Week 3: Spatial Diffusion of Disease

Diffuse

To disperse/diverge from a central point; to spread widely, disseminate

In Geographic al contexts, the term diffusion, has distinct applications:

1. Expansion diffusion

2. Relocation diffusion

3. Combined diffusion: (Expansion + Relocation)

These 3 known methods are the ways disease spreads through populations and regions.

Expansion diffusion

The process whereby a phenomenon of interest (e.g information, a material artifact, 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
  

Relocation Diffusion

Is a spatial spread process, but the items being diffused leave the areas where they originated as they move to new areas

• Cannot be in the same space at the same time

• eg. a thief has to move from house to house and cannot be in all houses at once

Combined Diffusion: Expansion + Relocation

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

• the vector/ means in which the disease travels is important (intensity of virus)

• level of immunity is also important

space vs place

space: A blank space, lacking meaning

place: A place with assigned meaning and culture

All 3 types of spatial diffusion

Expansion diffusion, relocation diffusion, and their combined processes:

• t1, t2, t3 denotes time 1, 2 and 3 respectively

Examples of combined diffusion

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

• began in Sulawesi (Indonesia) in 1961 and spread throughout
  → Asia
  → Middle East
  → Africa,
  → → Europe

Expansion diffusion occurs in 2 ways:

• Contagious spread

  • Transmission of disease through organisms

• Hierarchical spread

  • Constructed patterns in the spread

    • eg. from larger to smaller network

    • or richer to poorer

  • Cascade diffusion

contagious spread

• depends on direct contact.

• 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.

Hierarchal spread

• transmits through an ordered sequence of classes or places. example from large metropolitan centers 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 (subtype of hierarchical spread)

Refers to processes that always defaults to a downward spread,
from larger to smaller centers.

HIV/AIDS example

• example of expansion

• when considering social factors, obviously some aspect of hierarchy is involved

• consider cities, airports, highly populated areas

Inter-spatial vs Intra-spatial

Inter-spatial: In-between spaces

Intra-spatial: inside spaces

Torsten Hägerstand (1953) identified four (4) distinct stages in the passage of an innovation through an area

1. Primary stage

2. Diffusion stage

3. Condensing stage

4. Saturation stage

Technical Adoption

example:

• Introduction: Silicon Valley (San Francisco)

• Decline: when a phone stops working and the parts are not being made anymore

Primary Stage

Birthplace: marks the beginning of the diffusion process.

• Patient zero

• 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.

Diffusion stage

The diffusion stage signals the start of the actual spread process

• Patient 2, 3, 4

• 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.

• expansion spread, specifically contagious spread

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.

• example of paradigm shift

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.

The shape of a changing diffusion profile

The shape of the changing diffusion profile in time and space has been formally modeled.

• 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.
  

S-shaped curve

Red line: levels of diffusion

Blue line: levels of interest

technology example

• Condensation stage

• Everyone has a phone, lots of kinds of phones exist

Epidemic: Oxford Dictionary definition

‘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’

Epidemic: Beneson definition

“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.”

Beneson analysis

• Occurrence and not quantity

• 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

Disease vs epidemic

• every epidemic is a disease, but not every disease is an epidemic

• an epidemic causes unexpected chaos

Epidemics of communicable diseases are of two main types:

1. Propagated epidemic

2. Common-vehicle epidemic

Propagated Epidemic

• Results from the chain transmission of some infectious agent.

• 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).

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