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Microbiology
AN INTRODUCTION
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
• Your microbiome
• You have 1013 eukaryotic cells and 1014 prokaryotic cells
• Most of our gut enzymes from gut microbiome
• 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
Symbiosis 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
Symbiosis
Normal Microbiota and the Host:
• Locations of normal
microbiota (flora) on
and in the human
body
Figure 14.2
Normal Microbiota (Flora) 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
Figure 14.3 Koch’s Postulates: Understanding Disease.
1 Microorganisms are isolated
3 The microorganisms
2a The microorganisms are
from a diseased or dead
animal.
are injected into a healthy
laboratory animal.
grown in pure culture.
Colony
2b The microorganisms
4 Disease is reproduced
are identified.
in a laboratory animal.
5a The microorganisms are isolated
from this animal and grown in
pure culture.
5b Microorganisms
are identified.
The microorganism from the
diseased host caused the same
disease in a laboratory host.
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 A change in body function that is felt by a patient as a
result of disease
• Sign
A change in a body that can be measured or observed as
a result of disease.
• Syndrome A specific group of signs and symptoms that accompany
a disease.
Classifying Infectious Diseases
Diseases may be grouped by how spread
• Communicable disease
A 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:
Clostridium tetani
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 a short
time.
Worldwide epidemic.
• Pandemic disease
Herd Immunity
Herd immunity
Immunity in
most of a population.
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
inactive Shingles
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
A specific tissue type
• Bacteremia
Bacteria in the blood
• Septicemia
Growth of bacteria in the blood
• Toxemia
Toxins in the blood
• Viremia
Viruses in the blood
Extent of Host Involvement
• 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 infection) Hepatitis /
Typhoid Mary / Polio
Predisposing Factors
Make the body more susceptible to disease
• Inherited traits such as the sickle-cell gene
• Climate and weather
• Fatigue and Stress
• Age
• Lifestyle
• Chemotherapy
• Gender
• urethra shorter in females therefore more UTI’s
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
• Period of Convalescence
regain strength and recovery BUT may be
a reservoir
The Stages of a Disease
Figure 14.5
Time
Period of convalescence
Prodromal period (mild signs or symptoms)
Incubation period (no signs or symptoms)
Number of microbes
Figure 14.5 The stages of a disease.
Period of
illness
Period of
decline
Most severe signs
and symptoms
Signs and
symptoms
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
Anthroponotic diseases (anthroponosis)
• Animals contract from humans
• There are many more zoonotic
diseases (animal to human) than
anthroponotic diseases involving
animals (human to animal), but a
few examples of human
conditions that animals may
contract include the following:
• Avian influenza- human to
bird, (or vice versa)
• Strep throat- human to dogs
(or vice versa)
• Human influenza- ferrets
• Tuberculosis-human to dogs,
(rarely)
• Ringworm- (human to animal
and vice versa)
Transmission of Disease
•
Three main routes
1. Contact -
Direct or indirect
2. Vehicles - inanimate objects - e.g. waterways, air,
food or drugs
3. Vectors -
arthropods
1. Contact
•
Direct
Requires close association between
infected and susceptible host
•
Indirect
Spread by fomites (inanimate objects)
glass, toothbrush or clothing
•
Droplet
Transmission via airborne droplets
Transmission of Disease
Figure 14.6a & 8
Transmission of Disease
• 2. Vehicle
Transmission by an inanimate reservoir
(food, water) Shigella, cholera, airborne on
dust aerosol >3’, tapeworm Staphylococci,
Streptococci, tuberculosis, fungal spores -histoplasmosis, coccidiodomycosis
• 3. Vectors
Arthropods, especially fleas, ticks, and
mosquitoes
• Mechanical Arthropod carries pathogen on feet
• Biological
Pathogen reproduces in vector and
bites host. Dengue fever, Yellow fever,
Malaria, encephalitis, plague, Lyme disease
RMSF
Transmission of Disease
Figure 14.6b, c
Figure 14.7 Vehicle transmission.
Water
Food
Air
Figure 14.8 Mechanical transmission.
Figure 12.31 Mosquito.
Nosocomial (Hospital-Acquired) Infections
• Are acquired as a result of a hospital stay
• 5-15% of all hospital patients acquire nosocomial
infections
During the study period, 97 out of 333 patients acquired nosocomial infection. The frequency of nosocomial infection was 29.13%.
Respiratory tract infection was seen in 29 (30.1%), urinary tract infection in 38 (39.1%) and blood stream infection in 23 (23.7%)
patients. Other infections we identified were skin, soft tissue, wound and gastrointestinal tract infections.*
http://www.ncbi.nlm.nih.gov/pubmed/19999200
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, MRSA,
VRE, C. difficile enterics like Serratia
•
Control by aseptic techniques
•
Antibiotic abuse
Common Causes of Nosocomial Infections
Percentage of
Total Infections
Percentage
Resistant to
Antibiotics
Coagulase-negative
staphylococci
S. aureus
15%
89%
15%
80%
Enterococcus
10%
4–71%
15–25%
3–32%
13%
Not reported
Gram-negative rods
C. difficile
Relative frequency of nosocomial infections
Figure 14.10
Common Causes of Nosocomial Infections
Percentage of
nosocomial infections
Percentage resistant to
antibiotics
Gram + cocci
34%
28%-87%
Streptoccous and
Staphylococcus and
VRE (Vancomycin
resistant Enterococci)
Gram – rods
32%
3-34%
Enterics and
Pseduomonas
Clostridium difficile
17%
Fungi
10%
Clinical Focus 14.1 Nosocomial Infections
Nosocomial
• Hospital Associated infections
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
• Ebola?
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
Figure 14.10 a & b Epidemiological graphs.
40,000
35,000
Number of reported cases
30,000
25,000
20,000
15,000
10,000
5,000
0
1999
2000
(a) Lyme disease cases, 1999–2010
2001
2002
2003
2004
Apr
May
Jun
2005
2006
2007
2008
2009
600
500
Reported cases per
100,000 people
400
300
200
100
0
Jan
(b)
Feb
Lyme disease by month, 2009
Mar
Jul
Month
Aug
Sep
Oct
Nov
Dec
2010
Figure 14.10c Epidemiological graphs.
120
Reported cases per
100,000 people
100
80
60
40
20
0
1948
1958
1968
1978
1988
Year
Reported tuberculosis cases, 1948–2010
1998
2008
Epidemiology
John Snow
1848-1849
Mapped the occurrence
of cholera in London
Ignaz
Semmelweis
1846-1848
Florence
Nightingale
1858
Showed the hand
washing decreased the
incidence of puerperal
fever
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 John Snow
disease to local, state, and national
offices
Physicians are required to report
occurrence
• Nationally Notifiable
Diseases
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
Mortality rate - number of deaths from a disease/total
population in a given time