Photo by Helen Hazen

Photo by Helen Hazen Clear-cutting, South Australia

An Introduction to the Geography of Health

Chapter 2: Environmental Changeand Human Health

An ecological approach to human health considers

humans as part of a broader cycle of disease that requires

an understanding of how environments influence

human health.

2 An Introduction to the Geography of HealthAnthamatten and Hazen Chapter 2

Ecology of Human Health


Photo by Peter Anthamatten

The natural environment refers to components of the

environment such as soil, air, vegetation, and water.



The built environment refers to human-constructed parts of the

landscape such as buildings, dams, and roads.



The social environment refers to aspects of human behavior and

organization that comprise the social structures within which we live.

Can you think of one way in which each aspect of the human environment

(natural, built, and social) could influence human health?


7 Anthamatten and Hazen Chapter 2

People are continually exposed to organisms and substances that can

cause disease.

Pathogens Geogens Physical hazards

Bacteria Coal dust Trauma

Rickettsiae Asbestos Extreme heat/cold

Fungi Lead Radiation


Agents of Disease


Can you think of other examples of communicable

and non-communicable diseases?

What is the difference between an infectious and a

contagious disease?

Transmissibility

An Introduction to the Geography of HealthAnthamatten and Hazen Chapter 29

Human or animal source of a pathogen

Indirect transmission occurs via vectors, food, water, soil, or

fomites.

Direct transmission occurs via bodily

contact or airborne droplets. Human or

animal recipient of the pathogen

Transmission Routes

The concept of adaptation can also help explain spatial patterns of disease

in the context of human ecology. Adaptation can be a genetic,

physiological, or behavioral process.

Can you think of an example of each type of

adaptation?


Adaptation


Physiological adaptation can help us to understand the passage of an

epidemic through a population.

An S-shaped curve is often used to represent changes in the number of people

infected during the passage of an epidemic.


In the early part of an epidemic few people are infected and can act as a

source of the pathogen so the disease spreads slowly.


In the mid-part of an epidemic, the rate of new infections

increases as more and more people become infectious.

The majority of the population is susceptible because most people

have yet to come into contact with the pathogen.


Over time, the rate at which new infections occurs slows as much of the

population has either died or developed immunity (physiologically

adapted) to the pathogen.


Can you think of any historical examples of

infectious diseases that have followed this pattern?


The cultural ecology of disease is the study of how cultural practices influence the likelihood of a disease circulating in a particular population.

Cultural Ecology of Disease

Photo by Centers for Disease Control / Michael Schwarz, 1975

This image shows men and boys bathing in a communal bathing tank in

the Patuakhali District, Bangladesh.


Understanding the cultural ecology of disease can shed light on key

behaviors and practices that might slow or prevent the spread of disease.

For instance, the Hungarian doctor, Ignaz Semmelweis, recognized the

importance of hand washing to health in the nineteenth century.

Source: Doby (1860)


For example, Chagas disease was not a problem

in the Amazon until recently, perhaps due to

changing cultural practices.


An important aspect of human ecology is the way in which humans interact with

other species.

Many species act as vectors or reservoirs of diseases that

infect humans. Source: US Department of Agriculture (2001)

Disease Cycles


A disease cycle diagram offers a way to visualize and

analyze disease cycles.

Boxes represent organisms or environments in which the pathogen must reside to complete its lifecycle.Arrows indicate how the pathogen

moves between boxes.


This generalized disease cycle represents a hypothetical disease.

Pathogens such as bacteria are living agents of disease.

Hosts such as humans are the organisms they infect.

Vectors such as flies transmit the pathogen between hosts.

Reservoirs act as sources of infection and may be animate (e.g., a monkey)

or inanimate (e.g., a water body).


The disease cycle of dengue shows that the dengue virus (the pathogen)

is transmitted from human host to human host by a mosquito vector.


Disease cycle diagrams can indicate where interventions could be made to break the

disease cycle.

An Introduction to the Geography of Health24 Anthamatten and Hazen Chapter 2

Russian geographer E. N. Pavlovsky (1966) coined the term “landscape epidemiology” to refer to the ways in which

regions impart patterns to disease distributions via factors such as vegetation, geology, and climate.

Photo by Helen Hazen

Landscape Epidemiology

Mineral Springs, Yellowstone

Anthamatten and Hazen Chapter 225

Elevation Rainfall

Landscape epidemiology suggests that factors such as elevation and precipitation might play a key role in the distribution of a vector-borne disease such as malaria.

Temperature

Data Sources: Guerra et al. (2008), Legates and Willmott (1990)


The distribution of dengue fever is also related to climate, providing another example of landscape epidemiology. The

mosquito vector, Aedes aegypti, is only able to survive where year-round temperatures remain above 10oC.

Anthamatten and Hazen Chapter 226

Data Source: WHO (2008)



Of course, the natural environment cannot fully explain the range of a

disease.Schistosomiasis provides a good example of how landscape and

human behavior interact in ways that affect disease patterns.

Schistosomiasis affects about 200 million people worldwide and more

than 650 million people live in endemic areas (WHO 2008b).

This map shows the distribution of schistosomiasis in Egypt.

Image Source: Courtesy of the WHO (1987)


The schistosomiasis pathogen completes its

lifecycle in water, snails and vertebrate hosts such as people or water buffalo.


Hosts are infected when they come in contact with

free-swimming larvae.


How could the schistosomiasis disease

cycle be broken?



RegionProportion of urban

populationliving in slums (%)

Developing World Sub-Saharan Africa 72.5 Asia (excluding ) 39.7 Middle East and 35.8 South America 35.5 Cen. America / Caribbean 23.7 Total 43.4

Developed World Europe 6.2 North America 5.8 Oceania 3.5 Total 7.5

Overcrowding and overextended

infrastructure have become significant

problems in places that have experienced rapid

urbanization.

The Urban Environment

Data Source: WRI (2007)

Diarrheal diseases such as cholera and typhoid are significant problems in urban contexts, particularly in

slums where fresh water is lacking.

Photo by Heike Alberts



Outskirts of Lima, Peru

Outskirts of Esparza, Costa Rica


Urban environments have also proved to be ideal for some disease-spreading pests such as rats.

The mosquitoes that carry dengue fever thrive in urban contexts where trash and small containers provide an

abundance of small pools of water for breeding.


Changes in vegetation and the array of species living in an area can also alter disease distributions.

For example, malaria is spreading in parts of the Amazon Basin

where forest has been converted to cattle pasture, improving breeding conditions for the

mosquito vector.

Environmental Change

For example, Lyme disease may be spreading in the northeast US as

landscape change alters the species balance of the region and more settlements are built in peri-

urban settings where the tick vector is common.


Climate change may have a variety of impacts on human health associated with changes in temperature, precipitation,

vegetation, and atmospheric patterns.

How, more specifically, might climate change

influence human health?

Photo by Helen Hazen


Impacts of climate change on human health may include:

More frequent heat waves may increase heat-related deaths.

Changing rainfall patterns and related vegetation distributions may

alter the range of vectors.

Warmer temperatures may increase the altitudinal and latitudinal range

of certain vectors.

Changes in the composition of the atmosphere may increase

respiratory pollutants like ozone.

More extreme weather events may have direct impacts on health such

as through trauma, as well as lead to ecological disruption with associated

health impacts.


1. How is cultural ecology important in the transmission of dengue fever?2. Using specific examples, consider whether the built environment or the

natural environment influences human health more profoundly.3. Considering recent events, can you identify any evidence that suggests a

link between climate change and health?

Discussion Questions


ReferencesCenters for Disease Control and Schwarz, M. (1975) “Image ID# 12356” Public Health Image Library [Online]. Available: <http://phil.cdc.gov/phil/download.asp> (Accessed 10 Jan 2011).

Doby, J. Ignaz Phillip Semmelweis. Copper engraving.

Guerra, C. A., Gikandi, P. W., Tatem, A. J., Noor, A. M., Smith, D. L., Hay, S. I. and Snow, R. W. (2008) "The limits and intensity of Plasmodium falciparum transmission: Implications for malaria control and elimination worldwide", Plos Medicine, 5: 300–11.

Legates, D. R. and Willmott, C. J. (1990) "Mean seasonal and spatial variability in gauge-corrected, global precipitation", International Journal of Climatology, 10: 111–27.

United States Department of Agriculture. (2001). “Aedes aegypti (Linnaeus) yellow fever mosquito” Agricultural Research Service Image Gallery. [Online]. Available: <http://www.sel.barc.usda.gov/selhome/gbu/aedes.html> (Accessed 12 December 2010).

[WHO] World Health Organization. (1987) Atlas of the Global Distribution of Schistosomiasis. [Online]. Available: <http://www.who.int/wormcontrol/documents/maps/en/egypt.pdf> (Accessed 03 Aug 2010).

WHO. (2008) Schistosomiasis: A Major Public Health Problem [Online]. Available: <http://www.who.int/schistosomiasis/en/index.html> (Accessed 23 December 2009).

[WRI] World Resources Institute. (2007) EarthTrends Database: The Environmental Information Portal [Online]. Available: <http://earthtrends.wri.org/searchable_db/index.php?theme=4> (Accessed 31 December 2008).

Date post:	23-Feb-2016
Category:	Documents
Upload:	pepper
View:	34 times
Download:	0 times

Photo by Helen Hazen

Documents