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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

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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

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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

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Combined Expansion and Relocation Diffusion

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

Source: Hagget, 1998

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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

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An Example of a Combined Diffusion Wave

Source: Hagget, 1998

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

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Types of Expansion Diffusion

• Expansion diffusion occurs in two ways.

– Contagious spread

– Hierarchical spread• Cascade diffusion

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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.

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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.

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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 flat tening 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.

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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 …

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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 disease causing organism.

• E.g., cholera and typhoid