Download - MediPIET

Infectious Disease
Epidemiology
Tunisia, 27 October 2014
Prof. Dr. Alena Petrakova
Acknowledgments: MediPIET Consortium FIIAPP-ISCIII and ECDC
Learning Objectives
• Define Infectious/Communicable Disease (ID)
• Become familiar with concepts and basic ID
terminology
• List the ID transmission routes
• Understand the ID occurrence in population and
ID dynamics
• Describe the tools to control ID
2
John SNOW
John Snow (15 March 1813 – 16 June 1858)
was an English physician and a leader in the
adoption of anaesthesia and medical
hygiene. He is considered one of the
fathers of modern epidemiology, in part
because of his work in tracing the source of
a cholera outbreak in Soho, London, in
1854. His findings inspired fundamental
changes in the water and waste systems of
London, which led to similar changes in
other cities, and a significant improvement
in general public health around the world.
3
The risks affecting a population
shift over the time
Globalisation and its side-effects
 Demographic changes: Population growth, urbanization,
increasing life expectancy at birth, etc.
 Changes in healthcare provision:
 (+): Better laboratory tests, better diagnostics, better treatment, etc.
 (-): Increased use of antibiotics - antimicrobial resistance, etc.
 Globalization of the food supply: Global transport of food and
animals
 Changes in human behaviour: International travel, changes in
dietary habits, intravenous drug use, unsafe sex, etc.
 Changes in environment and climate: Floods, hurricanes,
droughts / increased human contact with reservoirs, insects
Concepts
 Epidemiology (Lecture of Prof. Dr. Nissaf Bouafif)
 Infectious disease / Communicable disease
 Infectious agent
 Reservoir
 Susceptible host
 Source of infection
 Case
 Incubation period
Epidemiology
The study of the distribution and determinants of healthrelated states or events in specified human populations, and
the application of this study to the control of health problems
John Last, Dictionary of Epidemiology (1983, 1988, 1995, 2001)
There are many definitions of epidemiology, but every epidemiologist will
know exactly what it is that he or she does. Defining epidemiology is
difficult primarily because it does not represent a body of knowledge, as
does, for example, anatomy, nor does it target a specific organ system, as
does cardiology. Epidemiology represents a method of studying a health
problem and can be applied to a wide range of problems, from transmission
of an infectious disease agent to the design of a new strategy for health care
delivery.
Infectious / Communicable disease
A clinically manifested disease of humans or animals resulting
from an infection
„Communicable disease” means an illness caused by an
infectious agent or its toxins that occurs through the direct or
indirect transmission of the infectious agent or its products
from an infected individual or via an animal, vector or the
inanimate environment to a susceptible animal or human host.
(CDC U.S.)
Infectious agent
An organism (virus, rickettsia, bacteria, fungus, protozoan or
helminth) that is capable of producing infection or infectious
disease
Reservoir
Any person, animal, arthropod, plant, soil or substance in which
an infectious agent normally lives and multiplies, on which it
depends primarily for survival, or where it reproduces itself in
such manner that it can be transmitted to a susceptible host
Susceptible host
A person or animal nor possessing sufficient resistance to a
particular infectious agent to prevent contracting infection or
disease when exposed to that agent
Source of infection
The person, animal, object or substance from which an
infectious agent passes to a host
Case
A person with an infectious disease of interest
Primary case: Person who brings a disease into a population
Secondary case: Persons who are infected by the primary case
Index case: The 1st case to be discovered during an outbreak
investigation
Incubation period
The time interval between initial contact with an infectious
agent and the first appearance of symptoms associated with
the infection
Some special features
 A case may also be an exposure
 Person with infection can also be source of infection
 People may be immune
 Having had an infection or disease could result to resistance to an
infection (immunity)
 A case may be a source without being recognized
as a case
 Asymptomatic/sub-clinical infections
 There is sometimes a need for urgency
 Epidemics may spread fast and require control measures
 Transmission dynamics important for prevention and control
Transmission routes
Direct transmission
Indirect transmission
Mucous to mucous membrane
Waterborne
Across placenta
Airborne
Transplants, blood, etc.
Foodborne
Skin to skin
Vectorborne
Sneezes, cough
Objects
Hello
Incubation period:
4-7 days (3-14 days)
Dengue fever
Flavivirus - Dengue virus 1-4
Human without
immunity
Human
Mosquito bite
Mosquito bite
Vector borne - Aedes
Hello
Typhoid fever
Incubation period:
8-14 days (3-60 days)
Salmonella typhi
Humans
Humans
Ingestion of
water or food
contaminated
Faeces and urine
Faecal-oral route
Hello
Incubation period:
10 days (2-30 days)
Leptospirosis
Leptospira interrogans
Humans.
Serovar Specific
immunity
Wild and domestic
animals: rats,
cattle, swine, etc
Urine of infected
animals
Skin/mucous/tissues
Contact /ingestion of urine
or contaminated water
Dynamics of disease and
infectiousness
Latent period
Infectious period
Incubation period
Infection
Clinical disease
Onset of
symptoms
Non-infectious period
Recovery
Resolution
of symptoms
Time
Dynamics of disease and
infectiousness (ii)
Second patient
Transmission
Latent period
Incubation period
First patient
Clinical disease
Transmission
Latent period
Incubation period
Infection
Infectious period
Infectious period
Clinical disease
Serial interval
or generation time
Time
Disease occurrence in populations
 Sporadic: occasional cases occurring at irregular intervals
 Endemic: continuous occurrence at an expected frequency
over a certain period of time and in a certain geographical
location
 Epidemic or outbreak: occurrence in a community or region
of cases of an illness with a frequency clearly in excess of
normal expectancy
 Pandemic: epidemic involves several countries or continents,
affecting a large population
Factors influencing disease transmission
Agent
Environment
• Infectivity
• Weather
• Pathogenicity
• Housing
• Virulence
• Geography
• Immunogenicity
• Occupational setting
• Antigenic stability
• Air quality
• Survival
• Food
Host
• Age
• Sex
• Genotype
• Behaviour
• Nutritional status
• Health status
Basic reproductive rate (R0)
• Average number of individuals directly infected by an
infectious case during her or his entire infectious period,
when she or he enters a totally susceptible population
– R0 < 1 - the disease will disappear
– R0 = 1 - the disease will become endemic
– R0 > 1 - there will be an epidemic
800
number of cases
700
600
500
400
300
200
100
0
1
2
3
4
generation
5
6
7
800
number of cases
700
600
500
400
300
200
100
0
1
2
3
4
generation
5
6
7
Herd immunity or Herd effect
also called Community immunity
• General immunity to a pathogen in a
population based on the acquired immunity
to it by a high proportion of members over
time (even if some transmission may still
occur)
• Presence of immune individuals protects
those who are not themselves immune
Herd immunity threshold
Minimum proportion (p) of population that needs to be
immunized in order to obtain herd immunity
p > 1 - 1/R0
e.g.
if R0 = 3, immunity threshold = 67%
if R0 = 16, immunity threshold = 94%
Important concept for immunization programs and eradication
of an infectious disease
Vaccination coverage for elimination
Critical proportion, Pc
Pc = 1-1/Ro
100%
80%
60%
40%
rubella
measles
20%
0%
0
2
4
6
8
10
12
14
Basic reproduction number, Ro
16
18
20
Tools for Control of Communicable
Diseases







Isolation
Treatment
Vaccination
Prophylaxis
Disinfection
Quarantine
Surveillance
Surveillance
High quality national surveillance is the
corner-stone of infectious disease
prevention and control.
- World Health Organisation, 2004
MediPIET: WHY? – WHEN? - HOW?
April 2012:
Technical Consultation Conference
May 2013: Preparatory Technical
Meeting (Kick-off-meeting)
June 2013 – September 2014:
Working together on deliverables
September 2014: Final Project
Meeting…Starting Implementation
MediPIET 2013 - 2017
Useful References:
• Heymann D: Control of Communicable Diseases Manual, 19th ed. 2008
• M´ikanatha NM, Lynfield R, Van Beneden ChA, de Valk H: Infectious
Disease Surveillance, First ed.2007
• Anderson RM & May RM, Infectious Diseases of Humans: Dynamics
and Control, 11th ed. 2006
• Barreto ML, Teixeira MG, Carmo EH: Infectious diseases epidemiology.
J Epidemiol Community Health 2006; 60; 192-195
• Giesecke J: Modern Infectious Disease Epidemiology,
2nd ed. 2002
• McNeill, WH: Plagues and Peoples, 3rd ed. 1998
• Beaglehole R, Bonita R: Basic Epidemiology, WHO Geneva, 1993,
reprinted in 2005
Thank you!
Wishing you all full success!
Prof. Dr. Alena Petrakova
[email protected]