chapter 14 infection, infectious diseases, and epidemiology
TRANSCRIPT
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Infection, Infectious Diseases, and Epidemiology
Chapter 14
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Mammals have elaborate physical, chemical, and immunological defenses that protect against disease-causing microbes.
However, every fortress has its weakness.
Pathogenic microbes exploit those weaknesses, and the result is disease.
The fundamental question of microbial pathogenesis is this:
How an organism too small to be seen with the naked eye can kill a human a million times larger.
Introduction
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Symbiotic Relationships Between Microbes and Their Hosts
• Symbiosis means "to live together"
• We have symbiotic relationships with countless microorganisms
• Types of symbiosis
• Mutualism
• Commensalism
• Parasitism
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Normal Microbiota in Hosts
Organisms that colonize the body's surfaces without normally causing disease
Also termed normal flora and indigenous microbiota
Two types:
Resident microbiota
Transient microbiota
Bacterial flora benefit host by preventing overgrowth of harmful microbes – microbial antagonism
Endogenous infections – occur when normal flora is introduced to a site that was previously sterile
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Normal Microbiota in Hosts
Resident microbiota
Are a part of the normal microbiota throughout life
Are mostly commensal
Less influenced by hygiene
Transient microbiotaRemain in the body for short period
Found in the same regions as resident microbiota
Competition from other microorganisms
Elimination by the body's defense cells
Influenced by hygiene
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Acquisition of normal microbiota
Development in womb free of microorganisms
Microbiota begin to develop during birthing process
Much of one's resident microbiota established during first months of life
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How Normal Microbiota Become Opportunistic Pathogens
Opportunistic pathogens: Normal microbiota that cause disease under certain circumstances
Conditions that provide opportunities for pathogens
1) Introduction of normal microbiota into unusual site in body
2) Immune suppression
3) Changes in the normal microbiota
4) Changes in relative abundance may allow opportunity for a member to thrive and cause disease
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Reservoirs of Infectious Diseases of Humans
Most pathogens cannot survive for long outside of their host
Reservoirs of infection: Sites where pathogens are maintained as a source of infection
Three types of reservoirs:
1) Animal reservoir
2) Human carriers
3) Nonliving reservoir
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Animal Reservoirs
Zoonoses: Diseases naturally spread from animal host to humans
Acquire zoonoses through various routes:
1) Direct contact with animal or its waste
2) Eating animals
3) Bloodsucking arthropods (Also review pages 372-373 in chapter 12)
Humans are usually dead-end host to zoonotic pathogens
Impossible to eradicate
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Human Carriers
Asymptomatic infected individuals can be infective to others
Some individuals eventually develop illness while others never get sick
Healthy carriers may have defensive systems that protect them
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Typhoid Mary (Mary Mallon)
Source: PBS/NOVA
Article in New York American, June 20, 1909
http://www.pbs.org/wgbh/nova/typhoid/
http://www.pbs.org/wgbh/nova/body/disease-detective.html
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Nonliving Reservoirs
Soil, water, and food can be reservoirs of infection
Presence of microorganisms often due to contamination by feces or urine
Airborne – droplet nuclei, aerosols
Vehicle – inanimate material, food, water, biological products, fomites
Fomites consist of both porous and nonporous surfaces or objects that can become contaminated with pathogenic microorganisms and serve as vehicles in transmission
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Exposure to Microbes: Contamination and Infection
Contamination: The mere presence of microbes in or on the body
Infection: When organism evades body's external defenses, multiplies, and becomes established in the body
Infectious disease: an infection that causes damage or disruption to tissues and organs
Endogenous agents
normal flora
Exogenous agents
originate from source outside the body
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Morbidity: diseased stateMortality: death
Contact
Invasion
Infection
Colonization with
microbiota
Effects of microbes
resulting in injury or tissue
damage
Immune defense prevails
Morbidity/Mortality can occur
Immunity/Damage repair
Carrier state
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Concept of an infectious dose (ID)
Infectious dose: Minimum number of microbes required for successful infection
Microbes with small IDs have greater virulence
Lack of ID will not result in infection
www.mass.gov
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Portals of Entry
Portal of entry: Sites through which pathogens enter the body
Three major pathways:
1) Skin
2) Mucous membranes
3) Placenta
Entry via the parenteral route circumvents the usual portals
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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 enter by burrowing into or digesting outer layers of skin
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Line the body cavities that are open to the environment
Provide a moist, warm environment hospitable to pathogens
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 : Mucous membranes
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Typically forms effective barrier to pathogens
Pathogens may cross the placenta and infect the fetus
Can cause spontaneous abortion, birth defects, premature birth
Portals of Entry : Placenta
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STORCH - Syphilis, Toxoplasmosis, Other diseases (hepatitis B, HIV-1/AIDS
and chlamydia), Rubella, Cytomegalovirus and Herpes simplex virus
Pathogens that infect during pregnancy
http://phil.cdc.gov/
N Engl J Med 2010; 362:833
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Not a true portal of entry
Means by which portals of entry can be circumvented
Pathogens deposited directly into tissues beneath the skin or mucous membranes
Portals of Entry : Parenteral route
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The Role of Adhesion in Infection
Process by which microorganisms attach themselves to cells
Required to successfully establish colonies within the host
Uses adhesion factors
Specialized structures
Attachment proteins
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The Role of Adhesion in Infection
Attachment proteins help in adhesion and found on viruses and many bacteria
Viral or bacterial ligands bind host cell receptors
Interaction can determine host cell specificity
Changing/blocking a ligand or its receptor can prevent infection
Inability to make attachment proteins or adhesins renders microorganisms avirulent
Some bacterial pathogens attach to each other to form a biofilm
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The Nature of Infectious Disease
Infection is the invasion of the host by a pathogen
Disease results if the invading pathogen alters normal body functions
Disease is also referred to as morbidity
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Manifestations of Disease: Symptoms, Signs, and Syndromes
Symptoms: Subjective characteristics of disease felt only by the patient
Signs :Objective manifestations of disease observed or measured by others
Syndrome: Symptoms and signs that characterize a disease or abnormal condition
Asymptomatic, or subclinical infections lack symptoms but may still have signs of infection
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Causation of Disease: Etiology
Etiology: Study of the cause of disease
Diseases have various causes
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Using Koch's postulates to determine etiology
Germ theory of disease
Infections by pathogenic microorganisms cause disease
Robert Koch developed a set of postulates one must satisfy to prove a particular pathogen causes a particular disease
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Koch’s postulates
1. Find evidence of a particular microbe in every case of a disease
2. Isolate that microbe from an infected subject and cultivate it artificially in the laboratory
3. Inoculate a susceptible healthy subject with the laboratory isolate and observe the resultant disease
4. Re-isolate the agent from this subject
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Exceptions to Koch's postulates
Some pathogens can't be cultured in the laboratory
Diseases caused by a combination of pathogens and other cofactors
Diseases can be caused by more than one pathogen
Ethical considerations prevent applying Koch's postulates to pathogens that require a human host
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Virulence Factors of Infectious Agents
Pathogenicity: Ability of a microorganism to cause disease
Virulence: Degree of pathogenicity
Virulence factors contribute to virulence
1) Adhesion factors
2) Biofilms
3) Extracellular enzymes
4) Toxins
5) Antiphagocytic factors
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Virulence Factors: Extracellular enzymes
Secreted by the pathogen
Dissolve structural chemicals in the body
Help pathogen maintain infection, invade, and avoid body defenses
Important to virulence of the pathogen
Mutant species that do not secrete the enzymes are often avirulent
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Chemicals that harm tissues or trigger host immune responses that cause damage
Toxemia refers to the presence of toxins in the bloodstream
The toxins are carried beyond the site of infection
Two types: Exotoxins and Endotoxins
Virulence Factors: Toxins
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A-B Exotoxins
The A (active) component is attached to the B (binding)component
The A component inhibits a cellular protein to cause the damage
Toxin precursor (inactive)
A chain
B chain
A
B
Binding site
B chain attaches to receptor
A
B
Host cell membrane with specific receptor
Endocytosis
Vacuole formation
A
B
B
A
A portion of toxin is released from vacuole
Toxin acts on ribosomes and blocks protein synthesis; causes cell death
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Factors prevent phagocytosis by the host's phagocytic cells
Allow pathogens to remain in a host for longer time
Bacterial capsule
Composed of chemicals not recognized as foreign
Slippery and difficult for phagocytes to engulf
Antiphagocytic chemicals
Prevent fusion of lysosome and phagocytic vesicles
Leukocidins directly destroy phagocytic white blood cells
Virulence Factors: Antiphagocytic factors
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Phagocytosis: Microbes That Evade ItPLAY
Phagocytosis: Microbes That Evade It
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The Stages of Infectious Disease
The disease process occurs following infection
Many infectious diseases have five stages following infection
1)Incubation period
2) Prodromal period
3) Illness
4) Decline
5)Convalescence
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The Movement of Pathogens Out of Hosts: Portals 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
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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:
1) Contact transmission
2) Vehicle transmission
3) Vector transmission
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Contact TransmissionDirect contact transmission
Usually involves body contact between hosts
Transmission within a single individual can also occur
Indirect contact transmission
Pathogens are spread from host to host by fomites
Droplet transmission
Spread of pathogens in droplets of mucus by exhaling, coughing, and sneezing
Fomites consist of both porous and nonporous surfaces or objects that can become contaminated with pathogenic microorganisms and serve as vehicles in transmission
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Vehicle Transmission
Airborne transmission
When pathogens travel more than 1 meter via an aerosol
Aerosols can occur from various activities
Sneezing, coughing, air-conditioning systems, sweeping
Waterborne transmission
Important in the spread of many gastrointestinal diseases
Fecal-oral infection
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Foodborne transmission
Spread of pathogens in and on foods
Inadequately processed, cooked, or refrigerated foods
Foods may become contaminated with feces
Bodily fluid transmission
Bodily fluids such as blood, urine, saliva can carry pathogens
Prevent contact with conjunctiva or breaks in the skin or mucous
membranes
Vehicle Transmission
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Vector Transmission
Biological vectors
Transmit pathogens and serve as host for some stage of the pathogen's life cycle
Biting arthropods transmit many diseases to humans
Mechanical vectors
Passively transmit pathogens present on their body to new hosts
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Classification of Infectious DiseasesLocalized infection
(boil)Systemic infection (influenza)
Focal infection
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Frequency of Disease
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
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Different terms for the occurrence of disease.
Pandemic – epidemic across continents
Sporadic – when occasional cases are reported at irregular intervals
Epidemic – when prevalence of a disease is increasing beyond what is expected
Endemic – disease that exhibits a relatively steady frequency over a long period of time in a particular geographic locale
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Epidemiological Studies: Descriptive epidemiology
Careful tabulation of data concerning a disease
Record location and time of the cases of disease
Collect patient information
Try to identify the index case of the disease
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Seeks to determine the probable cause, mode of transmission, and methods of prevention
Useful in situations when Koch's postulates can't be applied
Often retrospective
Investigation occurs after an outbreak has occurred
Epidemiological Studies: Analytical epidemiology
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Test a hypothesis concerning the cause of a disease
Application of Koch's postulates
Epidemiological Studies: Experimental epidemiology
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Hospital Epidemiology: Nosocomial Infections
Types of nosocomial infections
Exogenous: Pathogen acquired from the health care environment
Endogenous: Pathogen arises from normal microbiota due to factors within the health care setting
Iatrogenic: Results from modern medical procedures
Superinfections: Use of antimicrobial drugs reduces competition from some resident microbiota – other microbes may thrive
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The interplay of factors that result in nosocomial infections.
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Control of nosocomial infections
Requires aggressive control measures
Handwashing is the most effective way to reduce nosocomial infections
Universal precautions - stringent measures to prevent the spread ofnosocomial infections from patient to patient, from patient to worker, and from worker to patient
Based on the assumption that all patient specimens could harbor infectious agents, so must be treated with the same degree of care
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Epidemiology and Public Health
Sharing of data among public health organizations
Agencies at the local, state, national, and global level share information concerning disease
The United States Public Health Service
National public health agency: CDC is one branch
World Health Organization (WHO):Coordinates public health efforts worldwide
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Role of public health agencies in interrupting disease transmission
Public health agencies work to limit disease transmission
• Monitor water and food safety
• Work to reduce disease vectors and reservoirs
• Establish and enforce immunization schedules
• Locate and treat individuals exposed to contagious pathogens
• Establish isolation and quarantine measures
Public health education
Public health agencies campaign to educate the public on healthful choices to limit disease
http://www.cdc.gov/tuskegee/timeline.htm