Epidemiology Variables: Time, Place, Person

Descriptive Epidemiology

• Descriptive epidemiology involves characterizing disease occurrence by time, place, and person factors to identify associations and hypotheses about disease sources and routes of transmission
• This step is crucial in outbreak investigations to develop control measures and prevent further cases

Time Factors

• Epidemic curves are used to characterize disease occurrence over a short period of time for a specific population event
• Epidemic curves are histogram graphs that display the number of cases over time
• The shape of the epidemic curve can indicate the type of outbreak, such as a point source outbreak or a propagated outbreak
• Point source outbreaks have a sharp rise and decline, while propagated outbreaks have a more gradual increase and decrease in cases
• The incubation period of a disease can be determined from the epidemic curve, and knowing the incubation period can help identify the time of exposure

Seasonal and Secular Trends

• Seasonal cycles are repeating patterns of disease occurrence over a specific period of time
• Seasonal cycles can be caused by vectors, diet, temperature, and other environmental factors
• Secular trends are long-term patterns of disease occurrence over time
• Secular trends can be caused by changes in disease reporting, case definition, or diagnostic techniques
• Examples of seasonal cycles include foodborne diseases, leptospirosis, and norovirus
• Examples of secular trends include changes in cancer rates over time

Place Factors

• Characterizing disease occurrence by place involves identifying spatial patterns of disease distribution
• Place can be defined as country, state, county, city, neighborhood, household, or other geographic unit
• Spatial distribution can be influenced by population density, age distribution, breed or race distribution, exposure to occupations, and vectors
• John Snow's study on cholera in London is a famous example of using spatial distribution to identify the source of a disease outbreak

Person Factors

• Characterizing disease occurrence by person involves identifying demographic patterns of disease distribution

• Person factors include age, sex, occupation, and behavior

• Person factors can influence disease occurrence and transmission

• Examples of person factors include the distribution of disease among different age groups or occupations### Disease Spatial Distribution

• The difference in disease spatial distribution is related to soil type, level of moisture, and other environmental factors.

Characterizing Disease by Person Factors

• Person factors can include: age, sex, race, ethnic group, marital status, occupation, economic group, religion, habits, personal habits, travel history, disease symptoms, immunization status, and previous diseases.
• These factors can provide important clues as to why particular individuals are affected and others are not, and how transmission is occurring.

Importance of Characterizing Disease

• Characterizing disease by epidemiology variables (time, place, and person) can yield important information about disease causation and suggest ways to control the disease.
• This is true at the national, international, and local levels (e.g., specific food production facility, restaurant).

Example: Brucellosis

• Brucellosis cases tend to be male and of middle age.
• The age and gender distribution of brucellosis cases provided clues as to how individuals were coming to be infected (e.g., occupational exposure to infected animals).

Key Principles

• Disease occurrence usually involves an interaction of the host, agent, and environmental factors.
• Good descriptive epidemiology of time, place, and person factors is essential to understand how a disease can be transmitted and spread in a particular facility.