epidemiology Flashcards
Epidemiologists
- Study positive health states
- Discover agent of disease, environmental factors
- Determine patterns
- Identify segments of population at risk
Health programs or services to improve health
Cholera 1800s vs 2020
- Better knowledge of outbreak
- Germ theory in early phase
- Widespread, high mortality vs ~200,000 deaths/yr.
- Water from Thames vs separate water supply
Not efficient sewage system vs efficient
Epidemiological triangle
- Infectious agent - virus, bacteria, parasite etc.
- Host
○ Demographic factor - e.g. age, ethnicity
○ Biological and genetic factor - e.g. blood group, immunity
○ Behavioural and cultural - smoking, alcohol, diet - Environment
○ Physical, chemical, biological
○ Environment - geology, climate, vectors
Socioeconomic - occupation, income, access to services
- Host
Pathogen - effect of dose
- Shorter incubation period when increased does of Salmonella
Incubation period - time from pathogen exposure to onset of signs and symptoms
Risk
- Modifiable = potential to improve health
○ Usually environmental - climate change, agriculture, individual behaviours
Unmodifiable risk factors = less able to improve health - geneitic
mortality vs morbidity
amount of death due to disease
amount of illness due to disease
elimination vs eradication
reduction to zero in geographic location
complete and permanent reduction to zero
Disease burden
- Measure of impact of living with disease
- Measure of gap between current health and ideal health
- Financial cost, morbidity, mortality
Measurement = disability- adjusted life years (DALYs) - years healthy life lost due to illness and death
Prevalence
- Proportion of population with disease at specific time
- Snapshot
Number of ppl at a specific time with disease / total number of people in the population at risk
- Snapshot
Incidence
- Number of new cases over a period of time
- Presented as proportion or rate measurement
Indication of infection risk with a specific time
- Presented as proportion or rate measurement
Prevalence vs incidence
- Allows comparison among different populations
- Prevalence measures existing disease
- Incidence measure rate at which new cases occur
- If incidence increases, prevalence increases
- If prevalence is introduced (eg vaccine, quarantine), incidence will decrease
- If effective treatment is found, prevalence will decrease
levels of outbreak
Pandemic - all
Epidemic - outbreak
Endemic - expected level
Sporadic - occurs infrequently
sporadic level
occasional cases occurring at infrequent intervals (low level)
endemic level
Persistent occurrence - baseline - low to moderate
epidemic level
- Increase in expected level - can be rapid - large number of cases in short period
- Clusters
- Conditions
○ Agent +susceptible host in reasonable numbers
○ Change to infectious agent - increase in amount, fast incubation
○ New disease in naïve population
○ Mode of transmission
Opportunities for exposure - slow recognition
pandemic level
- Over several countries
Large number of people
Exposure
- Host and infectious agent encounter each other
- Possible outcomes
○ No infection
○ Death - clinical
○ Recovery + immunity - clinical
Immunity - subclinical
- Possible outcomes
Incubation period
- 3-5 days average
- From exposure to sign
- Pathogen development but no clinical manifestations
- Length
○ Virulence
○ Host health
○ Site
Reproduction number
Prodromal period
- Clinical manifestation
- Short period preceding illness
Can be contagious
- Short period preceding illness
Incubation period useful for:
- Predict disease severity
○ Shorter incubation = more severe- Quarantine duration
Interpret public health surveillance data
- Quarantine duration
Illness
- Onset of symptoms
- Most severe stage
Immune response still being formed
- Most severe stage
Decline
- Returning to normal
- symptoms reduce
Or death
- symptoms reduce
Convalescent period
- Recovery
Symptoms disappear
The chain of infection - expanding infection triangle
- Source / reservoir
- Agent
- Transmission
- Portal of entry
- Susceptible host
Portal of exit
Source
location from which pathogen is transmitted to host
- Animate (humans, plant, animals)
○ Zoonosis (disease of animals transmitted to humans)
- Inanimate - soil, water, food, air
reservoir
normal habitat where pathogen is normally found
- Sometimes same as source (human carrier of TB)
- Sometimes different from source (Hendra virus)
○ Reservoir - bats
○ Source of human infection = horses
Direct or indirect by 4 main routes
- Airborne
- Contact
- Vehicle
Vector
Direct
- person to person - secretions, cells, air
- Zoonosis
Vertical transmission via placenta
- Zoonosis
Indirect
one host to another via intermediate (vehicles)
Intermediate can be animate (e.g. mosquito) or non-living (e.g. Doorknob)
Airborne (Water droplets)
- Large respiratory droplets (5-10 micrometre diameter)
○ Singing, talking, coughing, sneezing- Aerosols (1-5micrometre in diameter
○ Talking and breathing
○ Long time (hours to days)
Reservoirs - air conditioning cooling towers e.g. legionnaires
- Aerosols (1-5micrometre in diameter
airborne (dust)
- Become airborne due to disturbance, survive long
- Fungal infections
Hospital acquired
- Fungal infections
Vehicles
- Surgical instruments
- Cosmetics
- Clothing
- Phones
- Food
- Soil
Blood products
Vector
- Insects - malaria
- Ticks - Lyme
- Fleas - plague
- External transmission - passive carriage on body surface - shigellosis
- Internal transmission - carried inside vector - malaria
Portal of entry
- Oral route - food borne, water borne
- Respiratory - dust, droplets
- Urogenital - genital
- Parenteral - injection to bloodstream - needles
- Conjunctival - surface of eyes
Dermal - skin - bite
primary vs index vs secondary case
Primary case - First case in population Index case - First case reported to authorities Secondary Cases resulting from subsequent transmission
Reproduction rate (R0)
- Indicate how contagious the disease is
- R0 assumes population is completely vulnerable (no immunity, no vaccine)
- If R0 = 1, each infection causes one new infection
- If R0 <1, disease declines
- If R0 >1, disease increases, may result in outbreak
The larger R0 the more contagious
Re and R0 factors
Re (effective) is used when there is some immunity or intervention measures R0 = not measurement of time or severity Factors - Duration - Infectivity - No. of contacts - Host factors - health Climatic factors
immunity
- The capacity of a person when exposed to an infectious agent to remain free of infection
- Can be acquired prior infection or immunisation
- Varies for different diseases
○ Measles- experience it once, immunity life long
Whooping cough - immunity can wain from previous infection
herd immunity
- Immunity of a group or community
- Resistance to infection of a high proportion of individuals of a group
- Resistance is a product of the number of susceptible people and the probability that those whose are susceptible will come into contact with an infected person
1-1/R0
Ten steps of treating an outbreak
- Establish existence of outbreak
- Research the disease, fieldwork
- Verify diagnosis
- Define and identify cases = case definition - speak to patients
- Use descriptive epidemiology - who, what, when, where, why, how
- Develop hypotheses
- Evaluate hypotheses
- Refine hypotheses
- Implement control and prevention measure - prevent additional cases
- Develop policies and communicate findings - health policies, education
