Transcript Sources and spread of infection

Sources and Spread of
Infection
Sources and spread of infection
Some definitions:
Epidemiology is the study of the
determinants and distribution of disease.
Incidence is the number of new specified
cases in a specified period in a specified
population.
Prevalence is the number of cases of a
specified disease in a population at a
particular point in time.
Sources and spread of infection
Carriage
Carrier/case ratio depends on the immunity
and general health and status of the
population and the virulence of the
organism.
C/C ratio is low in typhoid or dysentery
where cases hugely outnumber carriers.
C/C ratio is high say, for Staphyloccus where
carriers greatly outnumber cases
Sources and spread of infection
Endemic Disease. A disease is endemic
when it is consistently present in the
community. This depends on:
• The population of susceptible individuals.
(depending on immunity – ‘herd immunity’)
• The relative ease of transmission
• The size and physical location of the
population
Sources and spread of infection
If the Herd Immunity falls to say below say
50-75% this facilitates greater spread and
results in a paroxysmal increase in the
incidence of the disease. This is an
epidemic.
(Cf enzootic and epizootic)
A pandemic disease is one affecting and
extended area eg. a continent.
Sources and spread of infection
Epidemiology of Infectious Disease
Factors involved:
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b.
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d.
e.
Microbiology/Pathology
Drugs
Immunology
Public and Social Health
Statistics
Sources and spread of infection
Virulence – The ability to invade and spread
Pathogenicity – the ability to cause disease
Infectivity – the ability to invade and
establish at a primary focus.
Not all organisms can do all three.
Sources and spread of infection
Pathogenicity is achieved by a variety of
methods but principally by the production
of toxins.
Exotoxins are formed by the organism and
released into the environment or at the site
of infection eg. Clostridium botulinum,
Corynebacterium diphtheriae or the
enterotoxin produced by Vibrio cholerae
Sources and spread of infection
Endotoxins formed by organisms (mostly
Gram-negative bacilli) within the organism
and released on degeneration of the
organism.
Generally membranes must be destroyed or
disrupted for infection to be established
and this is usually achieved by exotoxins
Sources and spread of infection
Pathogenesis
Depends on:
Toxigenicity
Invasiveness
Toxins
Capsules
Aggressins
Organotropism
Sources and spread of infection
Pathogenicity
=
Transmissibility +
Infectivity +
Virulence
Sources and spread of infection
Establishment of an infection depends on:
• Numbers of organisms (inoculum)
therefore virulence and infectivity.
• Pathogenicity.
• Host susceptibility.
The host parasite relationship.
Sources and spread of infection
Microbes
Susceptible Host
Other Hosts
Sources and spread of infection
Microbial factors:
• Pathogenicity.
• Ability to survive or multiply in a given
inanimate environment.
• Infecting dose
• Effective portal of entry
• Means of dispersal
• Carriers
Sources and spread of infection
In the community the factors that put the host at risk are:
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Poverty
Overcrowding
Hypothermia
Poor personal
Hygiene
• Immigration and
population
movements
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War and strife
Drug addiction
Alcoholism
Smoking
Malnutrition
Sources and spread of infection
In individuals more specific factors may
enhance susceptibility to infection:
Disorders such as :
• Diabetes
• Hormonal upset
• Haematological eg Leukaemia or drug
induced neutropenia
Sources and spread of infection
Poor general resistance due to
Immunological Deficiency :
• Neoplasia
• Pregnancy
• Primary infections
• Extremes of age
• Malnutrition
• HIV
• Iatrogenic immunosuppression
Sources and spread of infection
Poor local resistance due to:
• Underlying renal disease
• Foreign bodies
• Imperfect blood supply (dead tissue, blood
clot or diabetes)
• Operations
• Instrumentation and anaesthesia
Sources and spread of infection
A reservoir of infection is where organisms
can survive and multiply – normal habitat.
These include:
• Active human carriers
• Human carriers of causative organism
• Animal cases of disease or carriers
• The inanimate environment
A source may be a reservoir but a reservoir is not
always the source.
Sources and spread of infection
Exogenous modes of spread
• Direct Contact
• Ingestion
• Inoculation (sharps and bites)
• Inhalation
• Inanimate environment – fomites,
showers, water & air, soil, (pathology
specimens)
Sources and spread of infection
Direct contact
Hands clothing, sexual contact
Indirect contact
Fomites
Ingestion
Faecal-oral route
Sources and spread of infection
Inoculation
Arthropod borne blood infections – parasites multiply in,
and spread by the vector
Mosquito - eg. malaria and yellow fever
Flea - plague
Louse – typhus
Tick – Rocky Mountain spotted fever. Lyme disease.
Mite – scrub typhus
Tsetse fly - trypanosomiasis
Sources and spread of infection
Inhalation:
Explosive coughing and sneezing
colds and flu
Organisms in the air eg. C. diphtheriae
Streptococci
Staphylococci
M. tuberculosis
Smallpox virus
Legionella
Sources and spread of infection
From soil
Tetanus
Gas gangrene
Madura foot
Sources and spread of infection
Examples of Zoonoses
• BrucellosisTB
• Rabies
• Psittacosis
• Plague
• Salmonella
• Leptospirosis
(seldom man to animal)
Sources and spread of infection
Vertical spread of infection - Mother to child
eg. Viruses or Listeria.
Horizontal spread of infection eg. STD
Sources and spread of infection
Factors affecting spread:
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Geography
Season/climate
Presence of vector (arthropod etc)
Presence of suitable host