From Infection to Pandemic in 48 hours? Factors of Disease Diffusion

By Mark Welford, University of Northern Iowa, USA & Brian Bossak, College of Charleston, USA

For the last couple of decades, we’ve lived in an interconnected global society in which humans (and their pathogenic biota) are rarely more than 48 hours from any other location on earthThe efficiency of global transportation networks have largely eliminated the few existing barriers to geographic expansion of emerging infectious diseases. In just the 2000’s alone, emergent viral diseases such as West Nile Virus, SARS, and Swine Flu illustrated just how well our global transportation networks facilitate dispersing new strains of disease to unexposed populations across space and time.

For reasons that include environmental interaction, slowly occurring natural mutations, and relatively fast genetic changes in pathogenic strains, emerging infectious diseases continue to ebb and flow within and between human (and non-human) hosts. For instance, Ebola virus continues to move between fruit bats1 and people leaving death in its wake. (Fruit bats are also thought to be the most likely zoonotic reservoir of Ebola virus, but this has not yet been confirmed.)Although the world is focused on COVID-19, an Ebola epidemic that erupted in August 2018 is still afflicting Uganda and the Democratic Republic of Congo, with at least 2,253 dying from the pathogen (as of early May, 2020). 

Why do some pathogens wreak havoc and infect and kill many across the globe, while others remain highly localized?

While molecular biology and the epigenetic characteristics of susceptible individuals play a role in the virulence of disease, it is the human factors, such as those associated with socio-economic and cultural factors that  affect disease transmission. The local, regional, and national characteristics of the built-environment, and crucially the interconnectedness of places, establish mechanisms for infectious diseases to pass from one infected host to another susceptible one. To add to this complexity, the nature of the disease’s transmission pathways (respiratory, GI, dermal, etc.), the length of the disease’s asymptomatic yet infectious period, and environmental persistence also critically affect the pathogen’s ability to move up a spatial scale from local infection to regional epidemic to global pandemic.

For instance, respiratory-associated pathways facilitate rapid transmission of diseases such as influenza, whooping cough, and of course, COVID-19. Rapid onset of severe symptoms, such as in traditional Ebola virus disease, can act to slow widespread transmission, as people who are infected and symptomatic frequently do not travel far and their symptoms elicit avoidance; until the recent 2014-16 Ebola virus outbreak, most cases were restricted in geographic scale to isolated villages or areas within a country.

What led to the virus expanding across West Africa, and in a few cases, imported Ebola virus cases in western countries where the disease is not endemic? The true story of how close the world came to a potential Ebola pandemic in 2015-2016 has yet to be written.  Covid-19, on the other hand, rocked the world in weeks, and has been found to be highly infectious. Its viral loading in the lungs and throat is quite high even when individuals appear healthy; it is environmentally persistent — viable virus material was found on the cruise ship Diamond Princess 17 days after the last passengers and crew exited the ship;. it exhibits a relatively long asymptomatic period, and in many cases, infected individuals are never aware they were infected. As we noted in our prior work, emerging diseases are often represented by their metrics in infectivity, lethality, and environmental persistence. Observations so far, suggest that Covid-19 is more transmissible than other coronaviruses such as SARS and MERS, but less lethal than both.

Historical comparisons

Just like the Black Death (1347-1353) and plagues before 1815, Covid-19 is rapidly transmitted between people in times and places of intense human co-mingling, raising the question: are pandemics a by-product of globalization, a consequence of human cooperation and commerce? Between 1347 and 1815, plagues thrived and infected people during fetes, fairs, markets, harvesting, religious pilgrimages, and wars, as well as along trade routes.  Large urban areas, especially port cities and capitals, and especially those villages, towns or cities unable to effectively isolate themselves from the plague suffered high plague mortalities. Simply put the larger and denser the city and its inhabitants, the greater the potential existed for   transmission and suffering from an emerging pathogen. Isolated villages in north-east Spain suffered few historical plague mortalities, although once plague entered a remote village, for instance Eyam in the UK, plague mortalities were still very high.

The perception of urban areas as spaces of contamination from which to flee is not relegated to the history books. In the current pandemic, many wealthy urbanites from California and New York have fled to exclusive and relatively-isolated communities in low-density states such as Arizona, Montana, Utah, and Wyoming – much to the concern of locals who fear they have or will bring Covid-19 with them. This rings an historical bell, one perhaps best encapsulated in Boccaccio’sDecameron, which describes people fleeing cities and towns as the Black Death closed in on them.

Today, across the world, it is the most interconnected and/or globalized urban settings where cities, ports and capitals are the principal transportation hubs of their countries or regions, that are  witnessing elevated Covid-19 infections, such as Lombardy and the Po Valley of Italy, where the country’s highest population densities exist. New York (and particularly New York City) has one of the highest population densities in the US and remains a hotspot for Covid-19. 

Because social distances are compressed, social interactions are high, and social contacts of long duration frequently occur in urban settings, disease transmission velocities are elevated, and urban areas are consistent hot spots for contagion. In cities like New York, London, Rio de Janeiro, Moscow and Chicago, people across most socio-economic groups use public transportation, eat out frequently, shop frequently, and attend public social events at much higher frequencies than citizens in more rural areas. Other potential high-density disease hot spots include military vessels/basesprisons, hospitals, and other care-facilities. Historically, these facilities were also hot spots in historical epidemics such as the Black Death, plagues, and the Spanish Flu of 1918-19.

Even within densely populated urban areas, socio-economic and racial divides determine the spatial expression of Covid-19. The rich and middle class have retreated to their homes and can telecommute, but the working poor, medical staff and police, all who have been unable to stop working outside their homes, have suffered the highest Covid-19 mortalities. Maintaining appropriate social distancing conventions prescribed during the pandemic is a herculean feat where survival depends on social and commercial interactions (corner stores for groceries, limited ingress and egress from housing, poor indoor ventilation in vertical housing structures, and the like). During the Black Death and subsequent plagues, it was the working poor and slum dwellers who also suffered the highest plague mortalities; the pattern repeats in urban slums throughout the world. As one example, Brazil is now sixth in infections and 5th in Covid-19 deaths worldwide (as of 22nd May 2020), but nearly all of the deaths and infections there are spatially restricted to the urban favelas of SE Brazil and Manaus.

Urban/rural divisions

As a result, the urban/rural divide offers states and countries a geographic perspective in which to attempt suppression and containment of Covid-19 while restarting economic activity. Five rural mid-western US states, among the most politically conservative states and those that consider themselves part of the heartland, choose to initially close all education facilities, all consumer shops, all spas and salons, and all restaurants except those offering take-outs, but did not issue a stay-at-home order and allowed all gas stations and grocery stores and all agricultural-processing plants to remain open. Despite existing within a rural geographical context, a number of meat-processing factories in the US mid-west have become Covid-19 hotspots (largely due to the human density of the work environment).

Historically, during the Black Death, rural East Anglia in the UK had the highest density of rural markets and suffered the highest Black Death mortalities of any region in the UK. However, responses based on rural/urban differences are complex and must consider not just geographic scales and transmission pathways, but cultural and ethnic traditions and preferences as well. For instance, it appears that supporters of the notion that Covid-19 is a hoax and those that support ‘re-opening the country’ as quickly as possible live predominantly in rural areas, in heartland states, and in Southern US states. Cell-phone data from armed ‘re-open’ movement protestors in Michigan show the majority returned home to rural areas following the protests, possibly taking Covid-19 with them.

Consider the difference in approaches between California, a state with extremely concentrated urban areas and vast rural landscapes, and South Dakota, a largely rural state with numerous Native American reservations that exist as sovereign nations. California is currently relaxing Covid-19 business and travel restrictions in rural areas, while city leaders and public health directors in LA and San Francisco are maintaining strict limitations on business and personal travel. In largely rural South Dakota, the Cheyenne River Sioux and Oglala Sioux tribes installed multiple checkpoints on roads leading to their reservations in early April, as part of their emergency response to minimize the spread of Covid-19. This has greatly irritated the Governor of South Dakota, who has threatened legal action over the checkpoints. Cultural spaces and geographies of identity are part of the human ecosystem and responses to perceived threats, including health threats, must consider differential conceptualizations in their planning and execution.  

Despite approaches to pandemic mitigation and suppression that vary between rural and urban spaces, all spatial scales remain at elevated risk of infections until effective vaccines are universally available (or this coronavirus is eradicated). Middle-class and rich urban and suburban elites and rural dwellers cannot pretend this is something they do not need to be worried about and assume that COVID-19 is only a disease of the urban working classes. As history has shown in prior pandemics, and as COVID-19 is demonstrating today, geographic variability in factors of exposure suggest that a one-size fits all approach may not be the best fit for a globally-interconnected world that varies by how close people live, work, and interact with each other. Rather, more intense social distancing measures in high human density/interaction environments and modified social distancing measures in less dense environments may be an approach that balances risk reduction with economic activity and activities of daily living.

As with most things in life (pandemics are no exception), geography matters.

About the authorsMark Welford is a nature-society geographer and head of geography at the University of Northern Iowa. His research interests include environmental change in, and conservation of, tropical montane environments; hurricanes and climate change; and the spatial dynamics of historical pandemics. Brian Bossak is a Medical Geographer with research interests in climate change, natural hazards, spatial analysis of infectious diseases, environmental health, and coastal risk assessment. 

Suggested further reading

Welford, M. and Bossak B. (2010) Revisiting the Medieval Black Death of 1347-1351: Spatiotemporal Dynamics Suggestive of an Alternate Causation. Geography Compass, 4, 561-575.

Bossak B, Welford, M. (2010) Spatio-Temporal Attributes of Pandemic and Epidemic Diseases. Geography Compass. 4: 1084-1096.

Füller, H. (2016), Pandemic cities: biopolitical effects of changing infection control in post‐SARS Hong Kong. Geographical Journal, 182: 342-352. doi:10.1111/geoj.12179

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