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Disease PrinciplesDisease Principles

• Hugh B. FackrellHugh B. Fackrell

• Filename:Filename: DiseasePrinciples.pptDiseasePrinciples.ppt

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OutlineOutline

• Infection vs DiseaseInfection vs Disease

• PathogenicityPathogenicity

• VirulenceVirulence

• Pathogenic factorsPathogenic factors

• Latency DormancyLatency Dormancy

• CommunicabilityCommunicability

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SaprophyteSaprophyte

• nutrient source is non-livingnutrient source is non-living

• can become parasitecan become parasite

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ParasitismParasitism

• host/microorganism interactionhost/microorganism interaction

• MutualismMutualism - both host and parasite benefit. - both host and parasite benefit.

• Commensalism Commensalism - the parasite does no damage - the parasite does no damage to the host.to the host.

• Pathogen Pathogen - the parasite damages the host.- the parasite damages the host.

• OpportunismOpportunism - the parasite takes advantage of - the parasite takes advantage of the weakened condition of the host.the weakened condition of the host.

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Microbial Infection vs DiseaseMicrobial Infection vs Disease

• Infection: colonization of the body with microbeInfection: colonization of the body with microbe– usually non pathogenicusually non pathogenic– indigenous or commensalindigenous or commensal– beneficialbeneficial

• Disease: breach of host defensesDisease: breach of host defenses– microbes infect tissues not normally exposedmicrobes infect tissues not normally exposed

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Infection: BenefitsInfection: Benefits• MetabolitesMetabolites

– E. coliE. coli makes vitamin K makes vitamin K

• Bacterial antagonismBacterial antagonism– E.coliE.coli blocks colonization of gut by blocks colonization of gut by S. aureusS. aureus– evidenceevidence: antibiotic sterilization before abdominal surgery: antibiotic sterilization before abdominal surgery– subsequent Staphylococcal infectionssubsequent Staphylococcal infections– enteritisenteritis

• Encourage immune systemEncourage immune system– axenic animals have poor immunityaxenic animals have poor immunity

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Infection: Adverse EffectsInfection: Adverse Effects

• Staphylococcal infectionStaphylococcal infection– Produces penicillinaseProduces penicillinase– Concurrent infection of Concurrent infection of Neisseria gonorrhaeaeNeisseria gonorrhaeae– Becomes disease gonorrheaBecomes disease gonorrhea

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Microbial DiseaseMicrobial Disease

• Microbial disease is the Microbial disease is the exceptionexception

– imbalance favours the microbeimbalance favours the microbe

• 3% of all microbes pathogenic3% of all microbes pathogenic– majority of known microbesmajority of known microbes

• >95% do not cause disease>95% do not cause disease– virtually unknownvirtually unknown

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Location of Microbe in Host Location of Microbe in Host • Location often decides outcome of Infection vs Location often decides outcome of Infection vs

disease disease • StreptococcusStreptococcus

– Infection: nasopharynxInfection: nasopharynx– Disease: heartDisease: heart– bacterimiae after tooth extractionbacterimiae after tooth extraction

• E. coliE. coli– Infection gutInfection gut– Disease: cystitis in urinary tractDisease: cystitis in urinary tract

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Contagious TransmissionContagious Transmission

• can be transmitted from one host to another can be transmitted from one host to another (communicable)(communicable)

• some infections acquired from indigenous some infections acquired from indigenous flora are categorized as communicable.flora are categorized as communicable.

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CommunicabilityCommunicability• Communicable: spread directly or indirectly Communicable: spread directly or indirectly

from one host to anotherfrom one host to another– chickenpox, measles, tuberculosis, typhoid feverchickenpox, measles, tuberculosis, typhoid fever

• Contagious: Contagious: easilyeasily communicable communicable– eg chickenpox,measles, sore throateg chickenpox,measles, sore throat

• Non communicable: Non communicable: Not spreadNot spread from host to from host to hosthost– tetanustetanus

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DormancyDormancy

• Latency = DormancyLatency = Dormancy: causative microbe : causative microbe remains inactive in the host for some time remains inactive in the host for some time but later becomes active to produce the but later becomes active to produce the signs and symptoms of the diseasesigns and symptoms of the disease

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Carrier StateCarrier State

• Individual infectedIndividual infected– results from a previous disease state (may be results from a previous disease state (may be

temporary)temporary)– the host is a true carrierthe host is a true carrier

• microbe in balance with that individualmicrobe in balance with that individual• No overt signs or symptomsNo overt signs or symptoms• reservoir for infection of othersreservoir for infection of others

–Typhoid Mary Typhoid Mary

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Koch’s PostulatesKoch’s PostulatesKoch’s PostulatesKoch’s Postulates• The organism should be found in all cases of The organism should be found in all cases of

the disease and its distribution in the body the disease and its distribution in the body should be in accordance with the lesions should be in accordance with the lesions observed.observed.

• The organism should be cultivated outside The organism should be cultivated outside the body of the host, in pure culture, for the body of the host, in pure culture, for several generations of the pathogen.several generations of the pathogen.

• The disease should be reproduced in The disease should be reproduced in susceptible animals.susceptible animals.

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EtiologyEtiologyEtiologyEtiology

Science of the causes of diseaseScience of the causes of disease• the nature of the host - speciesthe nature of the host - species• the condition of the hostthe condition of the host• the nature of the disease agentthe nature of the disease agent• the transmission of the agentthe transmission of the agent

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Etiology of DiseaseEtiology of DiseaseEtiology of DiseaseEtiology of Disease

• Specimen from patient containing infectious Specimen from patient containing infectious agent cultured.agent cultured.

• Pure culture obtained and described; identified Pure culture obtained and described; identified if possible.if possible.

• Inoculation and observation of test animal.Inoculation and observation of test animal.

• Many organs removed and cultured.Many organs removed and cultured.

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Etiology of DiseaseEtiology of Disease (2/2) (2/2)• Isolation and identification of the test Isolation and identification of the test

organism.organism.

• Inoculation of a second test animal.Inoculation of a second test animal.

• Culture of second test animal.Culture of second test animal.

• Antibodies in blood of human or animal.Antibodies in blood of human or animal.

• Immunity developed to the infecting agent in Immunity developed to the infecting agent in recently recovered animal.recently recovered animal.

• Animal protected by vaccine or toxoid.Animal protected by vaccine or toxoid.

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PathogenicityPathogenicity

• Ability or potential to cause disease Ability or potential to cause disease attributed to a genus or speciesattributed to a genus or species

• Dependent on ability toDependent on ability to

– enter the hostenter the host

– adapt and multiply in the hostadapt and multiply in the host

– exit from the hostexit from the host

– transmit to new hosttransmit to new host

Keppie Smith

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Portal of EntryPortal of Entry

• May multiply at entry site> lesionMay multiply at entry site> lesion

• Often enters multiplies elsewhereOften enters multiplies elsewhere

• 2/3 Respiratory2/3 Respiratory

• 1/3 intestine, urethra, conjunctiva and skin1/3 intestine, urethra, conjunctiva and skin

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Multiplication in HostMultiplication in Host

• Rate of multiplication different Rate of multiplication different in vivoin vivo and and in vitro

• time to overt symptoms

• Carrier state

• reservoir gall bladder

• Temperature– viruses– aspirin

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Exit from HostExit from Host

• Usually by same route a entryUsually by same route a entry

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Bacterial transmissionBacterial transmission

• DropletsDroplets• FomitesFomites• Direct contactDirect contact

– sexual sexual

– non sexualnon sexual

• BitesBites– insectsinsects

– animalsanimals

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Types of PathogensTypes of Pathogens

• pathogens characterized as “frank” - pathogens characterized as “frank” - SalmonellaSalmonella

• opportunistic pathogens - opportunistic pathogens - E. coliE. coli in urinary in urinary tracttract

• non-pathogensnon-pathogens - such as - such as Serratia marcescensSerratia marcescens may become infectious agentsmay become infectious agents

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VirulenceVirulence

• DEGREE of pathogenicity shown by a DEGREE of pathogenicity shown by a specific strain of an organismspecific strain of an organism– C. diptheriaeC. diptheriae > diptheria > diptheria

• variables include:variables include:– number of infecting bacterianumber of infecting bacteria– route of entry into bodyroute of entry into body– specific host defense mechanismsspecific host defense mechanisms– non-specific host defense mechanismsnon-specific host defense mechanisms– virulence factors of the bacteriumvirulence factors of the bacterium

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Virulence MeasurementVirulence Measurement

• Measures the pathogenicity of a isolateMeasures the pathogenicity of a isolate– variable among strainsvariable among strains

• Measure of VirulenceMeasure of Virulence– Median DoseMedian Dose– Minimum Effective DoseMinimum Effective Dose

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Median DoseMedian Dose

Per

cent

Sus

cept

ible

[Pathogen]

Amount that affects half the population

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Cause of Virulence VariationCause of Virulence Variation

• Dose of pathogenDose of pathogen

• Virulence/Pathogenic factorsVirulence/Pathogenic factors

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Dose of PathogenDose of Pathogen

• Typhoid FeverTyphoid Fever– S. typhosaS. typhosa contaminated water contaminated water– 1-100 bacteria no effect1-100 bacteria no effect

• BoilsBoils– Staphylococcus aureusStaphylococcus aureus– just on surface 7 x 10 just on surface 7 x 10 66cellscells– on suture or scratch ~1000 cellson suture or scratch ~1000 cells– ““stictch abcesses”stictch abcesses”

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““Pathogenic Factors”Pathogenic Factors”• Gene mutationGene mutation

– S. pneumoniae- S. pneumoniae- capsulecapsule– Shigella Shigella > dysentery> dysentery

• Lysogeny Lysogeny – scarlet feverscarlet fever– diptheriadiptheria

• CapsulesCapsules– KlebsiellaKlebsiella > polysaccharide > polysaccharide– Anthrax > protein capsuleAnthrax > protein capsule