Chapter 10 Field Epidemiology PDF

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Summary

Define field epidemiology.
Discuss the role of the epidemiologist in planning
and establishing an epidemiologic study for
assessing epidemics.
Describe the steps of a field investigation.
Formulate epidemiologic questions that may be
helpful in a field inves...

Description

Chapter 10 Field Epidemiology Introduction - An outbreak is similar to an epidemic, but confined to a smaller geographical area. - Field epidemiology is set under a specific set of conditions o The problem is unexpected o A timely response may be demanded o The intervention of epidemiologists are required to solve the problem o The investigation time is likely to be limited due to timely intervention - Differ from epidemiology in three ways: o Do not start with a clear hypothesis o Public health action often occurs o Field epidemiologists must decide when the available information is determined to take action. - Small numbers may greatly restrict statistical power, collecting biological specimens after may be impossible, and cooperation may be at a low level. Conducting a Field Investigation - Prepare for fieldwork - Establish the existence of an epidemic or outbreak o The presence of a disease outbreak can be difficult to detect o Attack rates – useful for rapidly occurring new cases of disease on a well-defined population - Confirm the Diagnosis o Lab tests are conducted to confirm diagnosis of cases o In some settings, it may not be possible to confirm all cases o False-positive tests are concerning o Some conditions can not be confirmed with lab tests o It is easier to diagnose a bacterial-caused disease than an environmental or occupational disorder - Establish criteria for case identification o Case definition – standard clinical criteria to determine whether a person has a disease  Made to ensure that every case is properly diagnosed. o A loose case definition may be provoked if a fatal communicable disease such as the plague erupts. - Search for Missing Cases o Healthcare facilities must be looked at to see if other people might have the disease or condition under investigation. o Asymptomatic persons or mild cases and their contacts should be evaluated. - Count Cases o Necessary to calculate attack rates in an at-risk population. - Orient the Data According to Person, Place, and Time

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o Person – some diseases/conditions include varying races/ethnicities o Place – where the agent resides and multiplies o Time – time of onset with an epidemic curve  Epidemic curve is a histogram that shows cases plotted by time of onset in an epidemic.  Incubation periods are also important Classify the Epidemic o Mode of transmission is assessed and determined if the disease outbreak is a common-source, propagated, or mixed epidemic o Common-source epidemic curves rapidly increase, peaks, then gradually decreases, occurring in one incubation period. o Propagated – produces successively larger peaks separated by one incubation period curve begins to fall when intervention measures take effect o Continuous source – increase may be more gradual, and the curve more symmetric, covering a longer period of time. Determine who is at risk of becoming a case o Clinical, medical, and lab findings need to be confirmed and analyzed in order for all cases to substantiate the diagnosis. o Searches for human and animal sources of infection in those that are at risk Formulate Hypotheses o Identify the most probable source of the epidemic o Use information earlier in the investigation including, but not limited to case counts, those at risk, sources of epidemic, time, place, or person factors, or attack rates o Grouping of data based on person, place or time characteristics o Hypotheses should be developed for  The source of infection, vehicle of infection, suspect foods, mode of transmission, type of pathogen, time factors, place factors, person characteristics, outside sources of infection, transmission of disease outside of study population, the exposed, unexposed, well, and ill cases/individuals. Test the Hypotheses o If hypothesis is not substantiated, either gather more information, or research hypothesis be rejected. Develop reports and inform those who need to know o Inform the public on communicable diseases o Many officials think that panic will be created, but should not be a reason to withhold information from the public or those at risk Execute control and prevention measures o Basic public health control and preventative measures be employed o Identifies the source and mode of transmission o Immunization programs are the first line of defense in prevention and control of some communicable diseases

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Administration and Planning activities o Government assistance and administration is needed

Investigation of a Food-borne Illness - Most people take care of the matter at home - Therefore, the doctors and medical community and public health department never know because the outbreak is short and individuals recover quickly - However, those caused by salmonella, giardia, and shigella, they do not recover quickly and medical intervention is usually needed - Food-borne illnesses – consumption of contaminated or spoiled foods and liquids o Food infections  Ingestion of pathogens o Food poisoning  Toxins that are present in foods before consumption o Chemical poisoning – chemical agents such as zinc, vitamin A, niacin, nitrite, MSG, or glutamate, pesticides, and cleaners can provoke an illness is consumed in large amounts - Any good epidemiologic investigation should interview both ill and healthy persons Basic Epidemiologic Questions - Each epidemic poses a new range of questions - In whom is the disease present or absent? - What are the sick people doing that healthy people are not? - Are risk factors only seen in certain locations or places? Disease Clusters - A disease cluster is an aggravated series of health events that are grouped together in time and space and are reported to a health agency o Results from a sudden introduction of physical stress, chemical or biological agent, or psychosocial condition. - Can involve accidents or injury of death - A cluster investigation involves looking over the unusual numbers presented of health-related states that are grouped together in time and space. - Involves the occurrence of seemingly unexpected events where no recognized causes exist, sentinel events are occurrences of unexpected health related states that occur from specific recognized causes - A sentinel health event was defined as an unnecessary disease, disability or death that is occupationally related, with its occurrence yielding evidence of a failure in prevention. Guidelines for Investigating Clusters - Four stages; 1. Initial response 2. Assessment 3. Major feasibility study 4. Etiologic investigation - Stages as proposed by the CDC: - Stage I: Initial Contact and Response

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o Collect relevant information from those reporting the disease cluster Stage II Assessment o Three phases – 1.) preliminary evaluation to determine whether an excess of the health issue has occurred 2.) case evaluation to see if disease is present 3.) further evaluation of all suspected cases Preliminary Evaluation o Determine quickly if a health-related event has indeed, occurred quickly and to describe characteristics of the cluster. o To identify unusual aggregations of health outcomes requires knowledge of what is usual based on the distribution of cases in the same location during an earlier time period or in other similar locations during the same time period. o Person-time evaluations are needed in some situations because it allows each person’s contribution to the denominator of the rate calculation to be only as much time as observation in the at-risk population. o Health related state excess is obtained through the P value Case Evaluation o Verify the diagnosis o Obtaining confirmation and reevaluation may not be possible o Ex: laboratory tests are not applied in certain conditions, injuries, or behaviourally caused occurrences. Occurrence Evaluation o Characteristics of the cluster are defined o Geographical and temporal boundaries should correspond to the entire area that could have been exposed to the suspected cause.  If selected boundary is too small, rate calculation will be overestimated  If selected boundary is too big, the rate calculation will be underestimated. o Classify diseases by type of outbreak  Diseases spread person-to-person are biologically based, whereas adverse health outcomes that are not communicable arise from physical or psychosocial stressors or chemical contaminants. Stage III Major Feasibility Study o Associate excess cases with exposure o Review literature and select study design, determine case and control data o Research hypothesis is formulated by identifying the most probable source for the cluster. Stage IV Etiologic Investigation o Criteria for establishing a cause and effect relationship o Epidemiologic report – data tests lab reports information and characteristics Statistical Challenges in Cluster Investigations o Post hoc hypotheses – formulation of a hypothesis after the event such as the excess of cancer

Significance tests are inappropriate Performing the study in a different location but with a similar exposure o Priori hypotheses – without prior knowledge of the level of health events in a specified population o Boundary shrinkage – rates have danger of being overestimated  ...