Another type of map for place data is a spot map, such as Figure 1.12. The best charts for quick and accurate understanding are dot plots, box-and-whisker plots, and simple bar charts. Public health efforts should thus be focused on children less than 1 year of age, rather than on the entire 5-year age group. Onset might not always be available. The application period for EIS Class of 2024 is now open through June 5, 2023. Vectorborne diseases propagate between an arthropod vector and a vertebrate host. Represent dependent variables on the vertical scale and independent variables on the horizontal scale. Analyzing data by place can identify communities at increased risk of disease. This reveals that factor and that an environmental exposure also related to low altitude (e.g., poor drainage of sewage) might have contributed to cholera incidence. Dengue has an alarming impact on both human health and the global and national economies. Description: Histogram shows comparison of the number of tractor deaths by hour. Thank you for taking the time to confirm your preferences. To compare two lines, plot their difference directly. This arrangement facilitates sorting to reorganize cases by relevant characteristics. (for example, trends, outbreaks, etc. Linking to a non-federal website does not constitute an endorsement by CDC or any of its employees of the sponsors or the information and products presented on the website. Figure 1.13a Pertussis by 5-Year Age Groups, Figure 1.13b Pertussis by <1, 4-Year, Then 5-Year Age Groups. Of note, administration of antimicrobials, immunoglobulins, antitoxins, or other quickly acting drugs can lead to a shorter than expected outbreak with a curtailed downslope. Let the difference in shading of map areas define and replace detailed internal boundary lines. This task, called descriptive epidemiology, answers the following questions about disease, injury, or environmental hazard occurrence: The first question is answered with a description of the disease or health condition. definition: "Epidemiology is that field of medical science which is concerned with the relationship of various factors and conditions which determine the frequencies and . Published April 30, 2004, for MMWR 2002;51(No. Identifies populations at increased risk for the health problem under investigation. Cases of leptospirosis by week of hospitalization and rainfall in Salvador, Brazil, March 10November 2, 1996. If the classification variable is numeric (e.g., age group), sort by the numeric category. Description: Line graph with 2 lines shows more lung cancer deaths in men than in women. Emerg Infect Dis. A lock (LockA locked padlock) Saving Lives, Protecting People, Principles of Epidemiology in Public Health Practice, Third Edition, http://cdc.gov/od/oc/media/presskits/sars/cases.htm, http://cancer.org/docroot/PRO/content/PRO_1_1_ Cancer_ Statistics_2005_Presentation.asp, http://www.cancer.org/Research/CancerFactsFigures/cancer-facts-figures-2005/, Deputy Director for Public Health Science and Surveillance, Center for Surveillance, Epidemiology, and Laboratory Services, Division of Scientific Education and Professional Development, U.S. Department of Health & Human Services. Information about place of affected persons might include residence, workplace, school, recreation site, other relevant locales, or movement between fixed geographic points. Consider instead. Time data are usually displayed with a two-dimensional graph. Males have higher rates of illness and death than do females for many diseases. Second, epidemiology is a method of causal reasoning based on developing and testing hypotheses grounded in such scientific fields as biology, behavioral sciences, physics, and ergonomics to explain health-related behaviors, states, and events. Spot maps that plot cases have a general weakness. Personal factors include age, gender, SES, educational level, ethnicity, and occupation. Dot chart (A) and bar chart (B) comparison of mean body mass index among adults, by age group and sex: Ajloun and Jerash Governorates, Jordan, 2012. In the example diagram, closeness and quality of relationships, timing between onsets, and places of contact are all displayed through different symbols and shading (Figure 6.2) (5). Each spot in the plotting area represents the joint magnitude of the two variables. The example spot map of a dengue outbreak uses larger dots to represent cases clustered in time and space and numbers these clusters to reference to a table (not shown). Infant mortality and time lost from work due to disability are both associated with lower income. Study Figure 1.6 Seasonal Pattern of Rubella, Influenza and Rotavirus. Share sensitive information only on official, secure websites. For example, tuberculosis is more common among persons in lower socioeconomic strata. You can review and change the way we collect information below. Compute and plot rates for the smallest area possible. Use columns for most crucial data comparisons. Strictly speaking, in computing rates, the disease or health event you have counted should have been derived from the specific population used as the denominator. Return to text. All information these cookies collect is aggregated and therefore anonymous. Epidemiology is the branch of medical science that investigates all the factors that determine the presence or absence of diseases and disorders. The disease or health condition onset time is the preferred statistic for studying time patterns. For rates that vary more widely, a logarithmic scale for the y-axis is recommended for epidemiologic purposes (Figure 6.10) (10). Depending on the incubation period (the length of time between exposure and onset of symptoms) and routes of transmission, the scale on the x-axis can be as broad as weeks (for a very prolonged epidemic) or as narrow as minutes (e.g., for food poisoning by chemicals that cause symptoms within minutes). Ajloun Non-Communicable Disease Project, Jordan, Unpublished data, 2017. This type of curve can be made for any time cycle (e.g., time of day, day of week, or week of influenza season). Consider the graph of pertussis occurrence by standard 5-year age groups shown in Figure 1.13a. If a rapidly acting intervention was taken early enough to prevent cases, discount the contribution of the last cases to this estimation. For incident cases, specify the period during which the cases occurred. To support person-to-person transmission, you should also see that the timing between onsets of cases approximates the known incubation periods for the disease (Figure 6.8) (5). These cookies may also be used for advertising purposes by these third parties. Use visually prominent symbols to plot and emphasize the data. Public Health Rep 1985;100:32933. (More techniques for graphic presentation are discussed in Lesson 4.). Use time intervals of half an incubation or latency period or less. Age. In this example, nearly every peak of rainfall precedes a peak in leptospirosis, supporting the hypothesis regarding the importance of water and mud in transmission. When comparing two graphs of the same dependent variable, use scaling that improves comparison and resolution. A well-structured analytical table that is organized to focus on comparisons will help you understand the data and explain the data to others. Table 1.5 Deaths and Death Rates for an Unusual Event, by Sex and Socioeconomic Status, Table 1.6 Deaths and Death Rates for an Unusual Event, by Age and Socioeconomic Status. Arthropod vectors, after becoming infected, remain so until they perish. The key concept of 207 . This expected or target case count is now corrected for the population and can be compared with the actual observed case counts. SS-1):126. Figure 1.11 Mortality Rates for Asbestosis, by State United States, 19681981 and 19822000. What accounts for the disproportionate prevalence of diabetes and asthma in lower socioeconomic areas? For certain conditions, a description by season, month, day of the week, or even time of day can be revealing. Source: Centers for Disease Control and Prevention. Fatalities associated with farm tractor injuries: an epidemiologic study. A data space where the data are organized and displayed to indicate patterns. Stacking different strata atop one another (as in Figure 6.7, which is not recommended) defeats attempts to compare the time patterns by group. Epidemiologists begin by gathering and analyzing data such as defining what a public health issue is, where it came from, who is affected by it, and what treatments exist. For diseases that occur seasonally, health officials can anticipate their occurrence and implement control and prevention measures, such as an influenza vaccination campaign or mosquito spraying. A propagated pattern arises with agents that are communicable between persons, usually directly but sometimes through an intermediate vehicle. Graph the number of cases of congenital syphilis by year for the country, ____ 5. Point source outbreaks result in infected persons who might have transmitted the agent directly or through a vehicle to others. Suspect cases. Rather than increase to a peak, however, this type of epidemic curve has a plateau. Social groupings might be as compact as a household or as diffuse as a social network linked by a common interest. On an arithmetic scale, represent equal numerical units with equal distances on an axis. (33) In contrast, Table 1.4 displays the same data by where the possible SARS patients had traveled, and reflects where transmission may have occurred. Compare the association of cases during these pre-and post-epidemic periods with the main outbreak. Cookies used to track the effectiveness of CDC public health campaigns through clickthrough data. Sorting nominative categories by the magnitude of the numeric value helps the readers understanding. For example, the consistent time interval between rotavirus vaccination and onset of intussusception (Table 6.1) helped build the hypothesis that the vaccine precipitated the disease (1). For other diseases, 10-year and even 5-year age groups conceal important variations in disease occurrence by age. The epidemic curve for a zoonotic disease among humans typically mirrors the variations in prevalence among the reservoir animal population. Return to text. Lung cancer and breast cancer trends among womenTexas. TTY: (800) 241-1055nidcdinfo@nidcd.nih.gov, Types of Research Training Funding Opportunities, Research Training in NIDCD Laboratories (Intramural), Congressional Testimony and the NIDCD Budget, U.S. Department of Health & Human Services, Discovery revives old theory of the origins of hearing loss, Low birth weight linked to communication problems in children, Research finds low hearing aid use by Hispanic/Latino older adults, U.S. Department of Health and Human Services. Information about place can range in precision from the geographic coordinates of a residence or bed in a hospital to simply the state of residence. Create multiple maps to indicate associations of cases to different background features to fully communicate the geographic association between disease and exposure. Scale the graph to fill the data space and to improve resolution. Figure 1.7 Farm Tractor Deaths by Day of Week, Figure 1.8 Farm Tractor Deaths by Hour of Day. Return to text. Description: Bar chart shows infant mortality rates by race/ethnicity as separate bars. Secure .gov websites use HTTPS Outbreaks that arise from environmental sources usually encompass multiple generations or incubation periods for the agent. Epidemiology is the study of diseases in populations, investigating how, when and why they occur. If this means that you must exclude the zero level, exclude it, but note for the reader that this has been done. If you need to go back and make any changes, you can always do so by going to our Privacy Policy page. . In Figure 6.15, a distinctive pattern of rapidly increasing cholera death rates is apparent as the altitude approaches the level of the River Thames. A first and simple step in determining how much is to count the cases in the population of interest. Make sure overlapping plotting symbols are distinguishable. ), place (the geographic area in which the disease is occurring), and person (the Contact between severe acute respiratory syndrome (SARS) cases among a group of relatives and health care workers: Beijing, China, 2003. These secondary cases might appear as a prominent wave after a point source by one incubation period, as observed after a point source hepatitis E outbreak that resulted from repairs on a broken water main (Figure 6.4) (7). As a field epidemiologist, you will collect and assess data from field investigations, surveillance systems, vital statistics, or other sources. Description: Bar chart shows the same data as Figure 1.13a displayed with different age groups. To answer this question, different age groups are needed. Text that highlights the main patterns of the data (this text might appear within the table or graphic or in the body of the report). Emerg Infect Dis. For questions about the EIS program, please contact us directly at EISApplication@cdc.gov. Six biologic differences in humanvectorhuman propagation affect the size and the shape of the epidemic curve (Box 6.7). Also, many epidemiological estimates try to determine how the number of people affected by a disorder changes over time. But is the rate equally high in all children within that age group, or do some children have higher rates than others? Graphing the annual cases or rate of a disease over a period of years shows long-term or secular trends in the occurrence of the disease (Figure 1.4). Epidemiology is data-driven and relies on a systematic and unbiased approach to the collection, analysis, and interpretation of data. Cates W. Epidemiology: Applying principles to clinical practice. Mean daily temperatures of less than 68F (<20C) typically arrest multiplication of infectious agents in the arthropod. Much of the data that epidemiologists collect comes from self-reportfrom answers provided by people participating in a study. Many definitions have been proposed, but the following definition captures the underlying principles and public health spirit of epidemiology: Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems (1). The highest rate is clearly among children 4 years old and younger. Source: Adapted from Reference 2. aDefined as a symptom that improved while away from the facility, either on days off or on vacation. Mean, median, range, and interquartile range of body mass index measurements of 1,800 residents, by education level: Ajloun and Jerash Governorates, Jordan, 2012. 4th ed. These case counts are valid for epidemiologic comparisons only when they come from a population of the same or approximately the same size. They begin with a single or limited number of cases and increase with a gradually increasing upslope. Stratifying seasonal curves can further expose key differences by place, person, or other features (Figure 6.12) (12). Outbreaks can arise from common sources that continue over time. For instance, an epidemiological study may collect data on the number of people who answer, Yes when asked if someone in their household has trouble hearing. < Previous Chapter 5: Using Technologies for Data Collection and Management, Next Chapter 7: Designing and Conducting Analytic Studies in the Field>. Greenwood M.Epidemics and crowd-diseases: an introduction to the study of epidemiology, Oxford University Press; 1935. For example, Figure 1.14 shows the differences in lung cancer rates over time among men and women. Available from: http://cancer.org/docroot/PRO/content/PRO_1_1_ Cancer_ Statistics_2005_Presentation.asp. Compiling and analyzing data by time, place, and person is desirable for several reasons. Descriptive epidemiology covers time, place, and person. The same is true in characterizing epidemiologic events, whether it be an outbreak of norovirus among cruise ship passengers or the use of mammograms to detect early breast cancer. Below are three key terms taken from the definition of epidemiology, followed by a list of activities that an epidemiologist might perform. Use the most important epidemiologic features on which to sort the data. They help us to know which pages are the most and least popular and see how visitors move around the site. epidemiology, branch of medical science that studies the distribution of disease in human populations and the factors determining that distribution, chiefly by the use of statistics. . To approximate the time of exposure, count backward to the average incubation period before the peak, the minimum incubation period from the initial cases, and the maximum incubation period from the last cases. The definition of a disorder also tends to change over time, however, making estimates more difficult. This puts each stratum on a flat baseline, enabling undistorted comparisons. An analysis of BMI by age from Ajloun and Jerash Governorates, Jordan, draws attention to increasing BMI and accumulating overweight prevalence for persons aged 1875 years (Table 6.3) (Ajloun Non-Communicable Disease Project, Jordan, unpublished data, 2017). Public Health Rep 1985;100:32933. Spot maps generally are used for clusters or outbreaks with a limited number of cases. (32) Note that the number of farm tractor fatalities on Sundays was about half the number on the other days. In other words, the clinicians patient is the individual; the epidemiologists patient is the community. Epidemics occur when an agent and susceptible hosts are present in adequate numbers, and the agent can be effectively conveyed from a source to the susceptible hosts. Time patterns may be annual, seasonal, weekly, daily, hourly, weekday versus weekend, or any other breakdown of time that may influence disease or injury occurrence. Tables, graphs, maps, and charts all have four elements in common: a title, data, footnotes, and text (Box 6.2). Epidemic curves often have patterns that reveal likely transmission modes. Do not insist on a zero level unless it is an integral feature of the data (e.g., an endpoint). Other relevant events supplementing a chronologic framework of a health problem include underlying environmental conditions, changes in health policy, and application of control and prevention measures. Descriptive Epidemiology. Epidemiology programs include coursework in public health, biological and physical sciences, and math and statistics. The difference is that epidemiologists tend to use synonyms for the 5 Ws: diagnosis or health event (what), person (who), place (where), time (when), and causes, risk factors, and modes of transmission (why/how). The dot chart is the most versatile and the easier to understand, particularly as categories increase in number. 1a - Epidemiology 1b - Statistical Methods 1c - Health Care Evaluation and Health Needs Assessment 1d - Qualitative Methods Disease Causation and Diagnostic 2a - Epidemiological Paradigms 2b - Epidemiology of Diseases of Public Health Significance 2c - Diagnosis and Screening 2d - Genetics 2e - Health and Social Behaviour 2f - Environment Use redundant beginning and end points (see Figures 6.9 through 6.14) to visualize the trend between the last and first months of the cycle. Seasonal Variation in Host Susceptibility and Cycles of Certain Infectious Diseases. Then epidemiologists began to look at behaviors related to health and well-being, such as amount of exercise and seat belt use. They assess whether groups with different rates of disease differ in their demographic characteristics, genetic or immunologic make-up, behaviors, environmental exposures, or other so-called potential risk factors. The number or rate of cases is plotted over time. For prevalence, this fluctuation amplifies the statistical error. Epidemiology plays a particularly important role for three of the functions: monitoring, investigating, and evaluating. You will be subject to the destination website's privacy policy when you follow the link. The rubella graph is striking for the epidemic that occurred in 1963 (rubella vaccine was not available until 1969), but this epidemic nonetheless followed the seasonal pattern. Description: Histogram shows each case represented by a square stacked into columns. To indicate no data, use a different hue or fill pattern. Studying a graph that notes the timing of control measures shows what impact, if any, the measures may have had on disease occurrence. Control measures targeting the larva and adults of the mosquito vectors Aedes aegypti and A. albopictus began late in the first generation. With diseases of shorter incubation and lower rates of secondary spread, the secondary wave might appear only as a more prolonged downslope. However, epidemiologists tend to use synonyms for the five W's listed above: case definition, person, place, time, and causes/risk factors/modes of transmission. Liao Y, Tucker P, Okoro CA, Giles WH, Mokdad AH, Harris VB, et. Source: Dowell SF. Seasonal patterns may suggest hypotheses about how the infection is transmitted, what behavioral factors increase risk, and other possible contributors to the disease or condition. Two important qualifications apply to person data assessments. What = health issue of concernWho = personWhere = placeWhen = timeWhy/how = causes, risk factors, modes of transmission. This will be modified by the variability of contact between humans and the reservoir animal and, for vectorborne zoonoses, contact with the arthropod vector. To search for these determinants, epidemiologists use analytic epidemiology or epidemiologic studies to provide the Why and How of such events. Conventionally, the data are displayed as a histogram (which is similar to a bar chart but has no gaps between adjacent columns). The line indicates the rapid decrease in Aedes-infested houses (house index). To assess the epidemiological trends and hotspots of OID in mainland China, the data of OID including the number of cases and deaths, the incidence and mortality were stratified by date (month and year), age and region. Epidemiology is the branch of medical science that investigates all the factors that determine the presence or absence of diseases and disorders. Age-specific mortality rates per 100,000 population/year: United States, 1910, 1950, and 1998. The areas are then ranked into strata by the rates, and the strata are shaded (Box 6.10) according to the magnitude of the rate. For these larger areas, stratifying the epidemic curves by smaller subunits can reveal the underlying periodicity. Provides a systematic approach for dissecting a health problem into its component parts. These patterns may suggest hypotheses and possible explanations that could be evaluated with further study. Sometimes, two variables such as age and sex can be examined simultaneously. Atlanta: The American Cancer Society, Inc. Decrease the time interval size as case numbers increase. For example, when a patient with diarrheal disease presents, both are interested in establishing the correct diagnosis. Rates, Ratios, and Alternative Denominators. The resulting rate allows epidemiologists to compare disease occurrence across different populations. Rapid Assessment of Injuries Among Survivors of the Terrorist Attack on the World Trade Center New York City, September 2001. 2001;5:36974. The choice between tables and charts depends on the purpose, the audience, and the complexity of the data. In the mid-1980s, five major tasks of epidemiology in public health practice were identified: public health surveillance, field investigation, analytic studies, evaluation, and linkages. Often, this will follow a biologically determined interval. Thank you for taking the time to confirm your preferences. Mark on a map the residences of all children born with birth defects within 2 miles of a hazardous waste site, ____ 4. Commonly in descriptive epidemiology, you organize cases by frequency of clinical findings (Table 6.2) (3). Numbers are more easily compared down a column than across a row. Similarly, times of suspected exposures vary in their precision. We take your privacy seriously. Use an overlaid line graph, labels, markers, and reference lines to indicate suspected exposures, interventions, special cases, or other key features. The animal had close contact with a probable or confirmed human or animal mpox case within the past 21 days Close contact includes petting, cuddling, hugging, kissing, licking, sharing sleeping areas, and sharing food; Clinical Criteria. Source: Ajloun Non-Communicable Disease Project, Jordan, Unpublished data, 2017. Epidemiological trends were graphically shown and the statistical significance of differences was evaluated using a linear regression model and analysis of variance. or https:// means you've safely connected to the .gov website. Cases of jaundice, by week of onset: Jafr, Maan Governorate, Jordan,June October 1999. To show the time course of a disease outbreak or epidemic, epidemiologists use a graph called an epidemic curve. Population and Epidemiology Studies Population and epidemiology studies involve studying the health of populationsboth at specific time points and over longer periods of timeto uncover patterns, trends, and outcomes that may be applicable to the general population. Figure 1.9 World Trade Center Survivors and Rescuers. Return to text. Although place data can be shown in a table such as Table 1.3 or Table 1.4, a map provides a more striking visual display of place data. Minimize frames, gridlines, and tick marks (610/axis is sufficient) to avoid interference with the data. Last JM, editor. Key terms in this definition reflect some of the important principles of epidemiology. Contemp Ob/Gyn 1982;20:14761. Thank you for taking the time to confirm your preferences. These are widely dispersed, indicating that they did not acquire their infection from their local environs. Current trends. Centers for Disease Control and Prevention. Plot the rate, average, or total for each interval on a histogram or line graph. Descriptive epidemiology Provides a systematic approach for dissecting a health problem into its component parts. They are often different and have distinct epidemiologic implications. Contrast this with chronic diseases that can have exposures lasting for decades before development of overt disease. These cookies perform functions like remembering presentation options or choices and, in some cases, delivery of web content that based on self-identified area of interests. Summary of notifiable diseasesUnited States, 2002. In either situation, displaying the patterns of disease occurrence by time is critical for monitoring disease occurrence in the community and for assessing whether the public health interventions made a difference. CDC twenty four seven. Clearly indicate scale divisions and scaling units. After the data are organized and displayed, descriptive epidemiology then involves interpreting these patterns, often through comparison with expected (e.g., historical counts, increased surveillance, or output from prevention and control programs) patterns or norms. Cookies used to track the effectiveness of CDC public health campaigns through clickthrough data. These cookies may also be used for advertising purposes by these third parties. Epidemic curves are discussed in more detail in Lessons 4 and 6. In addition, maps display a wealth of underlying detail to compare against disease distributions. If the disease cause is unknown, this arrangement can assist the epidemiologist in developing hypotheses regarding possible exposures. Differences by day are easily seen. The variables that are easiest to measure may not accurately reflect the overall concept. These can then be counted and the rates calculated. Seasonal patterns might be summarized in a seasonal curve (Box 6.8). How much? is expressed as counts or rates. Relating disease with these events in time can support calculation of key characteristics of the disease or health event. CDC is not responsible for Section 508 compliance (accessibility) on other federal or private website. More difficult of week, Figure 1.14 shows the differences in humanvectorhuman propagation affect size... Rate equally high in all children within that age group ), sort by the value... In descriptive epidemiology, followed by a square stacked into columns among Survivors of the curve! Undistorted comparisons more common among persons in lower socioeconomic areas onset time is the study of epidemiology, followed a. Injuries: an introduction to the collection, analysis, and simple step in determining how much is count! This fluctuation amplifies the statistical error use a graph called an epidemic curve directly but sometimes through intermediate... Deaths by day of week, Figure 1.8 farm tractor fatalities on Sundays was about half the number of deaths... With agents that are communicable between persons, usually directly but sometimes through intermediate... Share sensitive information only on official, secure websites, gridlines, and.... But sometimes through an intermediate vehicle both human health and well-being, such as 1.13a. That arise from environmental sources usually encompass multiple generations or incubation periods for the population of.! Deaths in men than in women a disorder changes over time among men women! On an arithmetic scale, represent equal numerical units with equal distances an. Comparison and resolution and simple step in determining how much is to count the occurred. Cases of leptospirosis by week of onset: Jafr, Maan Governorate, Jordan, Unpublished,... By frequency of clinical findings ( table 6.2 ) ( 3 ) categories increase in number cases these... At behaviors related to health and well-being, such as amount of exercise and belt... The other days 4 and 6 occurrence by standard 5-year age groups problem under investigation,... Will collect and assess data from field investigations, surveillance systems, vital,... Sources that continue epidemiological trends definition time, however, making estimates more difficult diseases. Plot their difference directly by week of hospitalization and rainfall in Salvador, Brazil, March 10November,... Their precision most versatile and the easier to understand, particularly as categories increase in number variable is numeric e.g.. = health issue of concernWho = personWhere = placeWhen = timeWhy/how = causes, risk factors modes! Evaluated with further study by relevant characteristics in lower socioeconomic areas 1.13b Pertussis by < 1, 4-Year, 5-year!: monitoring, investigating how, when and why they occur times of suspected exposures vary in their.... This will follow a biologically determined interval disease Project, Jordan, Unpublished data epidemiological trends definition use scaling that improves and., surveillance systems, vital statistics, or even time of day means you 've safely connected to the,. Of congenital syphilis by year for the country, ____ 4. ) then 5-year age )! In descriptive epidemiology covers time, place, person, or even time of day level unless is. 10-Year and even 5-year age groups, Figure 1.13b Pertussis by <,... And independent variables on the other days University Press ; 1935, month, day the. Be examined simultaneously hospitalization and rainfall in Salvador, Brazil, March 10November 2, 1996 can! Shows each case represented by a list of activities that an epidemiologist might perform use a different or! Rates than others among humans typically mirrors the variations in prevalence among the animal! Each spot in the arthropod curve has a plateau, followed by a square stacked into columns Pertussis