Descriptive Study - BSc Medical Sciences: Epidemiology PDF

Descriptive study BSc Medical Sciences: Epidemiology Assistant Prof. Dr. Soza Th. Baban Introduction and Review In session 6, we discussed: Explain the natural history of disease and the stages of disease progression and provided with case scenarios. Differentiate between communicable and n...

Descriptive study BSc Medical Sciences: Epidemiology Assistant Prof. Dr. Soza Th. Baban Introduction and Review In session 6, we discussed: Explain the natural history of disease and the stages of disease progression and provided with case scenarios. Differentiate between communicable and non-communicable diseases in the context of natural history. Discuss how social, economic, and cultural factors affect disease progression. Learning objectives i. Explaining descriptive studies in identifying disease patterns based on time, place, and person. ii. Define and differentiate between the types of descriptive epidemiological studies: case reports, case series, and cross-sectional studies. iii. Discuss the advantages and disadvantages of descriptive studies. iv. Analyse real-world examples of descriptive studies, evaluating their contribution to identifying public health trends and informing interventions. Let’s go through each of these points in detail!  Epidemiological studies Descriptive - describes the distribution of disease by time, place, and person; used to generate hypotheses of disease causation or for health planning. Analytic - measures and tests the association between a hypothesized risk factor and a disease.  Descriptive studies describes the occurrence and distribution of disease (or health- related characteristics within populations) by time, place and person, and identifying those characteristics associated with presence or absence of disease in individuals.  Descriptive studies Characteristics frequently examined in studies Time PLACE PERSON Year, season Climatic zones Age Birth order Month, week Country, region Gender Family size Day, hour of onset Urban/rural Marital state Height Local community Weight Duration Towns Occupation Blood pressure Cities Social status Blood cholesterol Institutions Education Personal habits  Descriptive studies (A) When is the disease occurring? TIME DISTRIBUTION The pattern of disease can be described by the time of its occurrence, i.e., by week, month, year, the day of the week, hour of onset, etc. It raises questions whether the disease is seasonal in occurrence; whether it shows periodic increase or decrease; or whether it follows a consistent time trend: ✓ Short-term fluctuations ✓ Periodic fluctuations ✓ Long-term or secular trends  Descriptive studies (A) When is the disease occurring? Time TIME DISTRIBUTION Year, season Month, week Day, hour of onset Two diseases that occur during the same season Duration each year include influenza (winter) and West Nile virus infection (August–September). In contrast, diseases such as hepatitis B and salmonellosis can occur at any time.  (A) When is the disease occurring? TIME DISTRIBUTION Human West Nile virus infection confirmed and probable cases, by month of illness onset, 2005-2010 How coronavirus lockdowns stopped flu in its tracks Eurosurveillance | Surveillance of West Nile Virus Disease  Descriptive studies (A) When is the disease occurring? TIME DISTRIBUTION 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) When is the disease occurring? TIME DISTRIBUTION Seasonal patterns of rubella, influenza, and rotavirus All three diseases display consistent seasonal distributions, but each disease peaks in different months — Rubella in March to June, Influenza in November to March, Rotavirus in February to April. 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. Source: Dowell SF. Seasonal Variation in Host Susceptibility and Cycles of Certain Infectious Diseases. Emerg Infect Dis. 2001;5:369–74.  (A) When is the disease occurring? TIME DISTRIBUTION Three kinds of time trends or fluctuations in disease occurrence: 1. Short-term fluctuations (Epidemic) refers to "the occurrence in a community or region of cases of an illness or other health-related events clearly in excess of normal expectancy".  (A) When is the disease occurring? TIME DISTRIBUTION Short-term fluctuations (Epidemic) Types of epidemics A. Common-source epidemics (a) Single exposure or "point-source" epidemics. (b) Continuous or multiple exposure epidemics B. Propagated epidemics (a) Person-to-person (b) Arthropod vector ( c) Animal reservoir C. Slow (modern) epidemics.  (A) When is the disease occurring? TIME DISTRIBUTION Short-term fluctuations (Epidemic) A. Common-source epidemics Example: Food Poisoning at a party. Features: a) Common-source, single exposure or "point-source" epidemics (i) the epidemic curve rises and falls rapidly, with no secondary waves. (ii) the epidemic tends to be explosive, there is clustering of cases within a narrow interval of time. (iii) more importantly, all the cases develop within one incubation period of disease.  (A) When is the disease occurring? TIME DISTRIBUTION Short-term fluctuations (Epidemic) A. Common-source epidemics: b) Common-source, continuous or repeated exposure: The exposure from the same source may be prolonged - continuous, repeated or intermittent - not necessarily at the same time or place. Example: The outbreak of respiratory illness, the Legionnaire's disease, in the summer of 1976 in Philadelphia (USA);  (A) When is the disease occurring? TIME DISTRIBUTION Common-source, continuous or repeated exposure Example: The outbreak of respiratory illness, the Legionnaire's disease, in the summer of 1976 in Philadelphia (USA); In 1976, an outbreak of Legionnaires' disease occurred during the American Legion's convention in Philadelphia, resulting in 34 deaths and 221 illnesses. The cause was later identified as Legionella pneumophila bacteria in the hotel's cooling tower, which had spread through the ventilation system.  (A) When is the disease occurring? TIME DISTRIBUTION Short-term fluctuations (Epidemic) B. Propagated epidemics: a) Person-to-person b) Arthropod vector c) Animal reservoir A propagated epidemic is most often of infectious origin and results from person-to-person transmission of an infectious agent (e.g., epidemics of hepatitis A and polio). The epidemic usually shows a gradual rise and tails off over a much longer period of time.  (A) When is the disease occurring? TIME DISTRIBUTION Short-term fluctuations (Epidemic) Transmission continues until the number of susceptibles is depleted or susceptible individuals are no longer exposed to infected persons or intermediary vectors. The speed of spread depends upon herd immunity, opportunities for contact and secondary attack rate. Propagated epidemics are more likely to occur where large number of susceptibles are aggregated, or where there is a regular supply of new susceptible individuals {e.g., birth, immigrants) lowering herd immunity.  (A) When is the disease occurring? TIME DISTRIBUTION Short-term fluctuations (Epidemic) Example: The course of a typical propagated epidemic in which the agent is transmitted by contact between individuals.  (A) When is the disease occurring? TIME DISTRIBUTION Periodic fluctuations (i) Seasonal trend : Examples: Measles is usually at its height in early spring and so is varicella. Upper respiratory infections frequently show a seasonal rise during winter months. Bacterial gastrointestinal infections are prominent in summer months because of warm weather and rapid multiplication of flies. The seasonal variations of disease occurrence may be related to environmental conditions (e.g., temperature, humidity, rainfall, overcrowding, life cycle of vectors, etc.) which directly or indirectly favour disease transmission.  (A) When is the disease occurring? TIME DISTRIBUTION Periodic fluctuations (i) Seasonal trend Example: A typical pattern of seasonal trend, the outbreaks of dengue starting by month of July and peaking in September, October and November, coinciding with late summer and rain.  (A) When is the disease occurring? TIME DISTRIBUTION Periodic fluctuations (ii) Cyclic trend Some diseases occur in cycles spread over short periods of time which may be days, weeks, months or years. Examples: Measles in the pre-vaccination era appeared in cycles with major peaks every 2-3 years and rubella every 6-9 years. This was due to naturally occurring variations in herd immunity. Influenza pandemics are known to occur at intervals of 7-10 years, due to antigenic variations.  (A) When is the disease occurring? TIME DISTRIBUTION Periodic fluctuations (iii) Long-term or secular trends Implies progressive increase or decrease over a long period of time, generally several years or decades. Examples: Coronary heart disease, lung cancer and diabetes which have shown a consistent upward trend in the developed countries during the past 50 years or so, followed by a decline of such diseases as tuberculosis, typhoid fever, diphtheria and polio.  Descriptive studies PLACE Climatic zones (A) Where is the disease occurring? PLACE DISTRIBUTION Country, region (Geographical pathology) Urban/rural Local community Describes the distribution of disease in different populations Towns and providing insights into differences (or variations) in Cities disease patterns not only between countries, but also within Institutions countries. Relative influence of genes vs environment Impact of migration on disease patterns Potential roles of diet and other etiological factors  Descriptive studies (A) Where is the disease occurring? PLACE DISTRIBUTION (Geographical pathology) Examples: Cancer exists all over the world. Cancer of the stomach is very common in Japan, but unusual in US. Cancers of the oral cavity and uterine cervix are exceedingly common in India as compared to industrialized countries. An international study of breast cancer showed that rates differ widely from country to country with the lowest prevalence in Japan and the highest in  Descriptive studies (A) Where is the disease occurring? PLACE DISTRIBUTION (Geographical pathology) International variations Examples: Cancer exists all over the world. Cancer of the stomach is very common in Japan, but unusual in US. Cancers of the oral cavity and uterine cervix are exceedingly common in India as compared to industrialized countries. An international study of breast cancer showed that rates differ widely from country to country with the lowest prevalence in Japan and the highest in the western countries.  Descriptive studies (A) Where is the disease occurring? PLACE DISTRIBUTION (Geographical pathology) National variations Variations in disease occurrence must also exist within countries or national boundaries. Example: The distribution of endemic leprosy, malaria, nutritional deficiency diseases have all shown variations in their distribution in India, with some parts of the country more affected and others less affected or not affected at all.  Descriptive studies (A) Where is the disease occurring? PLACE DISTRIBUTION (Geographical pathology) Rural-urban variations Example: Chronic bronchitis, accidents, lung cancer, cardiovascular diseases, mental illness and drug dependance are usually more frequent in urban than in rural areas. Skin and zoonotic diseases and soil-transmitted helminths may be more frequent in rural areas than in urban areas. Death rates, especially infant and maternal mortality rates, are higher for rural than urban areas.  Descriptive studies (A) Where is the disease occurring? PLACE DISTRIBUTION (Geographical pathology) Rural-urban variations These variations may be due to differences in population density, social class, deficiencies in medical care, levels of sanitation, education and environmental factors. The epidemiologist seeks to define groups which are at higher risk for particular diseases, and provides guidelines to the health administrator for their prevention and control.  Descriptive studies (A) Where is the disease occurring? PLACE DISTRIBUTION (Geographical pathology) Local distributions Inner and outer city variations in disease frequency are well known. These variations are best studied with the aid of 'spot maps' or 'shaded maps’. These maps show example, areas of high or low frequency, the boundaries and patterns of disease distribution. For example, if the map shows "clustering" of cases, it may suggest a common source of infection or a common risk factor shared by all the cases.  Descriptive studies (A) Where is the disease occurring? PERSON DISTRIBUTION Local distributions Inner and outer city variations in disease frequency are well known. These variations are best studied with the aid of 'spot maps' or 'shaded maps’. These maps show example, areas of high or low frequency, the boundaries and patterns of disease distribution. For example, if the map shows "clustering" of cases, it may suggest a common source of infection or a common risk factor shared by all the cases.  Descriptive studies (A) Where is the disease occurring? PERSON DISTRIBUTION Host factors: (a) Age : Measles in childhood, cancer in middle age and atherosclerosis in old age. (b) Gender : diabetes, hyperthyroidism and obesity are strikingly more common in women than in men, and diseases such as lung cancer and coronary heart disease are less frequent in women. (c) Ethnicity : Differences in disease occurrence have been noted between population subgroups of different racial and ethnic ongm. These include tuberculosis, essential hypertension, coronary heart disease, cancer, and sickle cell anaemia. Host factors: (d) Occupation : Persons working in particular occupations are exposed to particular types of risks. For instance, while workers in coal mines are more likely to suffer from silicosis, those in sedentary occupations face the risk of heart disease. (e) Social class : Certain diseases (e.g., coronary heart disease, hypertension, diabetes) have shown a higher prevalence in upper classes than in the lower classes. (f) Behaviour : cigarette smoking, over-eating. (g) Stress (h) Migration : Leprosy, filaria, and malaria, once rural issues, have become significant urban problems due to rural-to-urban migration.  Descriptive studies Case Reports and Case Series The main objective of case reports and case series is to provide a comprehensive and detailed description of the case(s) under observation. Case reports Indvidual-level observations can be documented in a case report, describing a particular clinical phenomenon in a single patient, or in a case series that describes more than one patient with similar problems.  Case Reports and Case Series Example: In 2015 ➔ outbreak of the Zika virus in Latin America. Zika virus is transmitted by Aedes mosquitoes and originally isolated from a rhesus monkey in the Zika forest in Uganda in 1947.  Case Reports and Case Series In early 2016➔ increasing numbers of infants born with microcephaly in Zika virus-affected areas The CDC published a descriptive case series from Brazil on the possible association between Zika virus infection and microcephaly, a condition in which the baby’s head is significantly smaller than expected, potentially due to incomplete brain development.  Case Reports and Case Series Another case report was published about the offspring of a Slovenian woman who lived and worked in Brazil and became pregnant in February 2015. She got ill with a high fever, followed by severe Musculo-skeletal and retro- ocular pain and an itching and generalized rash. No virologic testing for Zika virus was performed. She returned to Europe in the 28th week of gestation when ultrasonographic imaging showed fetal anomalies. The Zika virus was found in the fetal brain tissue.  Case Reports and Case Series Advantages: Case reports and case series are merely descriptive in nature with no reference group to make a strict comparison, the Brazilian case series was instrumental in the development of the CDC’s guidelines for the evaluation and testing, by health care providers, of infants whose mothers traveled to or resided in an area with ongoing Zika virus transmission during their pregnancies  Case Reports and Case Series Case reports and case series are key hypothesis-generating tools, especially when they are simple, inexpensive, and easy to conduct in the course of busy clinical settings. However, they have some important disadvantages: Cross-Sectional Studies: Studies that analyse data from a population at a single point in time, often used to assess 1. Lack of a comparison group to the prevalence of alink theordisease condition disease. to a specific cause. 2. Limited external validity (generalizability) ➔ all identified in clinical practice, they may not present the wider population, or people outside of that specific group or location.  Cross-Sectional Study Analyse data from a population at a single point in time, often used to assess the prevalence of a condition or disease. Cross-sectional study is the simplest form of an observational study. It is based on a single examination of a cross-section of population at one point in time - the results of which can be projected on the whole population provided the sampling has been done correctly. Cross-sectional study is also known as "prevalence study". Cross-Sectional Study Example d958077f8f5b59b7 (Session 8 – Outbreaks scenario) 1. Identification of AIDS in the Early 1980s 2. Smoking Prevalence and Anti-Smoking Campaigns 3. Zika Virus Outbreak in Brazil (2015-2016) 4. Cholera Outbreak in London (1854) – John Snow’s Study 5. Cross-Sectional Studies on Obesity and Public Health Response (Session 8 – Discussion prompts)  Recap and Q&A Key Points from Today’s Lecture: i. Understand the role of descriptive studies in identifying disease patterns based on time, place, and person. ii. Define and differentiate between the types of descriptive epidemiological studies: case reports, case series, and cross- sectional studies. iii. Discuss the advantages and limitations of descriptive studies in public health research. iv. Analyse real-world examples of descriptive studies, evaluating their contribution to identifying public health trends and informing interventions. Preparation for Next Session In the next session we'll explore the Case-control study studies (Analytical Epidemiology). Read Chapter 9 (Pages 210-230) from "Gordis Epidemiology" by Celentano DD and Szklo M. Review Chapter 4 (Pages 79-82) from "Parks Textbook of Preventive & Social Medicine" by K. Park  Descriptive studies Descriptive studies Time When is the disease Who occurring? d