Disease 11.15 Flashcards
What are the different types of disease diffusion?
Expansion diffusion
Relocation diffusion
Contagious diffusion
Hierarchical diffusion
What is expansion diffusion?
A disease has a source and spreads outwards into new areas.
Meanwhile carriers in the source area remain infected.
An outbreak of TB is an example of expansion diffusion
What is relocation diffusion?
Relocation diffusion occurs when a disease leaves the area or origin and moves into new areas. The cholera epidemic in Haiti in 2010 which killed 7000 people illustrates this as it originated in Nepal
What is contagious diffusion?
Contagious diffusion describes the spread of disease through direct contact with a carrier. It is strongly influenced by distance. The Ebola epidemic in West Africa in 2014 is an example of contagious diffusion
What is hierarchical diffusion?
Hierarchical diffusion is when a disease spreads through an ordered sequence of places, usually from the largest centres with the highest connectivity to smaller, more isolated centres.
Diffusion is also channelled along road, rail and air transport networks which facilitate contact between carriers.
In 2009 the H1N1 virus became a pandemic via international flight routes
What types of barriers to diffusion are there?
physical and socio-economic and political
How do physical barriers affect disease diffusion?
In general, the probability of a contagious disease spreading to an area is inversely proportional to the distance from its source. Other physical barriers which slow or halt diffusion include mountain ranges, seas, oceans, deserts and climate.
Climate is a major factor in the epidemiology and distribution of diseases such as malaria and sleeping sickness
Give three examples of socio-economic barriers to disease diffusion
global mobility
technology
education
How does global mobility limit disease diffusion?
Global mobility - Improved transport and communications linked to globalisation e.g. bigger aeroplanes; faster and more widespread rail and road networks has improved the ability of international Aid agencies and NGOs to respond quickly and efficiently to disease crisis areas. E.g. Cholera Haiti 2010 or Ebola 2014. e.g. Covid- 19 2020
How is technology a barrier to disease diffusion?
Technology - Linked to level of economic development and ability to invest resources into disease detection; monitoring; treatment and mitigation. In ACs, the resources are available to invest in screening; ultra-sound scans; keyhole surgery etc. UK – blood screening for HIV; screening and ultra-sounds to detect cancers; key hole surgery to remove tumours or do heart surgery to reduce CHD. Vehicles to transport vaccines and medicines have become far more sophisticated and hi-tech e.g. refrigerated containers/lorries to transport medicines and vaccines
How is education a barrier to disease diffusion?
Education - the WHO and Red Cross seek to educate people about how diseases are caught; transmitted and how they can be mitigated against. Particularly successful at grass roots level. Guinea Worm in Ghana. At national level, also successful. ABC programme in Uganda to educate public about HIV/AIDS and cancer awareness in UK and obesity in UK, Brazil and China.
How do political barriers affect disease diffusion?
Political borders check the international movement of carriers of infectious disease. The spread can also be controlled by imposing curfews to limit contact between people.
This happened on several occasions in Sierra Leones in 2015 in an effort to contain the spread of Ebola virus.
Quarantining of wester workers infected with Ebola on their return from West Africa also minimised the risks of the disease spreading to the UK and other countires. Other precautions often taken to check the spread of viruses include wearing facemasks in public or mass vaccination programmes
What does the hagerstrand model show?
It explores innovation diffusion and has been applied to the contagious diffusion of diseases
What are the important features of the Hagerstrand model?
The neighbourhood effect
The number of people infected by an epidemic approximates an S-shaped or logistic curve over time. After a slow beginning, the number infected accelerates rapidly until eventually levelling out, as most of the susceptible population have been infected
The progress and diffusion of a disease may be interrupted by physical barriers
What is the neighbourhood effect?
The probability of contact between a carrier and non-carrier is determined by the number of people living in each 5x5 grid square, and theri distance apart.
How does temperature and precipitation influence disease?
Are important drivers of vector-borne diseases and epidemics.
E.g. Malaria who’s epidemiology depends on warm, humid conditions endemic to the tropics and sub-tropics
Diseases influenced by climate often show seasonal variation as temp determines rates of vector development and reproduction
Precipitation in the tropics is often seasonal and creates aquatic habitats such as ponds which allow insects and disease vectors to complete their life cycles
How does relief affect global patterns of disease?
Altitude causes abrupt changes in climate and disease habitats. In Ethiopia malaria is concentrated in the humid lowlands but is largely absent in the cooler highlands
How does stagnant water affect patterns of disease?
Many diseases are water-borne, in the developing world millions of people rely on water from wells and surface supplies contaminated by sewage.
Bacteria responsible for cholera and other infectious diseases thrive in these conditions.
Unprotected and stagnant drinking water supplies also provide habitats for disease vectors - for example, copepod vectors which transmit the Guinea Worm
What is Dengue fever?
Dengue fever is widespread in the tropics. Annually it infects around 400 million people and is responsible for 25,000 deaths.
How does climate control dengue fever epidemiology?
Mosquitoes thrive in warm humid condition which in turn favours the outbreak of dengue. In the South Pacific, sustained temperatures of more than 32C and humidity levels above 95% trigger waves of dengue epidemics. These condition occur in the summer months, but short-term weather changes and exceptional rainfall events can also lead to the outbreaks
How does seasonal variations in northern hemisphere affect disease outbreaks?
In temperate regions in the northern hemisphere, epidemics of influenza peak in the winter months. Although the precise reason for the seasonality is not fully understood, it is know that the transmission of the flu virus is most efficient at lower temperatures e.g. 5c and when atmospheric humidity is low.
How does seasonal variations in the tropics and subtropics affect disease outbreaks?
Vector-borne diseases transmitted by mosquitoes, flies, ticks, fleas and worms often reach a peak during the rainy season.
For example diarrhoeal disease in South Asia surges in the pre-monsoon (March-April) and the end of monsoon (September-October) periods when fly populations are highest.
What factors related to climate change have provided conditions for emerging infectious diseases?
Warmer and wetter conditions in the tropics and sub tropics
Lyme’s disease, for warmer conditions
Sleeping sickness when temps are 20.7C-26.1C
How will climate change affect sleeping sickness?
sleeping sickness is endemic in sub-Saharan countries. It is transmitted by the tsetse fly. Outbreaks of the disease occur when average temperatures are in the range of 20-26C.
In the future climate change is likely to affect the vector’s growth rate and the geographical distribution as it is likely to spread to southern Africa and according to the WHO will affect up to 77million more people by 2090.
However it may disappear from East Africa as it is too hot for the tsetse fly to survive
