Preventive Medicine: Epidemiology 3

Surveillance of Disease

• Surveillance is the entire process of collecting, analyzing, interpreting, and reporting data on the incidence of death, diseases, and injuries and the prevalence of certain conditions.
• The Centers for Disease Control and Prevention (CDC) is the federal agency responsible for the surveillance of most types of acute diseases in the United States.

Creating a Surveillance System

• A surveillance system requires clear objectives regarding the diseases or conditions to be covered.
• The objectives for each surveillance item should be clear, and the criteria for defining a case of a reportable disease or condition must be known.
• The intensity of the planned surveillance (active vs. passive) and duration of the surveillance (ongoing vs. time-limited) must be known in advance.

Methods and Functions of Disease Surveillance

• Surveillance may be either passive or active.
• Passive surveillance is when physicians, clinics, laboratories, and hospitals report disease cases, while active surveillance involves periodic telephone calls, electronic contact, or personal visits to obtain data.
• The percentage of patients with reportable diseases that are actually reported to public health authorities varies considerably.

Establishment of Baseline Data

• Usual (baseline) rates and patterns of diseases can be known only if there is a regular reporting and surveillance system.
• Epidemiologists study the patterns of diseases by the time and geographic location of cases and the characteristics of the persons involved.

Evaluation of Time Trends

• Secular (long-term) trends in disease are usually different from those of outbreaks or epidemics and often carry greater significance.
• Seasonal variation is a common pattern in infectious diseases, with periods of highest incidence usually depending on the route of spread.

Identification and Documentation of Outbreaks

• An epidemic, or disease outbreak, is the occurrence of disease at an unusual (or unexpected) frequency.
• To determine that the level of a disease is unusual, the usual rates of the disease must be known, and reliable surveillance must show that current rates are considerably elevated.

Surveillance for Bioterrorism

• Epidemiologists have worried about the use of biologic agents for military or terrorist purposes.
• Special surveillance techniques are being developed to enable rapid detection of major increases in the most likely biologic agents.

Evaluation of Public Health and Disease Interventions

• The introduction of major interventions intended to change patterns of disease in a population should be followed by surveillance to determine if the intended changes were achieved.
• Surveillance can help detect the failure of a vaccine to produce satisfactory immunity or the failure of people to use the vaccine.

Setting of Disease Control Priorities

• Data on the patterns of diseases for the current time and recent past can help governmental and voluntary agencies establish priorities for disease control efforts.

Study of Changing Patterns of Disease

• By studying the patterns of occurrence of a particular disease over time in populations and subpopulations, epidemiologists can better understand the changing patterns of the disease.

Investigation of Epidemics

• Definition of an epidemic: an unusual occurrence of a disease, derived from Greek roots meaning "upon the population"
• Can be a single case of a disease that has been eliminated or eradicated from a region

Terms for Disease Patterns in Animals

• Epizootic: a disease outbreak in an animal population
• Enzootic: a disease deeply entrenched in an animal population but not changing much

Attack Rate

• Measures disease frequency
• Calculated as: (Number of new cases / Number of persons exposed) x 100
• Expressed as a percentage

Procedures for Investigating an Epidemic

• Establish the diagnosis
• Establish an epidemiologic case definition
• Determine if an epidemic is occurring
• Characterize the epidemic by time, place, and person

Characterizing an Epidemic

• Time: use an epidemic time curve to show the number of new cases over time
• Place: create a spot map to show the location of cases
• Person: gather information on age, gender, race, ethnicity, religion, and other personal characteristics of affected individuals

Developing Hypotheses

• Identify the source of infection
• Determine the pattern of spread (common-source, propagated, or mixed)
• Identify the mode of transmission### Modes of Transmission
• Respiratory transmission
• Fecal-oral transmission
• Vector-borne transmission
• Skin-to-skin transmission
• Transmission through exchange of serum or other body fluids
• Transmission through contact with fomites (objects that can passively carry organisms)

Laboratory Studies

• Cultures from patients and possible vehicles (e.g., food, water)
• Stool examinations for ova and parasites
• Serum tests for antibodies to suspected organisms
• Tests for nonmicrobiologic agents (e.g., toxins, drugs)

Case-Control Studies

• Assemble cases (people with the disease) and controls (people without the disease)
• Question both groups about specific foods eaten before the outbreak
• Calculate the percentage of controls who consumed each food item and subtract from the percentage of cases who consumed it
• Identify the food item with the greatest difference in consumption percentage between cases and controls as the most likely risk factor

Control Measures

• Sanitation: modify the environment to remove the pathogenic agent
• Prophylaxis: use a barrier to infection (e.g., vaccine, immunization)
• Diagnosis and treatment: treat infected individuals to prevent spread
• Control of disease vectors (e.g., mosquitoes, ticks)

Follow-up Surveillance

• Evaluate the effect of control measures
• Detect subsequent outbreaks
• Active surveillance is more reliable than passive surveillance

Investigation of an Outbreak

• Define the case criteria
• Conduct interviews with affected people
• Analyze the epidemic time curve
• Identify possible risk factors (e.g., food handlers working while ill)
• Consider past knowledge from similar outbreaks and disease surveillance
• Make recommendations based on the investigation's findings

Surveillance of Disease Activity

• Surveillance is the foundation of public health control of disease and can be active or passive.
• The functions of surveillance include determining baseline rates of disease, detecting outbreaks, and evaluating control measures.

Investigation of Disease Outbreaks

• Investigation of disease outbreaks is a primary function of public health agencies.
• Practicing physicians contribute to detecting and reporting acute outbreaks.
• The standard approach to investigating disease outbreaks involves:
  • Making a diagnosis
  • Establishing a case definition
  • Determining whether there is a definite outbreak

Characterizing Outbreaks

• If an outbreak is occurring, cases are characterized by:
  • Time (using an epidemic time curve)
  • Place (determining rates in people who live and work in different locations)
  • Person (determining personal characteristics and patterns of people involved in the outbreak)

Developing and Testing Hypotheses

• Hypotheses are developed regarding:
  • Source of infection
  • Pattern of spread
  • Mode of transmission
• Hypotheses are tested using:
  • Laboratory data (e.g., cultures, paired sera, analysis for toxins)
  • Research methods (e.g., case-control studies)

Control Measures and Follow-up

• Control measures and follow-up surveillance are initiated as soon as practical.
• This is done to prevent further transmission of the disease.

Learn about the process of disease surveillance, its importance, and how it promotes public health. Discover the role of Centers for Disease Control and Prevention (CDC) in the United States.