Transcript Document

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