Disease 11.15

Question 1

What are the different types of disease diffusion?

Answer 1

Expansion diffusion
Relocation diffusion
Contagious diffusion
Hierarchical diffusion

Question 2

What is expansion diffusion?

Answer 2

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

Question 3

What is relocation diffusion?

Answer 3

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

Question 4

What is contagious diffusion?

Answer 4

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

Question 5

What is hierarchical diffusion?

Answer 5

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

Question 6

What types of barriers to diffusion are there?

Answer 6

physical and socio-economic and political

Question 7

How do physical barriers affect disease diffusion?

Answer 7

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

Question 8

Give three examples of socio-economic barriers to disease diffusion

Answer 8

global mobility
technology
education

Question 9

How does global mobility limit disease diffusion?

Answer 9

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

Question 10

How is technology a barrier to disease diffusion?

Answer 10

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

Question 11

How is education a barrier to disease diffusion?

Answer 11

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.

Question 12

How do political barriers affect disease diffusion?

Answer 12

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

Question 13

What does the hagerstrand model show?

Answer 13

It explores innovation diffusion and has been applied to the contagious diffusion of diseases

Question 14

What are the important features of the Hagerstrand model?

Answer 14

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

Question 15

What is the neighbourhood effect?

Answer 15

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.

Question 16

How does temperature and precipitation influence disease?

Answer 16

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

Question 17

How does relief affect global patterns of disease?

Answer 17

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

Question 18

How does stagnant water affect patterns of disease?

Answer 18

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

Question 19

What is Dengue fever?

Answer 19

Dengue fever is widespread in the tropics. Annually it infects around 400 million people and is responsible for 25,000 deaths.

Question 20

How does climate control dengue fever epidemiology?

Answer 20

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

Question 21

How does seasonal variations in northern hemisphere affect disease outbreaks?

Answer 21

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.

Question 22

How does seasonal variations in the tropics and subtropics affect disease outbreaks?

Answer 22

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.

Question 23

What factors related to climate change have provided conditions for emerging infectious diseases?

Answer 23

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

Question 24

How will climate change affect sleeping sickness?

Answer 24

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

Question 25

How can climate change affect diseases spread by animals?

Answer 25

Climate change has forced some animal species into new habitats as their natural habitats disappear, and it has expanded the habitats of other animals. This movement of animals into new areas increases opportunities for contact between humans and animals and the potential spread of zoonotic diseases, as these examples show:

Wildlife carrying the rabies virus are expanding to new geographic areas of the country.
Arctic temperatures are rising more than twice as rapidly as the rest of the world. Warming temperatures in Alaska have led to increases in vole populations, which can spread diseases like Alaskapox to humans.

Question 26

How can climate change affect disease spread by fungi?

Answer 26

Rising temperatures have allowed certain disease-causing fungi to spread into new areas that previously were too cold for them to survive. For example, Valley fever – caused by a fungus that lives in the soil in hot and dry areas – has already spread into the Pacific Northwest

Question 27

How can climate change causing extreme weather events increase emerging infectious diseases?

Answer 27

Can lead to circumstances that facilitate the spread of disease among people, such as through crowded conditions in emergency shelters and the disruption of healthcare services, including delays in routine childhood immunizations.

Question 28

What factors can increase the probability of zoonotic disease being transmitted to humans?

Answer 28

Urbanisations creates suitable habitats for animals such as foxes
Hygiene and sanitation are poor; drinking water is contaminated by animal faeces, blood or saliva.
Manmade habitats e.g. surface pools encourage insect vectors to breed

Question 29

What factors can decrease the probability of zoonotic disease being transmitted to humans?

Answer 29

The movement of infected wild animals is restricted e.g. by mountain ranges
Controls on the movement of diseased domestic animals within countries
There is vaccination of pets and domestic livestock