Dynamics of Disease Transmission

Objectives

• At the end of lecture students will be able to :– Understand iceberg phenomenon of disease– Know definitions of important terms i.e.

epidemic, endemic and pandemic– Explain herd immunity– Understand steps of outbreak investigation

Spectrum of disease

• The idea that an exposure can lead to varying signs, symptoms and severity of the same disease in the population is the spectrum of disease.

• Why do we have varying degrees of severity?

• The outcome will depend on the interactions of host, agent and environmental factors.

Factors Affecting Disease Transmission

Host

Vector

Agent Environment

-- Susceptibility-- Immune response-- Resistance--- Portal(s) of entry

-- Virulence Toxigenicity-- Infectivity Resistance-- Pathogenicity Antigenicity

VECTOR-- Prevalence-- Portal(s) of entry

-- Balance of immune to susceptible individuals--- Opportunity for exposure (e.g. crowding)

Routes of transmission

Timeline for Infection

Infe

ctio

nSusceptible

Susceptible

Dynamics of infectiousness

Dynamics of disease

Incubation period

Symptomaticperiod

Non-diseased

Latentperiod

Infectious period

Non-infectious

Infe

ctio

n

Time

Time

Transmission

Cases

• Index – the first case identified

• Primary – the case that brings the infection into a population

• Secondary – infected by a primary case

• Tertiary – infected by a secondary case

Classification of diseases according to Classification of diseases according to spectrumspectrum

Examples: Tuberculosis, Polio, Hepatitis A, Meningitis, AIDS

(Low pathogenicity and low virulence)

Examples: Measles, Chickenpox

(High pathogenicity & low virulence)

Examples: Rabies, Hemorrhagic fevers caused by Ebola Examples: Rabies, Hemorrhagic fevers caused by Ebola and Murberg and Murberg viruses.viruses.

(High pathogenicity & high virulence)(High pathogenicity & high virulence)

Iceberg Phenomenon

• Cases of illness correctly diagnosed by clinicians in the community

often represent only the “tip of the iceberg.”

• Many additional cases may be too early to diagnose or may remain

asymptomatic.

• Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS.

• The risk is that persons with in-apparent or undiagnosed infections

may be able to transmit infection to others.

Iceberg Phenomenon

DiagnosedDiagnosed diseasedisease

Undiagnosed orUndiagnosed orwrongly diagnosed diseasewrongly diagnosed disease

Iceberg Phenomenon--application

• Persons with in-apparent or undiagnosed infections can

transmit infections to others.

• Control measures must not be directed solely for

clinically apparent cases.

• Control measures must be directed toward all infections

capable of being transmitted to others;

– both clinically apparent cases and

– those with in-apparent or undiagnosed infections.

Important terms

Reservoir• A host that carries a pathogen without injury to

itself and serves as a source of infection for other host organisms (asymptomatic infective carriers)

Endemic• The constant presence of a disease or infectious

agent within a given geographic area or population group; may also refer to the usual prevalence of a given disease within such area or group.

Epidemic• The occurrence of more cases of a disease than

expected in a given area or among a specific group of people over a particular period of time.

Pandemic• An epidemic occurring over a very wide

area (several countries or continents) at the same time and usually affecting a large proportion of the population.

e.g. Influenza, choleraOutbreak• A more or less localized epidemic affecting

large number of a group, in the community e.g. outbreak of food poisoning

Sporadic

• Cases occur irregularly, haphazardly from time to time and generally infrequently.

• Cases are few and separated widely in space and time showing no connection to each other.

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

Ca

ses

Epidemic vs Endemic

Time

EndemicEpidemic

Nu

mb

er o

f C

ases

of

a D

isea

se

Herd effect (Herd immunity)

Immunised individuals provide indirect protection to susceptible (unvaccinated, partially vaccinated) individuals:

Herd effect (Herd immunity)

• Most mass vaccination provides herd immunity

• Protection occurs even when vaccination coverage is less than 100% of the population

• The greater the infectivity (reproductive rate) of a disease, the higher the immunisation rate needed to achieve herd immunity

What is an outbreak ?

• Occurrence of more cases of disease than expected

– in a given area – among a specific group of people – over a particular period of time

Food-or waterborne outbreak (WHO definition)

• two or more persons

• similar illness

• after ingestion of the same type of food or water

• from the same source

• epidemiological evidence - the food or the water - the source of the illness

Why investigate outbreaks?

• Stop the outbreak

– Find and neutralise the source (cause)– Prevent additional cases

• Prevent future outbreaks

• Improve surveillance and outbreak detection

• Improve our knowledge

• Keep the public’s confidence

• Training

Steps of an outbreak investigation

• Confirm outbreak and diagnosis• Define a case • Identify cases & obtain information• Describe data collected and analyse• Develop hypothesis• Test hypothesis: analytical studies • Special studies• Communicate results,

– including outbreak report• Implement control measure

Co

ntro

l measu

res

Detection

Routine surveillanceClinical / LaboratoryGeneral publicMedia

Confirm outbreak and diagnosis

Is this an outbreak?• More cases than expected?• Surveillance data• Surveys: hospitals, labs, physicians

Caution!• Seasonal variations• Notification artefacts• Diagnostic bias (new technique)• Diagnostic errors (pseudo-outbreaks)

Cases of legionellosis by week of notificationFrance, January 1996 - August 1997

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1

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3

4

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6

7

8

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52

1996 Week of notification 1997

3

Cases of legionellosis by week of notificationFrance, January 1996 - August 1997

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1

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1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52

National meeting: legionellosis diagnosis and reporting

Confirm outbreak and diagnosis

• Laboratory confirmation • Contact (visit) the laboratories• Meet attending physicians• Examine some cases

Not always necessary to confirm all the casesbut confirm a proportion throughout the outbreak

Outbreak confirmed

Further investigation?

Immediate control measures?

- aetiological agent- mode of transmission- vehicle of transmission- source of contamination- population at risk- exposure causing illness

- prophylaxis- exclusion / isolation- public warning- hygienic measures- others

Outbreak confirmed, further investigations warranted

Form Outbreak Control Team?

Team coordinatesfield investigation

EpidemiologistMicrobiologistClinicianEnvironmentalistEngineersVeterinariansOthers

Descriptive epidemiology

- Who are the cases? (person)

- Where do they live? (place)

- When did they become ill? (time)

Case definition

• Simple, practical, objective

• Sensitive?

• Specific?

• Multiple case definitions– confirmed

– probable

– possible

Identify & count cases

notificationslaboratorieshospitals, GPsschoolsworkplace,cases, media, etc

Identify & count cases

Obtain information

Identifying information

Demographic information

Clinical details

Exposures and known risk factors

Identify & count cases

Obtain information

Analysis of descriptive data

Describe in

- time

- place

- person

Place

• Place of residence• Place of possible exposure

– work– meals– travel routes – day-care– leisure activities

• Maps– identify an area at risk

Person

• Distribution of cases by age, sex, occupation,etc (numerator)– 60 female– 50 male

• Distribution of these variables in population (denominator)– 600 females– 350 males

• Attack rates– female: 60/600– Males: 50/350

Develop hypotheses

- Who is at risk of becoming ill?

- What is the disease?

- What is the source and the vehicle?

-What is the mode of transmission?

Compare hypotheses with facts

Test specific hypotheses

Analytical studies- cohort studies- case-control studies

Testing hypothesis

• Cohort – attack rate exposed group– attack rate unexposed group

• Case control– % of cases exposed– % of controls exposed

Verify hypothesisSpecial investigations/studies

• Microbiological investigation

• Environmental investigation• Veterinarian investigation• Trace back investigations (origin of foods)• Entomological investigations

Implement control measures

1) Control the source of pathogen

2) Interrupt transmission

3) Modify host response

At first, general measures

According to findings, more specific measures

May (must) occur at any time during the outbreak!!

Outbreak report

• Regular updates during the investigation

• Detailed report at the end – communicate public health messages– influence public health policy– evaluate performance– training tool – legal proceedings

References

• http://www.dorak.info

• Tayal,S. Assistant professor King Saud University

• Leon Gordis. Epidemiology (3rd ed.)