infectious disease epid_prof bhisma
TRANSCRIPT
Epidemiology of Infectious Epidemiology of Infectious DiseasesDiseases
Prof. Bhisma Murti
Department of Public Health, Faculty of Medicine,
Universitas Sebelas Maret
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
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
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)
A case is a risk factor … Infection in one person can be transmitted to others
What is infectious disease epidemiology?
(www)
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?
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 …
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?
TetanusTetanus MeaslesMeasles Hepatitis BHepatitis B
Infectious Disease
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
Some Pathogens that Cross the Placenta
Modes of Disease Transmission
No infection Clinical Sub-clinical Carrier
Death Carrier Immunity No immunity
Outcome
Exposure to Infectious Agents
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
Timeline for Infection
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
ST
Transmission
Hypothetical Data
Measles Chickenpox Rubella
Children exposed
Children ill
attack rate
251
201
0.80
238
172
0.72
218
82
0.38
Attack rate = illexposed
Person-to-Person Transmission
Disease is the result of forces within a dynamic system consisting of:agent of infection
hostenvironment
Epidemiologic Triad
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
Factors Influencing Disease Transmission
•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
Predisposition to Infections(Host Factors)
Gender
Genetics
Climate and Weather
Nutrition, Stress, Sleep
Smoking
Stomach Acidity
Hygiene
Chain of Infection
Iceberg Concept of Infection
CELL RESPONSE HOST RESPONSE
Bacteria
Viruses
Fungi
Protoctists / Protozoa
Helminths
Infectious Agents
A host that carries a pathogen without injury to itself and serves as a source of infection for other
host organisms
(asymptomatic infective carriers)
Reservoirs
Humans{hepatitis}
Other Vertebrates{zoonoses}
Birds & Bats{histoplasmosis}
NOT vectors
Reservoirs
Vectors
A host that carries a pathogen without injury to itself and spreads the
pathogen to susceptible organisms
(asymptomatic carriers of pathogens)
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
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
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
Infectious Disease Process
Direct tissue invasion
Toxins
Persistent or latent infection
Altered susceptibility to drugs
Immune suppression
Immune activation (cytokine storm)
Mathematical Modelling in Infectious Disease
Epidemiology
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 < 1become endemic, if R = 1become epidemic, if R > 1
Reproductive Number, 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
Reproductive Number, 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
Reproductive Number, Reproductive Number, R0
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
Reproductive Number, Reproductive Number, R0
Use in STI Control
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, screen 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 What determines R0 ?
Immunity – herd immunity
If R0 is the mean number of secondary cases in a susceptible population, thenR 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 > 0.50If 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
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
R = 1
R > 1
R < 1
Endemic EpidemicNu
mb
er o
f C
ases
of
a D
isea
se
Time
Endemic vs Epidemic
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 occurrenceEpidemic or outbreak: occurrence clearly in excess of the expected level for a given time periodPandemic: epidemic spread over several countries or continents, affecting a large number of people