Lecture 1 & 2: Epidemiology introduction, history, and definitions; Epidemiology indices

Define Epidemiology (LMU-CVM definition)

The study of the health of populations

define health

a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity

define disease

o Absence of Health

A non-compensated perturbation of one or several functions of the host. Pathological condition occurring in a susceptible host.

what is the equation for attack rate?

AR = (# of new cases in the time period) / (population at risk at beginning) × 10^n

define crude mortality rate

the mortality rate from all causes of death for a population during a specified time period

define case fatality rate

The proportion of animals/persons with a particular condition who die from that condition

equation for calculating case fatality rate

(# of deaths among those cases)/(# of incident cases)

define cause-specific death rate

The number of deaths from a specified cause per person/animal-years at risk

equation to calculate cause-specific death rate

[resident deaths in a specific geographic area (country, state, county, etc.)]/years at risk

example:

The cause-specific death rate from colic on a particular farm was found to be 0.7 deaths/100 horse-years

define pandemic

o An epidemic that becomes very widespread and affects a whole region, a continent, or the world due to a susceptible population

o By definition, a true pandemic is associated with high morbidity and significant mortality

define endemic/outbreak

o "The occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time."

An outbreak may occur in a restricted geographical area, or may extend over several countries

Can also be a single case of a communicable disease long absent from a population...esp. one of high mortality

• Botulism, rabies, etc.

What is a case definition?

a standard set of criteria used to decide whether an individual has a particular health condition, especially in outbreak or epidemic situations

Why is a case definition necessary during an outbreak or epidemic?

It is necessary because the cause of symptoms may not always be immediately clear, and a case definition helps to identify who is included as a case in the investigation.

What key elements does a case definition define?

A case definition defines a case in terms of person, place, and time, using criteria specific to the investigation.

What are the characteristics that a case definition can be?

A case definition can be sensitive, specific, and influenced by the number of cases.

What is the importance of consistent case definitions?

Consistent case definitions ensure reliable comparisons and accurate identification of cases over time, and they help avoid confusion or misclassification.

How did the CDC change the case definition for AIDS between 1992 and 1993?

The CDC added a laboratory test result to the case definition of AIDS. If patients had a CD4 count less than 200 per microliter, regardless of symptoms, their illness met the new definition of AIDS.

What is the Epidemiological Triad?

The Epidemiological Triad is a model used to understand the factors that contribute to the spread of disease. It consists of three components: the host, the agent, and the environment.

What are the three components of the Epidemiological Triad?

1. Host - The individual or organism affected by the disease.

2. Agent - The microorganism or pathogen that causes the disease.

3. Environment - The external factors that influence the host's susceptibility or exposure to the disease.

What role does the host play in the Epidemiological Triad?

The host is the individual or organism that is susceptible to the disease. Host factors include genetics, immunity, and behaviors that can influence disease risk.

What is the agent in the Epidemiological Triad?

The agent is the microorganism or pathogen (e.g., bacteria, viruses, fungi) responsible for causing the disease. Its characteristics influence its ability to infect and cause harm.

How does the environment factor into the Epidemiological Triad?

The environment includes external factors that influence the occurrence and spread of disease, such as climate, sanitation, and social conditions.

How does the Epidemiological Triad help in disease prevention?

By understanding and addressing the interactions between the host, agent, and environment, interventions can be developed to prevent or control disease spread.

What is the natural history of a disease?

The natural history of a disease refers to the course of the disease from its initial onset to its final clinical endpoints, including recovery, chronic illness, or death.

Why is it important to understand the natural history of a disease?

While a complete understanding of the disease's natural history is not always necessary for treatment and control, it facilitates better interventions by identifying key stages for intervention.

Can treatment and control of a disease occur without a full understanding of its natural history?

Yes, treatment and control can occur without a complete understanding, but knowledge of the natural history enhances the ability to intervene effectively at critical stages.

How does the natural history of a disease aid in identifying opportunities for intervention?

By understanding the progression of the disease, healthcare providers can target key points in the disease's course—such as prevention, early detection, or treatment—to reduce morbidity or mortality.

what are common epidemiologic measures?

o Proportion

o Percentage

o Rate

All are ratios

define ratio

The value obtained by dividing one quantity by another

examples of ratios

Rates

Proportions

Percentages

example of ratio calculations

fetal death ratio= (# of fetal deaths)/(# of live births)

define proportion

A type of ratio in which the numerator is part of the denominator

proportion is commonly expressed as what?

a percentage

examples of proportions

Incidence (# new cases)

Prevalence (# cases)

Case fatality (#deaths)

define rate

a rate differs from a proportion because the denominator involves a measure of time

common rates in epidemiology

Incidence rate (# new cases/PAR)

Prevalence rate (# cases/PAR)

Case fatality rate (#death/PAR)

all are expressed as a measure over a specific period of time

factors that cause prevalence to increase

Increase in incidence

Longer duration of the case

In-migration of cases

Prolongation of life of patients without a cure

factors that cause prevalence to decrease

Decrease in incidence

Shorter duration of disease

In-migration of healthy people

Improved cure rate of disease

define prevalence

the number of existing cases of disease in the population during a defined period

define incidence

is the number of new cases of disease that develop in the population during a defined period

What is the definition of "Population at Risk" (PAR)?

Population at Risk refers to those individuals (animal or human) in a population who are capable of developing the disease or condition being studied. It is the denominator in rate calculations.

Why is the concept of "Population at Risk" important in epidemiology?

It is critical for correctly calculating rates and understanding who is susceptible to a disease. It helps identify the true scope of risk in a population.

How can the concept of Population at Risk be confusing?

It can be manipulated depending on how the population and susceptible individuals are defined. For example, determining who is "employable" when calculating unemployment rates.

How do you calculate the "Incidence Rate"?

Incidence rate is calculated by dividing the number of new cases by the population at risk during a specific time period, then multiplying by 10^n.

equation for incidence rate

Incidence rate = (# of new cases)/(total population at risk) × 10^n

What is an example of calculating the Incidence of heartworms over a year?

If 50 dogs were tested for heartworms at the start of 2018 and 10 tested positive by the end of the year, the incidence is 10 new cases, and the incidence rate is 20% (10/50).

what are the variations in prevalence?

Point prevalence

Period prevalence

Lifetime prevalence

How does "Point Prevalence" differ from "Period Prevalence"?

Point Prevalence refers to the number of cases at a single point in time.

Period Prevalence refers to the number of cases within a specific period.

How do you calculate "Attack Rate"?

Attack Rate is an incidence rate applied to a narrow population over a specific, short time period, typically used in epidemics.

How do you calculate morbidity rate?

Morbidity rate is the total number of illnesses due to a disease in a population over a specified period of time, divided by the average population at risk during that period.

equation for calculating morbidity rate

Morbidity rate = (# of illnesses) / (average population at risk) × 10^n

What does "Case Fatality Rate" (CFR) measure?

CFR measures the number of deaths due to a disease among people who are afflicted with that disease.

equation for case fatality rate

CFR (%) = (# of deaths due to disease X) / (# of cases of disease X) × 100

What is a "Risk Factor"?

A risk factor is a characteristic or condition that has value in predicting the likelihood of a disease or health condition occurring in an individual or population.

How are risk factors used in epidemiology?

Risk factors are used to identify characteristics that increase the likelihood of developing a disease, helping to guide prevention and treatment strategies.

What are some examples of risk factors for degenerative joint disease (DJD) associated with hip dysplasia in dogs?

Examples of risk factors include:

- Breed

- Activity level

- Weight

- Sex

- Age

- Nutrition

- Genetics

What was the key finding regarding weight and distractive index in relation to hip dysplasia risk?

Weight and distractive index were found to be significant risk factors for degenerative joint disease (DJD) in all breeds studied.

How does breed affect the risk of degenerative joint disease (DJD) in hip dysplasia?

For example, in the study of hip dysplasia, German Shepherd Dogs had a 4.95 times higher risk of developing DJD compared to dogs of other breeds such as Golden Retrievers, Labrador Retrievers, and Rottweilers.

What role does age play in the risk of degenerative joint disease (DJD)?

In all breeds studied, the probability of having degenerative joint disease (DJD) increased with age.

What was the conclusion regarding hip joint laxity and degenerative joint disease (DJD)?

The study concluded that the probability of having DJD increased with greater hip joint laxity, which can be measured using diagnostic imaging techniques.

What is "Absolute Risk"?

Absolute Risk refers to the rate of occurrence of a specific event (e.g., disease or health condition). It is essentially the same as incidence, representing the total number of cases within a defined population over a period of time.

How do you calculate Absolute Risk?

Absolute Risk is simply the number of cases observed in a population.Example: In 2018, 4 out of 23 cows in a herd developed mastitis. The Absolute Risk of mastitis is 4.

What is "Attributable Risk"?

Attributable Risk refers to the difference in incidence rates between the exposed and unexposed groups. It shows the additional risk that can be directly attributed to the exposure.

How do you calculate "Attributable Risk"?

Attributable Risk is calculated by subtracting the incidence rate in the unexposed group from the incidence rate in the exposed group.

equation for attributable risk

Attributable Risk = Incidence Rate (exposed) - Incidence Rate (unexposed)

What is the relationship between Absolute Risk, Incidence, and Attributable Risk?

Absolute Risk is the total rate of occurrence of a condition (same as incidence). Attributable Risk is the difference between the incidence rates of the exposed and unexposed groups, showing how much of the risk is due to the exposure.

How is "Relative Risk" calculated?

Relative Risk is calculated by dividing the incidence rate among the exposed group by the incidence rate among the unexposed group.

equation for relative risk

Relative Risk = Incidence Rate (exposed) / Incidence Rate (unexposed)