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?
• 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. 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
What is infectious disease
epidemiology?
(www)
5. A case is a risk factor …
Infection in one person can be transmitted to others
What is infectious disease
epidemiology?
(www)
6. The cause often known
An infectious agent is a necessary cause
What is infectious disease epidemiology then 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
What is infectious disease
epidemiology?
(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 …
8.
9. Definitions
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)
Infectious Disease
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
Ting-borne
Scarlatina
Exposure
A relevant contact – depends on the agent
Skin, sexual intercourse, water contact, etc
(www)
16. 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
S
T
Susceptible
Immune
Sub-clinical
Clinical
S
T
(www)
Transmission
17. 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)
Person-to-Person Transmission
18. Disease is the result of
forces within a dynamic
system consisting of:
agent of infection
host
environment
Epidemiologic Triad
19. Agent
Host
Environment
• Age
• Sex
• Genotype
• Behaviour
• Nutritional status
• Health status
• Infectivity
• Pathogenicity
• Virulence
• Immunogenicity
• Antigenic stability
• Survival
• Weather
• Housing
• Geography
• Occupational setting
• Air quality
• Food
(www)
Factors Influencing Disease
Transmission
20. 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
Epidemiologic Triad-Related Concepts
21. Predisposition to Infections
(Host Factors)
Gender
Genetics
Climate and Weather
Nutrition, Stress, Sleep
Smoking
Stomach Acidity
Hygiene
22.
23. Horton & Parker: Informed Infection Control Practice (www)
Chain of Infection
31. 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)
Reservoirs
33. 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.
36. 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
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)
41. 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
contacts per unit time
probability of transmission per contact
duration of infectiousness
Infection will ….. disappear, if R < 1
become endemic, if R = 1
become epidemic, if R > 1
(www)
Reproductive Number, R0
42. • 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)
Reproductive Number, R0
43. • 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
Reproductive Number, R0
44. p condoms, acyclovir, zidovudine
c health education, negotiating skills
D case ascertainment (screening,
partner notification), treatment,
compliance, health seeking
behaviour, accessibility of services
R0 = p • c • d
(www)
Reproductive Number, R0
Use in STI Control
45. 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)
What determines R0 ?
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
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
R = 1
R > 1
R < 1
(www)
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)
