6.1 Spatial diffusion
Spatial diffusion is the spread of people, goods or ideas across a given area. The concept is important in health applications of GIS, since it provides a theoretical background for spatial models of disease spread. In looking at patterns of disease spread, it is also important to understand the types of infectious disease that exist and their modes of transmission. Modes of infectious disease transmission help explain the spatial patterns of disease spread that emerge over time.
Several different types of diffusion are recognised. Hierarchical diffusion occurs where the spread of a disease follows population size. The cities with the largest populations are infected first and then the disease progressively spreads to settlements with smaller and smaller populations. Contagion diffusion involves the localised spread of disease from an initial infected individual. The spread of infection is closely related to distance, with cases at the start of the outbreak being highly localised around the initial case. Network diffusion is also related to distance, but transport infra-structure (roads, air travel, etc.) are incorporated into the distance calculations. In practice, any given disease will spread through a mixture of these 3 mechanisms. The relative importance of the 3 types of diffusion may vary as a disease outbreak progresses and often an initial period of hierarchical or network diffusion may be followed by contagion diffusion. All 3 types of diffusion can be described mathematically by means of a social gravity model.
Activity
Work through the orange box below, which describe some of the main types of diffusion and social gravity models.
What about network diffusion? Skim read the article by Juliao (1999) in the references below to familiarise yourself with cost surface modelling. How do you think GIS cost surface modelling tools described be used to look at network diffusion with a social gravity model?
Answer
The social gravity model is based on distance and population size in different settlements. Instead of using STRAIGHT-LINE distances, the effect of rail and road transport networks could be investigated using COST distances. Cost distances take into account the presence of transport networks and could be used in a social gravity model.
Hide
Ebola haemorrhagic fever and SARS: Next, read the references below, which describe outbreaks of two infectious diseases, Severe Acute Respiratory Syndrome (SARS) and Ebola haemorrhagic fever. When you have read the references, download the attached spreadsheet and complete the table of characteristics for SARS and Ebola haemorrhagic fever. For your own personal study, make some notes on which type of diffusion was most important for each disease and why you think this is the case.
References (Essential reading for this learning object indicated by *)
You can find a description of cost surface modelling in GIS within this article:
Delamater P, Messina J, Shortridge A, and Grady S (2012): Measuring geographic access to healthcare: raster and network-based methods. International Journal of Health Geographics 11: 15. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3511293/
Ebola haemorrhagic fever:
The latest Ebola outbreak has provoked lots of spatial analysis, such as this article, which looks at air traffic patterns out of West Africa:
* Gomes, M., et. al. (2014) Assessing the international spreading risk associated with the 2014 West African Ebola outbreak. PLOS Currents Sept 2, 6. ecurrents.outbreaks.cd818f63d40e24aef769dda7df9e0da. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4169359/
Also of interest is the potential use of data from mobile phones in tracking human mobility in and out of the outbreak-affected areas:
* Wesolowski, A., et. al. (2014) Commentary: Containing the Ebola Outbreak – the potential and challenge of mobile network data. PLOS Currents Sept 29, 6. ecurrents.outbreaks.0177e7fcf52217b8b634376e2f3efc5e. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4205120/
More historic outbreaks and discussion of the geographical spread of Ebola haemorrhagic fever can be found in the journal Emerging Infectious Diseases:
Peterson, A. T., Bauer, J. T., and Mills, J. N. (2004) Ecologic and geographic distribution of filovirus disease. Emerging Infectious Diseases 10, 40-47 http://www.ncbi.nlm.nih.gov/pubmed/15078595
Francesconi, P., Yoti, Z., Declich, S., Onek, P. A., Fabiani, M., Olango, J., et. al. (2003) Ebola hemorrhagic fever transmission and risk factors of contacts, Uganda. Emerging Infectious Diseases 9 (11), 339-343 http://wwwnc.cdc.gov/eid/article/9/11/pdfs/03-0339.pdf
This latter article has links to descriptions of specific Ebola haemorrhagic fever outbreaks.
SARS
The same journal has useful descriptions of SARS outbreaks, including the following:
* Hsueh, P. R., Chen, P. J., Hsiao, C. H., Yeh, S. H., Cheng, W. C., Wang, J. L., et. al. (2004) Patient data, early SARS epidemic, Taiwan. Emerging Infectious Diseases. Available from: http://wwwnc.cdc.gov/eid/article/10/3/03-0571_article
Liang, W., Zhu, Z., Guo, J., Liu, Z., He, X., Zhou, W., et. al. (2004) Severe acute respiratory syndrome, Beijing, 2003. Emerging Infectious Diseases. http://www.ncbi.nlm.nih.gov/pubmed/15078593
Affonso, D. D., Andrews, G. J., and Jeffs, L. (2004) The urban geography of SARS: paradoxes and dilemmas in Toronto’s healthcare. Journal of Advanced Nursing 45 (6), 568-578. http://www.ncbi.nlm.nih.gov/pubmed/15012634