Disease Diffusion
Diffusion of AIDSSource: Hagget 1998
Disease diffusion refers to the spread of disease from its source into new areas. It is well documented that the incidence of disease is likely to be affected by distance so that places closer to the source of a disease are most likely to see higher incidence. This can be seen in the graph to the left which shows the incidence of AIDs rates falling the further away from its source in San Francisco. It is superimposed over the typical pattern of contagious spread of disease. However, in a globalised world, increasingly connected by an integrated transport communication system there is also strong evidence to suggest that disease can spread quickly via roads at the national scale and airports at regional and global scale.
Disease diffusion has been classified into a number of different types.
Expansion DiffusionSource: Hagget 1998 |
Relocation DiffusionSource: Hagget 1998 |
Expansion and Relocation Diffusion
Mixed DiffusionSource: Hagget 1998
Two common types of disease diffusion have been classified as expansion and relocation diffusion. Expansion occurs when a disease spreads from one place to another. In this expansion process, the disease often intensifies, in the originating region. As the disease expands into new areas it is likely to weaken. This type of diffusion was recognised in the recent H1N1 flu virus that had its source in Mexico.
Relocation diffusion is a spatial spread process, whereby the disease leaves the areas in which it originated as it moves into new areas. An example of relocation of disease can be seen in the migration of disease carriers, whether it be a migrant with HIV or measles. The spread of cholera in Haiti in 2010, which killed 6000 people was thought to be brought into the country by aid workers from Nepal in the emergency response to the earthquake. It is quite common to see a mixture of two or more types of disease diffusion. The diagram to the left shows a mix between expansion and relocation.
Network Diffusion
Network diffusion occurs when a disease spreads via transportation and social networks. A good example of a disease to explain this path of diffusion is HIV. We can see how HIV is spread along important transport routes such as those countries with a developed road network in sourthern Africa and also within social (sexual) networks. A second example would be the recent H1N1 flu virus that quickly went global via the aviation network of flights and major international airports.
Contagious Diffusion
Contagious diffusion is illustrated below in model form and in the scanned map of the original map identifying the cluster and source or the cholera outbreak of Broad Street in 1852. The actual contaminated water pump has been marked with the red dot. Contagious spread depends on direct contact. The process is strongly influenced by distance because nearby individuals or regions have a much higher probability of contact than remote individuals or regions.
Source: Hagget 1998 |
Source: Gilbert 1958 |
Hirerarchial Diffusion
Hierarchical spread involves the spread of disease through an ordered sequence of classes or places, for example from large cities to remote villages. Cascade diffusion is a term used to describe a process assumed to be downwards from larger to smaller centres. The diagram to the left shows a number of different cascades. For example, the spread of disease from one large city to smaller towns and then into peripheral villages. This hierarchial spread can also be seen with HIV in regard to social status and sexual habits. HIV spreads is contained at first among a few carriers with high status and high concurrency of sexual relations. The disease is spread first to people with medium status and medium level of sexual concurrency. Finally the disease may then spread from the second tier of carriers to people with lower status and lower sexual concurrency. In the the use of the word status here I am referring to the personality traits and sexual appeal of individuals.
Barriers to the Spread of Disease
Barriers of disease diffusion can be classified in terms of natural physical barriers and human measures. The most important natural barrier is that of distance decay. The further a place is away from the source of incidence the lower the incidence of disease. Other natural barriers relate to remoteness. Remote regions such as rural peripheries, mountainous regions and regions of extreme climate experience relatively small amounts of in and out-migration. As a result the spread of disease into these regions is less likely. Mountains and oceans also act as major natural barriers to the spread of disease as they contain people and restrict migration. Human measures relate to socio-political structures such as political borders and migration control, which restrict or prevent the movement of people. US migration policy specifically prevents the immigration of foreigners who carry infectious diseases. At times of high risk borders can be completely closed, however for the economic impacts of such a measure it would need to be an extreme case. Other human controls relate to the management of disease and directly to the way in which a disease is transmitted. In the case of the H1N1 flu virus many measures were adapted in the UK. Initially, people who contracted the flu were isolated in their homes. This later became a bit of a farce as these same people were giving media interviews from their house windows. Isolation is of course an important management measure for many diseases though, and is essential for highly infectious diseases such as cholera. Other measures in the UK involved creating a heightened awareness of improved hygiene. People were advised to refrain from typical greeting customs such kisses and hand shakes and to wash their hands carefully. In public places like airports and railway stations people wore face masks. In Catholic ceremonies people refrained from drinking directly from the challice during the celebration of the Eucharist. Finally, authorities considered cancelling larger public events such as sports events and pop concerts. These events at the time all saw reduced attendance.
One of the ways I get the students to examine the application of the concept of barriers to prevent the spread of disease is to set up a class simulation of a disease outbreak that they need to manage. This could be set at a global scale or confined within a refugee camp. For example, students could investigate the management of the cholera outbreak in Haiti, following the earthquake of 2010.
The following PDF produced by International Medical Corps shows the pattern of incidence of cholera in Haiti as well as its geographical extent. The two videos provide an insight from Oxfam and UNFPA on how to respond and manage an outbreak of cholera to prevent the spread of the disease.
The Geographical Spread of Disease and its Impacts - Case Study: HIV/AIDS in Sourthern Africa
Approximately 34 million peopleare infected with HIV/AIDs in the world today.The gapminder chart of HIV in 2009 shows a very powerful visualisation of the geographical extent of HIV/AIDS. Firstly, we can see that the vast majority of people suffering with HIV/AIDs live in Sub-Saharan Africa. Secondly,there is a great deal of variation within Sub-Saharan Africa, with many countries in this region experiencing very low numbers of HIV/AIDs and low percentage data.The concentration of high percentage and high total numbers of HIV/AIDs is geographically confined to the region of Southern Africa.Finally, both total number of people and the percentage of the population for the majority of countries outside Sub-Saharan Africa is very low with the exception of low percentage but high numbers in India, USA, Brazil, Russia and Thailand. The following video from gapminder is simply outstanding. It explains a brief history of HIV/AIDs, its geographical spread and the causes of its high incidence in the region of Sourthen Africa.
The Spread of HIV.
The following map shows the geographical extent of HIV/AIDs as a percentage of the population for each country. As you can see for the vast majority of countries the percentage is below 1 percent of populaton. Russia and countries within Sub-Saharan Africa have higher rates, generally between 1 and 5 percent. However East African countries can be seen to have rates between 5 and 15 percent, with South Africa, Swaziland, Losotho and Botswana with rates between 15 and 28 percent of population.
At the same time that HIV/AIDS was being discovered in the US it was emerging as a real threat in several southern Africa countries, including Zimbabwe, Uganda and the Gambia.
HIV/AIDs first became noticed in the USA in 1981. Doctors identified an outbreak of a rare form of cancer among gay men in New York and California. This "gay cancer" as it was called at the time was later identified as Kaposi's Sarcoma, a disease that later became the face of HIV/AIDS. Little is known about the disease prior to this time but there is evidence suggest that is was a silent killer in Africa for up to a hundred years before.
As Hans Rosling has already pointed out in the video earlier we can see that HIV/AIDs has reached a steady state in all regions of the world and although the percentage of rates are much higher in Sub-Saharan Africa they also appear to be stabilising and falling in some countries. This could show one or two things. Firstly, that care and treatment of HIV/AIDs is improving, although as there is no cure for HIV, any falls in numbers means people have died. Secondly, it shows that there has been a fall in the number of people contracting the disease. The second graph shows the tracks of the same countries but rather than incidence of HIV as a precentage it shows annual HIV deaths. A number of interesting patterns can be seen. Firstly, among the tracked countries Zimbabwe, Uganda and Tanzania all experience high annual death rates that peak around 2004. This coincides with the fall in HIV rates in the earlier chart. There are two exemptions highlighted in Burundi and Liberia both of which experience rather stable and low death rates; both of which have relatively small population size anyway of 6.2 million and 3.3 million respectively. The other general pattern to comment on is that in the vast majority of countries HIV kills only a small number of people. Even in India where it appears to have very large numbers it only ralates to a tiny percentage of its vast population size now 1.1 billion people.
The Causes for the spread of HIV/AIDs
Firstly, it's important to recognise that in the 1980's, HIV/AIDs went global to the extent that nearly every country of the world has some incidence of the disease. However, it must also be stated that for the vast majority of countries HIV/AIDs is at very low level of incidence and although countries need to be on the constant guard in regard to educational programmes these numbers are quite stable. Outside the region of Sub-Saharan Africa, HIV spread among the hom*osexual community and through drug addicts using hypodermic needles and syringes. It later then spread to haemophiliac sufferers through blood transfusions. Through a combination of nationwide educational programmes to inform people about the transmission of HIV and the development of antiretroviral treatment both incidence of HIV and consequent deaths were quickly stabilised.
The impacts of HIV as already discussed have been hardest felt in Sub-Saharan Africa and in particular the southern region of Africa, namely South Africa, with its landlocked countries of Lesotho and Swaziland, Tanzania, Zimbabwe and Uganda and several others. There are many reasons for this and text books often cite poverty and conflict as key reasons. Conflict in Africa has been linked to HIV through the anecdotal evidence of prevalence of rape, used as a weapon of war. However, as Hans Rosling shows in his data visualisation conflict and HIV/AIDs don't correlate well. As you can see in the interactive map below, major conflict zones in Africa do not corrospond to the region of Southern Africa and East Africa that have the highest incidence of HIV/AIDs.
HIV Hot Spots
Source: Council of Foreign Relations
Poverty, at every scale is an important factor in determining the extent of the impacts of HIV/AIDs but it most prevalent at the macro or national scale. Poverty at the national scale determines the extent to which a country is educated. Education determines literacy and literacy has a mjor impact on the ease in which a country can effectively roll out educational programmes to prevent the spread of HIV. Furthermore, traditional and cultural belief systems and taboos are rife in illiterate populations and these taboos often contradict educational advice and have much stronger controls on individual decision making. Furthermore poverty at national scale has restricted countries from accessing the vital antiretroviral treatment (ATVs) that is so important to increasing survival rates. A combination of national poverty, high prices and government incompetence has prevented comprehensive use of ATVs. Today fewer than half of all Africans who need ATVs are actually receiving them. Poverty also impacts countries on a broader social level. Poor nutrition, sanitation, water shortage as well as the dominace of males over females all impact on HIV prevalence. Poverty makes people more vulnerable to disease. Afterall the chance of a healthy adult catching HIV through one-off heterosexual intercourse is one in one thousand, for hom*osexual intercourse it is increased to one in one hundred. The odds off contracting HIV for an unhealthy person are increased. Poverty is also responsible for forcing many people into unfortunate and compromising situations. Prostitution is a direct consequence of poverty and is responsible for the very high numbers of HIV/AIDs in both India and Thailand. Women across the world are the most marginalised and discriminated group in society and as a consequence are more at risk to HIV/AIDs.
The Pattern aof Poverty and HIV
Source: World Bank 2008
As we can see from the map of Africa showing GDP per capita the pattern of wealth in Africa doesn't quite corrospond with prevalence of HIV. Two of the weathiest countries, South Africa and Botswana have among the highest rates of HIV prevalence. East Africa has high HIV prevalence and is also among the income-poorest nations. However, DR Congo and Ethiopia are both income-poor countries and have relatively low HIV prevalence. The relationship between poverty and HIV prevalence is not as clear-cut as we might expect.
Within low-income countries there is also the unexpected reversal of the expected pattern between poverty and wealth. This can be seen in the chart below, which shows the incidence of HIV for different socio-economic groups in Tanzania. In this chart we can see that the wealthiest 20 percent have the highest incidence of HIV and the poorest 20 percent have the lowest HIV incidence. The second chart shows how HIV prevelance varies significantly within a country based on the geographical region. Most regions in Kenya show very modest levels of HIV with a clear concentration in one specific region, most likely its central region around Nairobi.
HIV in Adults based on Income groups in Tanzania | HIV in Adults by Region in Kenya |
So clearly, if HIV prevalence varies within regions and income groups it translates that the spread of HIV is more complex than simplifications of poverty, prostitution and conflict. Hans Rosling doesn't dismiss the possibility of the viral type in Southern Africa being different to others and suggests that in some way the virus may be stronger are more communicable. However there is also a geographical explanation that links to transport networks, urbanisation and migration. It seems logical and links with disease theory in regard to relocation, expansion, hirarchial and network diffusion of disease that HIV/AIDs is more prevalent in urban areas and along connecting transport routes than in remote rural areas where fewer people migrate. Migrants arriving in cities for short stays or for more permanency in the hope of finding work find themselves in a completely different environment to what they are familiar with. They are also often living away from loved ones and family support networks and are therefore more vulnerable. This vulnerability may lead migrants to seek comfort from prostitutes or seek female company. This especially translates to highly patriarchal societies where male dominance limits the empowerment of women. Without wanting to stereotype whole regions of Southern Africa or to be judgemental it is well known that some African traditions carry a strong patriachal structure, whereby concurrent sexual relations (when men have more than one sexual partner at the same time) and age dispirate sexual relations (older men have sexual relations with much younger girls) is very common. Concurrency is very likely to increase the liklihood of contact with HIV and age dispirate sex, places young girls in contact with men who have been sexually active for many years. Male dominance and traditional taboo further impacts women as contraception such as condoms are often frowned upon. Furthermore female migrants are often forced through poverty and need into prostitution.
The Management of Disease
There are many geographical factors that determine the relative emphasis placed by policy-makers in countries on either preventative or treatment of disease. Prevention generally refers to education of a population in regard to how disease is transferred and therefore develops both actions and behaviours that can be taken to limit the spread of disease. This may be related to lifestyle or the environmental management but can also be medicine, e.g. the flu jab. In most cases preventative management is crucial to reducing the incidence of disease and relatively low cost. Treatment of disease is reactive and is a response to a disease itself. Some treatments can cure disease entirely but others like antiretroviral treatment just limit the health impacts and therefore improve the quality of life and extend life. Treatment is by far the more expensive when compared to preventative management and so limite some countries' ability to access it.
Case Study: Preventative Management of Disease- Tanzania- Click on the PDF to open case study
Case Study: The Impacts of HIV/AIDs in Southern Africa - Click PDF to open case study
Case Study: The Management of HIV/AIDs in Tanzania - Click on PDF to open case study
