#4: Epidemiology and communicable diseases

Epidemiology

-Study of the causes, distribution, and control of diseases in a population.

-study of the distribution and determinants of disease or other health-related outcomes in human populations, and the application of that study to controlling health problems

-looks at cause and effect

Main epidemiological concepts

-Causality

-Risk

-Rates of occurrence

Causality

-One event is the result of another event

-Links between contributing factors and resulting states of health and illness

Criteria for causality

-Consistency of relationship

-Strength of association

-Specificity

-Temporal relationship

-Coherence with current knowledge

Consistency of relationship

-The association between the supposed cause and its effect is consistent and always occurs in the same direction.

-The condition must occur when the factor is present, and not occur when it is absent

Strength of association

-The greater the correlation between cause and effect (correlation is a statistical measure), the greater the possibility the relationship is a causal one.

-Look at the same situation again and again, and see if one thing is correlated with the other, again and again. If so, that would be a strong correlation and lead us to determine causality

Specificity

-The supposed cause always creates the same effect

-This is usually a very strong criteria for infectious diseases, but less so when we are trying to figure out causation for noninfectious diseases. Smoking can cause all sorts of problems, ie. Heart attacks, lung cancer, bladder cancer, etc

Temporal relationship

The supposed cause always occurs before the effect.

Coherence

-The supposition that one event causes another is coherent with the established body of scientific knowledge.

-What you think is the cause based on your data has to match up with other existing knowledge

Risk

-Probability that a given individual will develop a specific condition

-Risk may be absolute or relative

Absolute risk

Probability that anyone in a given population will develop a particular condition

Relative risk

Probability that someone in a group of people with a particular characteristic will develop the condition when compared to people without that characteristic

Rates of occurrence

-Statistical measures that indicate the extent of a health problem within a group

-Allows comparison between groups of different sizes or divide the instances of an event by the population at risk

-Multiply these proportions by 1,000 or 100,000 to make them easier to understand

Morbidity

-another term for illness.

-A person can have several co-morbidities simultaneously. Morbidities are NOT deaths.

-Rate of diabetes in a population

-Prevalence of diabetes

Mortality

-is about actual deaths that occur

-Number of deaths attributed to diabetes in a population

-Cause-specific death rate

Incidence

-Number of new cases of a particular condition in a specified time period

-Measure of disease RISK (proportion of population who will get the disease)

Prevalence

-Total number of people affected by a particular condition at a given point

-how many people have the illness or disease at a certain time

-Measure of disease BURDEN

-often used to determine the level of morbidity in a population.

Case fatality

-number of people who develop a health problem and will die as a result of it

-Shows how deadly a disease is

Prevalence is impacted by

-Incidence (how many new people got the disease)

-Duration (do people get cured or do they live with it forever)

-Deaths (how many people actually die of the disease)

Incidence rate

-measure of how often new cases of a disease or condition occur in a specific population over a defined period

-divide by total population then multiply by 100,000

Prevalence rate formula

total number of cases/total population at risk x 1,000

Prevalence rate

statistical measure that tells you the proportion of a population that is affected by a particular condition at a specific point in time

Crude death rate

the total number of deaths during the year, divided by the total population at midyear times

Cause-specific annual death rate

the number of deaths from a specific cause during a given year, divided by the total population at midyear times

crude death rate formula

total number of deaths during year/total population at midyear x 1,000

cause-specific annual death rate

numbers of deaths from specific cause during year/total population at midyear x 1,000

Case fatality rates

Percentage of persons with a condition who die of that condition

Survival rates

Proportion of people with a condition remaining alive after a specified period of time

Survival time

Average length of time from diagnosis to death

Epidemiologic process

-Defining the condition

-Determining the natural history of the condition (assessment and diagnosis)

-Identifying strategic points of control (planning)

-Developing control strategies (planning)

-Implementing control strategies (implementation)

-Evaluating the effects of control strategies (evaluation)

Descriptive epidemiology

It focuses on identifying patterns and trends in health issues to understand who is affected, where the condition is happening, and when it occurs

Components of descriptive epidemiology

-Person: the characteristics of people who develop the condition (age, gender, race and ethnicity, economic status, behaviors, etc.)

-Place: where the condition tends to occur (Cities or regions)

-Time: when the condition occurs (Seasonal patterns, etc)

Types of descriptive studies

-Prevalence surveys

-Cross-sectional studies

-Case reports

-Case series

-Surveillance studies

Analytic epidemiology

-Study of factors contributing to health states

-Suggest mechanisms of causation

-Generate etiologic (causal) hypotheses

-Test those hypotheses

Three categories of analytic epidemiology

-Ecological studies (quick, easy, uses existing data)

-Case-control studies (retrospective, case comparison)

-Cohort studies (prospective / retrospective)

Experimental epidemiology

-A subset of analytic epidemiology; exposure to certain variables is manipulated by the researcher to determine the results

-Used to test the effectiveness of interventions

-Intervention may be positive or negative

-Application of an intervention by the researcher is called a trial

Epidemiologic models (3)

-The epidemiologic triad/triangle

-The web of causation model

-Determinants-of-health models

Epidemiological triad

When data is collected with respect to a 3 things:

      -Host-or organism harboring the disease (the “who” of the Triangle)

      -Agent-or microbe that causes the disease (the “what” of the Triangle)

      -Environment-or those external factors that cause or allow disease transmission (the “where” of the Triangle)

Web of causation model

-Many complex factors that contribute to illness, or health, and there are direct and indirect causes of disease - (more than the 3 of the epidemiologic triangle)

-Allows the epidemiologist to map interrelationships among factors contributing to development (or prevention) of a particular health condition

-We can try to intervene at the different points of the web where there are causes of illness or blockages to health

Determinants of health modesl

-Model we use most often in population health

-Examine factors in specific categories and their contribution to the development of health-related problems

-Common framework used around the world to depict the various structural and intermediary level social determinants of health

-When it comes to population health and examining inequities, the social determinants of health are where we will find that interventions matter the most and improve health outcomes the most

Surveillance

-Systematic collection, analysis, and interpretation of data in order to protect the public's health

-Timely dissemination of the data to those who need to know in order to protect public’s health

Communicable disease

A disease can spread from one person or animal to another in several ways:

-directly, through physical contact with bodily fluids or secretions

-indirectly, through contaminated items like drinking glasses, toys, or water

-via vectors such as flies, mosquitoes, ticks, and other insects

-Many must be reported to local health department by law

Chain of infection

How the communicable disease gets around

Causative agent

biological organism capable of being directly or indirectly transmitted from person to person, animal to animal, animal to person, or the environment (air, water, food etc) to person

How to break causative agent chain

-Diagnosis and Treatment

-Antimicrobial stewardship

Reservoir

-Hosts or environments where causative agent lives and reproduces

-Dirty surfaces/equipment

-Soil

-Food

-Water

-People

-Animals/Insects

How do we break reservoir chain

-Cleaning, disinfection and sterilization

-Infection prevention policies

-Pest control

Portal of exit

Route that the agent takes to leave the reservoir

How do we break portal of exit chain

-Hand Hygiene

-PPE

-Control of aerosols and splatter

-Respiratory etiquette

-Waste disposal

Mode of transmission

How the causative agent makes it’s move

Types of transmission

-Contact (direct and/or indirect)

-Airborne – via air or particles

-Droplet

-Vector-borne – a vector (mosquito, tick) carries virus from reservoir to susceptible host

-Common vehicle-a shared source that can transmit infectious agents to multiple people (contaminated water supply, infected food from a farm distributed to stores)

How do we break the mode of transmission chain

-Hand hygiene

-PPE (Personal Protective Equipment)

-Food safety

-Cleaning, disinfection, sterilization

-Isolation

Portal of entry

Where pathogen enters next susceptible host to cause another infection

How do we break the portal of entry chain

-Hand hygiene

-PPE

-Personal hygiene

-First Aid

-Removal of catheters and tubes

Susceptible host

A person who is potentially vulnerable to that particular causative agent

Host factors that influence the outcome of an exposure include:

-the presence or absence of natural barriers

-the functional state of the immune system

-the presence or absence of an invasive device

How do we break the susceptible host chain

-Immunizations

-Treatment of underlying disease

-Health Insurance

-Patient Education

Stages of infection

-incubation period

-prodromal stage

-illness stage

-convalescent stage

Incubation period

Pathogen begins replication but does not yet cause symptoms

Prodromal stage

Symptoms begin to appear but can be non-specific

Illness stage

Pathogen reproduces and disseminates rapidly causing signs/symptoms

Convalescent stage

Affected tissues are repaired, and manifestations resolve

Endemic

A moderate ongoing occurrence in a given location

Epidemic

infectious disease that spreads rapidly, or the rate of disease is exceeding the usual endemic level

Pandemic

when an epidemic spreads beyond continents, across the entire globe, and takes more lives and affects more people than an epidemic

Types of immunity

-herd immunity

-natural immunity

-acquired immunity (either active or passive)

Live attenuated vaccines

-contain a version of the living microbe that has been weakened in the lab so it can’t cause disease but is still live

-closest thing to a natural infection

-considered the “teachers” of the immune system

Inactivated vaccines

-contain a dead version of the microbe, which was usually killed via chemicals, heat or radiation.

-The dead microbes can’t mutate back to their disease-causing state but the body recognizes their receptors and that still stimulates the immune response we want.

-Most inactivated vaccines stimulate a weaker immune system response than do live vaccines

Immunity

-protects us from the communicable disease.

-main primary prevention strategy

Herd immunity

-Protection due to the immunity of most community members, making exposure unlikely (previous infection / vaccination)

-level of disease resistance of a community or population

Natural/innate immunity

-Natural defense mechanisms of the body resist specific antigens or toxins

-When immune system is weak, opportunistic infections can develop from things that would otherwise not make a person sick

Acquired immunity

-Immunity we are NOT born with

-When pathogens are introduced to the body from a vaccine or a disease, the body learns to target those pathogens in the future by making new antibodies.

-can be either active or passive

-is our goal with most of the interventions in the realm of immunity.

Active acquired immunity

-When our body produces antibodies in response to infection or immunization

-Requires an exposure to agent

What are the two ways we can get acquired immunity?

1. When our body produces antibodies in response to getting an infection with the causative agent itself

2. Getting an immunization

Passive acquired immunity

-Antibodies are given/transferred to the body

       -Via mother to newborn and all the body does is store the antibodies that are given to it.

       -Through administration of antibodies, plasma proteins, or antitoxins

-Administered to someone (usually parenterally) who needs to be immune and we don’t have a way or enough time to let their body develop it’s own response.

-does not require an exposure to the infectious agent

Active immunity

Induced after exposure to foreign antigens. Slow onset. Long-lasting protection (memory)

Naturally acquired active immunity

-a person is exposed to a live pathogen, and develops a primary immune response, which leads to immunological memory

-Person is making his or her own antibodies

-Natural active immunity is the MOST effective and longest lasting type of immunity

Artificially acquired active immunity

a person can build resistance to a disease following an immunization

Passive immunity

Based on receiving preformed antibodies from another host. Rapid onset. Short lifespan of antibodies

Naturally acquired passive immunity

Ex: The placental transfer of Immunoglobulin G (IgG) from mother to fetus during pregnancy

Artificially acquired passive immunity

immunity protection acquired by giving a person an injection or transfusion of antibodies made by someone else

mRNA vaccines

-work by manufacturing a look-alike for the tiny spike protein on a virus’s outer shell

-immune system knows exactly how to respond and builds up the ability to protect against the virus with the exact antibodies.

-mRNA vaccines were developed specifically for viruses because a virus always has this coat of proteins made up of the messenger RNA

-First approved in COVID 19 vaccines

Primary prevention

Prevent occurrence of new infection

Secondary prevention

Find out who has the disease

Tertiary prevention

-Treat infection

-decrease complications

-rehabilitation