Spatial Diffusion of Disease

1
Spatial Diffusion of Disease

• Diffuse is defined as to disperse or be
  dispersed from a centre to spread widely,
  disseminate. (Oxford English Dictionary)
• In the geography, the term diffusion, has
  distinct usages
• Expansion diffusion
• Relocation diffusion
• Combined expansion and relocation diffusion

2
Expansion diffusion

• Expansion diffusion the process whereby a
  phenomenon of interest (this may be information,
  a material artefact, a disease), spreads from one
  place to another.
• In this expansion process, the item being
  diffused remains, and often intensifies, in the
  originating region, but new areas are also
  occupied by the item in subsequent time periods.
• t1, t2, t3 denotes time 1, 2 and 3 respectively

Source Hagget, 1998
3
Relocation Diffusion

• Relocation diffusion is a spatial spread
  process, but the items being diffused leave the
  areas where they originated as they move to new
  areas.

Source Hagget, 1998
4
Combined Expansion and Relocation Diffusion

• The diagram below how the two processes of
  expansion and relocation may be combined.

Source Hagget, 1998
5
Types of Spatial Diffusion

• Expansion diffusion, relocation diffusion, and
  combined expansion and
• relocation processes t1, t2, t3 denotes time 1,
  2 and 3 respectively

Source Hagget, 1998
6
An Example of a Combined Diffusion Wave

• The spread of the E1 Tor cholera epidemic, 1960-71

Source Hagget, 1998
7
Types of Expansion Diffusion

• Expansion diffusion occurs in two ways.
• Contagious spread
• Hierarchical spread
• Cascade diffusion

8
Contagious spread

• Contagious spread depends on direct contact.
• This process is strongly influenced by distance
  because nearby individuals or regions have a much
  higher probability of contact than remote
  individuals or regions.
• Therefore, contagious spread tends to occur in a
  centrifugal manner from the source region
  outward.

9
Hierarchical spread

• Hierarchical spread involves transmission through
  an ordered sequence of classes or places, for
  example from large metropolitan centres to remote
  villages.
• For example, within socially structured
  populations, innovations may be adopted first on
  the upper level of the social hierarchy and then
  trickle down to the lower levels.
• Cascade diffusion is a term reserved for
  processes that are always assumed to be downwards
  from larger to smaller centres.

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Stages of Diffusion Waves

• Torsten Hägerstand (1953) identified four
  distinct stages in the passage of an innovation
  through an area
• Primary stage
• Diffusion stage
• Condensing stage
• Saturation stage

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Primary stage

• The primary stage marks the beginning of the
  diffusion process.
• A centre of adoption is established at the
  origin.
• There is a strong contrast in the level of
  adoption between this centre and remote areas
  which is reflected in the steep decline of the
  level of adoption curve beyond the origin.

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Diffusion stage

• The diffusion stage signals the start of the
  actual spread process
• There is a powerful centrifugal effect, resulting
  in the rapid growth of acceptance in areas
  distant from the origin and by a reduction in the
  strong regional contrasts typical of the primary
  stage.
• This results in a flattening of the slope of the
  proportion of adopters curve.

13
Condensing stage

• In the condensing stage, the relative increase in
  the numbers accepting an item is equal in all
  locations, regardless of their distance from the
  original innovation centre
• The acceptance curve moves in a parallel fashion.
  

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Saturation stage

• The final saturation stage is marked by a slowing
  and eventual cessation of the diffusion process,
  which produces a further flattening of the
  acceptance curve.
• In this stage, the item being diffused has been
  adopted throughout the country, so that there is
  very little regional variation.

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The Shape of a Changing Diffusion Profile

• The shape of the changing diffusion profile in
  time and space has been formally modelled.
• The temporal build-up in the number of adopters
  of an innovation follows an S-shaped curve when
  plotted against time, with a logistic curve as
  the mathematical form most commonly adopted.

16
The Nature of Epidemics

• The Oxford English Dictionary defines an epidemic
  as a disease prevalent among a people or
  community at a special time, and produced by some
  special causes generally not present in the
  affected locality.
• In the standard handbook of human communicable
  diseases, Benenson defines an epidemic more fully
  as
• The occurrence in a community or region of cases
  of an illness (or an outbreak) clearly in excess
  of expectancy. The number of cases indicating
  presence of an epidemic will vary according to
  the infectious agent, size and type of population
  exposed, previous experience or lack of exposure
  to the diseases, and time and place of
  occurrence epidemicity is thus relative to usual
  frequency of disease in the same area, among the
  specified population, at the same season of the
  year.

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The Nature of Epidemics

• Benenson's account goes on to stress that what
  constitutes an epidemic does not necessarily
  depend on large numbers of cases or deaths.
• A single case of a communicable disease long
  absent from a population, or the first invasion
  by a disease not previously recognized in that
  area, requires immediate reporting and
  epidemiological investigation.
• Two cases of such a disease associated in time
  and place are taken to be sufficient evidence of
  transmission for an epidemic to be declared.

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The Nature of Epidemics
19
Epidemics of communicable disease

• Epidemics of communicable disease are of two main
  types.
• Propagated epidemic
• Common-vehicle epidemic

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Propagated epidemic

• A propagated epidemic is one that results from
  the chain transmission of some infectious agent.
• This may be directly from person-to-person as in
  a measles outbreak, or indirectly via some
  intermediate vector (malaria) or a microparasite.
  
• In some cases, indirect transmission may occur
  via humans (e.g., a mosquito-man-mosquito chain
  with malaria).
• In others, the survival of the parasite is
  independent of man (thus, Pasteurella pestis, the
  cause of bubonic plague is continually propagated
  through rodents and the infection of man by an
  infected flea is in this respect an accidental
  diversion).

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Common-vehicle epidemic

• Common-vehicle epidemic results from the
  dissemination of a causative agent.
• In this case, the epidemic may result from a
  group of people being infected from a common
  medium (typically, water, milk, or food) which
  has been contaminated by a diseasecausing
  organism.
• E.g., cholera and typhoid