Disease, Disease Transmission, and
Epidemiology
Chapter 14
Table 14.1 The Three Types of Symbiotic Relationships
Symbiotic Relationships Between Microbes and Their Hosts
• Normal Microbiota in Hosts
– Organisms that colonize the body’s surfaces
without normally causing disease
– Two types
• Resident microbiota
• Transient 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
• Introduction of normal microbiota into unusual site
in body (cuts and abrasions)
• Immune suppression (radiation therapy)
• Changes in the normal microbiota (antibiotic use)
What is a disease?
• Infection
– When contaminating organism evades body’s
external defenses and multiplies in the host
– Most infections are eliminated by the bodies
defenses
• Disease
– Infections may lead to disease
– Results when normal body functions are altered
• 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 laboratory setting
– Mycobacterium leprae
» Never been grown in lab
Infectious Disease
Symptoms and Signs
• Symptoms
– Subjective characteristics of disease felt only by the
patient
• Signs
– Objective manifestations of disease observed or
measured by others
• 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
– Acute disease (rapid onset, short period of time ex.
Common cold)
– Chronic disease (develop slowly, continual or
recurrent ex. Hepatitis C, TB)
– Latent 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
• Incubation period (time between infection and first symptoms
and signs of disease)
• Prodromal period (short time, mild symptoms)
• Illness (severe stage, signs and symptoms most evident)
• Decline (gradual decline of signs and symptoms as body returns
to normal due to immune response or drug treatment)
• Convalescence (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?
• Reservoir of infection
– Sites where pathogens are maintained as a source of
infection
• Reservoir may or may not be harmed by pathogen
• 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
• Three types of reservoirs
– Human reservoirs
– Animal reservoirs
– Nonliving reservoirs
• Human reservoirs
– The principle living reservoir- the human body
– People with signs and symptoms may
transmit
– Infected individuals who are asymptomatic
but infective to others (before or after onset
of illness)
• Carriers
• individuals remain asymptomatic and infective for
years (ex. Tuberculosis, syphilis, and AIDS)
• Animal Reservoirs
– Zoonoses or Zoonotic diseases (150 diseases)
• Diseases naturally spread from animal host to
humans
– Acquire zoonoses through various routes
• Direct contact with animal or its waste (E. coli
O157:H7)
• Animal bites (Rabies)
• Insect bites (Lyme Disease)
• Nonliving Reservoirs
– Soil, water, and food 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
Movement of a pathogen into a host
• Portals of Entry
– Sites through which pathogens enter the
body
– Four major pathways Broken
•
•
•
•
Skin
Mucous membranes
Placenta
Parenteral route
skin
Insect
bite
Ear
Conjunctiva
of eye
Censored
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..
The Movement of Pathogens Out of a
reservoir: Portal of Exit
• 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)
Censored
Censored
Anus
(feces)
Seminal vesicles
(semen and lubricating secretions)
In females:
Mammary glands
(milk, secretions)
Vagina
(secretions, blood)
Urethra
(urine)
Microbial Mechanisms of
Pathogenesis
• Pathogenicity- the ability of a microorganism
to cause disease
• Pathogenesis- The mechanism a
microorganism uses to cause disease
• Virulence- the relative ability of a pathogen to
infect a host and cause disease.
A pathogens goal
– Enter a host
– Adhere to host cells
– Gain access to nutrients
– Evade detection and removal by immune system
in order to colonize
– Leave host
Virulence factors
• Traits a pathogen uses to:
– Enter a host
• Portals of entry
Virulence factors
– Adhere to host cells
• Adhesion factors
– Bacteria and viruses have surface proteins that enable them to bind to
complementary host receptors
» Adhesins on bacteria
• Fimbriae
» Attachment proteins on viruses
– Capsule
– Biofilm production
• Host specific interactions
– Adhesins and receptors determines the specificity of pathogens for
particular hosts
– Neisseria gonorrhoeae
» Has adhesins on its fimbriae that adhere to cells lining urethra and
vagina of humans, can not infect other hosts
» Can attach to sperm cells as they swim by!
Virulence factors
• Traits a pathogen uses to:
– Gain access to nutrients
• Produce extracellular enzymes
– Dissolve structural chemicals in the body
– To further invade and gain access to nutrients
• Produce Toxins
– Exotoxins
» Secreted by pathogen to destroy host cells
• Cytotoxins-Kill host cells or affect their function
• Neurotoxins- interfere with nerve function
• Enterotoxins- affect cells lining gastrointestinal tract
• Membrane damaging toxins
– Disrupt plasma membranes
– Hemolysins
• Blood cells
• Endotoxins
– Also called lipid A
– Lipid portion of Gram negative LPS
– Released when G – cells die naturally, divide, or
are digested by phagocytic cells
– Cause fever, inflammation, diarrhea, shock
Virulence factors
• Traits a pathogen uses to:
– Escape detection and removal by immune systems
• Antiphagocytic factors
– Capsules
» Capsules are composed of chemicals found naturally in the body
• Immune cells do not recognize them as foreign antigen
» Capsules are slippery which makes it difficult for phagocytes to adhere
– Antiphagocytic chemicals
» Preventing the fusion of lysosomes with phagocytic vesicles.
• Pathogen is able to survive inside phagocyte
– Avoiding Antibodies
» Secretion of proteases that break down antibodies
• IgA in body secretions bind to bacterial adhesins on pili
» Rapid turn over of pili
» Antigenic variation- genetically alter the type of pili made
» Fc Receptors
– Gonorrhea produce chemicals which prevents fusion of lysosome with
phagosome and secretes a protease which breaks down IgA in mucus.
• Leave host to “find” new host
– Portal of exit
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, indirect, or droplet
– Vehicle transmission
• Airborne, waterborne, or foodborne
– Vector transmission
• Biological or mechanical
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
• Foodborne intoxication
– Ingestion of an exotoxin produced by an organism
growing in a food product.
– Toxin causes illness not the living organisms
– Staphylococcus aureus and Clostridium botulinum
Vehicle Transmission: Food
• Foodborne infection
– Requires the consumption of living organisms
• 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
–Biological Vector
–Mechanical Vector
1. Biological Vector
• An organism transmits and serves as
host.
• Vector involved in life cycle of
pathogen
– Ex: Malaria: Part of life cycle of protozoan,
Plasmodium, occurs inside the Anopheles
mosquito
2. Mechanical Vector
Vector transmits disease causing organism
through mechanical contact
– 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
•
A few cases randomly distributed
geographically
Example:
• Plague in U.S.
Endemic
• A native disease that prevails continuously in a
geographic region
• 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!
• Hand washing is the most effective way to reduce
nosocomial infections
• CDC reports that on average, health care workers
wash their hands before interacting with patients only
40% of the time.
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