Epidemiology: Understanding the Spread and Distribution of Disease

1. Epidemiology= the study of mechanisms and factors involved in the spread and distribution of disease (or injury) within and/or between populations.Asks “who, when, where, why and how” people get sick or injured.Epidemiologists partly rely on knowledge of pathology and pathologists. Pathology = the formal study of disease at the individual level. * diagnosis based on unique… - symptoms (patient feels) and - signs (physician measures); * identification of cause (= Etiology); * understanding the pathogenesis (= disease development) * determining the effects on the body;

2. Etiology: Studying the Cause of DiseaseKoch’s Postulates: 1) same pathogen; 2) isolate and grow in pure culture; 3) cause the same disease in a healthy host; and 4) re-isolate the same. Doesn’t work for human viruses; non-culturable microbes; consortial diseases; pneumonias and diarrheal diseases which both can be caused by the same bacterium; conversely each of these disease types can be caused by several different agents.

3. Types of Infectious Disease • Communicable:exogenous bacteria transmitted from one host to another by direct contact or indirectly contact. Contagious diseases are easily spread. • Non-Communicable: endogenous bacteria of host (normal microbiota) or bacteria in nature that only produce disease when introduced into the body. • Local (specific site) versus Systemic (Body-wide) • Bacteremia (septicemia), Toxemia, Viremia (All relate to something in the blood) • Primary versus Secondary Infections • Emerging Infectious Disease (SARS)

4. Stages of Disease Development Carrier? Carrier? Carrier? Susceptible to 2º infection

5. Carriers of Infectious Disease Causative agent is Salmonella typhi

6. Frequency of Disease Occurrence Incidence = number of people that develop disease (new cases) in a given time period (indicates spread). Prevalence = total numbers of infected people in a population at a given time.

7. Classification by Morbidity * Spordic: occasionally with low prevalence. * Endemic: constantly present. • Epidemic: sudden or gradual increase in occurrence above normal expectations in a population. • Common source (non-communicable; sudden peak in incidence at once; cases cluster in time and space) • Progressive (communicable; gradual exponential rise in incidence; increasing spatial distribution of cases) * Pandemic: basically a worldwide epidemic.

9. Decline (Control) of Incidence • How can an epidemic be contained or disease incidence return to expected levels? • Natural Process: “Herd Immunity” • Prevalence increases up to a some threshold level. • Much of the population has naturally acquired immunity. • Probability of contact of susceptible individuals is reduced. • Incidence of disease declines. • Artificial Processes: • Vaccination Programs • Chemo-therapeutics • Incidence of disease declines.

10. The purposes of epidemiology are: 1. to define distribution and size of disease problems within and between populations; 2. to understand reservoirs and transmission of infections; 3. to identify contributing factors in pathogenesis of the disease (who has predisposing factors and are most at risk?); and 4. to provide a basis for developing & evaluating preventive procedures and public health practices. Not just a study of the present human population!

11. History of Infectious Diseases: These Andean mummies (~2000 yBP) were shown to have suffered from Mycobacterium tuberculosis, the cause of TB. However, Andean populations did not suffer from other infectious diseases found widespread in contemporary populations of Northern Africa (Egypt) and Europe

12. Reservoirs of Infectious Agents(= Any continuous source of infectious agent) • Human: “carriers” (any infected but non-symptomatic individual); subclinical, latent disease, ill or convalescent people. • Animals: zoonoses (wild and domestic; mammal to insects) • Non-living reservoirs: soil, water, surfaces

13. Methods of Transmission • Contact Transmission: • Direct: • person-to-person contact (kissing, sex, casual contact); • person-to-animal contact (bites from bugs or animals) • Indirect: via nonliving object (fomite) that infectious persons were in contact with (money to surgical implements) • Droplet transmission (droplet nuclei): sneezing and coughing; short distance (1 m). • Vehicle Tranmission: • Foodborne; waterborne; airborne • Vectors (often arthropods; mechanical or biological)

14. Portals of Entry and Exit Entries: • Respiratory (airborne) • Mouth (water, food, kissing, other foreign objects • Penetration across epithelial barriers (bites, wounds, injections) • Sex • Catheterization • IV fluids and blood transfusions Exits: • Sloughed off skin • Insect bites • Pussing or weeping lesions or exposed infection • Sex • Urine • Feces • Respiratory and oral secretions (expectoration; aerosolization)

15. Nosocomial InfectionsDon’t get it until after arriving to hospital; 5-15% all patients. 1. Microorganisms: 36% rise over past 20 years; attributed largely to rise of antibiotic resistant strains of bacteria. 2. Compromised Host: (worst state of predisposition) Damaged Defenses: broken skin or mucus membrane (non-specific), suppressed immunity (specific) Invasive Procedures: surgeries 3. Chain of Transmission: from direct contact to food, needles, surgical implements, linens.

16. Since 1990s the major bugs are:GRAM POSITIVES: Staphylococcus aureus & Enterococcus spp. (surgical wounds, urinary, septicemia; pneumonia) 34%;GRAM NEGATIVES: Escherichia coli, Psuedomonas aeruginosa, Enterobacter spp., Klebsiella. pneumoniae (surgical wounds, pneumonia) 32%.Major Target Tissues of Infection:

17. Descriptive Epidemiology • To describe the occurrence of a disease, it’s necessary to answer the questions of who? where? and when? Mostly retrospective studies (lock back on what happened). • Person (Who)? • age, gender, race, culture, socioeconomic status, occupation, marital status, maternal age. • Place (Where)? • Natural (climate; environment); • Political (less useful as are more arbitrary relative to predisposing (risk) factors. • Time (When?) patterns to occurrence of disease over time: • Secular (long-term); cyclic (e.g. seasonal); short-term (epidemic).

18. Why is it important to look beyond annual data? Greater time resolution of incidence can show you what?

19. When was Typhoid Fever Epidemic versus Sporatic?

20. Why did Cholera Spread to the US?1991 pandemic of cholera by Vibrio cholerae O1 Inaba strain.Gold states had cases in 1992. Blue area show coastal waters with the identical strain of the bacterium. This bacterium is common to the marine environment. Typically transmitted by the fecal-oral route.

21. Analytical Epidemiology: • Deals with making comparisons between infected and unaffected populations with or without certain risk factors (age, gender, race, etc..). • The goal is to establish associations between risk factors & events; may even try to establish probable cause. • Two approaches to probable cause studies: 1. Case Control Method: find 2 pops. with/without disease; then compare pops. by specific factors; retrospective analysis 2. Cohort Method: find 2 pops. with/without a factor/event; then compare occurrence of disease; prospective analysis

22. Experimental Epidemiology: • Experimentation begins with a hypothesis based on preexisting data. • Human subjects are divided into a treatment population (drug) and a control population (placebo). A drug trial is a good example. • Although it has happened over history, the strategy of infecting human subjects with infectious disease for the purpose of experimentation is highly unethical.