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Infectious Disease Epidemiology
and
Dynamics of Disease Transmission
Dr. Pracheth R.
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Outline
• Definitions
• Dynamics of disease transmission
• Modes of transmission
• Immunity
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Infection
• Entry, development/multiplication : infectious
agent- body
• Body defends : immune response
• Colonization: S. aureus in skin
• Subclinical/inapparent: polio
• Latent infection: herpes simplex
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Contamination
• Infectious agent: body surface: clothes,
beddings, toys, surgical instruments
Infestation:
• Lodgement , development of arthropods: body,
clothing . Eg: Lice
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Continued….
• Epidemic
• Endemic
• Sporadic
• Pandemic
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Iatrogenic (Physician-induced)
• Untoward/adverse consequence:
diagnostic/therapeutic regimen: impairment,
handicap
• Reactions to penicillin
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Continued….
• Zoonoses
• Nosocomial infection
• Eradication
• Elimination
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Eradication
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Elimination
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Dynamics : disease
transmission
• Source: person, animal, object/substance:
infectious agent passes to host
• Reservoir: agent lives, multiplies , depends :
survival, reproduces-transmit to host
• Host:
Person/ animal (birds, arthropods) :substinence
/lodgement to infectious agent
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Human reservoir
• Cases: person: disease/disorder
Clinical case
Subclinical case: laboratory test
Latent infection: host not shed infectious agentdormant
Primary case: first case introduced
Index case
Secondary case
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Carriers
• Inadequate treatment/immune response:
disease agent not eliminated: carrier
• Infected person/ animal: harbours infectious
agent, absence of disease, infect others
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Continued….
• Temporary
• Chronic
• Incubatory: measles
• Convalescent: diphtheria
• Healthy: polio
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Animal reservoir
• Animals and birds
• Cases or carriers
• Rabies
Reservoir in non living things:
• Soil: tetanus
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Modes of transmission
Direct contact:
• Direct contact: STD, leprosy
• Droplet infection
• Soil
• Transplacental/vertical
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Indirect
(Flies, fingers, fomites, food, fluid)
Vehicle borne:
Water, food, milk and milk products
Diarrhoea, cholera, typhoid
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Vector borne
• Arthropod/living carrier: transports agentsusceptible individual
• Methods of transmission
Mechanical: flies
Biological: Replication/development /both in
vector
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Continued….
Propagative: multiplies, no change in form. Plague
bacilli in rats
Cyclo-propagative: form, number: malaria parasites
Cyclo-developmental: only development, no
multiplication. Microfilaria in mosquito
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Airborne
• Droplet nuclei:
Tiny particles; dried residues of droplets
1-10 microns
TB, influenza, chicken pox
• Dusts:
Larger droplets: settle dusts
Pneumonia, tuberculosis
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Fomite borne
• Inanimate objects other than water, food
• Soiled clothes, towels, linen, cups
• Diphtheria, typhoid, dysentry
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Unclean hands and fingers
• Staphylococcal, streptococcal
• Lack of personal hygiene
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Successful parasitism
• Portal of entry
• Site of election
• Portal of exit:
Dead end infection
• Survival in external environment
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Incubation period
• Time between invasion: infectious agent-sign
and symptom
• Median: time : 50% cases occur
• Latent period:
For non-infectious diseases
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Importance
• Trace source of infection
• Immunization
• Prognosis
• Determine period of surveillance
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Continued…..
• Serial interval:
 Gap between onset : primary and secondary
cases
• Generation time:
Interval: receipt of infection-maximal infectivity
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Continued…..
• Communicable period:
 Time : infectious agent: transferred : personperson
• Secondary Attack Rate:
(Number exposed persons-disease within IP/
Total number of contacts) X 100
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Questions
1. Explain modes of transmission of an infectious
disease in detail (6 marks)
2. What is a vector? Explain its role in
transmission of a disease (1+5= 6 marks)
3. What is a carrier? Mention it’s types (1+2=3
marks)
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Continued……
4. Explain the terms: epidemic, endemic, sporadic
and pandemic (3 marks)
5. Incubation period (3 marks)
6. Period of Communicability (3 marks)
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Continued……
7. Dead end infection (2 marks)
8. Serial Interval and Generation Time (2 marks)
9. Iatrogenic disease (2 marks)
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Summary
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THANKS
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