Disease, Disease Transmission, and
Epidemiology
Chapter 14
Symbiotic Relationships Between Microbes and Their Hosts
• Types of symbiosis
– Mutualism
– Commensalism
– Parasitism
Table 14.1 The Three Types of Symbiotic Relationships
Figure 14.1 Mutualism as shown by wood-eating termites
Symbiotic Relationships Between Microbes and Their Hosts
• Normal Microbiota in Hosts
– Organisms that colonize the body’s surfaces
without normally causing disease
– Two types
• __________microbiota
• __________microbiota
Symbiotic Relationships Between Microbes and Their Hosts
• Normal Microbiota in Hosts
– Acquisition of normal microbiota
• Development in uterus is free of microorganisms
• Microbiota begin to develop during birthing
process
• Much of your resident microbiota is established
during first month of life
Symbiotic Relationships Between Microbes and Their Hosts
• How Normal Microbiota Become Opportunistic
Pathogens
– Opportunistic pathogens
• Normal microbiota that cause disease under certain
circumstances
– Conditions that provide opportunities for pathogens
•
•
•
What is a disease?
• Infection
–
– Most infections are eliminated by the bodies
defenses
• Disease
– Infections may lead to disease
–
• Causation of Disease
– Germ theory of disease
• Disease caused by infections of pathogenic
microorganisms
– Robert Koch developed a set of postulates to
prove a particular pathogen causes a
particular disease
• Exceptions to Koch’s postulates
– HIV/AIDS
» Not ethical
– Syphilis
» Not culturable in lab
The Nature of Infectious Disease
• Manifestations of Disease: Symptoms and Signs
• Symptoms
–
• Signs
–
• Asymptomatic
– Infections that lack symptoms but may still have signs of
infection and can be transmitted
Classification of Infectious Diseases
• Terms used to classify infectious disease
– _________disease (rapid onset, short period of time
ex. Common cold)
– _________disease (develop slowly, continual or
recurrent ex. Hepatitis C, TB)
– _________disease (pathogen remains inactive for a
long period of time before becoming active ex.
shingles)
• The Stages of Infectious Disease
– Many infectious diseases have five stages
following infection
• ____________period (time between infection and first symptoms
and signs of disease)
• _____________period (short time, mild symptoms)
• ____________(severe stage, signs and symptoms most evident)
• ____________(gradual decline of signs and symptoms as body
returns to normal due to immune response or drug treatment)
• _____________(patient recovers no signs or symptoms)
– Patient can be infectious at any stage of disease
depending on the causative agent
Number of microorganisms or
intensity of signs or symptoms
Figure 14.10 The stages of infectious diseases
Incubation Prodromal
period
period
(vague,
(no signs or
general
symptoms)
symptoms)
Illness
(most severe signs
and symptoms)
Time
Decline
(declining signs
and symptoms)
Convalescence
(no signs or
symptoms)
How do we acquire a disease causing
organism?
• Most pathogens cannot survive for long
outside their host
– A successful pathogen needs to overcome the innate and
adaptive defenses long enough for the pathogen to
multiply and exit the host
• It is a disadvantage for the pathogen to actually kill a
host, since the opportunity to be transmitted may be
limited and it loses the source of nutrients
•
Reservoirs of infection
• Reservoirs of infection
–
• Reservoir may or may not be harmed by pathogen
• Three types of reservoirs
– __________reservoirs
– __________reservoirs
– __________reservoirs
• Human reservoirs
– The principle living reservoir- the human body
–
– Infected individuals who are ___________
but infective to others (before or after onset
of illness)
– Some individuals remain asymptomatic and
infective for years (ex. Tuberculosis, syphilis,
and AIDS)
•
• Animal Reservoirs
– __________or Zoonotic diseases (150 diseases)
• Diseases naturally spread from animal host to
humans
– Acquire zoonoses through various routes
• _____________with animal or its waste (E. coli
O157:H7)
• __________animals (tape worm)
• ____________(Rabies)
• ____________(Lyme Disease)
• Nonliving Reservoirs
– _______________________can be reservoirs
of infection
• Presence of microorganisms often due to
contamination by feces or urine
• Can be naturally occurring in environment (Tetanus
and botulism)
• Water (Example: parasitic eggs or E. coli)
• Meats and vegetables
The Movement of Pathogens Out of a
reservoir: ______________
• Pathogens leave host through portals of exit
• Many portals of exit are the same as portals of
entry
• Pathogens often leave hosts in materials the
body secretes or excretes
Figure 14.11 Portals of exit
Eyes
(tears)
Ear
(earwax)
Nose
(secretions)
Broken skin
(blood)
Mouth
(saliva, sputum)
Skin
(flakes)
In females:
Mammary glands
(milk, secretions)
Vagina
(secretions, blood)
Anus
(feces)
Seminal vesicles
(semen and lubricating secretions)
Urethra
(urine)
Movement of a pathogen into a host
• Portals of Entry
– Sites through which pathogens enter the
body
– Four major pathways Broken
•
•
•
•
skin
Insect
bite
Ear
Conjunctiva
of eye
Mouth
Placenta
Anus
Vagina
Penis
Urethra
• Portals of Entry
– Skin
• Outer layer of dead skin cells acts as a barrier to
pathogens
• Some pathogens can enter through openings
or cuts
• Others burrow into or digest outer layers of skin
• Portals of Entry
– Mucous membranes
• Line the body cavities that are open to the
environment
• Respiratory tract is the most common site of entry
– Entry is through the nose, mouth, or eyes
• Gastrointestinal tract may be route of entry
– Must survive the acidic pH of the stomach
• Portals of Entry
– Placenta
• Typically forms effective barrier to pathogens
• Pathogens may cross the placenta and infect the
fetus
– Can cause spontaneous abortion, birth defects,
premature birth
– Listeria (lunch meats)
• Portals of Entry
– Parenteral route
• Not a true portal of entry
– A way portals of entry can be bypassed
• Mechanically depositing pathogens directly into
tissues beneath the skin or mucous membranes
– Hypodermic needles
– Thorns, nails, etc..
Modes of Infectious Disease
Transmission
• Transmission is from a reservoir or a portal of
exit to another host’s portal of entry
• Three groups of transmission
–
•
–
•
–
•
Contact Transmission
Direct Contact
• One person physically touches another
– Handshake
– Sexual intercourse
– Fecal-oral
Contact Transmission
Indirect Contact
• Transfer of pathogens via inanimate
objects (called fomites)
– Clothing
– Doorknobs
– Glasses and eating utensils
Ex. Scabies: Bedding serves as fomite
involved in transmission
Contact Transmission: Droplet
• Transmission of a pathogen through inhalation of
respiratory droplets.
– Large microbe saturated droplets fall to the ground
no farther than 3 feet from release.
• Close proximity necessary
– Coughing
– Sneezing
• Examples of diseases:
– Cold, Flu
Vehicle Transmission: Airborne
• Transmission of a pathogen that is suspended in the
air and travels through the air to another host
• Very difficult to control!
• Remains suspended indefinitely
– Coughing
– Sneezing
• Examples of diseases:
– Tuberculosis, Measles
Vehicle Transmission: Food
• Usually digestive tract diseases
• Contamination can occur in a variety of different
ways
• Animals can harbor pathogens
– Salmonella
– E. coli O157:H7
• Inadvertently added during food preparation
– No handwashing
• Staphylococcus aureus
– Cross-contamination
• Cutting board used on meat and vegetables for your salad
• Meat and vegetable processing facilities
Vehicle Transmission: Food
•
–
– Toxin causes illness not the living organisms
– Staphylococcus aureus and Clostridium botulinum
Vehicle Transmission: Food
•
–
• Undercooked hamburger or chicken
– Symptoms do not usually appear for at least 1 day
• Diarrhea
– Thorough cooking before consumption will kill the
organisms
– E. coli O157:H7
– Salmonella
Vehicle Transmission: Water
• Can involve large numbers of people
• 1993 Cryptosporidium parvum (intestinal
parasite)
– Watery diarrhea 10-15L a day
– Infectious dose: less than 10 organisms
– Milwaukee, WI
• Infected ~400,000 people from municipal water source
Vector Transmission
–
–
1. Biological Vector
• An organism transmits and serves as
host.
•
– Ex: Malaria: Part of life cycle of protozoan,
Plasmodium, occurs inside the Anopheles
mosquito
2. Mechanical Vector
– Ex: Trachoma (Blindness): Chlamydia
trachomatis carried on feet of fly from
infected person's eye to eye of new host
Epidemiology
• Epidemiology focuses on the effect of a pathogen in a
population
– The study of where and when diseases occur and how they are
transmitted in a population.
• Why is this field important?
– Earth’s population is becoming over populated.
– Humans are relying more on mass food production and
distribution
– Travel to other countries very readily.
– Leads to higher incidence, number of new cases of a disease in
a population)
– Prevalence, the total number of cases, new and already existing
in a population
Occurrence of an infectious disease
can be classified in terms of
geographic distribution and frequency.
•
•
•
•
•
Sporadic
Endemic
Outbreak
Epidemic
Pandemic
Sporadic
•
Example:
• Plague in U.S.
•
Endemic
• Endemic disease can lead to epidemic
Example:
• Pertussis in U.S.
• Common cold
• Influenza
Year
Reported Cases*
2000
7,867
2001
7,580
2002
9,771
2003
11,647
2004
25,827
2005
25,616
2006
15,632
2007
10,454
2008
13,278
2009
16,858
2010
27,550
2011*
18,719
*Total reported cases include those with
unknown age.
Outbreak
• Implies a cluster of cases occurring during a
brief period of time and attacking a specific
population, usually food borne
• E. coli O157:H7 from contaminated food sources
Seventy-seven
people sick and one
died in a salmonella
outbreak caused by
contaminated
ground turkey:
Centers for Disease
Control 2011
Epidemic
• Affecting an unusually large number of
individuals within given region or population
• Epidemic may lead to pandemic
• H1N1 was epidemic in US in 2009
• Pandemic- epidemic on more
than one continent
– Example 2009 H1N1, also known as Swine Flu, breaks
out and quickly spreads to more than 70 countries.
Pandemic
• A disease affecting an increased proportion of
the population over a wide geographic area,
most often worldwide
Example:
• AIDS
• Local and state level health departments rely
on doctors and hospitals to report infectious
diseases.
• In some cases the disease occurrence is
nationally recorded to the Center for Disease
Control (CDC).
– Weekly updates can be viewed of the notable
diseases in the Morbidity and Mortality Weekly
Report (MMWR).
Epidemiology of Infectious Diseases
• Hospital Epidemiology: Nosocomial
Infections
– Control of nosocomial infections
• Disinfection, good housekeeping, bathing, sterile
procedures, and HAND WASHING!
•
• CDC reports that on average, health care workers
wash their hands before interacting with patients only
__________________
Figure 14.20 The interplay of factors that result in nosocomial infections
Factors that
influence
nosocomial
infections:>>>>>
Immunocompromised
patients
Presence of
microorganisms in
hospital environment
Infection can
result from any
one of these
factors but
usually it is a
product of all
three
Nosocomial
infection
Transmission of
pathogens between
staff and patients
and among patients
Emerging Disease
• What is an emerging Disease?
• Examples:
– West Nile virus
– HIV/AIDS
– Cat Scratch Fever
Factors that contribute to disease
emergence
• Microbial evolution
• Advances in technology
• Mass production, widespread distribution and
importation of food
• Climate Change
Reemerging Diseases
• Resurgence of old diseases due to:
–
–
–
Diseases include:
TB, Pertussis
Tuberculosis reemerging in US
• Mycobacterium tuberculosis
– Causes an infection of the lower respiratory
system
– Inhalation of airborne bacteria from infected
persons
– 9 million people each year develop TB
• 2 million deaths
• About 2 billion people world wide are infected
• 14,000 cases annually in US usually from foreign born
individuals. Increasing at rate of about 2% each year.
Pertussis commonly called whooping
cough
• Bordetella pertussis
• Causes an infection of the lower respiratory system
• Before DTP vaccination 50% of individuals would develop
pertussis.
• Vaccination significantly decreased this disease
– Since 1980s number of cases has increased significantly
affecting adults and adolescents.
• Individuals are not up to date on booster shots which are
intended to boost immunity
• Immunity to the DTP vaccine declines after a few years
– No protection by age 12