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- The reservoir of an infectious agent is the habitat in which the agent normally lives, grows, and multiplies. Reservoirs include humans, animals, and the environment. The reservoir may or may not be the source from which an agent is transferred to a host.
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Reservoir. The reservoir of an infectious agent is the habitat in which the agent normally lives, grows, and multiplies. Reservoirs include humans, animals, and the environment. The reservoir may or may not be the source from which an agent is transferred to a host.
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Therefore, we define a reservoir as one or more epidemiologically connected populations or environments in which the pathogen can be permanently maintained and from which infection is transmitted to the defined target population.
- Daniel T. Haydon, Sarah Cleaveland, Louise H. Taylor, M. Karen Laurenson
- 10.3201/eid0812.010317
- 2002
- Emerg Infect Dis. 2002 Dec; 8(12): 1468-1473.
- On This Page
- Box 3.1
- The Investigation
- Box 3.2
- Box 3.3
- Box 3.4
- Figure 3.1
- Figure 3.2
- Figure 3.3
- Box 3.5
•Background Considerations
•The Investigation
•Conclusion
•References
When a threat to the public’s health occurs, epidemiologists are ready responders who investigate the problem so they can identify causes and risk factors, implement prevention and control measures, and communicate with everyone involved. Epidemiologic field investigations are a core function of epidemiology and perhaps the most obvious way information is transformed into action to ensure public health and safety (see Chapter 1). This chapter describes the step-by-step process required in performing an epidemiologic field investigation. The 10 steps covered here build on and further refine the steps that have been taught traditionally in the Centers for Disease Control and Prevention’s (CDC) annual Epidemic Intelligence Service courses, in the three previous editions of this manual (the textbook Field Epidemiology), and in other CDC instructional programs. The 10 steps discussed here are similar to those found in other epidemiology instructional publications (1–5). Lists, take-home points, and examples are provided to clarify key aspects and improve the practical utility of the discussion. This chapter describes a field investigation in the context of a public health response to a presumed acute infectious disease outbreak, although this approach also applies to other scenarios and problems.
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How Case Definition And Outbreak Focus Change: Zika Virus Infection
Zika was first identified in nonhumans in 1947 and associated with mosquito transmission. Since then, researchers have continued to learn and adapt to new information about Zika transmission. Before 2007, when the first larger scale outbreak occurred, Zika was not a disease of special concern given the small number of people affected. In 2008, sexual transmission was suspected as a mode of transmission, but with so few cases, confirming it was not possible. Then Zika virus cases increased exponentially in 2015. Preliminary investigations indicated mother-to-child transmission among pregnant women, and a case of sexual transmission was confirmed. Each time researchers learned new information, case definitions had to be adapted and the focus of information gathering had to expand to account for multiple transmission modes.
Source: Adapted from Reference 7.
An outbreak is defined as “the occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time” (1). When there are clearly many more cases than usual that are distributed across a larger geographic area, the term epidemic can be used. An outbreak is a situation that usually needs a rapid public health response. Notification of a suspected outbreak can come from different sources, including astute clinicians, laboratory scientists, public health surveillance data, or the media.
After the decision is made to start an investigation, clearly defining the objective of the investigation is crucial. Field investigations of common outbreak scenarios have standard objectives and time-tested methods that can be implemented rapidly. For example, because transmission modes associated with foodborne and waterborne outbreaks are well-known (i.e., spread by contact with infected persons, animals, or contaminated food or water), epidemiologists have developed the National Hypothesis Generating Questionnaire (6), a standardized questionnaire to help develop hypotheses and collect information from ill persons regarding demographics and specific exposures. In contrast, at the time of initial recognition, many outbreaks have no obvious or known cause, which challenges the epidemiologist to establish a clear objective early—albeit one that is broad and can be revised as the investigation evolves—and to generate hypotheses (Box 3.1).
Finally, a certain urgency to field investigations and pressure to find an answer quickly will always exist. For example, rapid surveys or other study designs used in outbreak investigations might lack the level of statistical power or proof of causality that often are possible in prospectively planned research studies. Likewise, delays caused by waiting for all laboratory samples to be tested can delay determination of pathogens or modes of spread and, consequently, implementation of control measures. However, the goal is to be both timely and accurate. Because of these considerations, coordinating with all partners and establishing priorities early is key to a successful investigation.
Consider whether control measures can be implemented now.Identify and count cases (i.e., create a case definition and develop a line listing).Prepare for field work
Ten Steps of a Field Investigation
1.Prepare for field work.
2.Confirm the diagnosis.
3.Determine the existence of an outbreak.
4.Identify and count cases (i.e., create a case definition and develop a line listing).
5.Tabulate and orient the data in terms of time, place, and person (i.e., descriptive epidemiology).
Establishing A Baseline For Confirming An Outbreak
After the initial June 1981 Morbidity and Mortality Weekly Report of a cluster of cases of Pneumocystis pneumonia among men in Los Angeles, the ensuing investigation required approximately 6 months to establish surveillance and a baseline that confirmed the early phase of what eventually came to be known as the national epidemic of human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS).
Source: Adapted from References 10, 11.
Determining the existence of an outbreak is a sometimes difficult step that should be completed before committing program resources to a full-scale investigation. This step also is necessary to rule out spurious problems (e.g., pseudoepidemics or reporting increases caused by surveillance artifacts). As noted previously, pseudoepidemics might result from real clustering of false infections (e.g., inadvertent contaminants of laboratory specimens) or artifactual clustering of real infections (e.g., increases in the number of reported cases because of changes in surveillance procedures introduced by the health department or implemented by a healthcare delivery system) (9). Problems potentially associated with pseudoepidemics include risks related to unnecessary or inappropriate treatment and unnecessary diagnostic procedures.
To confirm the existence of an outbreak, the field investigation team must first compare the number of cases during the suspected outbreak period with the number of cases that would be expected during a nonoutbreak timeframe by
•Establishing a comparison timeframe in the suspected epidemic setting by considering, for example, whether it should be the period (e.g., hours, days, weeks, or months) immediately preceding the current problem or the corresponding period from the previous year;
Simple Versus Complex Case Definitions
Example of a Simple Case Definition
The 2007 Zika virus (ZIKV) outbreak in Yap used the following case definition:
Case definition: A patient with suspected disease had acute onset of generalized macular or papular rash, arthritis or arthralgia, or nonpurulent conjunctivitis during the period from April 1 through July 31, 2007.
Case classification: We considered a patient to have confirmed Zika virus disease if Zika virus RNA was detected in the serum or if all the following findings were present: IgM antibody against Zika virus (detected by ELISA), Zika virus PRNT90 titer of at least 20, and a ratio of Zika virus PRNT90 titer to dengue virus PRNT90 titer of at least 4. A patient was classified as having probable Zika virus disease if IgM antibody against Zika virus was detected by ELISA, Zika virus PRNT90 titer was at least 20, the ratio of Zika virus PRNT90 titer to dengue virus PRNT90 titer was less than 4, and either no Zika virus RNA was detected by RT-PCR or the serum sample was inadequate for the performance of RT-PCR.
Example of a Complex Case Definition
Fig3-1
Spot map of residents on the ninth floor of the Metropole Hotel, Hong Kong, February 21, 2003, who had symptoms later identified as severe acute respiratory syndrome.
Source: Reference 12. Reprinted with permission from the World Health Organization, February 5, 2018.
View larger image
Schematic map of village X, Sierra Leone, indicating cumulative Ebola virus infection household status and quarantine status, August 1– October 10, 2014.
Source: Reference 13.
Public Health Example: Controlling an Outbreak of Hepatitis A in a Child Day Care Setting
When the etiology and mode of spread, as well as interventions, are known at the time an outbreak is recognized, control measures can begin immediately. For example, before hepatitis A vaccine was routinely administered to children starting at age 1 year, a single case of hepatitis A in a child day care setting led to administration of immune globulin prophylaxis to an entire cohort of exposed children and staff. This was performed because of the known epidemiologic associations between asymptomatic and symptomatic cases; it directed efforts toward prophylaxis of exposed persons while minimizing the need for an extensive investigation to specifically identify infected persons. The response was predicated on routine policy and guidelines developed by experts on the basis of studies and previous outbreak experience and virtual certainty about the etiology of the problem and its mode of spread.
Source: Adapted from Reference 15.
Control measures include two categories of interventions: (1) those that can be directed at the source(s) of most infectious and other disease-causing agents (e.g., treating infected persons and animals or isolating infected persons who are contagious) and (2) those that can be directed at persons who are susceptible to such agents (administering postexposure prophylaxis, vaccinating in advance, or employing barrier techniques) (see Chapter 11 and Box 3.5). In concept, control measures are implemented only after the preceding and subsequent steps—including developing and testing hypotheses about the cause or mode of spread—have been implemented. In practice, however, decisions about control measures might be necessary at any step in the sequence, and preliminary control measures can be instituted on the basis of limited initial information and then modified as needed as the investigation proceeds. Control measures should be considered again after more systematic studies are complete.
In certain instances, descriptive epidemiologic findings alone, or results of cross-sectional survey data or other studies will be sufficient for developing hypotheses. Often, however, analytic epidemiologic methods—especially cohort or case– control studies—will be needed for identifying possible risk and other causative factors and for testing the strength of the association of the factors with the disease. The process of hypothesis testing, therefore, can entail multiple iterations of hypothesis generating and testing, serial studies, and collection, analysis, and management of considerable additional data. See Chapter 7 for description of how cohort and case– control studies can be used effectively in foodborne or waterborne disease outbreaks and other types of field investigations (Box 3.6).
Typically, statistically significant (e.g., small p value) findings of associations alone do not constitute an adequate body of evidence to support conclusions about the validity of hypotheses and to implement interventions to terminate an outbreak. Instead, all key information and investigative findings should be viewed as a whole in relation to such standards as the Bradford Hill tenets of causation (17) (Box 3.7).
Searches related to define reservoir epidemiology meaning in medical field test report
Sep 6, 2017 · According to the accepted definition of a reservoir proposed by Haydon et al., 4 we discuss the requirements in two parts: the pathogen’s maintenance in a potential population or community followed by a discussion on proof of a feasible transmission route. Although the two components are addressed separately, only together they demonstrate ...
- Luisa K Hallmaier-Wacker, Vincent J Munster, Sascha Knauf
- 10.1038/emi.2017.65
- 2017
- Emerg Microbes Infect. 2017 Sep; 6(9): e79.
Apr 19, 2012 · Therefore, we define a reservoir as one or more epidemiologically connected populations or environments in which the pathogen can be permanently maintained and from which infection is transmitted to the defined target population.
Reservoirs of infection play a key role in the transmission dynamics of infectious diseases by providing a continuous source of pathogens that can infect new hosts. When a pathogen resides in a reservoir, it can multiply and maintain its lifecycle until it encounters a susceptible host.
Jul 22, 2022 · A human acting as a reservoir of a pathogen may or may not be capable of transmitting the pathogen, depending on the stage of infection and the pathogen. To help prevent the spread of disease among school children, the CDC has developed guidelines based on the risk of transmission during the course of the disease.
