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Chapter 14
Infection, Infectious Diseases, and Epidemiology
Terms to Learn, words to know
Symbiosis Mutualism Commensalism Parasitism
Microbiota Immunocompromised
Axenic Infection Parenteral Adhesion
Ligands Virulent Avirulent Disease Etiology
Pathogenicity Exotoxin Endotoxin Phagocytosis Prodrome
Reservoir Zoonosis Epidemiology Nosocomial
Symbiotic Relationships
" Symbiosis means “to
live together”
" Describes the
relationship between microorganisms and their host
" 3 types
" Mutualism
" Commenalism
" Parasitism
Normal Microbiota
" Also termed normal
flora and indigenous microbiota
" Refers to the
organisms that colonize the body’s surfaces without normally causing disease
" Two types
" Resident
microbiota
" Transient
microbiota
Resident Microbiota
" Are a part of the
normal microbiota throughout life
" Most are commensal
Transient Microbiota
" Remain in the body
for only hours to months before disappearing
" Found in the same
regions as resident microbiota
" Cannot persist in
the body
" Competition
from other microorganisms
" Elimination
by the body’s defenses cells
" Chemical or
physical changes in the body
Acquisition of Normal Microbiota
" Development in the
womb is generally free of microorganisms (axenic)
" Microbiota begins
to develop during the birthing process
" Much of ones
resident microbiota established during the first months of life
Opportunistic Pathogens
" Normal microbiota
that can cause disease under certain circumstances
" Conditions that
provide opportunities for pathogens
" Immune
suppression
" Changes in
the normal microbiota- changes in relative abundance of normal microbiota may
allow opportunity for a member to thrive and cause disease
" Introduction
of normal microbiota into unusual site in the body
Contamination versus Infection
" Contamination- the
mere presence of microbes in or on the body
" Infection- results
when the organism has evaded the body’s external defenses, multiplied, and
become established in the body
Portals of Entry
" Sites through
which pathogens enter the body
" 4 major types
" Skin
" Mucous
membranes
" Placenta
" Parenteral
route
Skin
"
Outer layer of packed, dead, skin cells usually acts as
a barrier to pathogens
"
Some pathogens can enter through openings or cuts
"
Others enter by burrowing into or digesting the outer
layers of skin
Mucous Membranes
" Line the body
cavities that are open to the environment
" Provides a moist,
warm environment that is hospitable to pathogens
" Respiratory tract
is the most commonly used site of entry- entry is through the nose, mouth or
eyes
" Pathogens able to
survive the acidic pH of the stomach may use the gastrointestinal tract as a
route of entry
Parenteral Route
" Not a true portal
of entry but a means by which they can be circumvented
" Pathogens
deposited directly into tissues beneath the skin or mucous membranes
Adhesion
" Process by which
microorganisms attach themselves to cells
" Required to
successfully establish colonies within the host
" Uses adhesion
factors
" Specialized
structures
" Attachment
proteins
Attachment Proteins
" Found on viruses
(attachment proteins) and many bacteria (adhesins)
" Surface
lipoproteins or glycoproteins, called ligands, that bind host cell receptors
" Interaction of ligand
with host receptor can determine specificity for host cells
" Ability to change
or block the ligand or its receptor can prevent infection
" Inability to make
attachment proteins or adhesins renders the microorganisms avirulent
Infection versus Disease
" Infection is the
invasion of the host by a pathogen
" Disease results
only if the invading pathogen alters the normal functions of the body
" Disease is also
referred to as morbidity
Manifestations of Disease:
" Symptoms-
subjective characteristics of disease felt only by the patient
" Signs- objective
manifestations of disease that can be observed or measured by others
" Syndrome- group of
symptoms and signs that characterize a disease or abnormal condition
" Asymptomatic, or
subclinical, infections lack symptoms but may still have signs of infection
Etiology
" Study of the cause
of disease
" Germ theory of
disease- disease caused by infections of pathogenic microorganisms
" Robert Koch
developed a set of postulates one must satisfy to prove a particular pathogen
causes a particular disease
Exceptions to Koch’s Postulates
" Using Koch’s
postulates is not feasible in all cases
" Some
pathogens can’t be cultured in the laboratory
" Some
diseases are caused by a combination of pathogens and other cofactors
" Ethical
considerations prevent applying Koch’s postulates to pathogens that require a
human host
" Difficulties in
satisfying Koch’s postulates
" Diseases
that can be caused by more than one pathogen
" Pathogens
that are ignored as potential causes of disease
Virulence Factors of Infectious
Disease
" Pathogenicity-
ability of a microorganism to cause disease
" Virulence- degree
of pathogenicity
" Virulence
factors contribute to an organisms virulence
" Adhesion
factors
" Extracellular
enzymes
" Toxins
" Antiphagocytic
factors
Extracellular Enzymes
" Enzymes secreted
by the pathogen
" Dissolve
structural chemicals in the body
" Help pathogen
maintain infection, invade further, and avoid body defenses
Toxins
" Chemicals produced
by the pathogen
" Harm tissues or
trigger host immune responses that cause damage
" Toxemia refers to
toxins in the bloodstream that are carried beyond the site of infection
" 2 types
" Exotoxins
" Endotoxins
Antiphagocytic Factors
" Certain factors
prevent phagocytosis by the host’s phagocytic cells
" Bacterial
capsule
" Often
composed of chemicals found in the body and not recognized as foreign
" Can be
slippery making it difficult for phagocytes to engulf the bacteria
" Antiphagocytic
chemicals
" Some prevent
fusion of lysosome and phagocytic vesicles
" Leukocidins
directly destroy phagocytic white blood cells
The Stages of Infectious Disease
" Following
infection, sequence of events called the disease process occurs
" Many infectious
diseases have five stages following infection
Movement of Pathogen Out of Host
" Pathogens leave
host through portals of exit
Reservoirs of Infection
" Most pathogens
cannot survive long outside of their host
" Sites where
pathogens are maintained as a source of infection are termed reservoirs of
infection
" 3 types of
reservoirs
" Animal
reservoir
" Human carriers
" Nonliving
reservoir
Animal Reservoirs
" Zoonoses- diseases
that are naturally spread from their usual animal host to humans
" Acquire zoonoses
through various routes
" Direct
contact with animal or its waste
" Eating
animals
" Bloodsucking
arthropods
" Humans are usually
dead end host to zoonotic pathogens
Human Carriers
" Infected
individuals who are asymptomatic but infective to others
" Some individuals
will eventually develop illness while others never get sick
Nonliving Reservoirs
" Soil, water, and
food can be reservoirs of infection
" Presence of
microorganisms is often due to contamination by feces or urine
Modes of Infectious Disease
Transmission
" Transmission from
either a reservoir or portal of exit
" 3 groups
" Contact
transmission
" Vehicle
transmission
" Vector
transmission
Classification of Infectious
Diseases
" Many different
methods of classification
" The body
system they affect
" The
taxonomic groups of the causative agent
" Their
longevity and severity
" How they are
spread to their host
Terms Used to Classify Infectious Diseases
Table 14.5-
will be helpful to know these prefixes and suffixes
Epidemiology
" Study of where and
when diseases occur and how they are transmitted within populations
" Track occurrence
of diseases using two measures
" Incidence-
number of new cases of a disease in a given area during a given period
of time
" Prevalence-
number of total cases of a disease in a given area during a given period
of time
" Occurrence also
evaluated in terms of frequency and geographic distribution
Epidemiologists Study Disease
Dynamics Using 3 Approaches
" Descriptive
" Analytical
" Experimental
Descriptive Epidemiology
" Careful tabulation
of data concerning a disease
" Record
information about the location and time of the cases of disease
" Collect
patient information
" Try to identify
the index case (or first case) of the disease
Analytical Epidemiology
" Seeks to determine
the probable cause, mode of transmission, and methods of prevention
" Useful in
situations in which Koch’s postulates can’t be applied
" Often
retrospective- investigation occurs after on outbreak has occurred
Experimental Epidemiology
" Involves testing a
hypothesis concerning the cause of a disease
" Application of
Koch’s postulates is experimental epidemiology
Nosocomial Infections
" Infections
acquired while in a health care facility
" Types of
nosocomial infections
" Exogenous-
pathogen acquired from the health care environment
" Endogenous-
pathogen arise from normal microbiota due to factors within the health care
setting
" Iatrogenic-
results from modern medical procedures
Control of Nosocomial Infections
" Involves
precautions designed to reduce the factors that result in disease
" Universal
Precautions instituted by the CDC to limit exposure to pathogens
" Hand washing is
the most effective way to reduce nosocomial infections
Epidemiology and Public Health
" Agencies at the
local, state, national, and global level share information concerning disease
" The United
States Public Health Service is the national public health agency
" World Health
Organization (WHO) coordinates public health services internationally
" Public health
agencies work to limit disease transmission
" Monitor
water and food safety
" Public health
agencies campaign to educate the public on healthful choices to limit disease