Field Epidemiology – Flashcards
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Field Epidemiology (252)
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Has been defined as the application of epidemiology under a set of general conditions. The problem is unexpected, a timely response may be demanded, the intervention of epidemiologists and their presence in the field are required to solve the problem, and the investigation time is likely to be limited because of the need for a timely intervention.
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Field Investigations (252)
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Involve acute problems may differ from conventional epidemiologic studies in three important ways. 1st---field investigations often do not start with a clear hypothesis, gather descriptive data on person, place and time may be required before the hypothesis can be formulated and tested. 2nd---acute problems involve an immediate need to protect the public and resolve the concern, hence in addition to data collection and analyses, public health action often occurs. 3---field epidemiologist must decide when the available information is sufficient to take appropriate action.
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Field Investigations Involve Several Activities (252)
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Include abstracting information from a variety of sources, collecting specimens for laboratory test, conducting clinical exams to confirm cases, identifying the natural course of the disease, and producing reports and graphs, however field investigation may pose unique challenges beyond the scientific ideal. Ex....abstracted information can vary considerably in completeness and accuracy, small numbers may greatly restrict statistical power, collecting biological specimens "after the fact" may be impossible and cooperation may be at a low level. Nevertheless, they highest scientific quality possible should be sought under such limitations.
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Conducting a Field Investigation (252)
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Epidemiolgic field investigations generally involve disease outbreaks that are confined to localized areas and have been traced to a common source, outbreaks that have spread from person to person, or a combination of the two.
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Disease Outbreak (252)
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Investigated in the field are typically confined to a limited time period. It is a term used synonymously with epidemic and is technically more correct if the epidemic is confined to localized area. Several steps in the field investigation process are described in order (TABLE 10-1 p253) although some of these steps may be applied simultaneously.
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Establish the Existence of an Epidemic or Outbreak (252)
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First, the epidemiologist must verify that a disease outbreak exists. Local health officials will likely know if disease rates are about those that are normally expected, however, the presence of a disease outbreak can be difficult to detect. On the other hand, some perceived outbreaks may be not real. Misdiagnosed by physicians, for example can give the false impression of an outbreak.
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Attack rates (252-3)
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Appropriate statistics for investigating disease outbreaks because they describe rapidly occurring new cases of disease in a well defined population over a limited time period. Attack rates are cumulative incidence rates expressed as a percentage and are usually calculated with person characters (age, gender, race/ethnicity, and occupation) in order to identify high risk groups.
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Steps for Conducting a Field Investigation (253)
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Table 10-1 --Establish the existence of an epidemic or outbreak --Confirm the diagnosis --Establish criteria for case identification --Search for missing cases --Count cases --Orient the data according to person, place and time --Classify the epidemic --Determine who is at risk of becoming a case --Formulate hypotheses --Test hypotheses --Develop reports and inform those who need to know --Execute control and prevention measures --Carry out administration and planning activities
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Confirm the Diagnosis (253)
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Laboratory techniques employed by trained professionals are required to confirm clinical diagnosis of cases. Assessment of the clinical findings should be done to ensure the correctness and reliability of the findings, however, in some settings, it might not be possible to confirm all cases. If a swift public health response is needed and several people are confirmed cases, it may be sufficient to identify others as cases if they display the same signs and symptoms, nevertheless, this should only be done by an appropriately trained individual.
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Confirm the Diagnosis--False-Positive Test Results (253)
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Can cause considerable concern and give inaccurate information in a disease investigation. Some conditions, injuries, or behaviorally caused occurrences cannot be confirmed with laboratory test. It is easier to diagnose a bacteria caused disease than an occupational or environmental disorder or condition, but these too must be verified. On the other hand, some diseases or conditions are only verifiable with laboratory findings, and some exotic or unique diseases can be verified only through a limited number of specialized lags, including the Centers for Disease Control and Prevention (CDC).
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Establish Criteria for Case Identification (253-4)
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A case definition involves standard clinical criteria that are used to establish whether a person has a particular disease. Applying a standard case definition will guarantee that every case is consistently diagnosed, no matter when and or where the diagnosis occurs. For certain rare but very lethal (plague) communicable diseases that require a quick response, a loose case definition may be appropriate. On the other hand, in many epidemiologic studies where a quick response is less critical and identifying causal association is important, it may be more essential to be sure that people in the study have the disease. In this situation, a stricter set of criteria for establishing the presence of a disease might be in order. The identifying features (signs, symptoms, disease progression, place and type of exposure, lab findings) will depend on the condition and disease under investigation.
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Search for Missing Cases (254)
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The Epidemiologist should search for cases that have not be recognized or reported. Physicians, clinics, health maintenance organizations, hospital emergency departments, public health clinics, migrant health clinics, and related facilities should be canvassed to ascertain whether other people might have the disease or condition under investigation. Asymptomatic persons or mild cases and their contacts should be evaluated. Individuals are placed into appropriate categories, initially separating the suspected cases from probable cases.
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Count Cases (254)
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Disease frequency must be determined. This involves quantifying the occurrence of disease in the population being investigated. Identifying the size of the at risk population from which the cases derive is necessary to calculate attack rates.
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Orient the Data According to Person (254)
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The epidemiologist should quickly become acquainted with the person-related issues and characteristics associated with the disease under investigation. Line listing should include information that characterizes the population and that can be adjusted for in the analysis, including inherent characteristics of people (age, race/ethnicity, gender) acquired characteristics (immunity or marital status), activities (occupation, leisure, use of medications), and conditions (socioeconomic status, access to health care). The interactions of family, friends, fellow workers, and relatives need to be considered. Certain person characteristics will have more relevance to some diseases or conditions than other. For example, if diabetes is discovered to be occurring in epidemic proportions, the epidemiologist should include the characteristics of race in the analysis because certain races have higher rates of some diseases than other. Specifically, Native Americans have higher rates of diabetes, Blacks have higher rates of hypertension, and Asians have lower rates of cardiovascular diseases.
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Orient the Data According to Place (254)
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The concentration of cases needs to be determined with regard to residence, birthplace, place of employment, school district, hospital unit, country, state, county, census tract, street address, map coordinates, and so forth. This allows the epidemiologist to understand the geographic extent of disease, gain an understanding of where the agent that causes a disease resides and multiplies, and better understand what may carry or transmit, spread, and cause a disease. A spot map is often an effective way to present this data pictorially. If possible, the epidemiologist might also plot on a map the locations of exposures, the locations of each case at the time of exposure, or when those exposed were identified as a case.
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Orient the Data According to Time (255)
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Presenting each case by time of onset with an epidemic curve can provide important information about the disease outbreak. Recall from Chapter 4 that an "EPIDEMIC CURVE" is a histogram, in which cases are plotted by time of onset, that shows the course of an epidemic. It is important to be familiar with the incubation period and how time affects the modes and vehicles of transmission. Chronologic events, step by step occurrences, chains of events tied to time distribution of the onset of cases should be determined and plotted on charts and graphs. From the epidemic curve information, the nature of the course of the disease is determined, and the researcher can ascertain whether people were exposed and infected at about the same time or at different times. Look for clustering of disease by both time and place. Determine and fix the time of the index case and the time of onset of the outbreak. Use the information from incubation periods to determine time factors in the course of the disease peaks and valleys in the epidemic curve.
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Classify the Epidemic (255)
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The mode of transmission is assessed and a determination is made as to whether the disease outbreak is a "COMMON-SOURCE EPIDEMIC" (starting at a specific point through intermittent or continuous exposure to a source over days, weeks, or years), a "PROPAGATED EPIDEMIC" (spread gradually from person to person), or a result of a common source of exposure that is then spread secondarily from person to person. "MIXED OUTBREAKS" involve a combination of both types of outbreaks. They typically begin with a common source and then are propagated from person to person, Ex...in September 1973, diarrhea caused by Salmonella typhimurium developed in 32 individuals in a hospital in Maine. The source of the outbreak was raw egg beaten in milk and then drunk as eggnog but presumably acquired the infection through person to person spread of typhimurium.
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The following questions should be when classifying an epidemic (255)
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1. Is the outbreak from a single source or a single-poing exposure? 2. Is disease spread from person to person? 3. Is there continued exposure to a single source? 4. Is the outbreak from multiple sources and or exposures? 5. Is the outbreak airborne? Behaviorally or chemically caused? Does the outbreak involve multiple events or exposures? 6. Are the sources of infection from unapparent sources? 7. Is there a vector involved in the transmission? 8. Is there an animal reservoir of infection?
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Shape of the Epidemic (255)
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Curve for a point source epidemic typically rapidly increases, peaks and then gradually decreases. With a continuous common-source epidemic, the increase may be more gradual and the curve more symmetric, covering a longer period of time. The curve typically contains one primary peak. With a propagated epidemic, the epidemic curve is usually a series of successively larger peaks.
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Determine Who Is at Risk of Becoming a Case (256)
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The epidemiology must determine who is ill and who is well in the exposed group. The people in the group can be classified by their individual disease and exposure histories. Clinical, medical , and laboratory findings need to be confirmed, evaluated, and analyzed for all cases to substantiate the diagnosis. Asymptomatic individuals or mildly ill persons should be medically evaluated. Search should be made for human and animal sources of infection in those at risk. The people exposed are separated from those not exposed. the ill are separated from the well. The status of the health of each case must be determined by exposure.
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Formulate Hypotheses (256)
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Firmly establish the source and type of epidemic. Is the outbreak from a common source or is it propagated? Identify the most probable source for the epidemic-the event, infection, or exposure source. Establish the mode of transmission. Use and analyze the information acquired earlier in the investigation, including but not limited to: case counts, assessing those at risk, the sources of the epidemic, time, place, or person factors, and attack rates. For example, if it is a food borne epidemic, in addition to investigating those ill from the exposure, the source of food must be investigated along with the food handling, preparation, production, and preservation. If the outbreak is environmentally caused, the conditions of the environment in which the individuals spent time must be investigated (the air at a worksite or skin exposure to chemicals). Consider al possible sources, food packing houses, imported foods, and so on.
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Animal Sources of Infection (256)
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As well as human sources should be considered. Researchers should study attack rates for the well/unexposed and the ill/exposed. They should also evaluate all suspected vehicles of transmission and assess frequency and levels of exposures. Variations in prevalence and incidence should be evaluated. As information comes in, it should be evaluated and the data assembled. Pertinent grouping of data based on time, place, person characteristics, and attack rates must be completed. Findings of collateral investigators and personnel, such as physicians, laboratory personnel, and hospital healthcare providers, should be gathered and assessed. Overall, the epidemiologist should develop hypotheses concerning the source of the outbreak as well as the mode of transmission (if an infectious disease)
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Hypotheses (256-7)
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Should be developed for all aspects of the investigations. For example, in a food-borne outbreak, hypotheses should be developed for: -The source of infection -The vehicle of infection -The suspect foods -The mode of transmission -The type of pathogen(based on clinical symptoms, incubation periods) -The time factors in the outbreak and course of the disease -The place factors in the outbreak -The person characteristics and factors in the outbreak -The outside sources of the infection -The transmission of the disease outside of the study population -The exposed, unexposed, and ill cases/individuals
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Test the Hypotheses (257)
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As data and information are acquired, the various hypotheses should be evaluated. The hypotheses need to be tested and established and shown to be consistent or inconsistent with facts.If established facts or information cannot substantiate a hypotheses, either more information should be gathered, or the research hypotheses should be rejected.
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Develop Reports and Inform Those Who Need to Know (257)
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The report typically presents a narrative of the investigation and a review of the course of the epidemic in the form of a case study. Tables, graphs, charts or any useful and helpful illustrations are presented as well as any pertinent epidemiologic data, test, laboratory reports, information and characteristics. A good epidemiologic report compares the hypotheses with the established facts.
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Communicable Diseases (257)
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Bring with them a more urgent need to inform the public than do noncommunicable diseases. When a disease poses a risk or danger to the public, then those who are in a position to intervene and control the epidemic need to be informed first. Public health officials, related government agencies, physicians, hospitals, health maintenance organization, medical clinic, schools, universities, and any group of people at risk should be informed. Unfortunately many times public health officials know of a health concern but fails to inform those who need to know most- the population at risk. Public health officials have a responsibility to warn the public and the population at risk and should not hesitate to do so. Sometimes officials are fearful that such information will create a panic, but this is not a reason to withhold information form the public or at least those who are at risk.
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Execute Control and Prevention Measures (257)
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The aim of pubic health disease-control programs and of epidemiology is to stop the spread of disease and to stop epidemics and prevent them from starting. Immunization programs are the first line of defense in prevention and control of some communicable diseases. Risk factor prevention and health protection programs are the first in line of defense in behaviorally caused or environmentally founded chronic diseases. Epidemiologic investigations are conducted if prevention and control measures have failed or were never adequately implemented.
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Administration and Planning Activities (258)
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Public health goals are accomplished only through an organized effort carried out with government assistance and under government administration. For epidemiologic activities to occur and be successful, organization, coordination, communication, planning, and funding assistance are all necessary. Immunization clinics and programs must be established and implemented. In the case of an epidemic, administrative plans and measures to provide treatment and care for the victims of the epidemic must be considered. Unbiased investigations are best handled by an agency without vested interest.. Government agencies often have the experts and professionals to carry out appropriate investigations of diseases, conditions, and disorders
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Investigation of a Food-Borne Illness (258)
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Food poisoning, food-borne illnesses, and food-caused epidemics are quite common but most are not serious and people rarely see their physicians unless it is serious. Little public health attention is paid to such occurrences, however the 1993 Jack in the Box epidemic cause by Escherichia coli which received national media attention, brought concern for food protection and preparation into the living rooms of families across America. Hamburger meat contaminated in meat processing plants was identified as the possible source of infection.
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Epidemic of Staphylococcal Food Poisoning (258)
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Is occurring (acquired from a fast food restaurant), most people simply take care of the matter at home: have a bout of diarrhea, take some over the counter antidiarrhea drugs, and feel better the next day. Hundreds of persons could be involved, but the family doctor and the medical community-let alone the public health department- never know because the outbreak is short and individuals recover quickly. In more serious food borne and waterborne illnesses, such as amebic dysentery and those caused by salmonella, Giardia and Shigella, peopl do not recover so quickly, the symptoms are stronger and last longer, and medical intervention is usually needed.. These diseases are serious and sometimes cause death, this makes them more likely to be reported.
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Food-Borne Illnesses (258)
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Illnesses arising from consumption of contaminated or spoiled foodstuffs and liquids, that is, solid foods, liquid foods, milk, water, and beverages. Are usually classified three ways (1) food infections (2) food poisoning (3) chemical poisoning
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Food Infection (258)
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Result of the ingestion of disease causing organisms (pathogens) such as bacteria and microscopic plants and animals. Ex...food infections are salomonellosis, giardiasis, amebiasis, shigellosis, brucellosis, diptheria, typhoid fever and tularmia.
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Food Poisoning (258)
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Result of preformed toxins that are present in foods prior to consumption, these toxins are often the waste products of bacteria. The two most common forms of staphylococcus infection and botulism. Staphylococcal food poisoning produces cramps and a short bout of diarrhea about 6 hours after consumption and is a milder form of food poisoning. The most serious and deadly form of food poisoning is botulism. The amount of botulinum toxin that will fit on the head of a pin will cause death in humans. Obviously, this toxin is extremely poisonous.
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Chemical Poisoning (259)
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Chemical food-borne illness. Some chemicals agents that are beneficial and essential in the diet as nutrients can cause food-borne illness if consumed in large enough dosages (zinc, vitamin A, niacin). Chemical agents that preserve food, improve eating quality (nitrite, monosodium glutamate) or assure a clean and sanitary food handling environment (pesticides, cleaners) can likewise cause food-borne illness if consumed in large enough amounts.
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Public Health and Medical Personnel (259)
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As in any disease investigation, must work together as a team. Personnel involved in a major disease investigation could possibly include epidemiologists, sanitarians, physicians, nurses, and laboratory personnel such as microbiologists, medical technologist, medical laboratory technicians and chemists.
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Epidemiology Team Interviews (259)
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If possible, all persons who were present at the time of the ingestion of suspect foodstuffs. When large groups or populations are involved in an outbreak, it may be feasible to interview all suspected cases. it has been suggested that in groups of over 50 people, 50% be interviewed, and in groups over 100, 25% be interviewed. Standardized interviewing procedures should be used. Random sampling techniques can help determine who should be interviewed and tested. Questions should be standardized and the information collected on a standard form, all interviewers should use this form.
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Any Good Epidemiologic Investigation (259)
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Should interview both ill and well persons. Half of the study populations should come from each category. Tabulation and analysis of the data should be completed as presented in earlier chapters of this book. In food-borne disease outbreaks, certain rates should always be included in the analysis and reports. Those factors necessary to a good investigation will identify and include: -Who ate the food -Who did not eat the food -For each food, calculation of the attack rates among those who ate it -For each food, calculation of the attack rates among those who did not eat it -Computation of the relative risk (the ratio of the attack rate of those eating the food to those who did not eat the food).
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Examples of How Field Epidemiology Influenced Public Health (259-61)
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Three examples: (1) E.Coli O157:H7 Outbreak Associated With Contaminated Alfalfa Sprouts (2) E.Coli O157:H7 Outbreak Associated With Contaminated Spinach (3) Outbreak of Cryptospordium
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Example 1....E.Coli O157:H7 Outbreak Associated with Contaminated Alfalfa Sprouts (259-60)
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In the final week of June 1997, the Michigan Department of Community Health observed an increase in Laboratory reports of E. colit O157:H7 infection. Greater than two time the typical number of infected cases were reported compared with the previous month. The increase in cases continued through the first 2 weeks of July. Thirty-eight of confirmed O157:H7 infections meeting the case definitions from ten countries in the lower-peninsula of Michigan are presented in Figure 10-1. The field investigation identified consumption of contaminated alfalfa sprouts of the outbreak.
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Investigating a Food-Borne Disease Epidemic Table 10-2 (260)
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-Obtain a diagnosis and make a disease determination -Establish that an outbreak has taken or is taking place -Determine which foods are contaminated and which are suspect. -Determine whether toxigenic organism, or chemical toxins are involved. -Ascertain the source of contamination. How did the foodstuff become contaminated? Who contaminated it? Where was it contaminated? Was it contaminated by direct or indirect sources? -After determining the source of poison and contamination, ascertain how much growth or the extent of contamination that could occur. -Identify foods and people implicated in the contamination and intervene to stop further spread of the disease -Ensure medical treatment -Exercise intervention, prevention, and control measures Inform those who need to know-private citizens, appropriate leaders, and public officials -Develop and distribute reports.
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Example 2....E Coli O157:H7 Outbreak Associated with Contaminated Spinach (261)
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More recently (fall of 2006) and extensive field investigation of an E Coli O157:H7 outbreak involving 205 confirmed cases and three deaths was conducted. The investigation identified contaminated Dole brand Baby Spinach grown in California as the cause of the outbreak. The source of the outbreak appeared to be contaminated near the presence of the wild pigs and surface waterways exposed to feces from cattle and wildlife. This and other outbreaks led the U.S. Food and Drug Administration (FDA) to announce and initiative called "Leafy Greens" which focused on produce, contamination agents, and corresponding public health concerns. The FDA also provided recommendations in a publication entitled "Guide to mInimize Microbial Food Safety Hazards of Fresh cut Fruits and Vegetables" which discussed ways to prevent microbial contamination while processing fresh cut produce. Emphasis is placed on washing all produce throughly before eating. Although aggressive hand washing would not have prevented the E. coli outbreak involving spinach, the risk of contamination from several other sources can be reduced.
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Example 3: Outbreak of Cryptosporidium (261)
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Cryptosporidiosis is an infectious diarrheal disease caused by cryptosporidium parasite. Clinical manifestations include frequent, watery diarrhea, nausea, vomiting, abdominal cramps and low grade fever. For immunocompromised persons the illness may be more sever, causing weight loss, anorexia, malaise, sever abdominal cramps and debilitating diarrhea. The illness is transmitted from focally contaminated food and water, from animal to person contact, and from person to person contact. There have been several outbreaks of crytosporidiosis in the United States. One very large outbreak occurred in Milwaukee in 1993, affecting over 400,000 people. In 2007, over 500 cases were reported during July and August throughout Utah. In a normal year, about 30 cases are reported. The Utah Department of Health and local health departments took immediate action, asking public pool managers to take aggressive action to help stop the ongoing outbreak of the infection. Steps taken included super chlorinating the pools, installing better filtering devices, and limiting pool use to adults ages 18 and older
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The CDC provides a list of Six Guidelines People may follow to Stay Safe (261)
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-Do not swim when you have diarrhea and for 2 weeks after the disease has cleared -Do not swallow the pool water -Take a shower before swimming and wash your hands after using the toilet or changing diapers. -Take the kids on bathroom breaks and check diapers often -Change diapers in a bathroom and not a pool side. -Wash your child thoroughly (especially the rear end) with soap and water before swimming
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Basic Epidemiologic Questions (261-2)
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If a disease occurs only in the summer, the epidemiologist searches for the causative factors that would be available only in that time period. Is the increase in the disease a result of exposure ot new water sources, for example, drinking from a stream in the mountains or swimming in a contaminated public swimming pool or lake? Is it a vector borne disease? What vectors are available for disease transmission int the given time period and are missing at other times of the year or seasons? are vehicles of transmission present during the time period that is not present during other time periods? Are the cases/subjects exposing themselves during this time period to environments, situation, places, or circumstances not available at other times of the year or in other seasons, such as hiking or camping in the woods in the summer when insects are present that are implicated in vector borne diseases? Are certain fomites used during a certain time period that might not be used during other seasons, such as shared drinking glasses or containers? Are risk factors only seen in certain locations or places? Do they occur only at work, only at home, or at the site of recreation (mountains, beaches, public swimming pools, ect?).
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Table 10-3 (262)
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Some Investigative Epidemiologic Questions to Consider List of several questions....
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Disease Cluster (263)
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An unusual aggregation, real or perceived, of health events that are grouped together in time and space and that are reported to a health agency. It generally occurs in response to the sudden introduction into the human environment of a physical stress, chemical or biological agent, or psychosocial condition. Ex...of clusters can involve injury or death related to accidents (plan crashes, fires, worksite conditions), natural disasters (flooding, tropical cyclones, tornadoes, volcanic eruptions, earthquakes, drought), political and social upheaval (unavailable health care, wars, racial discrimination), food poisoning caused by improper food handling introducing bacterial contaminants, and cluster of birth defects or cancer associated with biological and chemical contaminants.
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Cluster Investigation (263)
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Involves reviewing unusual numbers of health related states or events, real or perceived, a group together in time and location. Cluster investigations are conducted to confirm reported disease cases, identify whether the number of causes is above what is expected and if possible identify causal relationships. For Ex... among 50 cluster investigations of childhood leukemia and lymphoma in the United States during 1961 and 1977, where the cases were confirmed and higher than normal levels of the disease established, chemicals were implicated in all but eight, which suggested an underlying infectious agent.
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Investigations of Noninfectious Disease Clusters (263)
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Have also been useful in identifying potential biological and environmental causes such as clusters of angiosarcoma found in vinyl chloride workers, neurotoxicity and infertility found in kepone workers, dermatitis and skin vagina found for women who consumed diethylstilbestrol during pregnancy. In each of these studies there were definable health outcomes, confirmed of an elevation of the problem through statistical methods, a suspected environmental agent, and a short term public health impact that was immediate and self evident. Some selected health outcomes and their associated environmental risk factors are presented in Table 10-4 on p264
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Sentinel Events (263-4)
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Are occurrences of unexpected health related states or events that occur from specific, recognized causes, the adverse health outcome has a know cause. To illustrate, nine individuals were diagnosed with elevated blood lead levels at local hospitals in Alabama during March through October 1991. A cluster investigation showed that all had recently drunk illicit distilled alcohol (moonshine) made in two automobile radiators containing lead soldered parts. The framework for investigating sentinel events in occupational settings was established in the early 1980s by David Rustein and Colleagues at National Institute for Occupational Safety and Health. The concept of a Sentinel Health Event (Occupational), SHE(O), was defined as an unnecessary disease, disability, or untimely death that is occupationally related, with its occurrence yielding evidence of a failure in prevention. An epidemiologists role in the investigation of sentinel health may be described as assisting in recognition of the event, participating in evaluation, often with the aid of an industrial hygienist, arranging for appropriate intervention and summarizing and disseminating relevant information from the investigation to prevent similar cases elsewhere and in the future.
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Selected Organ/System Events and Their Toxic Exposure Risks-Table 10-4 (264)
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Respiratory--Asbestos, radon, cigarette smoke, glues Dermatologic--Dioxin, nickel, arsenic, mercury, cement (Chromium) polychlorinated biphenyls, glues, rubber cement Liver--Carbon tetrachloride, methylene chloride, vinyl chloride Kidney--Cadmium, lead, mercury, chlorinated hydrocarbon solvents. Cardiovascular--Carbon monoxide, noise, tobacco smoke, physical stress, carbon disulfide, nitrates, methylene chloride Reproductive--Methylmercury, carbon monoxide, lead, ethylene oxide Hematologic--Arsenic, benzene, nitrates, radiation Neuropsychologic--Tetrachlorethylene, mercury, arsenic, toluene, lead, methanol, noise, vinyl chloride
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Guidelines for Investigating Clusters (264)
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A four state process to cluster investigation has bee proposed by the CDC: (1) initial response (2) assessment (3) major feasibility study (4) etiologic investigation (CDC 1990) These stages incorporate many of the steps for conducting a field investigation. They are to be tailored according to the specific setting where the cluster is under investigation. An advisory committee may also be selected to provide consultation at critical decision points of the investigation, a brief description of these stages is presented here.
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Stage I, Initial Contact and Response (264-5)
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The purpose of this stage is to collect relevant information from those reporting the possible cluster. The recommended procedure are as follows: -Obtain identifying information on the caller -Obtain initial data on the suspected cluster, including proposed health events, suspected exposure, number of cases, geographic area, and time period of concern. -Obtain identifying information on those affected -Discuss initial impressions -Obtain additional information on cases such as follow up time for contact, if necessary -Assure that a written response to their concern will be received -Keep a log of initial and follow up contacts -Notify the public affairs office in the local health agency about the contact. Some alternative explanations for a reported disease cluster are presented in Table 10-5 Although new environmental data may be required early in the cluster investigation, existing data, if available, may also be useful. If the cluster investigation shows the presence of a single and rare disease and identifies a plausible exposure or a plausible clustering, then we proceed to Stage II
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Stage II. Assessment (265)
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This stage has three phrases: (1) preliminary evaluation to determine whether an excess of the health problem has occurred (2) case evaluation to assure that a biological basis is present (3) a further evaluation of some or all of the suspected cases to describe the epidemiologic characteristics. These phases may be performed sequentially or concurrently.
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Preliminary Evaluation (265-6)
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The primary aim of the preliminary evaluation is to determine quickly whether an excess of the health related state or event has occurred and to describe the characteristics of the cluster. This process involves: (1) determining the geographic area and time period for study (2) ascertaining those cases within the established time and space boundaries (3) identifying an appropriate reference population (4) determining whether there is a sufficient number of cases for assessment and whether a denominator is available for calculating rates and other statistics (5) if small numbers prevent obtaining meaningful rates or if the denominator is not available, assessing space, time or space-time clustering.
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Geographic Information Systems Software (266)
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Now available for conducting surveillance of putative clusters according to spatial or space-time clustering and to evaluate statistical significance.
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Issues Frequently Associated with Reported Clusters Table 10-5 (265)
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-An assortment of unrelated diseases and disease processes means a common origin is unlikely -If only women or older persons are affected, for example, this might indicate that an environmental pollutant is an unlikely cause -Neoplasm, and implicated environmental carcinogen is only plausible if the affected residents have lived in the area for a sufficiently long period of time because of the long latency period that typically accompanies cancer -Deceased cases may not provide useful information for linking exposure and disease because exposure many not be available and confounding factors may be present. -A rare disease cluster may be result of chance and not related to a given exposure -New diagnostic procedure may explain a cluster -Changes in reporting practices may explain a cluster. -Misdiagnoses by physicians may explain a suspected cluster -Migration patterns (existence of a new military base, housing area, or retirement area) -Increased awareness in certain diseases may explain a cluster.
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Identify Unusual Aggression (266)
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Of Adverse 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. What is usual may be obtained from local health officials who often know whether more disease is occurring than is expected based on ongoing diseases surveillance data through local surveys or health data registries. although laboratory confirmations are ideal, they may not be initially available for some acute conditions: however there still may be sufficient evidence available to warrant an investigation.
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Unreported or Unrecognized Cases (266)
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Can sometimes be identified through physicians, clinics, health maintenance organizations, hospital emergency departments, public health clinics, migrant health clinics, and related facilities and should be canvassed to ascertain whether other people might have the disease or condition under investigation.
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Case Identification (266)
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Should be restricted according to a specific time period, geographic region, and diagnostic group. Applying a standard case definition guarantees that every case is consistently diagnosed regardless of where and when the diagnosis occurs, in general, work best when information is also available about a probable exposure (lead, radiation, cigarettes)
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Rates (266)
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Useful for determining whether the putative cluster is actually unusual, preferred to just counts because they take into account the population size and can be effectively compared with rates in other time periods or places, however, to avoid misleading rate, the population value in the rate calculation needs to be appropriately selected.
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Attack Rates (266)
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Typically used in cluster investigations, however the person-time rate may be appropriate in some situations because it allows each persons contribution to the denominator of the rate calculation to be only as much time as observation in the at risk population. Consideration of adjusting the rates for potential confounders may also make comparisons between among rates more meaningful
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Probability (266)
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That there is an excess of the health-related state or event being considered is obtained through statistical methods, this probability is the P value, as already discussed. Interpretation of the conventional P value is dependent, however, on whether the hypothesis is a priori or not, as is discussed more fully later on in the chapter. If an excess of the health problem is supported from the preliminary evaluation, the next step is to do a case evaluation.
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Case Evaluation (266)
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The purpose here is to verify the diagnosis. False-positive results are a concern in that they may cause considerable alarm and present the impression that a suspected cluster is real.
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Diagnosis Verification (266)
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Often requires obtaining a referral to the responsible physician and permission to examine the patient record, access to pathology and medical examiners reports if possible, and sometimes histological reevaluation. In reality, though obtaining confirmation and reevaluation may not be possible. Laboratory test are not applicable with certain conditions, injuries or behaviorally caused occurrences. Furthermore, occupational or environmental disorders or conditions are often difficult to diagnose.
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Excess (266)
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Health problems is supported by case confirmation, then proceed to occurrence evaluation, yet if not confirmed, the investigators may still proceed if biologic plausibility persists.
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Occurrence Evaluation (267)
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In this step, the characteristics of the cluster are defined which is typically requires a field investigation. The procedure is as follows: (1) identify the appropriate geographic and temporal boundaries (2) ascertain all potential cases according to the specified time and space boundaries (3) identify numerator and denominator data and their availability (4) identify appropriate epidemiologic and statistical methods for describing and analyzing the data (5) review the literature and consider biologic plausibility (6) assess whether an exposure-event relationship can be established (7) identify the public pulse (perceptions, reactions, needs) (8) complete the descriptive investigation
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Critical (267)
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Select appropriate geographic and temporal boundaries. As a rule, the boundaries should correspond to the entire area that could have been exposed to the suspected cause. If the selected boundary is too small, the rate calculation will be overestimated. On the other hand, if the selected boundary is too big, the rate calculation will be underestimated.
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Epidemiologist (267)
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Should consider whether the disease cluster is associated with a common source or propagated from person to person. We are unable to narrow the potential cause of the cluster by classifying it by type of outbreak. Ex...diseases that are spread from person to person are biologically based, whereas adverse health outcomes that are not communicable arise from physical or psychosocial stressors or chemical contaminants.
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Clusters (267)
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Should be determined with regard to residence, birthplace, place of employment, school district, hospital unit, census tract, street address, map coordinates, and so on. by doing so, the epidemiologist can understand the geographic extent of disease and possibly a better idea of the cause. A map may be an effective way to present this data pictorially, especially if the map includes the locations of exposures, the locations of each case at the time of exposure, or when those exposed were identified as being a case.
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Epidemic Curve (267)
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Commonly used to present each case by time of onset. Chronologic events, step-by-step occurrences, chains of events tied to time and time distribution of the onset of cases should be determined and plotted on charts and graphs. Epidemiologists should determine the nature of the course of the disease and ascertain whether people were exposed and infected at about the same time or at different times, look for clustering of disease by both time and place, and use the information from incubation or latency periods to determined time factors in the course of the disease peaks and valleys in the epidemic curve. If an excess of cases is confirmed, along with compelling epidemiologic and biologic evidence, then we proceed to the final stage.
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Stage III. Major Feasibility Study (267-8)
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The purpose of this stage is to associate the excess number of cases with the putative exposure. The steps are as follows: (1) Review the literature for putative exposures of the health event under consideration (2) Select the appropriate study design and consider the attendant cost, sample size, use of previously identified cases, area, and time dimensions, and selection of a control group (3) Determine the required case and control data needed, which should include laboratory and physical measurements (4) Consider the appropriate methods for assessment (5) Outline the logistics for collecting and processing the data (6) Determine the analysis plan (hypotheses to be tested, power to detect differences) (7) Consider the current social and political climate and the potential impact of decisions and outcomes (8) Consider the resource requirement of the study
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Exposure Assessment (268)
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When complete, the question arises of whether sufficient information is available to formulate a plausible hypothesis. Research hypothesis is formulated by first identifying the most probable source for the cluster. If it is thought to be biologically caused, consideration should be given to known or potential pathways by which the contaminants might impact the population at risk (air, water, soil, food). If the cluster is environmentally caused, the conditions of the environment in which the individuals spent time must be investigated (the air at a worksite or skin exposure to chemicals.
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Evaluating Dose-Response (268)
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Relationships within clusters requires accurate measurement of exposure according to appropriate time and place factors. Personal measurement of exposure on a continuous scale is best for assessing dose-response relationships. The next best type of exposure information is the direct measure of a concentration of toxic contaminants in a specific environment(air, water, soil and food). If direct measures of exposure are not available, then indirect proxy measures of exposure may be the best option (use of drinking water, distance from a contamination site, duration of residence, and residence of employment)
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Hypotheses (268)
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Should be developed for all aspects of the investigation and are supported by the study design, data, methods, logistics, and context. With sufficient resources and justification for an etiologic investigation, proceed to Stage IV
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Stage IV. Etiologic Investigation (268)
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Involves a standard epidemiologic study approach, with specific criteria for establishing a cause and effect relationship. When the results of the investigation are obtained, a report should follow, presenting a narrative of the investigation. The report should contain any pertinent epidemiologic data, test, laboratory reports, information and characteristics. a good epidemiologic report compares the research hypotheses with the established facts. When the links to the health problem are understood, then intervention can occur, the links can be broken, the course of the health problem can be stopped, and if necessary, environmental cleanup can begin. It is often where prevention and control measures have failed or were never adequately implemented that disease clusters arise.
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Data Challenges in Cluster Investigations (268)
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Require accurate diagnostic information, case information according to person, place, and time, length of time cases lived in the area in question, potential changes in diagnostic or reporting procedures, migration patterns, and increase public awareness of the disease is question. In addition, a sufficient number of cases are needed in order to rule out chance as an explanation for the cluster finding. Unavailable data limit all levels of a cluster investigation. Lack of availability to quality health tracking data may: (1) Cause long delays in cluster investigation (2) Prevent public health officials from identifying disease trends (3) Inhibit the identification of true disease clusters (4) Reduce the number of cluster investigations carried out by states, meaning that some clusters go uninvestigated (5) Deter communities from getting the information and help they need when a suspected cluster arises.
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Statistical Challenges in Cluster Investigation (269)
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The primary statistical challenge with cluster investigations involves the fact that most cluster analyses involve post hoc (also called posteriori) rather than a priori hypotheses.
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Post Hoc Hypotheses (269)
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Refer to a formulation of the hypotheses after observation of an event such as an excess of cancer.
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Priori Hypotheses (269)
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Hypotheses of this type are problematic because the conventional P value is only interpretable, that is those hypotheses established without prior knowledge of the level of the health events in a specified population. Selectively choosing a suspected cluster for statistical testing is equivalent to multiple testing because the probability of finding a significant result increases as we become highly selective in testing only a given area out of many. Suppose that there is a cluster reported in a region that has 20 subareas, and we conclude that the disease rate in the area is statistically significant at the 5% level. By selectively choosing this area out of 20, we have essentially simultaneously conducted 20 tests. If the null hypothesis is true, we would expect 1 in 20 independent tests to be significant by chance alone at the 5% level of significance, therefore, the chance occurrence in the random variation of disease may be the sole explanation for the unusual events.
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Boundary Shrinkage (269)
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A second challenge is that rates have the danger of being overestimated, the population where the cluster is presumed to exist. Consider a putative cluster of childhood leukemia in the United Kingdom around a nuclear processing site at Windscale (later called Sellafied) reported in 1983. A journalist purported that an excess of leukemia could be due to radiation discharge from the nuclear processing site. The journalist initially focused on the health among employees at the plant. during the investigation, however, he was informed about a number of childhood leukemia cases in Seascale, a village close to the plant. An investigation found that the rate of childhood leukemia in the area containing the village was significantly higher than the national rate, however, multiple comparison and boundary shrinkage because of post hoc testing of hypotheses made the study findings potentially misleading. The first problem was the journalist may have followed several leads before focusing on leukemia cases. Hence multiple interpretation of the P value invalid. Second, after the identification of the group of cases, the underlying population corresponding to the suspected cluster was selected. The narrower this underlying population is statistical significance. This situation has been compare by Rothman to the Texas sharp shooter who first fires his gun and then draws a target around the bullet hole
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With "Post Hoc Hypotheses" where significance tests are inappropriate, alternative method of assessment include the following: (269)
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(1) performing the study in a different location but with a similar exposure (2) Excluding the cases in the original cluster and using new cases in the test of significance assuming further case ascertainment occurred (3) Looking for factors that distinguish the cases from others in the cluster, other than their residence (4) Evaluating a dose-response relationship between the exposure and health event.
