Introductory terminology Microbial pathogenesis Pathogenesis: the - - PDF document

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Introduction to Infectious Diseases

Introductory terminology

Microbial pathogenesis

• Pathogenesis: the mechanisms of the

development of a disease

• Pathogen: a infectious agent which causes an

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 opportunistic pathogens & commensals
• Commensal, symbiotic, parasitic

Terminology Microbial pathogenesis

• Virulence the ability for an organism to

cause disease

• Organism of high virulence tend to always

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

Illustration of virulence components eg Salmonella typhi

Terminology Infections

• Infections generally

divided into several periods:

• Incubation (asymptomatic)
• Prodromal (vague symp)
• Invasive (symptomatic)
• Acme (peak symptoms)
• Decline (resolving)
• Convalescent (recovery)

Terminology Infections

• Infections result from a deleterious

interaction between a host and a microbe

• Endogenous source agent is present on/in

g g p 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
• Virulence factors may determine outcome
• f interaction with host (usually multiple factors)
• Also related to mode of transmission

microbes have evolved factors to assist invasion/avoidance of immune system

Chain of infection

The agent: 1st link

• Virulence factors include:
• Infectious dose eg 3CFU for

Rickettsia (scrub typhus) cw many thousands Salmonella

E t i t d

• Proteases
• Collogenases
• Enterotoxins
• Adhesins
• Exotoxins secreted

factors eg C.diptheriae bacteriocin

• Neurotoxins
• Haemolysins
• Endotoxins
• Antigenic variation
• Adhesins
• Chemotaxis
• Intracellular growth
• Capsules
• Avoidance

Chain of Infection

Transmission:the 2nd link

• 4 recognized routes of

transmission

C t t

• Contact
• Common vehicle
• Airborne
• Vector-borne

Chain of infection

Transmission: the 2nd link

• Contact:
• Means contact with

sources of infection

• Most common is

person:person

• Direct eg HCW with infected

finger touches wound on patient

• Indirect eg Droplet usually

>5µm and travel <1M

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

Chain of infection

Transmission: the 2nd link

• Airborne transmission
• Agent is truly airborne
• Particle size<5µm
• Particle size<5µm
• travel >1m
• 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

Chain of infection

The host: the 3rd link

• Host completes the chain
• Mode and site of entry

influence infection rate

• Eg vaccinated?
• Immunocompromised?
• Compromised? Eg:
• Susceptibility issues

p g

• Primary viral infection?
• Broad range antibiotics?
• Surgery?
• Smoking?
• Underlying illness
• Other?

Chain of infection

The host: the 3rd link

Portals of entry

Chain of infection

The host: the 3rd link

Portals of exit

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 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’

Herd immunity

• In case 1 where few

people are immune to an infectious disease ( 10%) (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

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

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