child came from a different
family. The total number of persons in the 7
affected families was 32. One incubation
period later, 5 family members of the 7
infected children also developed hepatitis
♦ We will calculate the attack rate in the child
care center and the secondary attack rate
among family contacts of those cases.
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20. 1. Attack rate in child care center:
– x = cases of hepatitis A among children in
child care center = 7
– y = number of children enrolled in the child
care center = 70
– Attack rate = x/ y × 100 = 7 /70 × 100 =
10%
2. Secondary attack rate:
– x = cases of hepatitis A among family contacts
of children with hepatitis A = 5
– y = number of persons at risk in the families
(total number of family members—children
already infected) = 32 − 7 = 25
– Secondary attack rate =(x/y )× 100 = (5/25)
× 100 = 20%
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21. Binomial Model of Transmission
Probability
♦ Let the probability of transmission during a
contact with a susceptible and an infectious
person is p.
♦ The probability of the susceptible person
escaping infection during contact is q (or 1 - p).
♦ If a susceptible makes ‘n’ contacts with an
infective or with different infectives and each
contact is independent of the others, the
probability of escaping infection from all n
potential infectives is
♦ The probability of being infected after n contacts
is
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22. Number of susceptible who become infected
Total number of contacts with infective
– Denominator include the total number of
potentially infectious contacts that susceptible
individuals make
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23. Binomial …
Example
♦ A Study of HIV transmission was conducted
in a population of 100 steady sexual couples.
At the beginning, one couple was already
infected. 25 became infected. The total
number of sexual contacts was 1500. what is
probability of being infected after 2 contacts.
– 1-(1-p)2= the probability being infected
– P=25/1500=0.016
– 0.034
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24. Time lines of infection
– Latent period: The period between exposure and the onset
of infectiousness .
– Infectious period: Time during which the host can infect
another host.
– Incubation period: The interval between the time of
contact and/or entry of the agent and onset of illness
(latency period).
– Symptomatic period: period in which symptoms of the
disease are present.
– Serial interval: the gap in time between the onset of the
primary and the secondary cases (also called generation
time).
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25. Natural History of Disease:
 It refers to the progression of disease
process, in the absence of intervention.
 It begins with exposure to causative agent
capable of producing disease.
 Without intervention, the process ends
with recovery, disability or death
 Helps to know the progress of disease and
to take appropriate intervention at each
stage
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26. Stages in the Natural History of Disease
1.Susceptibility
 Disease has not yet developed but there is risk factor
that favor the disease occurrence
 E.g. unvaccinated child is susceptible to measles
2.Pre-symptomatic /stage of sub-clinical
 No manifestations of the disease but pathologic
changes (damages) have started to occur in the body
 period of latency, biological onset of disease and
period of induction/incubation
 The disease can only be detected through special
tests ; E.g. Detection of antibodies against HIV in an
apparently healthy person
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27. Stages…
3.Clinical disease
 Person has developed signs and symptoms
of the disease
 The clinical stage of different diseases
differs in duration, severity and outcome
For example:
 Rabies has a relatively short but severe clinical
stage and almost always results in death
 Polio has a severe clinical stage and many
patients develop paralysis becoming disabled
for the rest of their lives.
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28. Stages…
4.Disability/death
 Some diseases run their course and then
resolve completely either spontaneously or by
treatment.
 In others the disease may result in a residual
defect, leaving the person disabled for a short or
longer duration. Still, other diseases will end in
death
 E.g. Trachoma may cause blindness
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29. Stages in the natural history of disease
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30. Levels of Disease Occurrence
 Diseases occur in a community at different
levels at a particular point in time.
 Some diseases are usually present in a
community at a certain predictable level,
this is called the expected level, but at
times disease may occur in excess of what
is expected.
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31. Expected levels
Endemic:
 A persistent level of low to moderate
occurrence
Hyper-endemic
 A persistently high level of occurrence
Sporadic
 Occasional cases occurring at irregular
intervals
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32. Levels of disease occurrence
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33. Excess of what is expected
Epidemic
 occurrence of disease in excess of what is
expected in a limited period.
Outbreak
 same as epidemic, often used by public
health officials because it is less offensive to
the public.
Pandemic
 an epidemic spread over several countries or
continents, affecting a large number of people
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34. LEVELS OF DISEASE PREVENTION
 There are three levels of prevention
 Primary prevention
• Those measures provided to individuals to
prevent the onset of a targeted condition.
• Keeps the disease process from becoming
established by eliminating causes of disease
or increasing resistance to disease
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35. Components of primary prevention
 Has 3 components.
Health promotion
Prevention of exposure
Prevention of disease.
 Health promotion
non-specific interventions that enhance health and
the body’s ability to resist disease;
E.g.Improvement of socioeconomic status,
provision of adequate food, housing, clothing, and
education
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36.  Prevention of exposure
 is the avoidance of factors which may cause disease
 provision of safe and adequate water supply,
 proper excreta disposal,
 vector control,
 safe environment, etc.
 Prevention of disease
 prevention of disease development after the individual
has become exposed to the disease causing factors
 such as immunization.
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37. Secondary Prevention:
 is described as measures as those that
“identify and treat asymptomatic persons who
have already developed risk factors or
preclinical disease but in whom the condition
is not clinically apparent.
 Objective- to stop or slow the progression
of disease so as to prevent or limit
permanent damage
 E.g. Early diagnosis and treatment and
screening
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38. Tertiary Prevention
 It is limitation of disability and rehabilitation.
 Objective- is to limit the physical, psychological,
social and economical impact of disability.
 Tertiary prevention activities involve attempts
made to restore to highest function, minimize the
negative effects of disease, and prevent
disease-related complications.
 Example; Rehabilitation after polio, blindness
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39. Methods of controlling
communicable diseases
1.Attacking the source
2.Interrupting the mode of transmission
3.Protecting the host
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40. 1. Attacking the source
Domestic animals as reservoirs
i. Immunization e.g. Brucellosis
ii. Destruction of infected animals e.g.
Rabies
Wild animals as reservoirs
– Post exposure prophylaxis e.g. Rabies
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41. Humans as reservoirs
i. Isolation of infected persons & separation
of infected persons form other for the
period of communicability
Not suitable when,
Large proportion are apparently
infected, or
In which maximal infectivity precedes
over illness.
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42. ii. Treatment
 Of cases (clinical) and carriers
 Mass treatment – where large proportion are
known to have a disease it is sometimes advisable
to treat everybody, without checking whether
individuals have disease or not
 Quarantine – the limitation of freedom of
movement of apparently healthy persons or
animals who have been exposed to a case or
infections disease.
 Cholera, plaque, and yellow fever are the 3
internationally quarantinable diseases by
international agreement. These diseases are very
infections, so cases shouldn’t be referred but
seniors must be requested to visit the health
center.
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43. 2. Interrupting transmission
For Transmission by in gestation
i.Purification of water
ii.Pasteurization of milk
iii.Inspection procedures designed to
ensure safe food supply
iv.Improve housing conditions
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44. Interrupting trans…
 For Transmission by inhalation
i. Chemical disinfections of air
ii.Use of UV-light
iii.Improving ventilation
Transmission by vector or intermediate
hosts
i. Vector – control measures
ii.Environmental manipulation
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45. 3. Measures that reduce host
susceptibility
i. Immunization
- Active immunization – when either the altered
organism or its products is given to a person to
induce production of antibodies , E.g. BCG
- Passive immunization – provision of ready – made
antibodies
- E.g. TAT
ii.Chemoprophylaxis – use of antibiotics for
known contacts to a case, E.g.
Ciprofloxacin for contacts to a case of M.
meningitis
iii.Better nutrition
Malnourished children get infections more easily &
suffer more severe complications
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46. ♦ N.B Effective control of disease is most
likely when a combination of methods –
attacking the source, interrupting
transmission, and protecting the host is
used at the same time.
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47. THANK YOU
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