Transcript Jan 17 Spatial Diffusion of Disease

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
1
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
2
Source: Hagget, 1998
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
3
Combined Expansion and Relocation
Diffusion
• The diagram below how the two processes of
expansion and relocation may be combined.
Source: Hagget, 1998
4
Types of Spatial Diffusion
•Expansion diffusion, relocation diffusion, and combined expansion and
relocation processes: t1, t2, t3 denotes time 1, 2 and 3 respectively
5
Source: Hagget, 1998
An Example of a Combined Diffusion Wave
•The spread of the E1 Tor cholera epidemic, 1960-71
6
Source: Hagget, 1998
Types of Expansion Diffusion
• Expansion diffusion occurs in two ways.
– Contagious spread
– Hierarchical spread
• Cascade diffusion
7
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.
8
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.
9
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
10
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.
11
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.
12
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.
13
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.
14
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.
15
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.
16
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.
17
The Nature of Epidemics …
18
Epidemics of communicable disease
• Epidemics of communicable disease are of two main
types.
– Propagated epidemic
– Common-vehicle epidemic
19
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
20
accidental diversion).
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
21