7.1 Environmental Epidemiology & Outbreaks

7.1 Environmental Epidemiology & Outbreaks

Environmental epidemiology is the study of how environmental exposures impact public health and the distribution of disease in populations. As a Registered Environmental Health Specialist (REHS), understanding how diseases spread and how to investigate outbreaks is fundamental to protecting public health.

Principles of Environmental Epidemiology

Epidemiology relies on the systematic collection and analysis of health data to identify patterns, causes, and effects of health and disease conditions. The core focus is on identifying the source of an outbreak, controlling its spread, and preventing future occurrences.

Disease Transmission Models: The Epidemiologic Triad

The traditional model for infectious disease causation is the Epidemiologic Triad, which consists of three components:

1. Agent: The microbe (e.g., bacteria, virus, parasite) or chemical that causes the disease.
2. Host: The organism (human or animal) harboring the disease. Host factors such as age, immune status, and behavior affect susceptibility.
3. Environment: External factors that allow disease transmission, such as climate, sanitation, and living conditions.

For a disease to occur, there must be an interaction between the agent and a susceptible host in an environment that supports transmission.

Modes of Transmission and Reservoirs

Understanding how diseases move from the source to a new host is critical for implementing control measures:

• Reservoir: The natural habitat where an infectious agent lives, grows, and multiplies (e.g., humans, animals, environmental sources like water).
• Vehicle: Inanimate materials or objects involved in disease transmission, such as water, food, or biological products (e.g., blood).
• Vector: A living organism, usually an arthropod (like mosquitoes or ticks), that transmits an infectious agent from an infected animal or human to another.
• Fomites: Inanimate objects that can carry infectious agents, such as doorknobs, cutting boards, or medical equipment.

Outbreak Investigation Steps (CDC 10-Step Protocol)

When an outbreak occurs, environmental health professionals follow a systematic approach to identify the cause and stop the spread. The CDC outlines a 10-step protocol for outbreak investigations:

1. Prepare for field work: Ensure you have the necessary supplies, laboratory equipment, and administrative approvals.
2. Establish the existence of an outbreak: Confirm that the observed number of cases exceeds the expected baseline.
3. Verify the diagnosis: Review clinical findings and laboratory results to ensure the disease has been properly identified.
4. Construct a working case definition: Establish standard criteria for deciding whether a person has a particular disease. A case definition includes clinical criteria, time, place, and person.
5. Find cases systematically and record information: Actively search for additional cases to define the true scope of the outbreak.
6. Perform descriptive epidemiology: Organize and characterize the data by time, place, and person (e.g., creating an epidemic curve).
7. Develop hypotheses: Based on descriptive data, propose potential sources and modes of transmission.
8. Evaluate hypotheses epidemiologically: Conduct analytical studies (cohort or case-control) to test the hypotheses.
9. Implement control and prevention measures: Take immediate action to control the source, break the chain of transmission, and protect susceptible hosts.
10. Communicate findings: Prepare a formal report and share findings with public health officials, stakeholders, and the public.

Epidemiological Calculations

Quantifying risk and disease frequency is a key component of epidemiological investigations. Environmental health specialists must be comfortable with the following calculations:

Attack Rate

The attack rate is a specialized form of incidence rate used during an outbreak. It calculates the proportion of people who became ill after a specific exposure.

Formula: Attack Rate = (Number of people who became ill) / (Total number of people exposed)

Relative Risk (RR)

Relative risk is used in cohort studies to compare the risk of disease in an exposed group to the risk in an unexposed group.

Formula: RR = (Incidence rate in the exposed group) / (Incidence rate in the unexposed group)

• RR = 1: No association between exposure and disease.
• RR > 1: Exposure is associated with an increased risk of disease.
• RR < 1: Exposure is associated with a decreased risk (protective factor).

Odds Ratio (OR)

The odds ratio is used in case-control studies (when the total population is unknown) to estimate the relative risk. It compares the odds of exposure among cases (sick individuals) to the odds of exposure among controls (healthy individuals).

Formula: OR = (Odds of exposure in cases) / (Odds of exposure in controls) = (a * d) / (b * c) (where a=exposed cases, b=exposed controls, c=unexposed cases, d=unexposed controls).

Incidence vs. Prevalence

It is critical to distinguish between these two measures of disease frequency:

By mastering these concepts, models, and calculations, you will be well-prepared to lead environmental health investigations and implement effective control measures to safeguard public health.