final

Screening

The presumptive identification of unrecognized disease or defects by the application of tests, examinations, or other procedures that can be applied rapidly.

• Occurs during pathogenesis phase

• Positive screening test → diagnostic test to confirm actual disease presence

Details of screening (how its different than diagnostic)

Asymptomatic people/subclinical cases of disease

Positive result indicates suspicion of disease

Detects disease indicators, not the disease itself

Done first

Ex. Mammogram, positive screening doesn’t guarantee you have breast cancer

Details of diagnostic (how its different than screening)

Establish the presence or absence of the disease Targets symptomatic people or asymptomatic people with positive screening test Usually more invasive/expensive, don’t want to put people through them if you don’t need to Done second Ex. biopsy following positive mammogram screening

Mass (population) screening

Screen entire population regardless of risk status

Ex. taking peoples temps before entering school in covid

Selective (targeted) screening

Screen specific groups who are at high risk for disease.

Ex. HIV screening for sex workers

Criteria for appropriateness to screen (3)

1. Social

2. Scientific

3. Ethical

Criteria for appropriateness to screen - social

health problem should be important for the community

• Follow-up and interventions must be available

• Favorable cost-benefit ratio

• Public acceptance must be high

Criteria for appropriateness to screen - scientific

sufficient understanding of the disease in question (cause, natural history - when symptoms arise/when best to detect for)

• Prevalence is high

Criteria for appropriateness to screen - ethical

can’t freak people out and leave them high and dry

• Need to have interventions available

• Don’t screen if treatment doesn’t change outcome

Characteristics of a good screening test (5)

1. Simple: easy to administer → especially if can complete at home

2. Rapid: quick to administer, fast results

3. Inexpensive: favorable cost-benefit ratio

4. Safe: don’t cause harm

5. Acceptable: people must be willing to use test

Why is screening important?

Catch disease early = better prognosis

Reliability

aka precision

consistently repeated results

Validity

aka accuracy

ability of the measuring device to give a true statement

Gold standard

Diagnostic test to say if you have the disease, not perfect but best available test that has a standard with known results

True positve

Screen +

Gold standard +

False positive

Screened +

Gold standard -

False negative

Screened -

Gold standard +

True negative

Screened -

Gold standard -

Sensitivity

Characteristic of the test —> how good is the screening test at its job

examining the true positives, how many people who test positive actually have the disease

x% sensitivty = x% of people who screen positive have the disease

Specificity

Characteristic of the test —> how good is the screening test at its job

examining the true negatives, how many people who test negative actually don’t have the disease

x% specificty = x% of people who do not have the disease will test negative

Positive predictive value

Clinical relevance/applications of a test

 true positives and all positives, Probability that following a positive test result, that individual will truly have that specific disease

Negative predictive value

Clinical relevance/applications of a test

Probability that following a negative test result, that individual will truly not have that specific disease

Accuracy

The degree of agreement between the test result and the gold standard

all the true negatives and positives divided by grand total

Prevelance

The number of existing cases of a disease or health condition in a population at a point or during a period of time

true positives + false negatives over grand total

Sporadic

random/isolated occurrences of cases of disease

Endemic

a disease that occurs at a steady rate among a population

Outbreak

a condition that occurs above endemic levels

Epidemic

The occurrence, in a community or region, of cases of an illness (or an outbreak) clearly in excess of expectancy

• Relative to “usual” frequency of the disease

Pandemic

health condition epidemic has spread globally

Infectious disease

disease caused by an infectious agent

Communicable disease

infectious disease that can be spread from person to person

(Ex. of infectious disease that isn’t communicable: tetanus, sepsis)

Why we should care about the spread of infectous disease (4)

• Significant cause of morbidity/mortality worldwide → global south

• Association with noncomm diseases (cancer)

• Increasing global connectivity (= increase transfer)

• Disease outbreaks

Events that start epidemics (5)

• Susceptible patients move into an endemic area

• Introduction of new infectious disease by humans/animals (area disease endemic → move to area not and people susceptible)

• Contamination of food/water/etc.

• Changes in social/behavioral/sexual/cultural practices causes greater exposure of susceptible people to the disease

• Modification of host susceptibility - immunosuppressants

Components of epi triangle (4)

host + environment + agent + time

Epi triangle

Factors in the pathogenesis of disease, used to explain etiology

Agent

microorganism capable of producing an infectious disease

• Must be present for infectious to occur

• Necessary but not sufficient to cause disease

Infectivity

the ability to enter and multiply in a healthy host and thus cause disease

important takeaway from iceberg of infection

The majority of interactions with viruses do not result in infection. Most infections are asymptomatic or sub-clinical

Pathogenicity

capacity of the agent to cause active clinical disease (have symptoms)

Phases of symptoms in clinical disease

mild → moderate → severe → death

Virulence

degree of pathogenicity, how bad disease is after infection occurs

Vertical transmission

mother → child disease transfer 

• Mammary glands/milk

• placenta/blood

• Vagina (birth canal)/secretion and blood

Horizontal transmission

infected individual → susceptible individual

direct and indirect

Direct horizontal transmission

person to person, direct contact (skin to skin, body fluids) or droplets (sneezing, coughing, talking)

• A → B

• A directly transmits agent to B

Direct contact

Droplets

Indirect horizontal transmission

Through intermediary source

• A → B → C

• A indirectly transmits agent to C through an intermediary source (B)

Airborne

Vector borne

Vehicle borne

Direct contact, direct horizontal transmission

skin to skin

body fluid transfer

Droplets, direct horizontal transmission

sneezing

coughing

talking

Airborne, indirect horizontal transmission

Infectious agents are carried by dust or droplet nuclei suspended in air

Vector borne, indirect horizontal transmission

a living insect or animal involved with transmission of the disease agent

Vehicle borne, indirect horizontal transmission

an inanimate object involved with transmission of the disease agent

– water

– food

– soil

– fomites

Fomites

Objects or materials which are likely to carry infection (doorknob, clothing, unsterilized medical equipment, etc.)

Host

person or animal that is susceptible to disease → characteristics of the host influence the severity of the disease + how well body can fight off infectious agent

• 2 defence mechanisms

Innate responce

immediate, nonspecific to pathogen entering

Adaptive response

longer time, takes over when innate response can’t take down germs

• Identifies germs/more specific

• Has memory = exposure to pathogen, learn/create antibodies, better/quicker response upon next exposure

Active immunity

develops in response to infection or vaccination

Natural active immunity

antibodies develop in response to previous infection by the agent → long lasting immunity

Artificial active immunity

 antibodies developed in response to a vaccination → long lasting immunity

Passive immunity

develops after you receive antibodies from someone/somewhere else

Natural passive immunity

• antibodies moved from mother to baby (ex. through breast milk/in utero)

  • RSV vaccine given to mom while pregnant confers some immunity to the child once born

Artificial passive immunity

• antibodies received from a medicine, steal from a person with antibodies to put in person without antibodies 

  • ex. Convalescent plasma therapy

Environment

The domain in which disease-causing agents may exist, survive, or originate (must be conducive to keeping the infectious agent alive so it can spread)

• physical environment (weather, temperature, humidity, etc.)

• social environment (behavioral and cultural characteristics of a group of people)

Reservoir

where the infectious agent hides out, lives/grows/multiplies

Environmental reservoir

plants + soil (carries tetanus) + contaminated food or water

Animal reservoir

hella animals ex. Mosquitos carry malaria

Human reservoir

ex. Humans carry viruses

Acute clinical cases

person infected with disease, becomes ill/have symptoms, and can then pass on infection

Carriers + problem

• person is infected but is not ill/does not have symptoms, still can pass on

  • Scary: may present more risk for disease transmission because their contacts are unaware of their infection and their activities are not restricted by illness.

Asymptomatic/healthy carriers

• inapparent infections, never develop illness but can transfer infection to others

  • Ex. polio

Incubatory carriers

• people going to become ill but begin transmitting their infection before their symptoms start

  • Ex. measles, HIV

Convalescent carriers

•  people continue to be infectious after their recovery from illness

  • Ex. salmonella

Chronic carriers

• people who continue to harbor infections for a year or longer after their recovery

  • Ex. Mary Mallon/typhoid mary

Super-spreaders

someone who is responsible for infecting many people

80/20 rule

in any given outbreak, 20% of the individuals within any given population are thought to contribute at least 80% to the transmission potential of a pathogen

Incubation period

(occurs in subclinical phase of disease) time interval between exposure to an infectious agent and the appearance of the first signs and symptoms of disease

• Longer incubation period = potential for people to be superspreaders

• Use to determine/narrow down agent + time and circumstance of exposure

Generation time

time interval between exposure to an infectious agent and the maximal infectivity of the host

Epi curves

graphic plotting of the distribution of cases by time of onset

• Provides info on what infectious agent may be present

• Goal to “flatten the curve”

Common source epidemic

focus on time aspect of an epidemic, outbreak due to exposure to noxious influence that is common to the individuals in the group

• Identify source/where health event is coming from, determine who got sick, determine if all were exposed to the agent

• wrong place at wrong time, group of people share common source of infection

• Only last 1 incubation period (i.e. due to 1 source and rapid identification of source people don’t typically get reinfected)

• Examining epi curve: sharp rapid clustering then quickly drop off

Common source —> point source

Persons are exposed to the same common source over a brief period (e.g., single meal or event).

• The majority of cases occur within one incubation period.

• Ex.  ecoli at chipotle

Common source —> continuous common source

outbreak lasts longer than single incubation period, common source of exposure harder to determine = more incubation periods

• Ex. john snow cholera

Common source —> intermittent source

Similar to continuous but exposure is intermittent

• Multiple peaks

• No relation to the incubation period (e.g. contaminated food product sold over period of time)

Propogated source

!person to person transmission! can’t always identify source of epidemic

• Curve: long in duration, lasts more than 1 incubation period, no external source identified

Attack rate

type of incidence rate, used when disease occurs over a very short period of time (ex. Food borne illnesses)

New cases

 initial case(s) + secondary cases

Initial cases

case that first comes to attention of public health authorities

Coprimaries

cases related so closely in time that they are considered to belong to same generation of cases (getting sick in the same incubation period as the index cases)

Secondary case

person(s) who become ill after a disease has been introduced into a population and who become infected from contact with a primary case

Secondary attack rate

type of incidence rate, measuring infectivity - An index of the spread of disease in a family, household, dwelling unit, dormitory or similar circumscribed group

Case fatality rate

how deadly the disease is, what proportion of the individual that get sick with the disease die (info on virulence of the disease within a specific population) → PEOPLE WHO HAD THE DISEASE IS THE DENOMINTOR

Cause specific mortality rate

How many people in the population died of disease X? → ENTIRE POPULATION IS DENOMINATOR

Basic reproductive rate (R0)

 how many people on average will a sick person go on to infect, looking at transmissibility/contagiousness of the disease

Herd immunity

a large proportion of the population is vaccinated/not at risk of the disease due to previous infection. Protects people who can't receive vaccine, at lower risk of coming in contact with someone who has the disease

• Percentage of population needed to be vaccinated depends on r naught of the disease

How/when pandemics end (5)

• No more susceptible individuals (everyone who was susceptible got the disease)

• No more exposure to the source 

• No more source of contamination

• People decrease susceptibility 

• Pathogen becomes less pathogenic

1. Establish the existence of an outbreak

• Done by counting cases → descriptive epidemiology

  • Are the cases sporadic or an epidemic outbreak?

  • Are the cases actually increasing because there is an outbreak or is there a different explanation

2. Verify the diagnosis

Review clinical findings

3. Construct a working case definition

• Standard set of criteria for being diagnosed with a specific disease

  • Includes clinical criteria

  • Does not include exposure or risk factor evaluating

4. Find cases systematically and record information

• Passive surveillance: sending letter describing the situation and asking for reports of similar cases

• Active surveillance: telephoning or visiting the facilities to collect information on any additional cases

• Creating line listings

5. Perform descriptive epi

Characterize based on person (race, sex, age), place (create spot maps), and time (creating epi curve + calculate incubation period)

6. Develop hypothesis

• Must be testable

• Could address source of the agent, the mode (and vehicle or vector) of transmission, exposures that caused the disease

7. Evaluate hypotheses epidemiologically

Compare with established facts

Use analytic epi 

• Cohort studies → relative risk & attack rates

• Case-control studies → odds ratio

8. Reconsider, refine, and re-evaluate hypothesis

number 8