Urbanization and infections: Hidden links to rural settings

Catherine Machalaba & Virginia Porter
EcoHealth Alliance, USA

When the Ebola virus struck Guinea in late 2013, it quietly went on to surpass the infection toll of all other Ebola outbreaks combined. It did this by becoming, for the first time in history, an urban epidemic. It moved from a rural village of Guéckédou to the town of Dabola to the capital city of Conakry via a man who was travelling for business. He died shortly after falling ill, at which point his family took his body to his home village of Watagala. Within a matter of days, the disease was on the move. Ultimately, the outbreak, which continues in Guinea, has to date resulted in over 28,600 cases– more than 60 times higher than the number of cases in the previous largest known Ebola outbreak.

Guinean Red Cross
Members of the Guinean Red Cross participating in an Ebola awareness campaign. Source: National Geographic, Cellou Binani/AFP/Getty Images

Population density in urban settings had a role in the spread of the virus, as did the limited healthcare and public health services in the country (which sadly were impacted even further through deaths from the illness in healthcare workers tirelessly serving on the front lines in treating patients). But the outbreak, as well as the recent appearance and spread of other emerging infectious diseases, also shows the connectivity of rural and urban settings in disease spread. They also demonstrate the links to pressures we are exerting on our ecosystems, and in the process, facilitating more opportunities for new diseases to appear.

Infectious Diseases in Urban Settings

The geographic spread of previous outbreaks of Ebola virus was limited by distance, and relatively limited access, to affected rural populations. Although the Ebola Zaire strain of the virus was not known to be present in the region before the outbreak, in terms of its spread in the population, population density and human movement played an important role in the trajectory of the West Africa Ebola outbreak. Indeed, scores of other infectious diseases have followed a similar course in urban populations, as seen with the amplified spread of tuberculosis and HIV/AIDS in city settings. Infectious diseases are one of many public health threats that cities are contending with, especially as air pollution, changing nutrition, and more sedentary lifestyle practices become prominent health risks. But infectious disease dynamics represent an interesting lens to consider the connectivity of our society, and increasingly our pressures on our ecosystems. And arguably an important one, too: despite reductions in their global burden on health, infectious and parasitic diseases still caused an estimated 8.7 million deaths per year in 2008, Some populations are disproportionately affected, particularly those in living in poverty and children under the age of five; in low and middle-income African countries, infections account for 45% of the disease burden.

Outbreaks
Disease outbreaks alerts reported over the week ending 14 November 2015. Source: HealthMap

As people flood into urban centers, there is often little time for urban planning or infrastructure development. Slums and shantytowns pop up on the edges of cities, lacking sanitation, clean water, electricity, and other basic amenities. This quickly becomes a breeding ground for many types of infectious and highly transmittable diseases. Areas like West Africa have exceptionally mobile populations, with migration rates 7 times higher than the rest of the world. Porous borders and the absence of effective reporting and surveillance methods mean outbreaks can become epidemics in a very short amount of time. Rapid travel can also quickly bring disease across continents into other urban centers, as seen with the imported Ebola virus cases into Italy, Spain, the U.S., and the U.K.

Human population density may assist in the efficient spread of diseases that transmit directly from human to human, especially in the case of respiratory infections (such as influenzas). But other transmission pathways may benefit from human density and movement, too. Dengue fever, caused by a virus, has a human-mosquito-human transmission cycle, meaning that the mosquito species that transmit it can acquire it from an infected human and then go on to infect other humans. The risk of Dengue is determined by temperature, rainfall and urban growth with insufficient public health infrastructure, which enable conditions for mosquito breeding and survival success. Forty-five years ago, Dengue was known to be nine countries; currently, it is found in over 100.

Once an outbreak occurs, various dynamics in a city may change, from public transportation, to how people greet one another, to access to food. Traditionally, Liberian taxis were shared among riders, often 7-8 people in one vehicle at a time. Shortly after the outbreak, the government issued warnings and began prosecuting anyone who overloaded vehicles or commercial buses. The customary, warm West African greeting of a handshake and finger snap was shunned as fear of personal contact grew. Quarantines were imposed on neighborhoods known to have cases, and markets were closed all over the affected areas.

Connections with Rural Settings

People in Hong Kong’s business district wearing surgical masks to help prevent infections during the SARS outbreak in 2003. Photo: CNN, Christian Keenan/Getty Images
People in Hong Kong’s business district wearing surgical masks to help prevent infections during the SARS outbreak in 2003. Source: CNN, Christian Keenan/Getty Images

As cities grow, so have pressures on our rural settings to meet growing and evolving population demands. As one might expect, this enables introduction and spread of known diseases; for example, the influx of workers in rural areas around natural gas extraction has been associated with an increase in sexually transmitted infections in parts of the U.S. But we are also seeing newly-detected diseases — so-called emerging infectious diseases – popping up. Think back to the societal disruption from the Severe Acute Respiratory Syndrome (SARS) outbreak in 2003, shutting down markets in Asia and affecting global travel, spreading to 29 countries within months. And while a now-global epidemic, we remember that HIV/AIDS crossed over to humans via hunting or butchering of Great Apes, with human-human transmission then aided by railway travel among population centers within Central Africa. The appearance of diseases like these two, which originated in animals and went on to cause illness in people, is on the rise. But it’s not unprecedented, as over 60% of pathogens infectious to humans have originated from animals, leading to over a billion human cases of zoonotic infectious diseases per year.

What’s bringing about these emerging diseases? As it turns out, we are. Anthropogenic forces, including land use change (e.g., mining, oil and gas extraction, timber logging, and conversion of land for palm oil plantations), changes in food production and agriculture, trade and travel, hunting and butchering of wild animals, climate change, and other factors are among the leading drivers of disease emergence, and they also overlap closely with the leading causes of biodiversity loss, ecosystem degradation, and climate change. Many of these pressures are related to products or services destined for urban uses. The practices associated with these anthropogenic demands fundamentally change the ecological or evolutionary dynamics, and facilitate novel or increased contact between humans and animals, enabling pathogens to transmit across species.

Travel time between major cities (air, rail and road). Source: New Scientist 2009
Travel time between major cities (air, rail and road). Source: New Scientist 2009

Demand for food to feed a growing global population is expected to drive conversion of more than 100 million hectares of land for agriculture by mid-century in developing nations. Assuming a ‘business as usual’ scenario to meet the needs and wants of a population of 9 billion by 2050, the world will see a tripling of natural resource extraction activities (for minerals, fossil fuel, ores, biomass, etc.), accompanied by deforestation and other environmental impacts. By 2050, road building is expected to add 25 million kilometers of corridors, primarily in developing nations with biodiversity-rich ecosystems. In addition to direct ecological disturbance of road building activities, we can expect these roadways to open up opportunities for human encroachment into remote areas once limited by their lack of accessibility, and new settlements appearing to serve travelers. This increasing connectedness and the movement of people and goods may also help disease move more freely from forest to city. Hunting of wildlife, once primarily for subsistence, is increasingly destined for demand in urban markets where it can garner higher prices.

While offering many efficiencies, cities account for an estimated three-fourths of carbon dioxide emissions globally, serving a major contributor to climate change. Projected changes in species ranges related to climate change may result in shifts in distributions of the pathogens they carry. Trade or travel may facilitate introduction of pathogens across borders and continents, and favorable climate, ecological, or host species conditions may allow them to persist in new populations, as seen with the introduction and subsequent establishment of West Nile virus in New York City in 1999. Climate change may exacerbate flooding and other natural disaster events in low-lying urban centers, potentially increasing risk of some vector, rodent- and water-borne diseases.

Promoting Health in an Urban World

While demands on natural resources and population density may be linked to disease transmission and spread, we can also appreciate that urban settings may offer economies of scale in reducing infectious disease burden. For example, centralized water sanitation systems can be developed to serve a large number of people with access to potable water, and vaccination campaigns may be more readily implemented where there are congregating opportunities.  Urban centers offer access to modern healthcare services as well as communication channels, in theory facilitating disease detection, diagnosis and reporting leading to more timely resource allocation. But in West Africa, in the midst of the residual effects of civil war and a developing economy, human population growth outpaced health services, with affected countries lacking reliable energy sources and basic healthcare capacity. Contrast this to control measures implemented in Nigeria that contained the spread, as well as the early detection and rapid action during a concurrent Ebola outbreak in DRC in 2014.

In September, the United Nations General Assembly adopted the Sustainable Development Goals. For the first time, there is a global mechanism and commitment to advance progress on cities, health and well-being, climate, poverty eradication, water and sanitation services, energy access, ecosystems, consumption patterns and other key areas of development under one umbrella. Coordinated investments and policy action across the seventeen goals and 169 targets have the potential to yield tremendous synergies toward health and other societal gains while alleviating pressures on natural resources. While the health targets focus on addressing known disease threats, concerted efforts can be taken to prevent ones on the horizon by addressing their underlying drivers- and at the same time reducing climate and biodiversity loss pressures.

The road to global sustainable development is long, but health improvements can be catalyzed on local levels. The process of urban planning can be expanded to include consideration of health impacts of meeting urban needs. Currently, health impact assessment is not routinely conducted prior to development projects, preventing a full picture of the anticipated impacts. Integrating health with environmental and social impact assessment may provide a more comprehensive understanding of the links, and also help to identify interdisciplinary solutions.

As a long-term approach, urban populations can shift consumption trends toward healthy and sustainable alternatives that reduce the underlying drivers of disease and ecological impacts. For example, farmland allocation could be prioritized for already-converted landscapes to minimize forest deforestation or fragmentation. Where human settlements are formed in remote areas for natural resource extraction, employers can provide workers with a reliable protein source to reduce wildlife hunting. Ideally, urban health infrastructure will be able to anticipate, and mitigate, effects of future needs, or at least co-evolve to meet the present needs of populations. To achieve this, the dependencies and connections between urban and rural settings must be appreciated in sustainable development agendas.

For further information:

Goats and Soda: The Changing Face of West Africa has Fueled the Ebola Crisis – from NPR’s All Things Considered, September 6, 2014.

Video: Ebola virus: how it spreads and what it does to you – from The Guardian, October 22, 2014.


Catherine

Catherine Machalaba is the Science Officer for the Future Earth ecoHEALTH core project, exploring the links between global environmental change and health. She is based at EcoHealth Alliance.
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Virginia

Virginia Porter is a graduate of James Madison Univ. with a degree in Health Sciences, Public Health. She has spent time in West Africa doing medical aid work, and is currently the Administrative Assistant to the Executive Vice President for Health and Policy at EcoHealth Alliance.
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Header image: Freetown, Liberia.  Credit: Virginia Porter

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