dr.ihsan alsaimary university of basdrah - college of medicine- iraq

1. EPIDEMIOLOGY OF
Infectious diseases:
Defenitions , Management and control
PROF.Dr.ihsan edan alsaimary
Department of microbiology – college of medicine – university of basrah - iraq

2. INTRODUCTION
INFECTION,
INFECTIOUS DISEASES,
AND EPIDEMIOLOGY

3. A case is a risk factor …
Infection in one person can be transmitted to others
What is infectious disease
epidemiology?
(www)

4. The cause often known
An infectious agent is a necessary cause
What is infectious disease epidemiology then used
for?
Identification of causes of new, emerging infections, e.g. HIV,
vCJD, SARS
Surveillence of infectious disease
Identification of source of outbreaks
Studies of routes of transmission and natural history of
infections
Identification of new interventions
What is infectious disease
epidemiology?
(www)

5. Definitions
 Infectious diseases
 Caused by an infectious agent
 Communicable diseases
 Transmission – directly or indirectly – from an infected person
 Transmissible diseases
 Transmission – through unnatural routes – from an infected person
Note
 Infections are often subclinical – infections vs infectious diseases!
 Antonyms not well-defined
 Non-communicable diseases – virus involved in pathogenesis of diabetes?
 Chronic diseases – HIV?
Tetanus Measles vCJD
(www)
Infectious Disease

6. Data from Dr. Simpson’s studies in England (1952)
Measles Chickenpox Rubella
Children exposed
Children ill
attack rate
251
201
0.80
238
172
0.72
218
82
0.38
Attack rate =
ill
exposed
(www)
Person-to-Person Transmission

7. Disease is the result of
forces within a dynamic
system consisting of:
agent of infection
host
environment
Epidemiologic Triad

8. Agent
Host
Environment
• Age
• Sex
• Genotype
• Behaviour
• Nutritional status
• Health status
• Infectivity
• Pathogenicity
• Virulence
• Immunogenicity
• Antigenic stability
• Survival
• Weather
• Housing
• Geography
• Occupational setting
• Air quality
• Food
(www)
Factors Influencing Disease
Transmission

9. Infectivity (ability to infect)
(number infected / number susceptible) x 100
Pathogenicity (ability to cause
disease)
(number with clinical disease / number infected) x 100
Virulence (ability to cause death)
(number of deaths / number with disease) x 100
All are dependent on host factors
Epidemiologic Triad-Related Concepts

10. Predisposition to Infections
(Host Factors)
Gender
Genetics
Climate and Weather
Nutrition, Stress, Sleep
Smoking
Stomach Acidity
Hygiene

12. Horton & Parker: Informed Infection Control Practice (www)
Chain of Infection

13. Epidemiology
• Study of where and when diseases occur and
how they are transmitted within populations
• Track occurrence of diseases using two
measures
– Incidence – number of new cases of a
disease in a given area during a given
period of time
– Prevalence – number of total cases of a
disease in a given area during a given
period of time
• Occurrence also evaluated in terms of
frequency and geographic distribution

14. Occurrence of Disease
Figure 14.15a-d

15. Nosocomial Infections
• Infections acquired while in a health
care facility
• Types of nosocomial infections
–Exogenous – pathogen acquired
from the health care environment
–Endogenous – pathogen arise from
normal microbiota due to factors
within the health care setting
–Iatrogenic – results from modern
medical procedures

16. Nosocomial Infections
Figure 14.19

17. Control of Nosocomial Infections
• Involves precautions
designed to reduce
the factors that result
in disease
• Hand washing is the
most effective way to
reduce nosocomial
infections

18. Both pathology and epidemiology can be
loosely defined as the study of disease, but
they involve different aspects of disease .
Whereas a pathologist studies the structural
and functional manifestations of disease and
is involved in diagnosing diseases in
individuals.
 epidemiologist studies the factors that
determine the
frequency, distribution, and determinants of
diseases in human populations.

19. The factors that determine the frequency, distribution,
and determinants of diseases in human populations.
susceptibility of different human populations
resulting from overcrowding,
 lack of immunization,
nutritional status,
inadequate sanitation procedures,
 other factors; locations (reservoirs) where
pathogens are lurking;
The various ways in which infectious diseases are
transmitted.

20. Communicable disease :
If the infectious disease is transmissible from one human
to another (i.e., person-to-person), it is called a
communicable disease.
Contagious disease :
is defined as a communicable disease that is easily
transmitted from one person to another.
Zoonotic disease :
Infectious diseases that humans acquire from animal
sources are called zoonotic diseases or zoonoses.

21. Incidence
The incidence is defined as the number of new cases of
that disease in a defined population over a specific period.
Prevalence
There are two types of prevalence:
1. period prevalence
2. point prevalence.
The period prevalence is the number of cases of the
disease existing in a given population during a specific
period (e.g., the total number of cases of gonorrhea
that existed in the U.S. population during 2002).
The point prevalence is the number of cases of the disease
existing in a given population at a particular moment
in time (e.g., the number of cases of gonorrhea in the
U.S. population at this moment).

22. Mortality Rate
Mortality refers to death. 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 period per a
specified population .
Sporadic Diseases
A sporadic disease is one that occurs only occasionally
(sporadically) within the population of a particular
geographic area.

23. 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.
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.

24. Pandemic Diseases
is a disease that is occurring in epidemic proportions in
many countries simultaneously—sometimes worldwide.
According to the World Health Organization (WHO),
infectious diseases are responsible for approximately half
the deaths that occur in developing countries; approximately
half of those are due to three infectious diseases HIV/AIDS,
tuberculosis, and malaria—each of which is currently
occurring in pandemic proportions. Together, these three
diseases cause more than 300 million illnesses and more
than 5 million deaths per year.

25. What Causes an Infectious Disease?
• Infection is caused by a
microorganism
• The microorganism may
be a bacteria, a virus or
a fungus
Bacillus anthracis bacteria– Public
Health Image Library

26. Spread of infection
• Three conditions must be met for
infection to spread from person to
person
• If any of these conditions are not met,
the chain is broken and the infectious
disease does not spread.

27. 3 conditions required for infection to
spread from one person to another
• 1. One person must be infected with a
microorganism
• 2. The other person must be susceptible
to infection with that microorganism
• 3. The microorganism must be able to
leave the body of the infected person and
enter the body of the susceptible person.

28. Susceptibility
• Ability to become infected with an
organism when exposed to it
• People may loose susceptibility (or
become immune) once they have
been:
–infected with the organism
–vaccinated
• Medications may reduce
susceptibility temporarily

29. Mode of transmission
• The way a microorganism is spread
from an infected person to a
susceptible person

30. Modes of Transmission
Direct
transmission
Indirect
transmission
Direct contact Airborne
Droplet spread Vehicle borne
Vector borne

31. Direct transmission
• The spread of an organism from
one person to another as a result
of the direct contact of an infected
and a susceptible person
• The modes of direct transmission
–direct contact
–droplet spread

32. Direct contact
• The organism is passed to a
susceptible person when he or
she has contact with an
infected person’s skin, mucus
membranes or body fluid
(blood, urine, feces).

33. Droplet spread
• The organism is spread to a susceptible
person in respiratory secretions that form
when an infected person breaths, coughs
or sneezes.
• A susceptible person can become infected
if the droplet enters his eyes, nose or
mouth directly or via contamination of
hands.

34. Droplet Spread
• Considered direct transmission
because droplets are heavy so they
do not tend to travel more than 6 feet
from the infected person
–A susceptible person who is not near an
infected person will not become infected
–The spread of disease by respiratory
droplets usually requires extended
contact

35. Indirect transmission
• Occurs when an organism is spread
from an infected person to a
susceptible person via an
intermediary or go-between.
–Via the air = airborne
–Via an insect = vectorborne
–Via an inanimate object = vehicleborne

36. Types of infection
• Colonization – infection present on surface
of body –
– Organism propagating at a rate sufficient to
maintain its numbers without producing
identifiable evidence of any reaction in host
• Inapparent or subclinical infection
– organism not only multiplying but also causes
a measurable reaction that is however not
clinically detectable
• Symptomatic infection
– Organism causes clinically detectable reaction

37. Time parameters of interaction
• Latency Period = the time between
infection agent and onset of
infectiousness
• Incubation Period = the time
between infection and onset of
symptoms
• Latency period may not be the
same as the incubation period

38. Time parameters of interaction
Webber,Communicable Disease Epidemiology
and Control, 1996. Figure 2.1

39. INTERACTIONS AMONG PATHOGENS,
HOSTS, AND THE ENVIRONMENT
The infectious disease occurs depends on many
factors :
1. Factors pertaining to the pathogen:
■ Virulence of the pathogen (virulence as a measure
or degree of pathogenicity; some pathogens are
more virulent than others).
■ Is there a way for the pathogen to enter the body
(i.e., is there a portal of entry?).
■ Number of organisms that enter the body (i.e., will
there be a sufficient number to cause infection?).

40. 2. Factors pertaining to the host
(i.e., the person who may become infected):
■ Health status (e.g., is the person hospitalized?
does he or she have any underlying illnesses? has
the person undergone invasive procedures or
catheterization? does he or she have any prosthetic
devices?).
■ Nutritional status.
■ Other factors pertaining to the susceptibility of
the host (e.g., age, life style [behavior],
socioeconomic level, travel, hygiene, substance
abuse, immune status, etc.).

41. 3. Factors pertaining to the environment:
■ Physical factors such as geographic location,
climate, heat, cold, humidity, and season of the
year.
■ Availability of appropriate reservoirs ,
intermediate hosts , and vectors .
■ Sanitary and housing conditions; adequate
waste disposal.
■ Availability of potable (drinkable) water.

42. Factors that influence the kind and number
of microorganisms at anybody site are:
1. availability or unavailability of oxygen;
2. the availability of appropriate receptor sites for
attachment;
3. the pH of the host site (stomach, vagina);
4. the availability of nutrients;
5. the influence exerted by other microorganisms at
the site;
6. the immunological response of the host to the
presence
of the microbe.

46. Modes Of Transmission
Communicable diseases—infectious diseases that are transmitted
from person to person—are usually transmitted in the following
ways:
■ Direct skin-to-skin contact.
■ Direct mucous membrane-to-mucous membrane contact by kissing
or sexual intercourse. Most STDs are transmitted in this manner.
STDs include syphilis, gonorrhea, and infections caused by
chlamydia,
■ Indirectly via airborne droplets of respiratory secretions, usually
produced as a result of sneezing or coughing.
■ Indirectly via contamination of food and water by fecal material.
■ Indirectly via arthropod vectors.
■ Indirectly via fomites that become contaminated by respiratory
secretions, blood, urine, feces, vomitus, or exudates from
hospitalized patients.
■ Indirectly via transfusion of contaminated blood or blood products
from an ill person .

48. Definitions of Epidemiologic Terms Relating to
Infectious Diseases
Control
The WHO defines control of an infectious disease as ongoing
operations or programs aimed at reducing the incidence and/or
prevalence of that disease
Elimination
The WHO defines elimination of an infectious disease as the
reduction of case transmission to a predetermined very low level
(e.g., to a level below one case per million population)
Eradication
The WHO defines eradication of an infectious disease as
achieving a status where no further cases of that disease occur
anywhere and where continued control measures are unnecessary

49. Sources of Water Contamination
Rainwater falling over a large area collects in lakes and
rivers and, thus, is subject to contamination by soil
microbes and raw fecal material
In many communities, untreated raw sewage and
industrial wastes are dumped directly into local waters.
a storm or a flood may result in contamination of the
local drinking water with sewage .
In some very old cities, where cracked underground
water pipes lie beside leaking sewage pipes, sewage can
enter the water pipes, thus contaminating the water just
before it enters people’s homes.

50. In the laboratory, water can be tested for
fecal contamination by checking for the
presence of coliform bacteria (coliforms).
Coliforms are E. coli and other lactose-
fermenting members of the Family
Enterobacteriaceae, such as Enterobacter and
Klebsiella spp. These bacteria normally live in
the intestinal tracts of animals and humans; thus,
their presence in drinking water is an indication
that the water was fecally contaminated. Water
is considered potable (safe to drink) if it
contains 1 coliform or less per 100 mL of water.

51. Nosocomial Infections

52. Definitions
Infectious diseases (infections) can be divided into two categories:
(1) those that are acquired within hospitals or other healthcare facilities
(called hospital acquired infections or nosocomial infections)
(2) those that are acquired outside of healthcare facilities (called
community-acquired infections).
A hospitalized patient may have either type of infection. According to
the Centers for Disease Control and Prevention (CDC), community-
acquired infections are those that are present or incubating at the time of
hospital admission. All other hospital-associated infections are
considered nosocomial, including those that erupt within 14 days of
hospital discharge.
Iatrogenic infections (iatrogenic literally meaning “physician-
induced”) or diseases are the result of medical or surgical treatment and
are, thus, caused by surgeons, other physicians, or other healthcare
personnel. Examples of iatrogenic infections are post-surgical wound
infections and urinary tract infections that result from urinary
catheterization of patients.

53. Pathogens Most Often Involved in Nosocomial Infections
The following seven bacteria or groups of bacteria are the
most common causes of nosocomial infections :
■ Gram-positive cocci (during 1990–1996, the following
three Gram positive cocci caused 34% of the nosocomial
infections : Staphylococcus aureus
Coagulase-negative staphylococci
Enterococcus spp.
■ Gram-negative bacilli (during 1990–1996, the following
four Gram negative bacilli caused 32% of the nosocomial
infections:
Escherichia coli
Pseudomonas aeruginosa
Enterobacter spp.
Klebsiella spp.

54. Although some of the pathogens that cause nosocomial infections
come from the external environment, most come from the patients
themselves—their own indigenous microflora that enter a surgical
incision or otherwise gain entrance to the body. Urinary catheters, for
example, provide a “superhighway” for indigenous microflora
organisms to gain access to the urinary bladder.
Approximately 70% of nosocomial infections involve drug-resistant
bacteria, which are common in hospitals and nursing homes as a result
of the many antimicrobial agents that are used there. The drugs place
selective pressure on the microbes, meaning that only those that are
resistant to the drugs will survive. These resistant organisms then
multiply and predominate.
Pseudomonas infections are especially hard to treat, as are infections
caused by multi-drug-resistant Mycobacterium tuberculosis (MDRTB),
vancomycin resistant Enterococcus species (VRE), and methicillin-
resistant strains of Staphylococcus aureus (MRSA) and Staphylococcus
epidermidis (MRSE).

55. Most Common Types of Nosocomial Infections
The four most common types of nosocomial
infections, listed in descending order of frequency,
are:
1. Urinary tract infections (UTIs)
2. Surgical wound infections
(also referred to as post-surgical wound infections)
3. Lower respiratory tract infections
(primarily pneumonia)
4. Bloodstream infections (septicemia)

56. Patients Most Likely to Develop
Nosocomial Infections
■ Elderly patients.
■ Women in labor and delivery.
■ Premature infants and newborns.
■ Surgical and burn patients.
■ Diabetic and cancer patients.
■ Patients receiving treatment with steroids, anticancer drugs,
antilymphocyte serum, and radiation.
■ Immunosuppressed patients (i.e., patients whose immune systems are
not functioning properly).
■ Patients who are paralyzed or are undergoing renal dialysis or
catheterization; quite often, these patients’ normal defense
mechanisms are not functioning properly.

57. Major Factors Contributing to Nosocomial
Infections
The three major factors that combine to cause nosocomial infections are:
■ An ever-increasing number of drug-resistant pathogens.
■ The failure of healthcare personnel to follow infection control
guidelines.
■ An increased number of immunocompromised patients.
Additional contributing factors are:
■ The indiscriminate use of antimicrobial agents, which has resulted in
an increase in the number of drug-resistant and multi–drug-resistant
pathogens.
■ A false sense of security about antimicrobial agents, leading to a
neglect of aseptic techniques and other infection control procedures.
■ Lengthy, more complicated types of surgery.

58. ■ Overcrowding of hospitals and other healthcare
facilities as
well as shortages of staff.
■ Increased use of less-highly trained healthcare
workers, who
are often unaware of infection control procedures.
■ Increased use of anti-inflammatory and
immunosuppressant
agents, such as radiation, steroids, anticancer
chemotherapy,
and antilymphocyte serum.
■ Overuse and improper use of indwelling medical
devices.

59. Mathematical Modelling in
Infectious Disease
Epidemiology

60. A measure of the potential for transmission
The basic reproductive number, R0, the mean
number of individuals directly infected by an
infectious case through the total infectious period, when introduced to a
susceptible population
R0 = p • c • d
contacts per unit time
probability of transmission per contact
duration of infectiousness
Infection will ….. disappear, if R < 1
become endemic, if R = 1
become epidemic, ifR > 1
(www)
Reproductive Number, R0

61. • Useful summary statistic
• Definition: the average number of
secondary cases a typical infectious
individual will cause in a completely
susceptible population
• Measure of the intrinsic potential for an
infectious agent to spread
(www)
Reproductive Number, R0

62. • If R0 < 1 then infection cannot invade a
population
– implications: infection control mechanisms
unnecessary (therefore not cost-effective)
• If R0 > 1 then (on average) the pathogen will
invade that population
– implications: control measure necessary to
prevent (delay) an epidemic
Reproductive Number, R0

63. p condoms, acyclovir, zidovudine
c health education, negotiating skills
D case ascertainment (screening,
partner notification), treatment,
compliance, health seeking
behaviour, accessibility of services
R0 = p • c • d
(www)
Reproductive Number, R0
Use in STI Control

64. p, transmission probability per exposure – depends on the
infection
 HIV, p(hand shake)=0, p(transfusion)=1, p(sex)=0.001
 interventions often aim at reducing p
 use gloves, screene blood, condoms
c, number of contacts per time unit – relevant contact depends on
infection
 same room, within sneezing distance, skin contact,
 interventions often aim at reducing c
 Isolation, sexual abstinence
d, duration of infectious period
 may be reduced by medical interventions (TB, but not
salmonella) (www)
What determines R0 ?

65. Immunity – herd immunity
If R0 is the mean number of secondary cases in a susceptible population, then
R is the mean number of secondary cases in a population where a proportion, p,
are immune
R = R0 – (p • R0)
What proportion needs to be immune to prevent epidemics?
If R0 is 2, then R < 1 if the proportion of immune, p, is > 0.50
If R0 is 4, then R < 1 if the proportion of immune, p, is > 0.75
If the mean number of secondary cases should be < 1, then
R0 – (p • R0) < 1
p > (R0 – 1)/ R0 = 1 – 1/ R0
 If R0 =15, how large will p need to be to avoid an epidemic?
p > 1-1/15 = 0.94
The higher R0, the higher proportion of immune required for herd immunity
(www)

66. Endemic - Epidemic - Pandemic
 Endemic
 Transmission occur, but the number of cases remains
constant
 Epidemic
 The number of cases increases
 Pandemic
 When epidemics occur at several continents – global
epidemic
Time
R = 1
R > 1
R < 1
(www)