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9/10/2010 Dr. Salwa Tayel 19/10/2010 Dr. Salwa Tayel 1
Dr. Salwa Tayel 2Dr. Salwa Tayel 2
Associate Professor Associate Professor Family and Community Medicine DepartmentFamily and Community Medicine Department
King Saud UniversityKing Saud University
ByBy
Infectious Disease Infectious Disease EpidemiologyEpidemiology
Dr. Salwa TayelDr. Salwa Tayel 339/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 33
Learning ObjectivesLearning Objectives
At the end of this lecture you (will) be able to:At the end of this lecture you (will) be able to:
ExplainExplain common definitions and basic concepts used in common definitions and basic concepts used in
epidemiology.epidemiology.
Describe natural history and spectrum of infectious Describe natural history and spectrum of infectious
diseases and their diseases and their implicationsimplications for public health. for public health.
Dr. Salwa TayelDr. Salwa Tayel 44Dr. Salwa TayelDr. Salwa Tayel 44
Infection:Infection:
It is the entry, development and multiplication of an It is the entry, development and multiplication of an
infectious agent in the body of man or animal.infectious agent in the body of man or animal.
Outcome of infection varies.Outcome of infection varies.
Infectious disease:Infectious disease:
A clinically manifest disease of man or animal A clinically manifest disease of man or animal
resulting from infection.resulting from infection.
In-apparent infection:In-apparent infection:
The infection does not become manifest at any The infection does not become manifest at any
stage.stage.
Dr. Salwa TayelDr. Salwa Tayel 559/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 55
Dr. Salwa TayelDr. Salwa Tayel 669/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 66
An illness due to a specific infectious agent or its An illness due to a specific infectious agent or its
toxic products that arises through toxic products that arises through transmissiontransmission
of that agent from reservoir to susceptible host.of that agent from reservoir to susceptible host.
Communicable Disease (typhoid, influenza,..Communicable Disease (typhoid, influenza,..
Non- Communicable Disease (DM, cancer,..Non- Communicable Disease (DM, cancer,..
Communicable DiseaseCommunicable Disease
Dr. Salwa TayelDr. Salwa Tayel 779/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 77
Natural historyNatural history
NaturalNatural historyhistory of disease refers to the progress of a of disease refers to the progress of a
disease process in an disease process in an individualindividual over time, in the over time, in the
absenceabsence of intervention. of intervention.
The natural historyThe natural history of a disease of a disease describes the course describes the course
of the disease in an individual starting from the moment of the disease in an individual starting from the moment
of of exposureexposure to the causal agents till one of the possible to the causal agents till one of the possible
outcomesoutcomes occurs. occurs.
Dr. Salwa TayelDr. Salwa Tayel 889/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 88
Dr. Salwa TayelDr. Salwa Tayel 999/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 99
Spectrum of diseaseSpectrum of disease
The idea that an exposure can lead to varying signs, The idea that an exposure can lead to varying signs,
symptoms and severity of the same disease in the symptoms and severity of the same disease in the
populationpopulation is the spectrum of disease. is the spectrum of disease.
Why do we have varying degrees of severity? Prognosis?Why do we have varying degrees of severity? Prognosis?
The outcome will depend on the interactions of host, agent The outcome will depend on the interactions of host, agent
and environmental factors.and environmental factors.
Dr. Salwa TayelDr. Salwa Tayel 10109/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 1010
Classification of diseases according to clinical Classification of diseases according to clinical severity (spectrum)severity (spectrum)
Dr. Salwa TayelDr. Salwa Tayel 11119/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 1111
Examples: Tuberculosis, Polio, Hepatitis A, Meningitis, AIDS
Class A: Inapparent infection
Dr. Salwa TayelDr. Salwa Tayel 12129/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 1212
Examples: Measles, Chickenpox
Class B: Classic casesClass B: Classic cases
Dr. Salwa TayelDr. Salwa Tayel 13139/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 1313
Examples: Examples: Rabies, HemorrhagicRabies, Hemorrhagic feversfevers caused by Ebola caused by Ebola and Murberg viruses. and Murberg viruses.
Class C: Severe or Fatal infections
Dr. Salwa TayelDr. Salwa Tayel 14149/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 1414
Implications for public healthImplications for public health
Dr. Salwa TayelDr. Salwa Tayel 15159/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 1515
The relation of severity of illness to disease statistics.
Dr. Salwa TayelDr. Salwa Tayel 16169/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 1616
The pyramid and iceberg of diseaseThe pyramid and iceberg of disease
11 Diseased, diagnosed & controlledDiseased, diagnosed & controlled
22 Diagnosed, uncontrolledDiagnosed, uncontrolled
33 Undiagnosed or wronglyUndiagnosed or wronglydiagnosed diseasediagnosed disease
44 Risk factors for diseaseRisk factors for disease
55 Free of risk factors Free of risk factors
Diagnosed Diagnosed diseasedisease
Undiagnosed orUndiagnosed orwrongly diagnosed diseasewrongly diagnosed disease
Dr. Salwa TayelDr. Salwa Tayel 17179/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 1717
Cases of illness correctly diagnosed by clinicians in the Cases of illness correctly diagnosed by clinicians in the
community often represent only the “community often represent only the “tip of the icebergtip of the iceberg.”.”
Many additional cases may be too early to diagnose or may Many additional cases may be too early to diagnose or may
remain asymptomatic.remain asymptomatic.
Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS.Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS.
The riskThe risk is that persons with in-apparent or undiagnosed is that persons with in-apparent or undiagnosed
infections may be infections may be able to transmitable to transmit infection to others. infection to others.
Iceberg phenomenon Iceberg phenomenon
Dr. Salwa TayelDr. Salwa Tayel 18189/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 1818
Applications of the concepts of natural history Applications of the concepts of natural history and spectrum of diseaseand spectrum of disease
Persons with in-apparent or undiagnosed Persons with in-apparent or undiagnosed
infections can transmit infections to others.infections can transmit infections to others.
Control measures must be directed toward all Control measures must be directed toward all
infections capable of being transmitted to others;infections capable of being transmitted to others;
– both clinically apparent cases and both clinically apparent cases and
– those with in-apparent or undiagnosed infections.those with in-apparent or undiagnosed infections.
Dr. Salwa TayelDr. Salwa Tayel 191923/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 1919
Chain of infectionChain of infection
Dr. Salwa TayelDr. Salwa Tayel 202023/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 2020
Chain of infectionChain of infection
A process that begins when an agent leaves its
reservoir or host through a portal of exit, and is
conveyed by some mode of transmission, then
enters through an appropriate portal of entry to
infect a susceptible host.
Dr. Salwa TayelDr. Salwa Tayel 212123/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 2121
AgentAgent
Susceptible HostSusceptible Host ReservoirReservoir
Mode of transmissionMode of transmission
Cycle of infectionCycle of infection
Portal of ExitPortal of ExitPortal of InletPortal of Inlet
IP
PC
Dr. Salwa TayelDr. Salwa Tayel 222223/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 2222
The requisites (essentials) for the The requisites (essentials) for the perpetuation of communicable diseases:perpetuation of communicable diseases:
The elements of the cycle of infection:The elements of 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.
Dr. Salwa TayelDr. Salwa Tayel 232323/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 2323
1. Agent: 1. Agent:
Microorganisms are responsible for disease Microorganisms are responsible for disease
production (viruses, bacteria, protozoa, parasites, production (viruses, bacteria, protozoa, parasites,
fungi,.. fungi,..
Dr. Salwa TayelDr. Salwa Tayel 242423/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 2424
Mechanisms of disease production Mechanisms of disease production
(pathogenesis):(pathogenesis):
• Invasiveness: (Pneumococcosis, measles).
• Toxicity: (Tetanus, Botulism).
• Hypersensitivity: (Tuberculosis).
• Others: (Immune suppression; AIDS).
Dr. Salwa TayelDr. Salwa Tayel 252523/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 2525
Agent factors that affect disease transmission:Agent factors that affect disease transmission:
Infectivity, Infectivity,
Pathogenicity, Pathogenicity,
Virulence,Virulence,
AAntigenicity,…ntigenicity,…
Dr. Salwa TayelDr. Salwa Tayel 262623/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 2626
InfectivityInfectivity
The ability of an agent to invade and multiply (produce The ability of an agent to invade and multiply (produce
infection) in a susceptible host.infection) in a susceptible host.
How to measure How to measure ((Infectivity);Infectivity); ease & spread of infection?ease & spread of infection?
Secondary Attack RateSecondary Attack Rate
The proportion of exposed susceptible persons who The proportion of exposed susceptible persons who
become infected.become infected.
Examples: High infectivity: Measles, ChickenpoxExamples: High infectivity: Measles, Chickenpox
Low infectivity: LeprosyLow infectivity: Leprosy
100sec
xessusceptiblofNumber
casesondaryofNumberrateattackSecondary
Dr. Salwa TayelDr. Salwa Tayel 272723/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 2727
PathogenicityPathogenicityIs the ability of the organisms to produce specific clinical Is the ability of the organisms to produce specific clinical
reaction after infectionreaction after infection
ItIt refers to the proportion of infected persons who develop refers to the proportion of infected persons who develop clinical disease.clinical disease.
Examples: Examples:
High pathogenicity: Measles, Chickenpox (Class B)High pathogenicity: Measles, Chickenpox (Class B)
Low pathogenicity: Polio, Tuberculosis, Hepatitis A, Low pathogenicity: Polio, Tuberculosis, Hepatitis A, Meningitis, AIDS (Class A)Meningitis, AIDS (Class A)
It can be measured by:It can be measured by:
caseslSubclinica
casesClinical
Ratio of clinical to sub-clinical caseRatio of clinical to sub-clinical case= =
Dr. Salwa TayelDr. Salwa Tayel 282823/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 2828
VirulenceVirulence
It refersIt refers the ability of organisms to produce severe pathological the ability of organisms to produce severe pathological reaction.reaction.
It is the proportion of persons with clinical disease who become It is the proportion of persons with clinical disease who become severely ill or die.severely ill or die.
Examples: Examples: Rabies, Hemorrhagic feversRabies, Hemorrhagic fevers caused by Ebola and caused by Ebola and Murberg viruses. (Class C)Murberg viruses. (Class C)
100disease that of cases ofnumber Total
disease a from deaths ofnumber Total ratefatality Case x
It can be measured by: Case fatality rate
Dr. Salwa TayelDr. Salwa Tayel 292923/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 2929
Antigenicity (Immunogenicity)Antigenicity (Immunogenicity)
The ability of the organism to produce specific immunity
(antibodies or antitoxin).
It can be measured by:
Second attack frequency:Second attack frequency:
Second attacks are rare in measles, mumps and measles, mumps and
chickenpox. chickenpox.
Re-infection occurs as in case of common cold,common cold, syphilis syphilis
and gonorrhea.and gonorrhea.
Dr. Salwa TayelDr. Salwa Tayel 303023/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 3030
Human reservoirsHuman reservoirs
2. Reservoir of infection2. Reservoir of infection
The The reservoir reservoir of an agent is the of an agent is the habitathabitat in which an in which an
infectious agent normally lives, grows, and multiplies. infectious agent normally lives, grows, and multiplies.
Types of reservoirs:Types of reservoirs:
Humans, animals, and the environment.Humans, animals, and the environment.
Two types of human reservoir exist:Two types of human reservoir exist:• • Cases:Cases: persons with symptomatic illness persons with symptomatic illness• • CarriersCarriers
Dr. Salwa TayelDr. Salwa Tayel 313123/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 3131
Pat
ho
gen
Res
ervo
irs
Pat
ho
gen
Res
ervo
irs
Humans are the most important
reservoir of human
infectious disease.
Dr. Salwa TayelDr. Salwa Tayel 323223/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 3232
CarrierCarrier
A person or animalA person or animal withoutwithout apparent disease who apparent disease who
harbors a specific infectious agent and is harbors a specific infectious agent and is
capable of transmitting the agent to others.capable of transmitting the agent to others.
Dr. Salwa TayelDr. Salwa Tayel 333323/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 3333
Carriers are dangerous because:Carriers are dangerous because:
1. They do not show any clinical manifestation so they
carry normal life.
2. The carrier and his contacts are not aware of their
condition so, they take no precautions.
3. It is difficult to discover them.
4. It is not always possible to deal with them.
5. Chronic carriers can remain infectious for a long time
leading to repeated introduction of the disease to
contacts.
Dr. Salwa TayelDr. Salwa Tayel 343423/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 3434
Types of Carriers:Types of Carriers:1. Asymptomatic (In-apparent) carrier: The carrier state that may occur in an individual with an
infection that is in-apparent throughout its courseExamples: Polio virus, meningococcus, hepatitis A Examples: Polio virus, meningococcus, hepatitis A virusvirus
2. Incubatory, Convalescent, Post-Convalescent carriers:
The carrier state may occur during the incubation period, convalescence, and post convalescence of an individual with a clinically recognizable disease.Examples of Examples of Incubatory carrierIncubatory carrier: Measles, chickenpox: Measles, chickenpox
Dr. Salwa TayelDr. Salwa Tayel 353523/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 3535
Convalescent carriers:Convalescent carriers:
Examples:Examples: Diphtheria, hepatitis B viruses and Salmonella speciesDiphtheria, hepatitis B viruses and Salmonella species
****According to duration of carriage:According to duration of carriage:
The carrier state may be (The carrier state may be (transienttransient carrier or carrier or chronicchronic carrier). carrier).
Chronic carriers:Chronic carriers:
They continue to harbour an agent for anThey continue to harbour an agent for an extended time (months extended time (months
or years) following the initial infection.or years) following the initial infection.
Examples: HExamples: Hepatitis B virus and Salmonella typhiepatitis B virus and Salmonella typhi
Dr. Salwa TayelDr. Salwa Tayel 363623/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 3636
Animal reservoirsAnimal reservoirs
Zoonoses:Zoonoses: Infectious diseases that are transmissible
under normal conditions from vertebratevertebrate animals to
humans. (with humans as incidental hosts)
ZoonoticZoonotic diseases include:diseases include:
brucellosis (cows and pigs), brucellosis (cows and pigs),
anthrax (sheep),anthrax (sheep),
plague (rodents), plague (rodents),
rabies (bats, dogs, and other mammals).rabies (bats, dogs, and other mammals).
Dr. Salwa TayelDr. Salwa Tayel 373723/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 3737
ZoonosesZoonosesZoonoses are
Human Diseases with
Animal Reservoirs.
Dr. Salwa TayelDr. Salwa Tayel 383823/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 3838
ZoonosesZoonosesToxoplasmosis
Dr. Salwa TayelDr. Salwa Tayel 393923/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 3939
Environmental reservoirs:Environmental reservoirs:
Soil, and waterSoil, and water
• Soil:Soil: Agents Agents live and multiply in the live and multiply in the soilsoil..
Examples: Examples:
-Tetanus-Tetanus spores spores and and
- Fungal agents; (those causing - Fungal agents; (those causing histoplasmosishistoplasmosis))
• Pools of waterPools of water are the primary reservoir of are the primary reservoir of
Legionnaires’ bacillus.Legionnaires’ bacillus.
Dr. Salwa TayelDr. Salwa Tayel 404023/10/201023/10/2010 Dr. Salwa TayelDr. Salwa Tayel 4040
ThanThank youk you
Bibliotheca Alexandrina
Website http://faculty.ksu.edu.sa/73234/default.aspx