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Chapter 14: Principles of Disease and Epidemiology Principles of Disease and Epidemiology • Pathology Study of disease • Etiology Cause of a disease Development of disease • Pathogenesis • Infectious disease Caused by microorganisms Normal Microbiota and the Host • Locations of normal microbiota on and in the human body Figure 14.2 Normal Microbiota and the Host • Normal microbiota: permanently colonize the host ─ Prevent growth of harmful microbes ◦ Microbial antagonism: competition among microbes −Compete for nutrients, space −Affect pH, oxygen availability −Some produce bacteriocins (antibiotic proteins) • Some normal microbiota are opportunistic pathogens ─ Cause disease only in abnormal environments/ circumstances Normal Microbiota and the Host: Symbiosis Symbiosis: Relationship between normal microbiota and host Commensalism: One organism benefits, and the other is unaffected Mutualism: Both organisms benefit Normal microbiota Parasitism: One organism benefits at the expense of the other Opportunistic pathogens Normal Microbiota and the Host: Opportunistic Pathogens • Antibiotic-associated diarrhea (AAD) ─ Clostridium difficile ◦ Release of toxins leads to colon inflammation ◦ Also nonpathogenic strains www.textbookofbacteriology.net ─ One of the most common nosocomial infections around the world ◦ ~20% of US hospitalized patients acquire C. difficile ─ Result of normal intestinal microbiota disturbance by antibiotics ─ Can progress to more serious conditions (severe colitis) Koch’s Postulates: Determining infectious disease etiology • Koch's Postulates ─ Set of requirements that must be met to prove that a specific microbe causes a specific disease Application of Koch’s Postulates: Isolation Pure culture; Identification ◦ Framework for studying infectious disease etiology ─ B. anthracis causes anthrax Inoculation Figure 14.3.1 Koch’s Postulates: Determining infectious disease etiology Disease reproduction Original microbe re-isolation Figure 14.3.2 Stages of Disease Development Overt signs and symptoms Infection can be spread Figure 14.5 The Spread of Infection: Reservoirs of Infection • Reservoirs of infection: sources of disease organisms ─ Humans — AIDS, gonorrhea ◦ Carriers may have inapparent infections or latent diseases (incubation or convalescence periods) ─ Animals — Rabies, Lyme disease ◦ Some zoonoses may be transmitted to humans ─ Nonliving — Botulism, tetanus ◦ Soil, water, foods The Spread of Infection: Routes of Disease Transmission • Contact transmission • Vehicle transmission • Vector transmission The Spread of Infection: Routes of Disease Transmission • Contact transmission: spread of pathogen by ─ Direct contact (person-to-person) ◦ Touching, kissing, sexual intercourse ─ Indirect contact (via fomites) ◦ Fomites: nonliving objects involved in the spread of infection (bedding, eating utensils, thermometers) ─ Droplet transmission ◦ Sneezes, coughs ◦ Up to 20,000 droplets per sneeze! The Spread of Infection: Routes of Disease Transmission • Vehicle transmission: transmission by an inanimate reservoir (food, water, air) • Vector transmission: Animals transmit pathogens ─ Often arthropods (fleas, ticks, mosquitoes) ─ Arthropods can carry pathogens on their feet Nosocomial (Hospital-Acquired) Infections • Nosocomial infections: acquired as a result of a hospital stay ─ 5-15% of all hospital patients acquire nosocomial infections ─ Eighth leading cause of death in U.S. (~20,000/yr) Percentage of nosocomial infections Gram + cocci 34% (S. aureus, etc.) Gram – rods 32% Clostridium difficile 17% Fungi 10% Common Causes of Nosocomial Infections • Factors affecting nosocomial infection acquisition: ─ Presence of microorganisms in hospital ─ Compromised health/immune status of the host Broken skin/mucous membranes, immunosuppression ─ Effective chain of transmission Person-to-person, fomites, ventilation system, etc. • CDC: Handwashing is the single most important method of preventing the spread of infection Figure 14. 9 Epidemiology • Epidemiology: the study of where and when diseases occur ─ Identify the source and mode of transmission of a disease ─ Extensive population data collection and statistical analyses • Centers for Disease Control and Prevention (CDC) ─ Collects and analyzes epidemiological information in the U.S. ◦ Notifiable diseases: physicians are required by law to report cases to the US Public Health Service Table 14.7 Outbreak: 34 cases of C. tropicalis candiduria in 5 months in SICU 76 environmental swab samples 6 C. tropicalis isolates, all along urine disposal route All six matched DNA ‘fingerprint’ of patient isolates Reservoirs of infection identified: improperly disinfected urinals, urine collection bucket, med waste disposal sink Outbreak (patient) isolates Non-outbreak (unrelated) isolates