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TORTORA • FUNKE
• CASE
Microbiology
AN INTRODUCTION
EIGHTH EDITION
B.E Pruitt & Jane J. Stein
Chapter 14
Principles of Disease and
Epidemiology
Principles of Disease
and Epidemiology
• Pathology
• Disease
•
•
•
•
Study of disease
in a state of not being healthy,
change from health
Pathogens
disease causing organisms
Etiology
Study of the cause of a disease
Pathogenesis Development of disease
Infection
Colonization or invasion of
pathogens, may be microbes in
the
wrong place (E. coli in the
urinary
tract)
Normal Microbiota (Flora) and the
Host
• Normal Flora - the normal bacteria in you and on you
– You have 1013 eucaryotic cells and 1014 prokaryotic cells
– Within 8 - 12 hours of life you are colonized by normal flora
(microbiota).
• Breast feeding versus bottle - different organisms
• Transient microbiota may be present for days, weeks,
or months
• Microbial antagonism
– Normal microbiota overwhelm pathogens - no place for them
to colonize
• Intestines and vagina - excessive antibiotics disrupts balance
– Vagina normally pH ~ 4 with Lactobacillus spp without can lead to
Candida infections
• Symbiosis is the relationship between normal
microbiota and the host
Normal Microbiota and the Host:
• In commensalism, one organism is benefited and the other
is unaffected.
• In mutualism, both organisms benefit.
• Be able to give a few examples of mutualistic bacteria
• In parasitism, one organism is benefited at the expense of
the other.
• Some normal microbiota are opportunistic pathogens.
– E. coli and urinary tract
– Pneumocystis carinii and respiratory system
– Streptococcus pneumoniae and pneumonia
Normal Microbiota and the
Host:
• Locations of
normal
microbiota on
and in the
human body
Figure 14.2
Normal Microbiota and the Host:
• Microbial antagonism is competition between
microbes.
• Normal microbiota protect the host by:
– occupying niches that pathogens might occupy
– producing acids
– producing bacteriocins
• Probiotics are live microbes applied to or
ingested into the body, intended to exert a
beneficial effect.
– Lactobacillus spp
Koch’s Postulates
• Koch's
Postulates
are used to
prove the
cause of an
infectious
disease.
Figure 14.3.1
Koch’s Postulates
• Koch's Postulates are used to
prove the cause of an infectious
disease.
• Problems with Koch’s Postulates:
• Not all diseases have bacterial
etiologies
– Genetic
– Degenerative
– Congenital
• Exceptions
– Not culturable
• Treponema / Rickettsia / Chlamydia /
viruses
– Some pathogens cause many
different diseases
Figure 14.3.2
Classifying Infectious Diseases
• Symptom
that is
result of
• Sign
be
result
• Syndrome
A change in body function
felt by a patient as a
disease
A change in a body that can
measured or observed as a
of disease.
A specific group of signs and
symptoms that
accompany a
disease.
Diseases may be
grouped by Diseases
how spread
Classifying
Infectious
• Communicable diseaseA disease that is
spread
from one
host to another.
• Contagious disease
A disease that is
easily
spread from
one host to
another.
• Noncommunicable disease A disease that is
not
transmitted
from one host
to
another. Example:
By occurrence of Disease
• Incidence
Fraction of a
population that
contracts a disease during a
specific time.
• Prevalence
Fraction of a population
having
a specific disease
at a given time.
• Sporadic disease Disease that occurs
occasionally in a population.
• Endemic disease Disease constantly present
in a
population.
• Epidemic disease Disease acquired by many
hosts in a given area in
By Severity or Duration of a
Disease
• Acute disease
Symptoms develop
rapidly
• Chronic disease
Disease develops
slowly
• Subacute disease Symptoms between acute
and
chronic
• Latent disease
Disease with a period
of no
symptoms when
the patient is
By Extent of Host Involvement
• Local infection
Pathogens limited to a
small area
of the body
• Systemic infection An infection throughout the
body
• Focal infection
Systemic infection that
began as
a local infection
• Bacteremia
Bacteria in the blood
• Septicemia
Growth of bacteria in the
blood
Extent of Host Involvement
• Toxemia
Toxins in the blood
• Viremia
Viruses in the blood
• Primary infection
Acute infection that
causes the
initial illness
• Secondary infection
Opportunistic infection
after a
primary
(predisposing) infection
Pneumocystis pneumonia and AIDS
• Subclinical disease No noticeable signs or
symptoms
(inapparent
Predisposing Factors
• Make the body more susceptible to disease
– Short urethra in females
– Inherited traits such as the sickle-cell gene
– Climate and weather
– Fatigue and Stress
– Age
– Lifestyle
– Chemotherapy
– Gender
Development of Disease
•
•
•
•
Stages of Disease
Incubation
infection up to first symptoms
may or may not
be variable
Prodromal
short period of early mild
symptoms -- malaise
Period of Illness
overt signs -- fever and chills,
swollen lymph
nodes, GI
disturbance increase in WBC’s
Period of Decline
signs and symptoms subside susceptible to
2˚ infections
The Stages of a Disease
Figure 14.5
Reservoirs of Infection
• Reservoirs of infection are continual sources of
infection.
– Human — AIDS, gonorrhea
• Carriers may have inapparent infections or latent diseases.
• Carriers may be in pre-symptom stage or recovery of a disease no symptoms
– Animal — Rabies, Lyme disease
• Some zoonoses may be transmitted to humans
– Plague / psittacosis / swine flu / bird flu
– Nonliving — Botulism, tetanus
• Soil
• Water - rivers, lakes, snow, oceans and laundry water
Transmission of Disease
• Three main routes
– Contact - Direct or indirect
– Vehicles - inanimate objects - e.g. food or drugs
– Vectors - arthropods
Contact
– Direct
Requires close association
between
infected and
susceptible host
– Indirect
Spread by fomites (inanimate
objects)
glass, toothbrush or
clothing
Transmission of Disease
Figure 14.6a & 8
Transmission of Disease
• Vehicle
Transmission by an inanimate
reservoir
(food, water)
Shigella, cholera, airborne on
dust aerosol >3’, tapeworm Staphylococci,
Streptococci, tuberculosis, fungal
spores -histoplasmosis,
coccidiodomycosis
• Vectors
ticks, and
Arthropods, especially fleas,
mosquitoes
– Mechanical Arthropod carries pathogen on feet
Transmission of Disease
Figure 14.6b, c
•
•
Nosocomial
(HospitalAre acquired as a result of a hospital stay
Acquired)
Infections
5-15% of
all hospital patients
acquire
nosocomial infections
Figure 14.7, 9
Nosocomial
Hospital acquired
• 5 - 15% acquire >20,000 per year die
•
why?
•
a) microbes in environment -- (lots of sick
people)
•
b) already sick or wounded -- compromised
host
•
c) close to people - chain of transmission
• Also resistant strains - E. coli, Pseudomonas,
enterics like Serratia
Relative frequency of
nosocomial infections
Figure 14.10
Common Causes of
Nosocomial
Infections
Percentage of nosocomial
Percentage resistant to
infections
antibiotics
Gram + cocci
Streptoccous and
Staphylococcus
34%
28%-87%
Gram – rods
Enterics and Pseduomonas
32%
3-34%
Clostridium difficile
17%
Fungi
10%
Emerging Infectious Diseases
• Diseases that are new, increasing in
incidence, or showing a potential to increase
in the near future.
• Contributing factors:
– Evolution of new strains
• V. cholerae O139
– Inappropriate use of antibiotics and pesticides
• Antibiotic resistant strains
– Changes in weather patterns
• Hantavirus
– Spread of human populations and travel
Emerging Infectious Diseases
• Contributing factors:
– Modern transportation
• West Nile virus
– Ecological disaster, war, expanding human
settlement
• Coccidioidomycosis
– Animal control measures
• Lyme disease
– Public Health failure
• Diphtheria
Epidemiology
• The study of
where and when
diseases occur
Figure 14.11
Epidemiology
John Snow
1848-1849
Mapped the occurrence of cholera
in London
Ignaz Semmelweis
1846-1848
Showed the hand washing
decreased the incidence of
puerperal fever
Florence Nightingale
1858
Showed that improved sanitation
decreased the incidence of
epidemic typhus
• Descriptive
Collection and analysis of data
regarding occurrence of disease
Snow
• Analytical
Comparison of a diseased group and a
healthy group
Nightingale
• Experimental
Study of a disease using controlled
experiments
Semmelweis
• Case reporting
Health care workers report specified
disease to local, state, and national
offices
• Nationally Notifiable
Diseases
Physicians are required to report
occurrence
Table 14.7
Centers for Disease Control and
Prevention (CDC)
• Collects and analyzes epidemiological
information in the U.S.
• Publishes Morbidity and Mortality Weekly
Report (MMWR) www.cdc.gov
Morbidity: incidence of a specific notifiable
disease
Mortality: deaths from notifiable diseases
Morbidity rate = number of people affected/total
population in a given time period