MMBIO 221: Epidemiology Part 2 - Week 2
MMBIO 221: Epidemiology Part 2 - Week 2 MMBIO221
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This 7 page Class Notes was uploaded by Kirsten Notetaker on Friday September 9, 2016. The Class Notes belongs to MMBIO221 at Brigham Young University taught by Donald Breakwell in Fall 2016. Since its upload, it has received 9 views. For similar materials see General Microbiology in Biology at Brigham Young University.
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Date Created: 09/09/16
Epidemiology Part 2 Cycle of Infection 1. Infectious Microbe -> 2. Reservoir -> 3. Portal of Exit -> 4. Modes of Transmission -> 5. Portal of Entry -> 6. Susceptible Host -> back to Infectious Microbe 1. Bacteria, fungi, virus, prion, protozoa 2. Place where microbe lives & replicates (i.e. people, equipment, water, food, animals) 3. Place where microbe leaves reservoir (i.e. coughing, sneezing, bleeding, feces) 4. Contact (i.e. hands, sharps injury), airborne vehicle (DUWL), insect vector 5. Entry point (i.e. wound, opening in skin or mucosa of mouth, sutures, catheters, IV lines) 6. Organism which is not immune to the microbe (i.e. humans, animals, etc.) Example: Bubonic Plague Infectious agent: Yersinia pestis Host: humans & other mammals Portal of exit: skin Reservoir: rats, prairie dogs Transmission: fleas Portal of entry: skin Example: Measles Infectious agent: measles virus Host: humans Portal of exit: mucus Reservoir: humans Transmission: respiratory droplets Portal of entry: nasopharynx, mucous membranes Robert Koch Studied transmission & process of disease Studied anthrax, tuberculosis, cholera, malaria Koch’s postulates 1. The suspected pathogenic organism should be present in all cases of disease and absent from healthy animals Problems: some hosts are carriers but healthy, some diseases are caused only by multiple microbes at once or could be caused by several different microbes, and some aren’t even caused by microbes, suspected pathogen may not be found in the blood 2. The suspected organism should be grown in pure culture Problems: viruses need a living cell in order to reproduce, conditions in a petri dish may not be similar enough to the body to allow growth 3. Cells from a pure culture of the suspected pathogen should cause disease in a healthy (susceptible) animal Problems: some microbes require a certain portal of entry (i.e. skin vs. blood) 4. The organism should be re-isolated and shown to be the same as the original Problems: same as #2 Despite these problems, this was groundbreaking for the time (~2 centuries ago) and these postulates are still discussed Factors affecting the severity of an infectious disease N = dose of infecting microorganisms V = virulence R = resistance of host to infection S = severity of disease S = (N x V)/ R (equation is only conceptual, as a way to visualize this – not actually used) As N or V increases, S increases As R increases, S decreases This is NOT a quantitative analysis – it is just a way to think about it, you couldn’t actually calculate this Virulence and Host Resistance Effectiven Immunized Pathogens Non pathogens ess host Normal host Many commensals or Mostly of opportunists saprophytes Defense No Mechanis diseas ms e Mild or subclinical disease Compromise Clinical d host disease High Moderate Low None Level of virulence of microorganisms The host Environmental factors Anything that leads to poor hygiene, improper food preparation, lack of sanitation, and improper medical care Genetic factors Any genetic predisposition for a disease Social factors Stress, sexual behaviors, excessive drinking Phases of an infectious disease Incubation = time between exposure to pathogen & the initial onset of any symptoms Prodrome = initial onset of symptoms (prodromal symptoms) Clinical = typical signs & symptoms of disease Decline = symptoms begin to subside Convalescence = recovery or regaining strength (individual is considered healthy again) Disease Transmission Vehicle Conta Vector Water ct Mechani Food Drople cal Air t Biologica Direct l Vehicle Transmission Water – inadequate sewage treatment or water filtration Food – improper preparation and storage Infection – ingesting bacteria which reproduce & produce toxins inside you Intoxication – ingesting toxins produced by bacteria (not the actual live bacteria) Air – droplet nuclei from long distances (size is ~ 5 micrometers) In other words: Air droplet (vehicle) ≤ 5 micrometers, long distances Droplet (contact) ≥5 micrometers, short distances, victim is directly hit Droplet nuclei Contain mucus, debris, microbes Large particles settle rapidly, dry to become dust Evaporation decreases size and settling rate increases (they take longer to fall from the air) Dust can be suspended in the air Drying rate depends on temperature and humidity Transmission may occur over long distances Inhaled droplets (bigger than 10 micrometers) are trapped in the nose and usually do not make it to the bronchi, but droplet nuclei smaller than 5 micrometers may make it all the way to the alveoli in the lungs Producing droplet nuclei (DN) Cough – 1 good cough produces ~500 DN, ~230 are left in the air after 30 minutes Speech – counting from 1 to 100 produces ~1800 DN Prevention of vehicle transmitted diseases Proper storage of food at proper temperatures Thorough cooking of food Frequent and thorough handwashing, especially after bathroom use Adequate sewage treatment and water filtration/chlorination systems Disinfection of frequently touched surfaces to prevent indirect contact transmission Increase public awareness of safe practices Contact transmission Droplet – large droplets at short distances Direct – person-to-person (or animal) physical contact Indirect – fomites (inanimate objects which were contaminated and then touched) Droplet contact Generated by cough, sneeze, or talking Droplets contact eyes, nose, or mouth Droplets that contact a host are too large to be airborne for long periods of time, and quickly settle out of the air Direct contact – physical contact between infected person and susceptible person (or contact with oral or body secretions) Ex: touching, kissing, sexual contact, contact with body lesions Pathogens spread by direct contact are often unable to survive for significant periods of time away from a host Fomites – could be anything really, any inanimate contaminated surface Nosocomial bacteria on surfaces survive longer than we would like to think – for weeks! Vector Transmission Mechanical – microbes are ON an arthropod’s body Transmitted by contact with exterior of arthropods Biological – microbes are IN an arthropod’s body Arthropod = insect Biological vectors Transmit diseases by biting, stinging, or infesting tissues Most important disease vectors (more than 85% of animal species are arthropods) Mosquitoes and ticks are the most significant Others include fleas, lice, and sand flies
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