Epidemiology of Infectious disease 1. Symbiotic Relationships Between Microbes and Their Hosts 2...

Epidemiology of Infectious disease

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Symbiotic Relationships Between Microbes and Their Hosts

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Symbiosis means “to live together”We have symbiotic relationships with

countless microorganismsTypes of symbiosis

MutualismCommensalismParasitism


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InfectionMutualsim: Each one benefitsCommensual: One organism benefits,

the other is not harmed.Parasitism: Advantage to one,

disadvantage to the other.

Obligate relationship: one depends on the other for survival (parasites are obligate pathogens)

Normal Flora: Organism is always on or in us; GI, GU, oral mucosa.

Sterile areas of the body: Blood, CSF. Any organism here indicates something is wrong. The first indicator is decreased glucose, increased protein

Opportunistic Pathogens: A normal flora organism that becomes a pathogen when the host’s immune defense decreases.

Trauma, Surgery, Burns: overcomes barriers of defense

Catheterization: allows increased access and decreased defense

Chemotherapy: e.g. ablative (for CA) destroys bone marrow,

therefore myeloid and lymphoid immunity goes down.

Co-infection: e.g. haemophilic influenza is 2° to viral influenza.

Tissue TropismTissue Tropism (organisms are selective of

tissues they infect): influenza does not affect your hand; gonorrhea only effects mucosa. Tropism can also be species, organ, or non-specific.

HIV integrates a provirus into our chromosome. Antibodies actually assist the HIV organisms because opsinization helps to internalize the virus. Because of almost no cell mediated immunity (T-cells are infected), AIDS patients die from TB, cytomegalovirus (CMV), Candida, pneumocystis. When a person has a second infection, it is called a CO-INFECTION.

OBLIGATE PATHOGENSWill always cause infection and

disease whenever and wherever they are encountered (e.g. Yersinia pestis plague, anthrax, smallpox) unless you are immune. However, they don’t want to kill their host.

Virulence VIRULENCE: Ability to cause infection and disease. MULTIPLICITY OF INFECTION (INNOCULUM): the number of

organisms needed to cause disease. There is an inverse relationship

between virulence and Multiplicity of Infection. If only a few bacterial

cells cause the disease, it is said to be very virulent.

Obligate pathogens are virulent, even with a low inoculum, because

they are effective. Most innoculum needs 103-5 organisms. Only 10

organisms can form an innoculum in a virulent organism such as

anthrax.

Anthrax avoids immune defenses by exploiting the very mechanisms

used to fight it. It becomes activated after phagocytosis. Mycobacterium will coat the wall of vacuoles so lysozymes can’t fuse. Rickettsia is on a timer, and escapes the vacuole to invade the

nucleus.

PORTALS OF ENTRY (POE)CEPHALIC (7 PORTALS): Mouth, nose,

eyes, ears.CORPOREAL: Mammary, Vaginal,

Urethral, RectalTRAUMA/MEDICAL: Burn, Compound

Fracture, surgical/catheter, injury/IVDA, abnormal mucosa (CA chemo).

MODES OF TRANSMISSION (MOT)

AEROSOL: Airborne micro-particles (soil aerosol containing endospores)

DROPLET NUCLEI: Mucoid micro-droplet via cough or sneeze, durable on surfaces, mainly transmitted by contact. Sneeze and cough into your elbow!

DIRECT CONTACT: Direct object/tissue-to-tissue contact.o CASUAL TRANSMISSION: handshake, clothingo ASPIRATION: inhalation of oral, GI, or food-borne

organismso FECAL-ORAL: autoinoculation or contamination (poor hygiene)o SEXUAL TRANSMISSION (STD): sexual/bodily fluid

contact

AGENTS OF TRANSMISSIONVECTOR: a living organism that spreads

disease from one host to another. (mosquitoes, ticks, rats, humans)FOMITE: inanimate object that spreads

disease from one host to another. (toothbrush, water glass, toys, handles)

HOST-PATHOGEN RELATIONSHIP (HPR)

INFECTION: presence of organism DISEASE: adverse symptoms due to infection NORMAL FLORA: organism typically found on healthy individual PATHOGEN: organism causing disease (etiologic agent) IMMUNOCOMPROMISED: suppressed or deficient immunity NOSOCOMIAL: acquired in hospital setting COMMUNITY-ACQUIRED: acquired in routine, day-to-day activities OPPORTUNISTIC PATHOGEN: normal flora + compromised pathogen OBLIGATE PATHOGEN: always causes Dz if present PATHOGENESIS (Px): events leading to Dz (POEDz) MULTIPLICITY OF INFECTION (MOI): inoculum required for infxn INCUBATION (INC): time from transmission to Dz VIRULENCE: measure of Dz capability VIRULENCE FACTOR (VF): ultrastructure, exoenzyme, or exotoxin

BACTERIAL SYMPTOM TERMINOLOGY

BACTEREMIA: bacteria in bloodstream BACTERURIA: bacteria in urine (normal or pathogenic flora) SEPTICEMIA: Gram negative bacteremia in the bloodstream PYEMIA: Gram + bacteremia PYROGENIC: fever-producing infection or bacterial components PURULENT: pus-producing, reaches an opening to drain out. The pus is a

collection of dead neutrophils, which are the 1° immune cells when there is pus. GRANULOMATOUS: infxn where macrophages are 1° immune cells (no pus) DIFFERENTIAL DIAGNOSIS (DDx): all possible causes of the disease. UROSEPSIS: Septicemia from UTI FACULTATIVE INTRACELLULAR PARASITES: are not obligate pathogens.

They can survive temporarily in white cells, such as neutrophils, Monocytes/ Macrophages.

EXOTOXINS: A toxin made and then secreted, e.g. pesticin, hemolysins, shigatoxins, choleragen, enterotoxins, plasminogen activating factor, coagulase, etc.

ENDOTOXINS: A component of the cell wall is toxic to us, e.g. LPS and LOS.


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Normal Microbiota in HostsAlso termed normal flora and indigenous microbiota

Organisms that colonize the body’s surfaces without normally causing disease

Two typesResident microbiotaTransient microbiota


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Resident microbiotaAre a part of the normal microbiota

throughout lifeAre mostly commensal

Transient microbiotaRemain in the body for short periodFound in the same regions as resident

microbiotaCannot persist in the body

Competition from other microorganismsElimination by the body’s defense cellsChemical or physical changes in the body


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Acquisition of normal microbiotaDevelopment in womb free of

microorganisms (axenic)Microbiota begin to develop during

birthing processMuch of one’s resident microbiota

established during first months of life


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How Normal Microbiota Become Opportunistic PathogensOpportunistic pathogens

Normal microbiota that cause disease under certain circumstances

Conditions that provide opportunities for pathogensIntroduction of normal microbiota into

unusual site in bodyImmune suppressionChanges in the normal microbiota

Changes in relative abundance may allow opportunity for a member to thrive and cause disease

Inflammation

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Host defense summary

19What can you catch from a toilet seat?

Cell surface markers

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Designated as “CD”Cluster of Differentiationused to identify specific structures on a cell

~200 different designations

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Reservoirs of Infectious Diseases of Humans

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Most pathogens cannot survive for long outside of their host

Reservoirs of infectionSites where pathogens are maintained as a source of infection

Three types of reservoirsAnimal reservoirHuman carriersNonliving reservoir


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Animal ReservoirsZoonoses

Diseases naturally spread from animal host to humans

Acquire zoonoses through various routesDirect contact with animal or its waste

Eating animalsBloodsucking arthropods

Humans are usually dead-end host to zoonotic pathogens


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Human CarriersInfected individuals who are asymptomatic but 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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Nonliving Reservoirs Soil, water, and food can be reservoirs of infectionPresence of microorganisms often due to contamination by feces or urine

The Movement of Microbes into Hosts: Infection

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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


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Portals of Entry Sites through which pathogens enter the body

Four major pathwaysSkinMucous membranesPlacentaParenteral route (intravenous)

Routes by which humans acquire parasitic infections

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Routes of entry for invading pathogens

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Portals of exit

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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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Portals of Entry Mucous membranes

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 entryMust survive the acidic pH of the stomach


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Portals of Entry Placenta

Typically forms effective barrier to pathogens

Pathogens may cross the placenta and infect the fetusCan cause spontaneous abortion, birth defects, premature birth


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Portals of Entry Parenteral route

Not a true portal of entryPathogens deposited directly into tissues beneath the skin or mucous membranes (needle injection or i.v.)


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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

Adhesion factors includeSpecialized structures (acts like Velcro)Attachment proteins (acts like glue)The presence of adhesions are considered a virulence factor.


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The Role of Adhesion in Infection Attachment proteins help in adhesion

Found on viruses and many bacteriaViral or bacterial ligands (proteins) bind host cell receptorsInteraction 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

The Nature of Infectious Disease

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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 SyndromesSymptoms

Subjective characteristics of disease felt only by the patient

Signs Objective manifestations of disease

observed or measured by othersSyndrome

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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Cause of Disease: EtiologyStudy of the cause of diseaseGerm 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

Koch’s Postulates

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Causation of Disease: EtiologyExceptions to Koch’s postulates

Some pathogens can’t be cultured in the laboratoryDiseases caused by a combination of pathogens

and other cofactorsEthical considerations prevent applying Koch’s

postulates to pathogens that require a human hostDifficulties in satisfying Koch’s postulates

Diseases can be caused by more than one pathogen

Pathogens that are ignored as potential causes of disease


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Virulence Factors of Infectious AgentsPathogenicity

Ability of a microorganism to cause disease

VirulenceDegree of pathogenicity (how good is it at

its job?)Virulence factors contribute to virulence

Adhesion factorsBiofilmsExtracellular enzymesToxinsAntiphagocytic factors


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Virulence Factors of Infectious AgentsExtracellular enzymes

Secreted by the pathogenDissolve structural chemicals in the body

Help pathogen maintain infection, invade, and avoid body defenses


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Virulence Factors of Infectious AgentsToxins

Chemicals that 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

Two typesExotoxins (made and secreted)Endotoxins (part of the bacterial cell wall)


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Virulence Factors of Infectious AgentsAntiphagocytic factors

Factors prevent phagocytosis by the host’s phagocytic cellsBacterial capsule

oComposed of chemicals not recognized as foreign

oSlippery; difficult for phagocytes to engulf bacteria

Antiphagocytic chemicalsoPrevent fusion of lysosome and phagocytic

vesiclesoLeukocidins directly destroy phagocytic white

blood cells


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The Stages of Infectious DiseaseThe disease process occurs following infection

Many infectious diseases have five stages following infectionIncubation periodProdromal periodIllnessDeclineConvalescence

The stages of infectious disease

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The Movement of Pathogens Out of Hosts: Portals of Exit

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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

Portals of exit

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Modes of Infectious Disease Transmission

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Transmission is from a reservoir or a portal of exit to another host’s portal of entry

Three groups of transmissionContact transmission

Direct, indirect, or dropletVehicle transmission

Airborne, waterborne, or foodborneVector transmission

Biological or mechanical

Classification of Infectious Diseases

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Diseases can be classified in number of waysThe body system they affect

GI system, nervous system, bloodTaxonomic categories

Viral disease, Gram +Their longevity and severity

Chronic vs. acute; mild vs. severeHow they are spread to their host

STD, blood borne, foodThe effects they have on populations (rather than on individuals): global/pandemic vs. epidemic

Classification of Infectious Diseases

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Terms used to classify infectious diseaseAcute disease (rapid onset, short duration)

Chronic disease (long duration)Subacute disease (slower onset, medium duration)

Latent disease (present but not obvious system)

Communicable (Contagious)

Epidemiology of Infectious Diseases

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INFECTION: Presence of microbeDISEASE: Symptomatic consequence of infection

EpidemiologyEpidemiology is the study of the patterns,

causes, and effects of health and disease conditions in defined populations. It is the cornerstone of public health by identifying risk factors for disease and provides information for preventive healthcare.

Major areas of epidemiological study include disease etiology, outbreak investigation, disease surveillance and screening, biomonitoring, and comparisons of treatment effects such as in clinical trials.

John Snow Dr. John Snow is

known as the father of modern epidemiology. He showed that cholera was water-borne, and debunked the Miasma theory, which stated that cholera came from bad air.

John Snow John Snow was a British physician in London in

1854, where a major cholera outbreak struck the nearby city of Soho.

In three days, 127 people who lived near Broad Street died of Cholera. Within a week, 500 people had died and one in seven people who developed cholera eventually died from it.

John Snow suspected contamination of the water, but couldn’t find any organic matter in it. Almost all the cases of cholera occurred close to the Broad Street Water Pump.

Snow mapped out the cases of cholera in relation to the water pump.

The evidence Snow presented convinced the local council to remove the pump handle, halting the spread of the disease. It wasn’t until 1885, when Robert Koch identified V. cholerae as the bacillus causing the disease that he had proof of his theory. He was right, but wasn’t around to see this discovery himself. John Snow died four years later, at the age of 45.


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Frequency of DiseaseTrack occurrence of diseases using two measuresIncidence

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

The incidence and estimated prevalence of AIDS

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Epidemiologists report data in a number of ways

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Different terms for the occurrence of disease

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Epidemics Defined Relative to Expected Number of Cases

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Epidemiological StudiesDescriptive epidemiology

Careful tabulation of data concerning a diseaseRecord location and time of the cases of disease

Collect patient informationTry to identify the index case (or first case) of the disease


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Epidemiological StudiesAnalytical epidemiology

Seeks to determine the probable cause (etiology), 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


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Epidemiological StudiesExperimental epidemiology

Involves testing a hypothesis concerning the cause of a disease

Application of Koch’s postulates is experimental epidemiology

Endemic1: characteristic of or prevalent in a particular field, area, or environment2: restricted or peculiar to a locality or region“Endemic" means "in the population." "Endemic" is often used to characterize diseases that are generally found in a particular area; malaria, for example, is said to be endemic to tropical and subtropical regions. This use differs from that of the related word "epidemic" in that it indicates a more or less constant presence in a particular population or area rather than a sudden, severe outbreak within that region or group. The word is also used by biologists to characterize the plant and animal species that are only found in a given area.

EpidemicAffecting or tending to affect

a DISPROPORTIONATELY large number of individuals within a population, community, or region at the same time

DemographicsThe changes (such as the number of births,

deaths, marriages, and illnesses) that occur over a period of time in human populations; also : a set of such changes.

The demographics of a particular disease would include the ratio of males to females that have the disease, the average age, the geographical distribution, and the incidence (the likelihood of a person getting the disease) in a particular Country.


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Hospital Epidemiology: Nosocomial InfectionsTypes of nosocomial infections

Exogenous Pathogen acquired from the health care

environmentEndogenous

Pathogen arises from normal microbiota due to factors within the health care setting

Iatrogenic Results from modern medical procedures

The interplay of factors that result in nosocomial infections

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Hospital Epidemiology: Nosocomial InfectionsControl of nosocomial infections

Precautions designed to reduce factors that result in disease

Hand washing is the most effective way to reduce nosocomial infections


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Epidemiology and Public HealthAgencies at the local, state, national, and global level share information concerning diseaseThe United States Public Health Service

World Health Organization (WHO)Public health agencies work to limit disease transmissionMonitor water and food safety

Public health agencies campaign to educate the public on healthful choices to limit disease

Basic Principles of Microbial Control

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Epidemiology of Infectious disease 1. Symbiotic Relationships Between Microbes and Their Hosts 2...

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