What are the elements of the epidemiological triangle?
- Agent (the microbe that causes the disease)
- host (the organism harbouring the agent)
- environment (external factors that allow disease transmission)
What are characteristics of an agent that influence transmission?
- ability to live outsife the host
- ability to survive in unfavourable environments
- ability to shift antigenecity
What are the modes of transmission?
Direct
- direct contact
- droplets
Indirect
- vehicle borne (contaminated materials)
- vector borne (insect/animal carrying the agent)
- airborne (droplets/dust)
What are characteristics of the host thst affect exposure, susceptibility or response to an agent?
- behavioural (Sexual practices, hygiene, eating habits)
- biological (genetic composition, immunity)
- sociodemographic (age, sex)
What characteristics of the environment influence disease transmission?
- Physical (geology, climate, habitat)
- biological (human population, flora, fauna)
- socioeconomic (occupation, urbanisation, availability of health services)
What are the three markers of an infectious disease?
infectivity - ability to infect (persons who become infected ÷ total number infected)
pathogenicity - ability to cause disease (number infected & exhibiting disease ÷ total number infected)
virulence - ability to cause severe illness/death (number with severe disease ÷ total number with disease)
Howbdo we measure the infectivity of an agent?
Reproduction Number (R0)/Basic Reproductive Ratio
= the average number of secondary infections caused by one individual in a fully susceptible population
R0>1 - infection will spread exponentially
R0=1 - infection soread remains stable
R0<1 - infection spread slowing down and will eventually disappear
What is an outbreak vs a cluster?
- an **unusual* infectious disease event
- an increase in cases compared to what is expected normally for a given time and place
- a change in disease pattern (f.e.x more severe)
outbreak = 2 or more cases with a common source
cluster = 2 or more cases with with an epidemiological link to warrant further investigation
What is a case definition and what types are there?
= a standard set of criteria for deciding if a person should be linked to the outbreak under investigation
Clinical definition: symptoms
Epidemiological definiton: symptoms + lab confirmed + linkage to confirmed cases
What are the components of a case definition?
time - the period during which cases are at risk of exposure
place - geographic area/fscility associated with outbreak
person
- demographics: age, sex, occupation, exclusion criteria
- clinical featuresn: signs & symptoms, symptom duration
laboratory criteria - specimen type (type of sample), specific pathogenic organism, subtype/strain typing, local or specislized lab testing required
Sensitivity vs. Specificity
the less specific a case definition is, the more people will be picked up by it
- sensitivity = how good a system is at detecting true cases (produces many true positives)
- specificity = how good the system is at identifying false cases (produces few false positives)
Depends on stage of investigation:
- sensitive - case finding, describe outbreak, target control measures
- specific - epi study to investigate suspected source
Case definition can include levels of specificity
- differentiate confirmed and probable cases
What is the role of descriptive epidemiology in an outbreak investigation?
- to search for patterns by examining characteristics of perdon, place & time
- to generate hypotheses about the disease determinants by looking for differences, similarities & correlations in those characteristics
How does descriptive epidemiology investigate person?
- describe the demographics of who is affected
- depicted as age-sex pyramid
How does descriptive epidemiology investigate time?
- analyzing trends & identifying seasonality/cyclical changes
- evaluating impact of interventions
- identifying when observed cases exceed expected values
Depicted with epidemic curves
How does descriptive epidemiology investigate place?
- mapping identified cases
- overlaying exposures of interest (like potentially contaminated water sources)
Depicted with incidence maps or combined maps (mapping both incidences and potential contamination locations)
What do epi curves tell us?
- time trend of outbreak (i.e. distribution of cases over time)
- outliers (often representing index (source) case or secondary spread)
- outbreak magnitude
- most likely time of exposure
- mode of spread
What are the three modes of spread and what do they look like in epi curves?
Point Source
- when people are exposed briefly to the same source
- number of cases rises rapidly snd falls gradually
- majority of cases occur within one incubation period
continuous Common Source
- when people are exposed to the same source over prolonged time
- curve rises gradually and might plateau
Person-to-person Spread/propagation
- no common source
- graph will show progressively taller peaks, each one incubation period apart
How do you identify the most likely time period of exposure for point source outbreaks?
- Identify peak of the outbreak (time period with largest number of cases) - Dec. 10th
- Count back from the peak the average incubation period - Dec. 6th
- Identify earliest case and count back minimum incubation period - Dec. 7th
- Identify last case and count back maximum incubation period - Dec. 3rd
-> most likely exposure period: Dec. 3rd - 7th
What are the time stages of an infectious disease?
1. Susceptibility stage
= the time where a host is exposed & factors are sufficient for disease process to begin
2. Subclinical stage/incubation period
= time between start of disease process and onset of symptoms, where disease is still asymptomatic or inapparent
-> incubation period helps identify agent, source & period of infectiousness
3. Clinical stage
= time when symptoms start to show
- Latent period = time between infection and becoming infectious to others
- communicability period: interval where person is herself infectious
What different teams exist in Health Protection and what domains do they belong to?
Health Protection Team:
- nurses, doctors, surveillance and administrative staff
- works closely with environmental health departments, microbiologists, infection & disease control teams, GPs, community specialists, educational institutions
Incident Management Team:
- assembled during initial outbreak investigation
- consists of health protection, epidemiology, microbiology, nursery, primary care, microbiologists, local authorities, communications & admin support
Outbreak Control Team: once outbreak is defined, IMT automatically becomes OCT triggering additional resources
IPC teams:
- manage IPC programmes in facilities
- include infection control doctors & IPC nurses, microbiologist, secretarial & data technology
IPC committee:
- coordinate IPC programmesinclude clinical & nursing staff, estates & facilities department, hospital/community management, health protection representatives
Strategic Coordinating Group (SCG):
- for acute incidents (EPRR)
- convened by the police
Scientific & Technical Advice Cell:
- advises the SCG in emergency responses
- similar members to OCT, with environmental, chemists or toxicologists replacing microbiologists
What are index, primary and secondary cases?
index: first case to come to attention
primary: case that introduces the disease into group/population
secondary: case that contracted infection from primary
What are the 8 steps of an outbreak investigation?
1. Case ascertainment: compare observed & expected cases & compare cases against standard definition of disease
2. Case identification & outbreak confirmation: confirm epidemiological link & establish case definition
3. Case identification: identify unreported cases
4. Conduct descriptive epidemiology: including epidemic curves, determine mode of spread etc.
5. Generate hypothesis
6. Test hypothesis: usually through case-control or cohort studies)
7. Interpret results & consider wider public health issues: produce intelligence for control measures
8. Write report & communicate
When to declare an outbreak over?
Typical: 2 incubation periods without new cases
For shorter incubation periods, 3
What are the 4 steps of a public health risk assessment?
- Hazard analysis
- Exposure analysis
- Risk assessment
- Risk communication
