The course of infections
BelCubitaMicrobiology FUNDAMENTALS A Clinical Approach Third Edition
Marjorie Kelly Cowan
&
Heidi Smith
with
Jennifer Lusk
BSN RN CCRN
©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw-Hill Education.
Chapter 11
Interactions Between Microbes and Humans
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Learning Outcomes Section 11.1
Differentiate among the terms colonization, infection, and disease.
Enumerate the sites where normal biota is found in humans.
Discuss how the Human Microbiome Project is changing our understanding of normal biota.
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The Human Microbiome(1)
Humans and other mammals have the form and physiology that they have due to having been formed in intimate contact with their microbes
Human microbiome:
The sum total of all microbes found on and in a normal human
Critically important to the health and functioning of its host organism
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Colonization, Infection, Disease
For the most part, our resident microbiota colonize us for the long term and do not cause disease
Infection: microbes get past host defenses, enter tissues, and multiply
Disease: deviation from health; pathologic state that results when cumulative effects of infection damage or disrupt tissues and organs
Infectious disease: a pathogenic state caused directly by microorganisms or their products
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The Human Microbiome(2)
Human Microbiome Project (HMP):
A worldwide research effort under way since 2008
Characterize microbes living on human bodies when healthy
Determine how the microbiome differs in various diseases
Utilizes powerful techniques of genome sequencing and “big data” tools
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Important and Surprising Results of HMP
Human cells: 21,000 protein-encoding genes
Microbiota: 8 million protein-encoding genes
Microbes are found in locations previously thought to be sterile
100 million viruses per gram of human feces
All healthy people seem to harbor potentially dangerous pathogens in low numbers
The makeup of one’s intestinal biota can influence overall health
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Sites with Microbiota
Sites previously known to harbor normal microbiota:
Skin and adjacent mucous membranes, upper respiratory tract, gastrointestinal tract (including mouth), outer portion of urethra, external genitalia, vagina, external ear canal, external eye (lids, conjunctiva)
Additional sites now thought to harbor at least some normal microbiota (or their DNA):
Lungs (lower respiratory tract), bladder (and urine), breast and breast milk, amniotic fluid and fetus
Sites in which DNA from microbiota has been detected:
Brain, bloodstream
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Acquiring the Microbiota
Human body has a variety of environmental niches (variations in temperature, pH, nutrients, and oxygen tension) and supports a wide range of microbes
Microbial antagonism:
Normal biota are unlikely to be displaced by incoming microbes
Limited number of attachment sites
Chemical or physiological environment created by resident biota is hostile to other microbes
Normal biota is beneficial or, at worst, commensal to the host in good health with a functioning immune system
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Factors That Weaken Host Defenses and Increase Susceptibility to Infection
Old age and extreme youth (infancy, prematurity)
Genetic defects in immunity and acquired defects in immunity (AIDS)
Surgery and organ transplants
Underlying disease: cancer, liver malfunction, diabetes
Chemotherapy/immunosuppressive drugs
Physical and mental stress
Pregnancy
Other infections
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Colonization of the Fetus
Until recently, the uterus and its contents were thought to be sterile during embryonic and fetal development:
Analysis of newborns’ stools sampled before their first meal show a diversity of bacteria
This indicates that their intestines are colonized in utero
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Colonization of the Newborn
Important source of microbiota for a newborn is its trip through the vagina:
Lactobacillus provides the baby with the necessary enzymes to digest milk
Other species protect the baby from skin disorders and other conditions
Human milk contains around 600 species of bacteria and sugars the baby cannot digest but that are digested by healthy gut bacteria
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Where Babies Get a Microbiome
©Jim Connely (ultrasound); ©Adam Gault/SPL/Getty Images (birth); ©Jose Luis Pelaez Inc/Blend Images LLC (breast feeding); ©Pixtal/SuperStock (bottle);
©Marc Romanelli/Blend Images (family) ©Kwame Zikomo/ Purestock/SuperStock (with dog); ©alexmak72427/iStock/Getty Images (scar)
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Concept Check (1)
Which of the following body sites previously thought to be microbe-free harbor at least some normal microbiota?
Skin and mucous membranes
External genitalia
Gastrointestinal tract
Breast milk
Upper respiratory tract
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Learning Outcomes Section 11.2(1)
Differentiate between a microbe’s pathogenicity and its virulence.
List the steps a microbe has to take to get to the point where it can cause disease.
Explain the significance of polymicrobial infections.
List several portals of entry and exit.
Define infectious dose, and explain its role in establishing infection.
Describe three ways microbes cause tissue damage.
Compare and contrast major characteristics of endotoxin and exotoxins.
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Learning Outcomes Section 11.2(2)
Provide a definition of virulence factors.
Draw a diagram of the stages of disease in a human.
Differentiate among the various types of reservoirs, providing examples of each.
List several different modes of transmission of infectious agents.
Define healthcare-associated infection, and list the three most common types.
List Koch’s postulates, and discuss when they might not be appropriate in establishing causation.
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When Colonization Leads to Disease
Pathogen: a microbe whose relationship with its host is parasitic and results in infection and disease
Pathogenicity: an organism’s potential to cause disease
True pathogens: capable of causing disease in healthy persons with normal immune defenses
Opportunistic pathogens: cause disease when the host’s defenses are compromised or when the pathogens become established in a part of the body that is not natural to them
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Virulence
Virulence:
Relative severity of a disease caused by a particular microbe
Degree of pathogenicity
Virulence of a microbe is determined by its ability to:
Establish itself in a host
Cause damage
Virulence factor: any characteristic or structure of the microbe that contributes to its ability to establish itself in the host and cause damage
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Polymicrobial Infections
Majority of infections are polymicrobial, with contributions from more than one microbe
Influenza infection frequently leads to pneumonia
Several types of skin infections are caused by either Staphylococcus or Streptococcus species:
When these two are cultivated with Moraxella, the three of them together lead to disease symptoms
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Step One: Becoming Established—Portals of Entry(1)
Portal of entry: the route that a microbe takes to enter the tissues of the body to initiate an infection
Exogenous: microbe originating from a source outside the body from the environment or another person or animal
Endogenous: microbe already existing on or in the body—normal biota or a previously silent infection
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Step One: Becoming Established—Portals of Entry(2)
The majority of pathogens have adapted to a specific portal of entry, one that provides a favorable habitat for further growth and spread
If certain pathogens enter the “wrong” portal, they will not be infectious:
Inoculation of the nasal mucosa with the influenza virus will result in the flu, but if the virus contacts the skin, no infection occurs
Occasionally, an infectious agent can enter by more than one portal:
Mycobacterium tuberculosis can enter through both the respiratory and gastrointestinal tracts
Streptococcus and Staphylococcus can enter through the skin, urogenital tract, and the respiratory tract
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Portals of Entry(3)
Portal of Entry | Organism/Disease | How Access Is Gained |
Skin | Staphylococcus aureus, Streptococcus pyogenes, Clostridium tetani | Via nicks, abrasions, punctures, areas of broken skin |
Herpes simplex (type 1) | Via mucous membranes of the lips | |
Helminth worms | Burrow through the skin | |
Viruses, rickettsias, protozoa (i.e., malaria, West Nile virus) | Via insect bites | |
Haemophilus aegyptius, Chlamydia trachomatis, Neisseria gonorrhoeae | Via the conjunctiva of the eye | |
Gastrointestinal tract | Salmonella, Shigella, Vibrio, Escherichia coli, poliovirus, hepatitis A, echovirus, rotavirus, enteric protozoans (Giardia lamblia, Entamoeba histolytica) | Through eating/drinking contaminated foods and fluids Via fomites (inanimate objects contaminated with the infectious organism) |
Respiratory tract | Bacteria causing meningitis, influenza, measles, mumps, rubella, chickenpox, common cold, Streptococcus pneumoniae, Klebsiella, Mycoplasma, Cryptococcus, Pneumocystis, Mycobacterium tuberculosis, Histoplasma | Via inhalation of offending organism |
Urogenital tract | HIV, Trichomonas, hepatitis B, syphilis, Treponema pallidum, Neisseria gonorrhoeae, Chlamydia trachomatis, herpes, genital warts | Enter through the skin/mucosa of penis, external genitalia, vagina/cervix, urethra; may enter through an unbroken surface or through a cut or abrasion |
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Quantity of Microbes in the Inoculating Dose
Infectious dose (ID):
The minimum number of microbes necessary to cause an infection to proceed
Microorganisms with smaller infectious doses have greater virulence
ID for Q fever is a single cell
ID for tuberculosis, giardiasis, and coccidioidomycosis is about 10 cells
ID for gonorrhea is 1,000 cells
ID for typhoid fever is 10,000 cells
ID for cholera is 1,000,000,000 cells
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Step Two: Becoming Established—Attaching to Host Cells
Adhesion:
Process by which microbes gain a more stable foothold on host tissues
Dependent on binding between specific molecules on both the host and pathogen
Pathogen is limited to only those cells (and organisms) to which it can bind
Firm attachment is almost always a prerequisite for causing disease, since the body has so many mechanisms for flushing microbes from tissues
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Adhesion Mechanisms
Bacterial, fungal, and protozoal pathogens attach by:
Fimbriae (pili)
Surface proteins
Adhesive slimes or capsules
Viruses attach by specialized receptors
Parasitic worms fastened by suckers, hooks, and barbs
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Step Three: Becoming Established—Surviving Host Defenses
Phagocytes: cells that engulf and destroy host pathogens by means of enzymes and antimicrobial chemicals
Antiphagocytic factors:
Virulence factors used by some pathogens to avoid phagocytes
Leukocidins: kill phagocytes outright
Extracellular surface layer (slime or capsule) makes it difficult for the phagocyte to engulf the pathogen
Some bacteria survive inside the phagocyte
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Step Four: Causing Disease
Virulence factors are simply adaptations a microbe uses to establish itself in a host
Three ways that microorganisms cause damage to their host:
Directly through the action of enzymes or toxins (both endotoxins and exotoxins)
Indirectly by inducing the host’s defenses to respond excessively or inappropriately
Epigenetic changes made to host cells by microbes
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Three Ways Microbes Damage the Host
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Extracellular Enzymes
Exoenzymes:
Enzymes secreted by microbes that break down and inflict damage on tissues
Dissolve the host’s defense barriers to promote the spread of disease to other tissues
Examples of enzymes:
Mucinase: digests the protective coating on mucous membranes
Hyaluronidase: digests the ground substance that cements animal cells together
Coagulase: causes clotting of blood or plasma
Kinase: dissolves fibrin clots
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Bacterial Toxins: A Potent Source of Cellular Damage
Toxin: a specific chemical product of microbes that is poisonous to other organisms
Toxins are named according to their target:
Neurotoxins act on the nervous system.
Enterotoxins act on the intestine
Hemotoxins lyse red blood cells
Nephrotoxins damage the kidneys
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Exotoxins
Hemolysins: class of bacterial exotoxin that disrupts the cell membrane of red blood cells
Cause the RBC to hemolyze, to burst and release hemoglobin pigment
©McGraw-Hill Education/Lisa Burgess, photographer
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Endotoxin
Lipopolysaccharide (LPS), part of the outer membrane of gram-negative cell walls
Has a variety of systemic effects on tissues and organs
Causes fever, inflammation, hemorrhage, and diarrhea
Blood infections by Salmonella, Shigella, Neisseria meningitidis, and Escherichia coli are particularly dangerous and can lead to shock
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Origins and Effects of Circulating Exotoxins and Endotoxin
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Will Disease Result?
©Dave and Les Jacobs/Kolostock/Blend Images
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Definitions of Infection Types
Type of Infection | Definition | Example |
Localized infection | Microbes enter the body, remain confined to a specific tissue | Boils, warts, fungal skin infections |
Systemic infection | Infection spreads to several sites and tissue fluids—usually via the bloodstream—but may travel by other means such as nerves (rabies) and cerebrospinal fluid (meningitis) | Mumps, rubella, chickenpox, AIDS, anthrax, typhoid, syphilis |
Focal infection | Infectious agent spreads from a local site and is carried to other tissues | Tuberculosis, streptococcal pharyngitis |
Mixed infection (polymicrobial infection) | Several agents establish themselves simultaneously at the infection site | Human bite infections, wound infections, gas gangrene |
Primary infection | The initial infection | Can be any infection |
Secondary infection | A second infection caused by a different microbe, which complicates a primary infection; often a result of lowered host immune defenses | Influenza complicated by pneumonia, common cold complicated by bacterial otitis media |
Acute infection | Infection comes on rapidly, with severe but short-lived effects | Influenza |
Chronic infection | Infection that progresses and persists over a long period of time | HIV |
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Warning Signals of Disease
Sign: objective evidence of disease as noted by an observer
Symptom: subjective evidence of disease as sensed by the patient
Syndrome: a disease identified by a certain complex of signs and symptoms
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Signs and Symptoms of Inflammation
Symptoms of inflammation:
Fever
Pain
Soreness
Swelling
Signs of inflammation:
Edema: the accumulation of fluid in afflicted tissue
Granulomas and abscesses: walled-off collections of inflammatory cells and microbes in the tissues
Lymphadenitis: swollen lymph nodes
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Signs of Infection in the Blood
Signs of infection:
Leukocytosis: increase in white blood cell levels
Leukopenia: decrease in white blood cell levels
Septicemia: a general state in which microorganisms are multiplying in the blood and are present in large numbers
Bacteremia or viremia: bacteria or viruses are present in the blood but not multiplying
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Infections That Go Unnoticed
No noticeable symptoms are produced
Microbe is active in host tissues
Host does not seek medical attention
These infections are known as asymptomatic, or subclinical (inapparent)
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Step Five: Vacating the Host—Portals of Exit
Portal of exit:
Specific avenue by which pathogens exit
Shed through secretion, excretion, discharge, or sloughed tissue
High number of microbes in these materials increases the likelihood that the pathogen will reach other hosts
Portal of exit is usually the same as the portal of entry, but some pathogens use a different route
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Major Portals of Exit of Infectious Diseases
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Steps Involved When a Microbe Causes Disease in a Host
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Long-Term Infections and Long-Term Effects
Latency: a dormant state of microbes in certain chronic infectious diseases
Viral latency: herpes simplex, herpes zoster, hepatitis B, AIDS, Epstein-Barr
Bacterial/protozoan latency: syphilis, typhoid fever, tuberculosis, malaria
Sequelae: long-term or permanent damage to tissues or organs caused by infectious disease
Meningitis: deafness
Strep throat: rheumatic heart disease
Lyme disease: arthritis
Polio: paralysis
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Course of an Infection(1)
Incubation period:
The time from initial contact with the infectious agent to the appearance of symptoms
Agent is multiplying at the portal of entry but has not caused enough damage to elicit symptoms
Varies according to host resistance, degree of virulence, and distance between the target organ and the portal of entry
Ranges from several hours to several years
Majority of infections range from 2 to 30 days
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Course of an Infection(2)
Prodromal stage:
1- to 2-day period when the earliest notable symptoms of infection appear
Vague feeling of discomfort: head and muscle aches, fatigue, upset stomach, general malaise
Acute phase:
Infectious agent multiplies at high levels, exhibits its greatest virulence, becomes well established in its target tissue
Marked by fever and other prominent and specific signs and symptoms
Extremely variable in length of this period
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Course of an Infection(3)
Convalescent period:
Patient begins to respond to the infection and symptoms decline
Patient’s strength and health gradually return due to the healing nature of the immune response
Many patients stop taking antibiotics during this period, even though pathogens are still in their system, leading to antibiotic resistance
Continuation phase:
Only some infections have this phase
Either the organism lingers for months, years, or indefinitely after the patient is well or the organism is gone but symptoms continue
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Stages in the Course of Infection and Disease
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Reservoirs: Where Pathogens Come From
Reservoir:
Primary habitat in the natural world from which a pathogen originates
Often a human or animal carrier
Also soil, water, and plants
Transmitter: individual or object from which an infection is acquired
Syphilis: reservoir and transmitter are the same
Hepatitis A: reservoir is a human, transmitter is food
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Reservoirs and Transmitters
Living Reservoirs | Transmission Examples |
Animals (Other than humans and arthropods) Mammals, birds, reptiles, etc. | Pathogens from animals can be directly transmitted to humans, as in the example of bats transmitting rabies to humans can be transmitted to humans via vectors, as with fleas passing the plague from rats to people can be transmitted through vehicles such as water, as in the case of leptospirosis, which is often transmitted from animal urine to human skin via bodies of water |
Humans Actively ill | A person suffering from a cold contaminates a pen, which is then picked up by a healthy person. That is indirect transmission. Alternatively, a sick person can transmit the pathogen directly by sneezing on a healthy person. |
Humans Carriers | A person who is fully recovered from his hepatitis but is still shedding hepatitis A virus in his feces may use suboptimal hand-washing technique. He contaminates food, which a healthy person ingests (indirect transmission). Carriers can also transmit through direct means, as when an incubating carrier of HIV, who does not know she is infected, transmits the virus through sexual contact. |
Arthropods Biological vectors | When an arthropod is the host (and reservoir) of the pathogen, it is also the mode of transmission. |
Nonliving Reservoirs | Transmission Examples |
Soil, Water, Air The built environment | Some pathogens, such as the TB bacterium, can survive for long periods in nonliving reservoirs. They are then directly transmitted to humans when they come in contact with the contaminated soil, water, or air. |
Top: ©Thinkstock/Getty Images (writing); ©Ingram Publishing (pen in mouth); Middle: Source: CDC; Bottom: ©McGraw-Hill Education/Christopher Kerrigan, photographer
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Living Reservoirs
A great number of infections that affect humans have their reservoirs in other humans
Persons or animals with symptomatic infection are obvious sources:
Carrier: an individual who inconspicuously shelters a pathogen, spreads it to others without any notice, and who may not have experienced disease due to the microbe
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Carrier States
©Lane Oatey/Blue Jean Images/Getty Images (dancing); ©UpperCut Images/SuperStock (man); ©Siri Stafford/Digital Vision/Getty Images (child);
©John Lund/Marc Romanelli/Blend Images LLC (cook); ©UpperCut Images/SuperStock (patient)
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Animals as Reservoirs and Sources
The majority of animal reservoir agents are arthropods such as fleas, mosquitoes, flies, and ticks
Larger animals can also spread infection:
Mammals: rabies
Birds: psittacosis
Lizards: salmonellosis
Biological vector: actively participates in a pathogen’s life cycle, serving as a site in which it can multiply or complete its life cycle
Mechanical vector: carries the microbe more or less accidentally on its body parts
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Zoonosis
Zoonosis: an infection indigenous to animals but naturally transmissible to humans
Humans are essentially dead-end hosts that do not contribute to the natural persistence of the microbe
Some zoonotic infections have multihost involvement
Some have complex life cycles in the wild
Zoonotic spread of disease is promoted by close associations between humans and animals
Make up a full 70% of all new emerging diseases worldwide
Impossible to eradicate without also eradicating the animal reservoir
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Common Zoonotic Infections
Disease | Primary Animal Reservoirs |
Viruses | |
Rabies | Mammals |
Yellow fever | Wild birds, mammals, mosquitoes |
Viral fevers | Wild mammals |
Hantavirus | Rodents |
Influenza | Chickens, birds, swine |
West Nile virus | Wild birds, mosquitoes |
Bacteria | |
Rocky Mountain spotted fever | Dogs, ticks |
Psittacosis | Birds |
Leptospirosis | Domestic animals |
Anthrax | Domestic animals |
Brucellosis | Cattle, sheep, pigs |
Plague | Rodents, fleas |
Salmonellosis | Mammals, birds, reptiles, and rodents |
Tularemia | Rodents, birds, arthropods |
Miscellaneous | |
Ringworm | Domestic mammals |
Toxoplasmosis | Cats, rodents, birds |
Trypanosomiasis | Domestic and wild mammals |
Trichinosis | Swine, bears |
Tapeworm | Cattle, swine, fish |
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Nonliving Reservoirs
Microorganisms have adapted to nearly every habitat in the biosphere:
Thrive in soil, water, air
Surfaces in homes, offices, and other structures in the “built environment”
Most are saprobic and cause little harm and considerable benefit
Some are opportunists
A few are regular pathogens
Because humans are in regular contact with environmental sources, acquisition of pathogens from nonliving reservoirs is always a possibility
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Transmission of Infectious Agents(1)
Communicable: a disease in which an infected host can transmit the infectious agent to another host and establish infection in that host
Infectious is sometimes used interchangeably with communicable, but this is not precise usage
Contagious: a disease that is highly communicable, especially through direct contact
Influenza and measles are highly contagious
Hansen’s disease (leprosy) is only weakly communicable
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Transmission of Infectious Agents(2)
Noncommunicable: an infectious disease that does not arise through transmission of the infectious agent from host to host
Compromised person is invaded by their own microbiota
Individual has accidental contact with a microbe that exists in a nonliving reservoir
Infected persons do not become a source of disease to others
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Patterns of Transmission in Communicable Diseases(1)
Direct or indirect contact with animate or inanimate objects
Horizontal transmission: disease is spread through a population from one infected individual to another
Three major modes of transmission:
Direct contact - close contact between people
Indirect transmission - an object or substance carries the agent from one person to another
Vector transmission - arthropods that harbor an infectious agent and transfer it to a human
Vertical transmission: disease transmitted from parent to offspring via ovum, sperm, placenta, milk
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Patterns of Transmission in Communicable Diseases(2)
Vertical - transmission is from parent to offspring via the ovum, sperm, placenta, or milk
Horizontal - disease is spread through a population from one infected individual to another
Direct (contact) transmission
Indirect transmission
Fomite - inanimate object
Vehicle - natural, nonliving material like air, water, soil, and food
Vector transmission
©Guas/Shutterstock (kissing); Source: CDC/James Gathany (sneezing); ©silverlining56/Getty Images (opening door);
©Caia Images/Glow Images (playing soccer); ©IT Stock Free/Alamy Stock Photo (fly); ©abadonian/Getty Images (mosquito)
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Healthcare-Associated Infections
Healthcare-associated infections (HAIs) or nosocomial infections:
Infectious diseases acquired or developed during a hospital or health care facility stay
From 0.1 to 20% of all admitted patients, with an average of 4%
750,000 cases a year, resulting in 75,000 deaths
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Circumstances Make HAIs Unavoidable
Factors tied to healthcare-associated infections:
Compromised patients
Collection point for pathogens
Lowered defenses permit normal biota to enter the body.
Infections acquired directly or indirectly from fomites, medical equipment, other patients, medical personnel, visitors, air, and water
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Health Care Itself Increases Likelihood of Infectious Agent Transfer
Healthcare processes that lead to healthcare-associated infections:
Treatments using reusable instruments such as respirators and thermometers
Indwelling devices such as catheters, prosthetic heart valves, grafts, drainage tubes, and tracheostomy tubes form ready portals of entry
High proportion of the hospital population receives antimicrobial therapy, so drug-resistant microbes are selected for at a much higher rate
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Most Common HAIs
Most common HAIs include pneumonia, gastrointestinal illness, urinary tract infections, bloodstream infections, and surgical site infections
Five most common hospital pathogens:
Clostridium difficile: GI infections
Staphylococcus aureus: pneumonia, surgical site infections, bloodstream infections
Klebsiella species: surgical site infections, urinary tract infections, pneumonia
Escherichia coli: urinary tract infections, surgical site infections, bloodstream infections
Enterococcus species: surgical site infections, urinary tract infections, bloodstream infections
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Most Common Healthcare-Associated Infections
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Control of HAIs
Infection control officer: implements proper practices and procedures, tracks potential outbreaks, identifies breaches in asepsis, and trains health care workers in aseptic techniques
Training for nurses and caregivers - regularly exposed to needlesticks, infectious secretions, blood, and physical contact with patients; they need to be especially aware of infection control
Most hospitals have adopted universal precautions that recognize that all secretions from all persons in the clinical setting are potentially infectious and that transmission can occur in either direction
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Which Agent Is the Cause? Using Koch’s Postulates
Etiologic/causative agent: the cause of infection and disease
Koch’s postulates:
A series of proofs that became the standard for determining causation of infectious disease
Continue to play an essential role in modern epidemiology
Reliable for many diseases, but cannot be completely fulfilled in certain situations
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Koch’s Postulates
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Exceptions to Using Koch’s Postulates
Some infectious agents cannot be readily isolated or grown in the laboratory
Some infections cannot be elicited in animals; viruses have a limited host range, human viruses will only cause disease in humans, etc.
Not possible to determine causation in polymicrobial diseases
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Concept Check (2)
For each of the descriptions below, determine if it pertains to an exotoxin or an endotoxin.
Toxic in minute amounts
Causes systemic effects such as fever and inflammation
Released by a cell via shedding or during lysis
Composed of small proteins
Composed of lipopolysaccharide
Can be converted into a toxoid
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Learning Outcomes Section 11.3
Summarize the goals of epidemiology, and differentiate it from traditional medical practice.
Explain what is meant by a disease being “notifiable” or “reportable,” and provide examples.
Define incidence and prevalence, and explain the difference between them.
Discuss the three major types of epidemics, and identify the epidemic curve associated with each.
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Epidemiology: The Study of Disease in Populations
Epidemiology:
Study of frequency and distribution of disease and other health-related factors in defined populations
Involves many disciplines: microbiology, anatomy, physiology, immunology, medicine, psychology, sociology, ecology, and statistics
Considers all forms of disease: heart disease, cancer, drug addiction, and mental illness
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Florence Nightingale
Laid the foundations of modern epidemiology
Before the discovery of the germ theory, she understood that filth contributed to disease
Instituted revolutionary methods in military field hospitals, including separate linens and towels for each patient, cleaning of floors, and unclogging of sewage pipes
Kept meticulous notes and demonstrated that more men died of disease than traumatic injuries
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Techniques of Epidemiologists
Identify causative agents, using adaptations of Koch’s postulates
Track behaviors such as exercise and smoking
Collect clues on the causative agent, pathology, sources, and modes of transmission
Track the numbers and distribution of cases of disease in a community
Outcomes of these studies help public health departments develop prevention and treatment programs and develop a basis for predictions
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Tracking Disease in the Population
Reportable or notifiable diseases:
Certain diseases must be reported to authorities
Other diseases are reported on a voluntary basis
A network of individuals and agencies at the local, district, state, national, and international levels keeps track of infectious diseases
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Reportable Diseases in the U.S.(1)
Anaplasma phagocytophilum
Anthrax
Babesiosis
Botulism
Brucellosis
California serogroup virus neuroinvasive disease
Cancer
Chancroid
Chlamydia trachomatis infections
Cholera
Coccidioidomycosis
Cryptosporidiosis
Cyclosporiasis
Dengue fever
Diphtheria
Dengue fever
Diphtheria
Ehrlichiosis
Encephalitis/meningitis, arboviral
Encephalitis/meningitis, California serogroup viral
Encephalitis/meningitis, eastern equine
Encephalitis/meningitis, Powassan
Encephalitis/meningitis, St. Louis
Encephalitis/meningitis, western equine
Encephalitis/meningitis, West Nile
Foodborne disease outbreak
Giardiasis
Gonorrhea
Haemophilus influenzae invasive disease 3
Hansen’s disease (leprosy)
Hantavirus pulmonary syndrome
Hemolytic uremic syndrome
Hepatitis, viral, acute
Hepatitis A, acute
Hepatitis B, acute
Hepatitis B virus, perinatal infection
Hepatitis C, acute
Hepatitis, viral, chronic
Chronic hepatitis B
Hepatitis C virus infection (past or present)
HIV infection
Influenza-associated pediatric mortality
Lead poisoning
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Reportable Diseases in the U.S.(2)
Legionellosis
Leptospirosis
Listeriosis
Lyme disease
Malaria
Measles
Meningococcal disease
Mumps
Novel influenza A infections
Pertussis
Pesticide poisoning
Plague
Poliomyelitis, paralytic
Poliovirus infection
Powassan virus diseases
Psittacosis
Q fever
Rabies
Rabies, animal
Rabies, human
Rubella
Rubella, congenital syndrome
Salmonellosis
Severe acute respiratory syndrome–associated coronavirus (SARS-CoV) disease
Shiga toxin–producing Escherichia coli (STEC)
Shigellosis
Silicosis
Smallpox
Spotted fever rickettsiosis
Streptococcal toxic shock syndrome
Streptococcus pneumoniae, invasive disease
Syphilis
Syphilis, congenital
Tetanus
Toxic shock syndrome
Trichinellosis
Tuberculosis
Tularemia
Typhoid fever
Vancomycin-intermediate Staphylococcus aureus (VISA)
Vancomycin-resistant Staphylococcus aureus (VRSA)
Varicella
Vibriosis
Viral hemorrhagic fevers
Yellow fever
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Epidemiological Statistics(1)
Prevalence: total number of existing cases in a given population; snapshot
Total number of cases in population total number of persons in population 100 %
Example: The prevalence of smoking among adults in the U.S. is 17% currently
Incidence: the number of new cases over a certain time period
Number of new cases in a designated time period÷ total number of susceptible persons (usually reported per 100,000 persons):
Example: The incidence of new Lyme disease cases in the U.S. in 2014 was 8.6 per 100,000
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Epidemiological Statistics(2)
Statistics of concern to the epidemiologist are rates of disease with regard to sex, race, or geographic region
Mortality rate:
Measures the total number of deaths in a population due to a certain disease
Overall death rate from infectious diseases has dropped, although the number of persons afflicted with infectious rates (morbidity rate) has remained high
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Depictions of Epidemiological Data
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Epidemics
Common-source epidemic: results from common exposure to a single source of infection over a period of time
Propagated epidemic: results from an infectious agent that is communicable from person to person and is sustained over time in a population
Point-source epidemic: infectious agent came from a single source, and all of its “victims” were infected at once
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Epidemic Curves
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Additional Epidemiology Terms
Index case: may not be the first case of the disease, but it is the first case that brought the epidemic to the attention of officials
Endemic: an infectious disease that exhibits a relatively steady frequency over a long time period in a particular geographic locale
Sporadic: occasional cases are reported at irregular intervals at random locales
Epidemic: when statistics indicate that the prevalence of an endemic or sporadic disease is increasing beyond what is expected for a population
Pandemic: spread of an epidemic across continents
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Concept Check (3)
Which of the following terms describes the total number of persons afflicted with a particular infectious disease in the entire population?
Morbidity
Prevalence
Mortality
Incidence
Endemic
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Appendix of Image Long Descriptions
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Where Babies Get a Microbiome - Appendix
Where babies get a microbiome. In utero: previously thought to be sterile the womb has its own microbiota. Birth: vaginal and C-section births contribute different initial microbiomes to baby. Milk: Breast milk and formula have differing microbes in them. Caregivers: family, siblings, and others share microbes with the baby. Environment: baby can pick up microbes from anything she comes in contact with.
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Exotoxins - Appendix
Beta-hemolysis, in the lower right, results in complete clearing of the red blood cells incorporated in the agar. Alpha-hemolysis, on the lower left, refers to incomplete lysis of the red blood cells.
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Origins and Effects of Circulating Exotoxins and Endotoxins - Appendix
Exotoxins: Toxic in minute amounts; Specific to a cell type (blood, liver, nerve) and induces TNF production resulting in fever; Small proteins; Unstable with heat denaturation at 60°C; Can be converted to toxoid; Stimulate antitoxins; Usually does not stimulate fever; Secreted from live cell; Typical sources are A few gram-positive and gram-negative. Endotoxin: Toxic in high doses; Systemic: fever, inflammation; Lipopolysaccharide of cell wall; Stable (does not denature) at 60°C; Cannot be converted to toxoid; Does not stimulate antitoxins; Stimulates fever; Released by cell via shedding or during lysis; Source is All gram-negative bacteria.
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Will Disease Result? - Appendix
In the first row the microbe's virulence is low, only 20% of the optimal infectious dose is present, and it is missing the correct portal of entry. The host has a genetic profile that is pretty resistant to the microbe (nonspecific defenses), previous exposure to the microbe (specific immunity) and a generally high level of health. The result is that the microbe passes through unnoticed. In the second row the microbe's virulence is moderate, 50% of the optimal infectious dose is present, and the correct portal of entry is available. The host has a genetic profile that resists the microbe (nonspecific defenses), previous exposure to the microbe (specific defenses), and a high general level of health. The result is that the microbe passes through unnoticed or the microbe becomes established without disease (colonization or infection). In the third row, the microbe's virulence is moderate, the percentage of the optimal infectious dose is low, and it is missing the correct portal of entry. The host has a genetic profile that resists the microbe (nonspecific defenses) and a high general level of health, but does not have previous exposure to the microbe. The outcome is that the microbe passes through unnoiticed or that the microbe becomes established without disease (colonization or infection). And in the fourth row is a microbe with a moderately high virulence, about 80% of the optimal infectious dose, and the correct portal of entry. The host has a moderately low resistance to the microbe (nonspecific defenses), no previous exposure to the microbe (specific immunity), and only a moderate level of health. The outcome is that the microbe causes disease in the host.
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Steps Involved When a Microbe Causes Disease in a Host - Appendix
1. Finding a Portal of Entry: Skin, GI tract, Respiratory tract, Urogenital tract, Endogenous biota. 2. Attaching Firmly: Fimbriae, Capsules, Surface proteins, Viral spikes. 3. Surviving Host Defenses: Avoiding phagocytosis, Avoiding death inside phagocyte, Absence of specific immunity. 4. Causing Damage (Disease): Direct damage via enzymes or toxins, Inducing excessive host response, Causing epigenetic changes in host chromosome. 5. Exiting Host: Portals of exit: Respiratory tract, salivary glands; Skin cells; Fecal matter; Urogenital tract; Blood.
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Stages in the Course of Infection and Disease - Appendix
Incubation period is followed by prodromal stage. Next comes the acute phase with the height of infection. Symptoms go away at the end of the convalescent period or level out during a continuation period.
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Reservoirs and Transmitters - Appendix
A. Living Reservoirs. 1. Animals (other than humans and arthropods) such as Mammals, birds, reptiles, and so on - Animals harboring pathogens can directly transmit them to humans (bats transmitting rabies to humans); vectors can transmit the pathogens from animals to humans (fleas passing the plague from rats to people); vehicles such as water can transmit pathogens which originated in animals, as in the case of leptospirosis. 2. Humans who are Actively ill - A person suffering from a cold contaminates a pen, which is then picked up by a healthy person. That is indirect transmission. Alternatively, a sick person can transmit the pathogen directly by sneezing on a healthy person. 3. Carriers - A person who is fully recovered from his hepatitis but is still shedding hepatitis A virus in his feces may use suboptimal hand-washing technique. He contaminates food, which a healthy person ingests (indirect transmission). Carriers can also transmit through direct means, as when an incubating carrier of HIV, who does not know she is infected, transmits the virus through sexual contact. 4. Arthropods (biological vectors) - When an arthropod is the host (and reservoir) of the pathogen, it is also the mode of transmission. B. Nonliving Reservoirs (Soil, Water, Air, the Built Environment). Some pathogens, such as the TB bacterium, can survive for long periods in nonliving reservoirs. They are then directly transmitted to humans when they come in contact with the contaminated soil, water, or air.
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Carrier States - Appendix
Several situations can produce the carrier state. This table describes the various carrier states and provides examples of each. Asymptomatic carriers are Infected but show no symptoms of disease. Examples are Gonorrhea and genital herpes with no lesions. Incubating carriers are Infected but show no symptoms of disease. The example is infectious mononucleosis. Convalescent carriers are Recuperating patients without symptoms; they continue to shed viable microbes and convey the infection to others, as in the case of Hepatitis A. Chronic carriers are those Individuals who shelter the infectious agent for a long period after recovery because of the latency of the infectious agent. Examples include tuberculosis and typhoid fever. And passive carriers are Medical and dental personnel who must constantly handle patient materials that are heavily contaminated with patient secretions and blood risk picking up pathogens mechanically and accidentally transferring them to other patients, causing various healthcare-related infections.
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Patterns of Transmission in Communicable Diseases(2) - Appendix
Vertical Transmission: Transmission is from parent to offspring via the ovum, sperm, placenta, or milk. Horizontal Transmission: 1. Direct (contact) transmission - Involves physical contact between infected person and that of the new infectee. Types: Touching, kissing, sex; Droplet contact, in which fine droplets are sprayed directly upon a person during sneezing or coughing; Parenteral transmission via intentional or unintentional injection into deeper tissues (needles, knives, branches, broken glass, etc.). Examples: Contact: kissing and sex (Epstein-Barr virus, gonorrhea) and Droplets (colds, chickenpox). 2. Indirect transmission Infectious agent must pass from an infected host to an intermediate conveyor (a vehicle) and from there to another host Infected individuals contaminate objects, food, or air through their activities. Types: Fomite - inanimate object that harbors and transmits pathogens (doorknobs, telephone receivers, faucet handles); Vehicle - a natural, nonliving material that can transmit infectious agents; Air - smaller particles evaporate and remain in the air and can be encountered by a new host; aerosols are suspensions of fi ne dust or moisture particles in the air that contain live pathogens; Water - some pathogens survive for long periods in water and can infect humans long after they were deposited in the water; Soil - microbes resistant to drying live in and can be transmitted from soil; Food - meats may contain pathogens with which the animal was infected; foods can also be contaminated by food handlers. Special Category: oral-fecal route - using either vehicles or fomites. A fecal carrier with inadequate personal hygiene contaminates food during handling, and an unsuspecting person ingests it; alternatively a person touches a surface that has been contaminated with fecal material and touches his or her mouth, leading to ingestion of fecal microbes. 3. Vector transmission - Types: Mechanical vector - insect carries microbes to host on its body parts; Biological vector - insect injects microbes into host; part of microbe life cycle completed in insect.
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Most Common Healthcare-Associated Infections - Appendix
Pie chart of the most common HAIs: pneumonia 22%, gastrointestinal illness 17%, urinary tract infections 13%, bloodstream infections 10%, and surgical site infections 22%. “Other” makes up 16% of HAIs.
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Koch’s Postulates - Appendix
This figure illustrates Koch's Postulates. Postulate #1: Find evidence of a particular microbe in every case of a disease. Postulate #2: Isolate that microbe from an infected subject and cultivate it in pure culture in the laboratory; perform full microscopic and biological characterization. Postulate #3: Inoculate a susceptible healthy subject with the laboratory isolate and observe the same resultant disease. And Postulate #4: Reisolate the same agent from this subject.
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