1. Epidemiology of Infectious Diseases
M. Tevfik DORAK
(www)

2. Infectious Disease Epidemiology: Major Differences
A case can also be an exposure
Subclinical infections influence epidemiology
Contact patterns play major role
Immunity
There is sometimes a need for urgency
(www)

3. What is infectious disease epidemiology?
Deals with one population
Risk  case
Identifies causes
Infectious disease epidemiology
Two or more populations
A case is a risk factor
The cause often known
(www)

4. What is infectious disease epidemiology?
Two or more populations
Humans
Infectious agents
Helminths, bacteria, fungi, protozoa, viruses, prions
Vectors
Mosquito (protozoa-malaria), snails (helminths-schistosomiasis)
Blackfly (microfilaria-onchocerciasis) – bacteria?
Animals
Dogs and sheep/goats – Echinococcus
Mice and ticks – Borrelia
(www)

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

6. What is infectious disease epidemiology?
The cause often known
An infectious agent is a necessary cause
What is infectious disease epidemiology 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
(www)

7. Concepts Specific to Infectious Disease Epidemiology
Attack rate, immunity, vector, transmission, carrier, subclinical disease, serial interval, index case, source, exposure, reservoir, incubation period, colonization, generations, susceptible, non-specific immunity, clone, resistance, repeat episodes …

9. Infectious Disease Definitions Note 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)

10. Routes of transmission
Direct
Skin-skin
Herpes type 1
Mucous-mucous
STI
Across placenta
toxoplasmosis
Through breast milk
HIV
Sneeze-cough
Influenza
Indirect
Food-borne
Salmonella
Water-borne
Hepatitis A
Vector-borne
Malaria
Air-borne
Chickenpox
Exposure
A relevant contact – depends on the agent
Skin, sexual intercourse, water contact, etc
(www)

11. (www)

12. Some Pathogens that Cross the Placenta

13. Modes of Disease Transmission

14. Exposure to Infectious Agents
No infection Clinical Sub-clinical Carrier
Death Carrier Immunity No immunity
Outcome
(www)

15. Timeline for Infection
Susceptible
Dynamics of
infectiousness
Dynamics of disease
Incubation period
Symptomatic
period
Non-diseased
Latent
Infectious
Non-infectious
Time
(www)

16. 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 P S T
Susceptible
Immune
Sub-clinical
Clinical
(www)

17. Person-to-Person Transmission
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)

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

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

20. Epidemiologic Triad-Related Concepts
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

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

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

24. Infection Cycle of Schistosomiasis
Peters: Tropical Medicine and Parasitology, 2001 (www)

25. Infection Cycle of Leishmaniasis
Lipoldova & Demand, 2006 (www)

26. Iceberg Concept of Infection

27. Protoctists / Protozoa
Infectious Agents
Bacteria
Viruses
Fungi
Protoctists / Protozoa
Helminths
Algae
Prions
(proteinaceous infectious agents)

28. Phylogenetic Classification of Bacteria
Oxford Textbook of Medicine

29. Phylogenetic Classification of Viruses
Oxford Textbook of Medicine

30. (PRI-ons: proteinaceous infectious agents)
Mabbott & MacPherson, Nat Rev Microbiol 2006 (www)

31. (asymptomatic infective carriers)
Reservoirs
A host that carries a pathogen without injury to itself and serves as a source of infection for other host organisms
(asymptomatic infective carriers)
(www)

32. Reservoirs Humans Other Vertebrates Birds & Bats NOT vectors
{hepatitis}
Other Vertebrates
{zoonoses}
Birds & Bats
{histoplasmosis}
NOT vectors

33. (asymptomatic carriers of pathogens)
Vectors
A host that carries a pathogen without injury to itself and spreads the pathogen to susceptible organisms
(asymptomatic carriers of pathogens)

34. Vectors
Other parasites have life cycles that involve intermediate organisms, or vectors, which carry disease-causing microorganisms from one host to another. The protozoan blood parasite that causes sleeping sickness, or trypanosomiasis, infects humans, cattle, and other animals. It uses the tsetse fly as a vector to carry it from one host to the next. When a tsetse fly bites an infected animal, it picks up the parasite when it sucks blood. When an infected fly bites another animal, the parasite enters the bloodstream and begins to reproduce in the new host.

35. Arthropod Vectors Pathogen - Vector Viruses (Arbovirus) - Mosquitoes
Bacteria (Yersinia) - Fleas
Bacteria (Borrelia) - Ticks
Rickettsias (R. prowazeki) - Lice, ticks
Protozoa (Plasmodium) - Mosquitoes
Protozoa (Trypanozoma) -Tsetse flies
Helminths (Onchocerca) - Simulium flies

36. Koch’s Postulates The same organism is present in every case
It is isolated or grown in pure culture
The disease can be reproduced in healthy animals after infection with pure culture
The identical pathogen is reisolated from the experimental animals

37. Koch’s Postulates

38. Ecological Factors in Infections
Altered environment
{Air conditioning}
Changes in food production & handling
{intensive husbandry with antibiotic protection; deep-freeze; fast food industry}
Climate changes
{Global warming}
Deforestation
Ownership of (exotic) pets
Air travel & Exotic journeys / Global movements
Increased use of immunosuppressives/ antibiotics
American Museum of Natural History Exhibition:
Epidemic! The World of Infectious Disease (www)

39. Infectious Disease Process
Direct tissue invasion
Toxins
Persistent or latent infection
Altered susceptibility to drugs
Immune suppression
Immune activation (cytokine storm)

40. Mathematical Modelling in Infectious Disease Epidemiology

41. Reproductive Number, R0 R0 = p • c • d
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
probability of transmission per contact
duration of infectiousness
contacts per unit time
Infection will … disappear, if R < 1
become endemic, if R = 1
become epidemic, if R > 1
(www)

42. Reproductive Number, R0 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)

43. Reproductive Number, R0
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

44. Reproductive Number, R0 R0 = p • c • d
Use in STI Control
R0 = p • c • d
p condoms, acyclovir, zidovudine
c health education, negotiating skills
D case ascertainment (screening,
partner notification), treatment,
compliance, health seeking behaviour, accessibility of services
(www) 8

45. What determines R0 ?
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)

46. 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)

47. Endemic - Epidemic - Pandemic
Time
Cases
R > 1
R = 1
R < 1
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
(www)

48. Endemic vs Epidemic
Number of Cases of a Disease
Epidemic
Endemic
Time

49. Levels of Disease Occurrence
Sporadic level: occasional cases occurring at irregular intervals
Endemic level: persistent occurrence with a low to moderate level
Hyperendemic level: persistently high level of occurrence
Epidemic or outbreak: occurrence clearly in excess of the expected level for a given time period
Pandemic: epidemic spread over several countries or continents, affecting a large number of people
(www)