Pathogenesis & Virulence of Infectious Diseases

Why Exposure May NOT Lead to Infection

• Agent lands in/on wrong body part• Host cells lacks specific receptors• Antimicrobial compound destroys pathogen• Normal flora outcompetes or kills pathogen• Host immune system destroys pathogen

PathogenesisSteps involved in the development of disease

Infection: colonization by a microorganism

Primary, SecondaryIncubation: pathogen

arrival to symptomsProdromal: early

symptoms, usually general

Illness: typical disease-specific symptoms

Convalescence: recovery or disease regress

Common Steps of InfectionEntry: contact, injection, inhalation…Attachment: pathogen sticks to cells in bodyMultiplication: growth - leading, possibly, to…

Variable Steps of InfectionInvasion: localized to systemic, related to…Evasion: pathogen hides from immune system Damage: pathogen AND/OR host-mediated

Kinds of Diseases/InfectionsAcute: rapid onset and recovery. Sub-acuteChronic: slow onset and recoveryLatent: cycles of symptoms, no symptoms

Virulence FactorsPathogen traits that cause infection and disease

Some Facilitate Attachment

Surface Proteins: bind to host cell receptors

Pili/Fimbrae: bind to host epithelial surfaces

Capsules: bind to host epithelial surfaces

Some Facilitate InvasionFlagella: allows pathogen to moveFibrinolysin: “clot-busters,” digest fibrinFlesh-Eating Enzymes: burrow into tissuesCoagulase: coat self with host fibrin

Extracellar enzymes

• Hyaluronidase -connective tissue

• Collagenase -collagen

• Lecithinase -muscle, membranes

Hemolysins

Many also serve to evade the immune system.

Some Facilitate EvasionIntracellular Growth: hide IN cells undetectedAntiphagocytic: capsules, flagella, waxy wallsAg Variation: changing surface gene productsMimicry: look like host, autoimmune problems

Evasion

Some Facilitate Damage

Some Facilitate DamageEndotoxins: LPS proteins, less damagingExotoxins: secreted proteins, more

damagingHemolysin: lyse RBC, release iron for

growth