2. Etiology, Diagnosis, and Prevention of Infectious Diseases

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2. Etiology, Diagnosis, and Prevention of Infectious Diseases
2011-07-26 02:37:17
PH162A midterm2

public health microbiology midterm 2 lecture 2
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  1. etiology
    • the cause of a disease in general - cause of all cases of a disease in a population
    • can be singular or mixed
  2. diagnosis
    identification of a disease in a single case
  3. problems with postulate: the microorganism must be present in every case of the disease and absent in healthy host
    • microorganism may be present but difficult to detect, may need better techniques for detection
    • does not mean that disease must be present in every infection with the microorganism - iceberg concept, many people can carry an organism but not be sick
  4. problems with postulate: the microorganism but be isolated from the diseased host and grown in pure culture
    • pure culture means on an agar plate, where a single colony can be selected
    • not possible with viruses and some bacteria - obligate intracellular parasites, must be grown in cells so not pure
  5. problems with postulate: the specific disease must be reproduced when a pure culture of the microorganism is inoculate into a healthy, susceptible host
    may be difficult to find an animal model - must be susceptible and exhibit same disease, not ethical to use humans
  6. problems with postulate: microorganism must be recoverable once again from the experimentally infected host
    may be more difficult to recover from animal
  7. immunological postulates
    • there is an increase in either the cell-mediated or humoral immune response to the organism after onset of the disease
    • persons with no antibody or cell-mediated immune responses are susceptible to the disease, and persons with a high antibody level or strong cell-mediated response to the organism are resistant
    • the disease can be prevented by vaccination with a preparation of the organism or one with similar antigenic specificity
  8. Epstein-Barr virus (EBV)
    • first two immunological postulates showed EBV is the cause of infectious mononucleosis
    • - tested Yale students
    • - those developed IM were EBV antibody negative whne they entered and became EBV antibody positive after developing disease
    • - students who entered EBV antibody postive never developed disease
  9. Mareck's disease
    • 3rd immunological postulate showed that Mareck's disease is caused by Gallid herpesvirus 2
    • unsure of cause so created a vaccine that was antigenically similar to the virus
    • inoculated chickens were protected from disease
  10. symptoms
    • subjective evidence of a disease stated by the patient, but which cannot be verified by another person
    • malaise - a general feeling of discomfort
    • chill - a sensation of cold, usually occurs at initial stage of an infectious disease
    • nausea - an unpleasant sensation in the stomach, often leading to vomiting
  11. signs
    • objective evidence of disease recognizable by another person
    • lymphadenopathy - enlargement of the lymph nodes
    • rigor - visible shivering
    • vomiting - forcible expulsion of stomach contents through the mouth
    • diarrhea - increased frequency and liquidity of fecal discharges
    • cutaneous manifestations - abnormal appearance of the skin
  12. medical history
    • record of a patient's medical experiences and characteristics throughout life
    • age, race, diseases, medications, family history
    • occupation - may expose patient to a certain reservoir
    • exposure to pets - can result in zoonoses
    • underlying diseases
    • geographic areas of residence and travel
    • exposure to others known to have an infectious disease
  13. types of diagnosis
    • clinical
    • physical
    • differential
    • etiological
  14. clinical diagnosis
    • symptoms
    • signs
    • medical history
  15. physical diagnosis
    • made on the basis of objective physical measurements
    • palpation - examination by touching with hands
    • fever - body temperature
    • imaging techniques
  16. differential diagnosis
    distinguishes a disease from others with which it might be confused
  17. etiologic diagnosis
    • identification of a disease in terms of its cause
    • - for ID, means lab identification of the microorganism
  18. importance of etiologic diagnosis
    • proper treatment
    • clue about an underlying disease
    • controlling epidemics
    • required for reportable diseases
  19. reportable diseases
    • almost all are infectious
    • reasonably severe
    • most are diseases which have promise of being controlled by public health measures
  20. laboratory diagnosis
    • on the basis of morphology and structure
    • on the basis of biochemical properties
    • immunologic and molecular identification
    • host's immune response
    • DNA sequencing
    • strain typing
    • animal models
  21. morphology and structure
    • primary method for infections with protozoa, helminths and arthropods
    • used for fungal infections after growing fungus on agar
    • important for bacteria
    • morphology of host cells used for some viruses
  22. biochemical properties
    • intracellular reactions
    • - metabolic pathways - ability to use a particular carbon and end products, cell walls (gram stain)
    • extracellular products (bacterial enzymes)
    • - hemolysins - enzymes that lyse erythrocytes, detected by growing bacteria on agar media with sheep erythrocytes
  23. types of hemolysis
    alpha - partial lysis, greenish tint

    beta - complete lysis, clear

    gamma - no lysis
  24. immunological identification
    • a reagent antibody made in lab reacts specifically with a microorganism
    • antibody is labeled with a marker substance for detection
    • adheres tightly to microorganism and is not removed during washing
    • antibody from lab, antigen from patient
  25. molecular identification
    molecular probe: nucleic acid sequence complementary to sequence of microorganism, labeled with marker for detection, probe will hybridize and adhere if microorganism is present

    polymerase chain reaction: DNA extracted from specimen, nucleic acid primers specific for the suspected microorganism are reacted with the DNA, will amplify even a few copies

    DNA sequencing of 16S ribosomal RNA gene to determine genus/species
  26. utility of molecular and immunological probes
    • main use is on direct smear - taken directly from the patient
    • useful for difficult to culture organisms
    • useful for slow growing organisms
  27. serological tests
    • use host serum, react it with known laboratory antigen from a microorganism
    • measure a humoral immune response (antibodies)
    • patient provides antibody, lab provides antigen
  28. skin tests
    • measure the reaction of the host to antigens
    • injected between layers of skin
    • usually measures cell-mediated immunity
  29. elisa (enzyme-linked immunosorbent assay)
    • antigen attached to well
    • patient serum added (with antibody)
    • secondary antibody with enzyme added - binds patient antibody
    • substrate added, causes color producing reaction
  30. utility of serological and skin tests
    • causative agent cannot be cultured
    • causative agent is expensive to culture
    • causative agent is dangerous to culture
    • causative agent is slow growing
  31. strain typing
    antibiotic sensitivity - can indicate strains and be used to trace epidemics, help guide proper therapy

    phage sensitivity (lytic phages) - which phages a bacteria is sensitive to is strain specific, can use to trace epidemics

    • PCR and restriction based methods - organism specific
    • Pulsed-field gel electrophoresis - whole genome restriction
    • multilocus sequence typing - DNA sequencing of 7 housekeeping gene loci
    • gold standard - full genome sequencing
  32. strain typing utility
    • important to distinguish to the strain level to =
    • determine vehicle of spread for an epidemic
    • determine whether there is a common source for an epidemic
    • determine whether new cases of disease are part of same epidemic or new one
    • evaluate the effectiveness of control measures
  33. animal testing
    • used primarily to test virulence
    • animal injected, watched for morbidity and mortality
    • most animal tests are no longer used - more human methods
  34. primary prevention
    prevention of the infection from ever occurring in the first place
  35. secondary prevention
    in a person already infected, prevention of disease or sequelae (pathological condition resulting from disease)
  36. means of prevention
    • quarantine
    • isolation
    • body hygiene
    • lifestyle
    • source-reduction in reservoir
    • vaccination