CHAPTER 14
PRINCIPLES OF DISEASE AND EPIDEMIOLOGY
Pathology
is the scientific study of disease. It includes:
1.
Etiology---the cause of the disease--disease-causing organisms are called
pathogens
2.
Pathogenesis---the manner in which the disease develops
3.
Structural and functional changes brought about by the disease
4.
Final effects of the disease on the body
Infection---invasion or colonization of the body by
pathogenic microorganisms. This term is also used to describe the presence of
normal flora organisms in an abnormal location.
Disease---an abnormal state in which part or all of
the body is not properly adjusted or is incapable of performing its normal
function--change from the state of health.
These
2 terms do not mean exactly the same thing. A disease can occur without the
involvement of microorganisms. And an infection can occur without disease
actually developing.
The
location of microorganisms can be important. Escherichia coli in the intestine are considered normal and would
not be classified as an infection. If some of the same organisms are
transferred to the urinary tract, they usually cause disease and then their
presence would be considered an infection.
NORMAL
MICROBIOTA (NORMAL FLORA)
All
animal bodies have a large population of microorganisms that are considered
normal inhabitants. These are more or less permanent residents and do not
produce disease under normal conditions. These microbes are known as normal
flora or normal microbiota. The number of microbes in the normal flora is often
greater that the total number of human cells.
Animals
developing in utero are normally free of microorganisms. During the birth
process and immediately after birth, characteristic microbial populations begin
to appear. Lactobacilli of the vagina
are usually acquired first. Breathing and feeding bring in other types.
In
addition to the normal flora, which are usually present at all times, transient
microbes may appear and remain for days, weeks, or months, and then disappear.
Microbes
in the form of normal flora are expected to be present in the following parts
of the body:
Skin-secretions from sweat & oil glands may
inhibit growth of some microbes, but large numbers of
microbes that tolerate these factors
are always present
Outer part of eye within the conjunctival sac
Nose & throat—some of these are potential pathogens, but
competition and other factors keep the
numbers of potentially dangerous
microbes relatively low
Mouth—normal mouth flora are mostly gram
positive, but in an unhealthy mouth the balance shifts to
gram negative
Large intestine—contains the largest numbers of resident
microbes in the body
Urinary system—lower urethra in both sexes often contains
some microbes, but the
urinary bladder should be sterile
Reproductive systems—no microbes in males except for lower
urethra; large population of lactobacilli in
the vagina which create an
environment hostile to digestive tract bacteria
RELATIONSHIPS
BETWEEN NORMAL MICROBIOTA AND HOST
Normal
flora organisms are desirable because they can prevent the growth of large
numbers of pathogens. This is called microbial antagonism and is mainly due to
competition among microbes. If pathogens come along, normal flora may affect
them in several ways:
1. Compete for nutrients
2. Produce substances harmful to the
pathogens
3. Influence pH or oxygen levels in a way
harmful to the pathogens
When
the balance is upset, pathogens which arrive or which have already been present
but in numbers too small to cause disease take the opportunity to multiply. An
example of this is the diarrhea or yeast infections often associated with
taking antibiotics. The antibiotics may have a greater effect on the normal
flora organisms than on the pathogens, thus upsetting the balance.
In
the mouth, streptococci (not the species that causes “strep throat,” but
others) produce compounds that prevent the growth of most other gram-positive
and gram-negative cocci. In the large intestine, Escherichia coli organisms produce bacteriocins, which inhibit the
growth of other related bacteria, such as Salmonella
and Shigella.
Also
in the large intestine, an organism called Clostridium
difficile is normally found but in very small numbers. Normal flora
organisms keep the C. difficile numbers
down, but when the balance is disturbed the C.
difficile may greatly outgrow the others, producing symptoms ranging from
mild diarrhea to severe or fatal colitis.
The
relationship between normal flora and the host is called symbiosis, which means living together. More specific
types of symbiosis include:
·
Commensalism—relationship in which one organism is
benefited and the other is unaffected. In this type of relationship with humans,
bacteria live on secretions and sloughed-off cells but do not benefit or harm
the host. Some bacteria normally found in the eye, the ear, and on the external
genitals have this type of relationship.
·
Mutualism—both organisms benefit. Bacteria that live in the large
intestine benefit by receiving shelter and a constant supply of nutrients. In
return, they synthesize vitamins that humans cannot synthesize. Even though the
bacteria are making these vitamins for their own use, the extra amounts that
are produced are absorbed and used by the host.
·
Parasitism---one organism benefits and the other is harmed. Pathogens fit
into this category.
Under
certain circumstances, relationships between organisms can change. For example,
common strains of Escherichia coli are
harmless or beneficial in the large intestine. However, E. coli is a very common cause of urinary tract infections if it
gains access to that area. Although unusual, it also can cause infections in
the lungs, spinal cord, or wounds. Opportunistic pathogens are organisms that
do not ordinarily cause disease in their natural habitat in a healthy person,
but in case of broken skin, damaged mucous membranes, or a weakened host, these
same organisms can cause serious disease. Most AIDS patients die of infections
caused by opportunistic pathogens that take advantage of the weakened immune
system. One type of pneumonia, caused by Pneumocystis
jiroveci, is seldom seen except in AIDS patients, although we have all
probably been exposed to the organism.
Many
people carry organisms that are regarded as pathogens, but may not be causing
their disease in that particular host. This includes echoviruses, adenoviruses,
Neisseria meningitidis, and Streptococcus pneumoniae.
In
some situations, microbes cooperate with each other in causing disease.
Synergism means that the effect of two microbes together is greater than the
effects of either acting alone. For example, pathogens that cause periodontal
disease may attach to receptors on the streptococci that are considered normal
flora of the mouth, not to the teeth or gums themselves.
ETIOLOGY OF
INFECTIOUS DISEASES (THE WAY THEY ARE CAUSED)
Although
not all diseases are infectious (caused by microorganisms), those that are due
to microbes are the topic of this section. The requirements for establishing a
specific pathogen as the causative agent of a particular disease are known as
Koch Postulates:
1. The same pathogen must be present in
every case of the disease.
2. The pathogen must be isolated from the
diseased host and grown in pure culture.
3. The pathogen from the pure culture must
cause the disease when it is inoculated into a healthy, susceptible lab animal
(or apple!).
4. The inoculated animal must develop the
disease, and the original pathogen must be isolated from it.
Characteristics
of some microorganisms and some diseases make it impossible to exactly follow
Koch’s postulates:
1. Some microbes cannot be grown on
artificial lab media. Treponema pallidum
(syphilis) and Mycobacterium leprae (leprosy)
are examples. Rickettsias and viruses also cannot be grown on artificial media.
For these, Koch’s postulates must be modified. The suspected pathogen may have
to be grown in cell culture or in a laboratory animal.
2. Some diseases are not always caused by
the same pathogen. Nephritis, pneumonia, meningitis, and peritonitis may be
caused by a number of different pathogens, but still present similar signs and
symptoms.
3. Some pathogens can cause more than one
disease condition. Mycobacterium
tuberculosis, for example, can infect the bones instead of the lungs. Streptococcus pyogenes commonly causes
“strep throat,” but it can also cause
scarlet fever or puerperal (childbed) fever.
4. Some agents do not grow in any way except
a human host. This brings up a question of ethics.
Diseases
cause:
Symptoms---changes not apparent to an observer
Signs---changes that can be observed or measured
A syndrome is a group of signs and symptoms that
always accompanies a particular disease.
Infectious
diseases may be classified in several ways:
1. How the disease is spread
a. Communicable disease—a disease that spreads from one host to
another, either directly or indirectly
b. Contagious disease---easily
spread from one person to another
c. Noncommunicable disease---not spread from one host to another
2. Occurrence of the disease
a. Incidence
of a disease—the fraction
of a population that contracts the disease in a particular time period
b. Prevalence of a disease—the fraction of a population
having the disease at a specified time
c. Frequency of occurrence
1) Sporadic disease---occurs only occasionally
2) Endemic disease—always present in a particular population
3) Epidemic disease---many people come down with the disease
within a short time period
4) Pandemic disease—worldwide epidemic
3. Severity or duration of the disease
a. Acute disease---develops rapidly and lasts only a short
time
b. Chronic disease---develops more slowly and may continue or
recur for long periods
c. Subacute disease---intermediate between acute and chronic
d. Latent disease---causative agent enters the body but
remains dormant for some period of time. Later the agent becomes active and
causes symptoms
Immunity
of the population can determine the rate at which a disease spreads and the
number of individuals involved. Vaccination can produce a large number of
immune persons in a population, making it unlikely that even susceptible
persons will come in contact with the disease-causing agent. When many immune
people are present in a community, this is called herd immunity. The fewer sick
people there are, the less likely the disease will spread to susceptible
people.
CLASSIFICATION OF INFECTIONS ACCORDING TO THE
EXTENT OF HOST INVOLVEMENT
Local infection—limited to a relatively small area of the
body
Systemic (generalized)
infection—microbes or their
products are spread throughout the body by blood or lymph
Focal infection—agents of infection enter a blood or
lymphatic vessel and spread to other specific parts of the body where they
remain confined
Sepsis—toxic condition arising from the spread of
microbes or their toxins from a focus of infection
Bacteremia—presence of bacteria in the blood
Septicemia--the bacteria are not only present in the
blood but are multiplying there
Toxemia--the presence of toxins in the blood
Viremia—presence of viruses in the blood
Primary infection—acute infection causing the initial illness
Secondary infection—caused by an opportunistic pathogen after
the primary infection has weakened the body’s defenses
Subclinical infection—no noticeable illness but carrying the microbe—people with
this type of infection can spread the microbe to others, some of whom may
develop clinical cases of the disease
Sequence
of events:
1.
Existence of reservoir
2.
Transmission of pathogen to susceptible host
3.
Invasion of the host by the microbe
4.
Multiplication of the microbe
5.
Injury to the host
A
predisposing factor is one that makes the host more susceptible to the disease.
1.
Gender 11.
Occupation
2.
Genetic background 12.
Chemotherapy
3.
Climate & weather
13. Emotional disturbance
4.
Inadequate nutrition
5.
Fatigue
6.
Age
7.
Environment
8.
Habits
9.
Lifestyle
10.
Preexisting illness
1. Incubation period—time between initial infection and first
appearance of signs or symptoms. This can vary due to:
a. Microorganism involved
b. Virulence of infecting microbes
c. Original number of invaders
d. Resistance of the host
2. Prodromal period—early, mild symptoms
3. Illness—patient shows signs & symptoms of the
disease. Usually the resistance of the host overcomes the pathogen and recovery
begins.
4. Decline—patient improves and signs and symptoms
taper off. Secondary infections may appear during this phase.
5. Convalescence—recovery occurs and person returns to
original state of health.
The
infected person can spread the disease through all of these periods of the
infection.
1. Human reservoirs—many people carry pathogens and transmit
them to others, directly or indirectly. These people may have signs and
symptoms of the disease; or they may be in a period without signs and symptoms,
either before or after the disease develops. Others may be carriers without
ever developing signs or symptoms of the disease--these are called asymptomatic
carriers.
2. Animal reservoirs—zoonoses are diseases which are primarily
diseases of animals, but can be transmitted to humans. (See Table 14.2 P. 432).
About 150 zoonoses have been identified, and these diseases spread to humans in
various ways:
a. Direct contact with infected animals
b. Direct contact with waste of pets
(cleaning kitty box, for example)
c. Contamination of food or water
d. Contact with contaminated hides,
feathers, or fur
e. By consuming infected animal products
f. By insect vectors
3. Nonliving reservoirs—the two main examples are soil and water.
a. Soil—fungus organisms (ringworm &
systemic infections); Clostridium
botulinum, C. tetani are common
b. Water—Vibrio cholerae, Salmonella typhi, other gastrointestinal diseases. Usually the water has become contaminated by
feces of humans or animals.
c. Improperly prepared or stored foods
1. Contact transmission—spread of an agent of disease by direct
contact, indirect contact, or droplet.
a. Direct contact---person-to-person transmission with no
intermediate object involved. Respiratory diseases, staphylococcal infections,
hepatitis A, measles, smallpox, STDs including AIDS. Rabies & anthrax
spread from animals to people.
b. Indirect contact—a nonliving object transmits the disease
from its reservoir to a new susceptible host. The term for an object involved
is fomite. Examples would be hankies, bedding,
diapers, eating & drinking utensils, money, thermometers, contaminated
syringes, etc.
c. Droplet—discharged by coughing, sneezing, talking,
laughing, etc. Droplets travel less than 1 meter from reservoir to host.
Examples of diseases are influenza, pneumonia, and whooping cough.
2. Vehicle transmission—transmission of disease by water, food,
air, blood & other body fluids, etc.
a. Waterborne transmission—pathogens usually originate in untreated or
poorly treated sewage. Diseases: cholera, shigellosis, leptospirosis.
b. Foodborne transmission—transmission in foods that are incompletely
cooked, poorly refrigerated, and contaminated in preparation. Food poisoning
and tapeworm infestations.
c. Airborne transmission—spread of agents by droplets that travel
more than 1 meter from the reservoir to the new host. Measles and tuberculosis are examples.
d. Dust particles—staphylococci, streptococci, fungal spores.
3. Vectors—these are animals that carry pathogens from
one host to another. Often they are arthropods (insects, ticks, mites). A few
examples of diseases transmitted by vectors are: malaria, yellow fever, Lyme
disease, and Rocky Mountain spotted fever.
a. Mechanical transmission—pick up the pathogen and carry it on feet
or other body parts.
b. Biological transmission—pathogen must undergo a necessary stage of development
inside the body of the vector.
These
are infections acquired in the hospital.
5 - 15% of hospital patients are affected (about 2 million people per
year) and about 20,000 people per year die of these infections. Several factors
acting together are involved:
1. Microorganisms in the hospital
environment—in spite of
major efforts to control
them,
large numbers of pathogens live in hospitals. Common organisms include:
a. Staphylococcus
aureus
b. Escherichia
coli
c. Pseudomonas
aeruginosa
d. Enterococcus
Unfortunately,
many of the strains found in hospitals are resistant to antibiotics.
2. Compromised host---patients weakened by other illnesses or
conditions, including broken skin and a suppressed immune system. Invasive
procedures such as anesthesia and surgery also increase the risk.
3. Chain of transmission in the hospital
a. Direct contact transmission from
hospital staff to patient
b. Patient to patient
c. Fomites
d. Ventilation system
Accredited
hospitals are have an infection control committee, an infection control nurse,
or an epidemiologist. Their job is to identify problem sources, check equipment
for contamination, check sterilization procedures, etc.
Some
means of controlling infection include:
1.
Education of staff
2.
Handwashing!!!!! and gloves
3.
Aseptic techniques where appropriate
4.
Careful handling of contaminated materials
5.
Isolation where appropriate
These
are “new” diseases, identified in one of
these ways:
1. The disease presents symptoms that are
clearly distinctive from all other diseases
2. Improved diagnostic techniques may allow
the identification of a new pathogen
3. A local disease becomes widespread
4. A rare disease becomes common
5. A mild disease becomes more severe
6. An increase in life span permits a new
disease to develop
Remember,
the causative agent itself is usually not new, but just one that has not
previously been recognized. Examples ( Table
14.6 P. 439 for others):
|
MICROORGANISM |
YEAR |
DISEASE |
|
BACTERIA |
|
|
|
Borrelia burgdorferi |
1975 |
Lyme disease |
|
Legionella pneumophila |
1976 |
Legionnaire’s
disease |
|
Escherichia coli O157:H7 |
1982 |
Food poisoning
involving production of dangerous toxins |
|
FUNGI |
|
|
|
Pneumocystis carinii |
1981 |
Pneumonia, esp.
in AIDS patients |
|
PROTOZOA |
|
|
|
Cryptosporidium parvum |
1976 |
Cryptosporidiosis
|
|
VIRUSES |
|
|
|
HIV |
1983 |
AIDS |
|
Hepatitis C virus |
1989 |
Hepatitis |
|
Hantavirus |
1993 |
Hantavirus pulmonary syndrome |
|
Ebola virus |
1975 |
Ebola fever |
|
West Nile virus |
1999 |
West Nile
encephalitis |
|
PRIONS |
|
|
|
Bovine spongiform
encephalitis agent |
1996 |
Mad cow disease |
Factors
which may contribute to the emergence of new infectious diseases include:
1. Familiar microorganisms change--new
strains appear
2. The widespread use of antibiotics and
pesticides encourages the growth of resistant microbes and resistant insects
and ticks that carry them
3. Global warming may change the distribution
and survival of organisms and vectors
4. Modern rapid transportation encourages
the spread of disease
5. Previously unrecognized infections may
appear when ecological changes occur
6. Animal populations may affect the
incidence of disease
7. Lack of proper vaccination and failure to
report infectious disease may affect handling of a disease outbreak
The
Centers for Disease Control and Prevention (CDC) and the World Health
Organization (WHO) have plans related to emerging diseases:
1. To detect, promptly investigate, and
monitor emerging infectious diseases
2. To expand research on ecological and
environmental factors, microbial changes and adaptations, and host interactions
3. To enhance communication of public health
information and prevention strategies
4. To establish plans to monitor and control
emerging diseases worldwide
EPIDEMIOLOGY
This
is the science that studies when and where diseases occur and how they are
transmitted. This science began when Dr. John Snow traced an outbreak of
cholera to contaminated water in London in 1848-1849.
In
modern medicine, epidemiologists collect all possible data related to an
outbreak of disease:
1. Etiology (causative agent)
2. Age & sex of patients
3. Occupation
4. Personal habits
5. Socioeconomic status
6. History of immunization
7. What do these people have in common?
8. Where did each one come in contact with
the agent?
9. Is the disease seasonal, yearly,
etc.?
The
next step is controlling the disease, which may include