Lecture 17 Introduction to Infectious Diseases

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Lecture 17Introduction to Infectious Diseases

• Lecture aims
• To understand some key terms in microbial
  pathogenesis (disease causing microbes) and the
  study of infections
• To understand the nature of the infectious
  process
• To understand how vaccination/immunity may
  influence the spread of infections
• References Black Chp 14p384-401, Chp15p402-422

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Introductory terminology Microbial pathogenesis

• Pathogenesis the mechanisms of the development
  of a disease
• Pathogen a infectious agent which causes an
  infection. In more specific terms an organism
  which when isolated from clinical material is
  always considered significant
• As cw opportunistic pathogens commensals
• Commensal, symbiotic, parasitic

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TerminologyMicrobial pathogenesis

• Virulence the ability for an organism to cause
  disease
• Organism of high virulence tend to always cause
  disease in a susceptible host
• Eg Small pox no known carrier status
• Cw S.epidermidis low virulence may cause disease
  in compromised host

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Illustration of virulence components eg
Salmonella typhi
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TerminologyInfections

• Infections generally divided into several
  periods
• Incubation (asymptomatic)
• Prodromal (vague symp)
• Invasive (symptomatic)
• Acme (peak symptoms)
• Decline (resolving)
• Convalescent (recovery)

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TerminologyInfections

• Infections result from a deleterious interaction
  between a host and a microbe
• Endogenous source agent is present on/in the host
  which is source of infection
• Exogenous source agent comes from external source
  may be nosocomial or community acquired
• May result from different modes of transmission
  including live and innate

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Chain of Infection Development of an infection
is often dependent on a chain of events

• The infectious agent 1st link
• Virulence factors may determine outcome of
  interaction with host (usually multiple factors)
• Also related to mode of transmission microbes
  have evolved factors to assist invasion/avoidance
  of immune system

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Chain of infectionThe agent 1st link

• Virulence factors include
• Infectious dose eg 3CFU for Rickettsia (scrub
  typhus) cw many thousands Salmonella
• Exotoxins secreted factors eg C.diptheriae
  bacteriocin
• Neurotoxins
• Haemolysins
• Endotoxins
• Antigenic variation

• Proteases
• Collogenases
• Enterotoxins
• Adhesins
• Chemotaxis
• Intracellular growth
• Capsules
• Avoidance

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Chain of InfectionTransmissionthe 2nd link

• 4 recognized routes of transmission
• Contact
• Common vehicle
• Airborne
• Vector-borne

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Chain of infection Transmission the 2nd link

• Contact
• Means contact with sources of infection
• Most common is personperson
• Direct eg HCW with infected finger touches wound
  on patient
• Indirect eg Droplet usually gt5µm and travel lt1M

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Chain of infection Transmission the 2nd link

• Common vehicle spread
• Refers to contamination by inanimate objects
• Eg Water borne HepA
• Eg HBV by sharing needles

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Chain of infection Transmission the 2nd link

• Airborne transmission
• Agent is truly airborne
• Particle sizelt5µm
• travel gt1m
• May in droplet nuclei, dust particles, skin
  layers
• Eg Legionella pneumophila

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Chain of infection Transmission the 2nd link

• Vector borne
• Living organism carries agent
• Insects most important vector
• Can be passive eg Flies
• Biological vectors agent may change in vector eg
  Malaria

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Chain of infection The host the 3rd link

• Host completes the chain
• Mode and site of entry influence infection rate
• Susceptibility issues

• Eg vaccinated?
• Immunocompromised?
• Compromised? Eg
• Primary viral infection?
• Broad range antibiotics?
• Surgery?
• Smoking?
• Underlying illness
• Other?

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Chain of infection The host the 3rd
linkPortals of entry
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Chain of infection The host the 3rd
linkPortals of exit
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Question Do vaccinations or level of immunity
in a community influence the rate of infections?

• The spread of infection is dependent on several
  factors including
• The chain of infection aspects which include the
  susceptible status of the host
• An observation at the community level is that the
  level of immunity within a community influences
  the spread of disease
• This phenomenon is termed herd immunity

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Herd immunity

• In case 1 where few people are immune to an
  infectious disease (eg 10)
• The incidence of acute infections in susceptible
  hosts is relatively high

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Herd immunity

• In case 2 where the level of immune persons is eg
  50
• The incidence of acute infections in susceptible
  hosts is sporadic and relatively low

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Herd immunity

• In case 3 where the level of immunity is is high
  eg 90
• The incidence of new cases in susceptible hosts
  is very low

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Herd immunity summary

• Herd immunity then is a concept which suggests
  that the population may maintain a resistance to
  an infectious disease because a large proportion
  of the population is immune to the disease
• Eg vaccinated against rubella?, the risk to non
  vaccinated persons is lower if the general
  population has a high rate of vaccinates