Epidemiology

Question 1

What is epidemiology

Answer 1

• Study of the distribution and patterns of health events
• Health characteristics and their causes or influences in well defined populations
• Cornerstone method of public health research and practice
• Critical to understand how to treat and prevent diseases
• Diseases are described in terms of prevalence, incidence, virulence or relative pathogenicity

Question 2

How do we control disease

Answer 2

• Rely on surveillance, observation, recognition and reporting of diseases as they occur
• Ability to trace spread of disease to identify its origin and mode of transmission

Question 3

What is incidence vs prevalence of disease

Answer 3

• Incidence: Number of new cases of the disease in a given period of time
• Prevalence: Total number of new and existing cases in a population in a given time

Question 4

What is the difference between sporadic, endemic, epidemic and pandemic

Answer 4

• Sporadic: Random outbreak of a limited number of cases of infection within a defined area / region
• Endemic: Disease is constantly present in a population, usually at low incidences, infected individuals causing disease are reservoirs (human / non)
• Epidemic: Disease occurs in a large number of people in a population at same time
• Pandemic: Larger number of cases that are widespread within a country, but cases are observed worldwide

Question 5

What are the different stages of infection

Answer 5

• Infection: Organism invades and colonises the host
• Incubation Period: Time between infection and onset of symptoms
• Acute Period: Disease is at its height
• Decline Period: Disease symptoms are subsiding
• Convalescent Period: Patient regains strength and returns to normal

Question 6

What is mortality vs morbidity and DALY

Answer 6

• Mortality: Incidence of death in a population
• Morbidity: Incidence of disease, including fatal and nonfatal diseases in a population
• DALY: Quantitatively measures disease burden in terms of lost years due to disease, disability due to disease and premature death

Question 7

What is coevolution of the host and pathogen

Answer 7

• Virulence of parasite in host to host transmission diminishes, resistance of host increases
• Pathogen that kills its host before it can infect another host may become extinct
• If a pathogen does not rely on host to host transmission it may remain extremely virulent (E.coli)

Question 8

What is direct vs indirect transmission

Answer 8

• Direct: Infected individual transmits a disease directly to a susceptible host without assistance of an intermediary, examples include flu, common cold, STI’s and ringworm
• Indirect: Occurs when transmission is facilitated by a living (vector) or nonliving agent (fomite)

Question 9

What is zoonosis

Answer 9

• Any disease that primarily infects animals but is occasionally transmitted to humans
• Transmission occurs through vectors, air / food borne, close proximity to nature, direct contact
• Control of a zoonotic disease in the human population may not eliminate disease as a potential public health problem
• Some have complex life cycles involving an obligate transfer from a nonhuman host to humans followed by transfer back to the nonhuman host (malaria)

Question 10

What is a carrier of disease

Answer 10

• Pathogen-infected individuals showing no signs of clinical disease
• Potential sources of infections
• May be individuals in incubation period of disease
• Can be identified using diagnostic techniques, including culture and immunoassays
• Typhoid Mary (cook who infected 50 individuals)

Question 11

What are the different ways that carriers can be classified

Answer 11

• Type: Incubatory (short time, asymptomatic), convalescent (recovering, no sterilising immunity, carry infectious pathogens), healthy (asymptomatic with infectious pathogen)
• Duration: Temporary or permanent
• Portal of Exit: Released via a specific exist depending on the location of infection (urinary, intestinal, respiratory)

Question 12

What are the key methods for control and spread of disease

Answer 12

• Against Vehicles: Food laws, water purification
• Against Reservoirs: Animals (immunised / destroyed), humans (quarantine, immunised, treated)
• Against Transmission: Immunisation, quarantine, surveillance, contact tracing

Question 13

What is R0

Answer 13

• Number of expected secondary cases of a given disease from each single case
• Mathematical model assumes an entirely susceptible population
• Different diseases have different R0
• Estimate of how many people will be infected by an average individual with the disease

Question 14

Why is R0 important in epidemiological studies

Answer 14

• Epidemiological studies involve a systemic approach
• Attempt to describe “what, who, where, when, and why/how” of a situation
• Counts cases or health events, and describes them in terms of time, place, and person
• Divides the number of cases by an appropriate denominator to calculate rates
• Compares these rates over time or for different groups of people
• Involves a case definition (confirmed, probable, suspected)

Question 15

What is descriptive epidemiology

Answer 15

• Identifying information
• Demographic information (characterise population at risk)
• Clinical information (epidemic curve created, spectrum of illness)
• Risk factor information (tailored)
• Extent and pattern of public health problem being investigated
• Detailed description of the health of a population that can be easily communicated
• Identify areas or groups within the population that have high rates of disease
• Provides important clues to causes of disease, and thus a testable hypotheses

Question 16

What is the epidemic curve and how do we flatten it

Answer 16

• Graph of the number of cases by their date of onset
• Simple visual display of outbreak’s magnitude and time trend
• Shows course of epidemic / pandemic
• May enable estimation of probable time period of exposure
• May enable inferences to be drawn about the epidemic pattern
• Social distancing, washing hands, wearing PPE, self isolation and preventing mass gathering
• 80% compliance with these measures allows rate of infection to slow and ‘flattening curve’

Question 17

What is analytic epidemiology

Answer 17

• Searches for causes and effects, or the why and the how of a specific public health problem
• Use analytic epidemiology to test a hypothesis through using a comparison group
• Experimental studies (cohort, case control or cross sectional)

Question 18

What is an experimental study

Answer 18

• Investigator determines through a controlled process the exposure for each individual (clinical) or community (community)
• Then tracks individuals or communities over time to detect effects of exposure
• Types include cohort, case control and cross sectional studies

Question 19

What is a cohort study

Answer 19

• A cohort of patients is chosen randomly from the general population
• Cohort is divided into two treatment groups (unexposedorexposed)
• Track participants to see if they develop disease of interest

Question 20

What is a case control study

Answer 20

• Two cohorts
• Group of people with disease and without (controls)
• Compare previous exposures between two groups
• Control group provides an estimate of baseline or expected amount of exposure in that population
• If exposure among case group is substantially higher than the control group, then the illness is said to be associated with that exposure

Question 21

What is a cross-sectional study

Answer 21

• Sample of persons from a population is enrolled and their exposures and health outcomes are measured simultaneously
• Weaker compared to above two
• Assess presence (prevalence) of health outcome at that point of time without regard to duration
• Cannot disentangle risk factors for occurrence vs survival of disease
• May have patients within control group that display delayed onset of clinical symptoms, may not identified until late in the study

Question 22

What are two ways an epidemic can arise

Answer 22

• Common Source: Arises from contamination of water or food, rapid spike and decline in incidence of disease, includes cholera
• Host to Host: Disease shows a slow, progressive rise and gradual decline, includes influenza and chicken pox

Question 23

What is contact tracing

Answer 23

• Infected person shows symptoms (tests positive), infected person asked about contacts
• Contacts alerted of possible exposure
• Contacts self quarantine and monitor symptoms to prevent spread