Global populations are booming — particularly within the world’s ever expanding “megacities”. This trend towards rapid population growth is most notable within developing nations. However, with rapid population expansion comes the potential for dramatic shifts in disease epidemiology, allowing infectious diseases the ability to spread to millions of individuals at unprecedented speed.

Rapid urban development is an inevitable outcome of the population growth of countries. As more and more individuals flock to the cities seeking economic opportunity, urban areas become ever more densely populated.. This presents ample opportunity for infectious diseases to spread. In busses and trains, a single infected individual could pass the illness onto countless individuals on the daily commute, potentially spreading the illness.

By the year 2030 there will be 39 cities that qualify as being megacities, according to a forecast from Euromonitor International — to qualify as a megacity these cities must have a population of over ten million. These cities are projected to house nine percent of the world’s population, account for fifteen percent of the world’s GDP and take up around three percent of the world’s landmass.

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New megacities across the developing world

The Euromonitor study predicts that by 2030 there will be five new megacities across the developing world.

Bogotá, the Colombian capital, has already joined the ranks of the world’s megacities with a projected economy of $109 billion and a population now nearing eleven million. Luanda, the capital of Angola will become Africa’s second megacity. Dar es Salaam — the most populous city in Tanzania — is also expected to make the list, though has the smallest gross domestic product (GDP) of all the projected megacities.

The remaining megacities are both located in Asia. Chennai is projected to be the most densely populated of the new megacities. Its economy, however, is set to be one of the smallest, at only $50 billion. Baghdad is the final city on the list, with a population already exceeding eight million.

Only one city in a Western nation has made the list, although it serves as a valuable comparison point against the cities in developing nations. Chicago has been close to the status of megacity for a number of years, and is projected to cross the line before 2030. Its economy is in a significantly better state than the other cities, with a projected economy of $596 billion by 2030.


Urban metropolises: a haven for infectious disease?

Modern aerial transport has created a situation where the concerns of one city are shared with the rest of the globe. David Heymann, head of the centre on global health security at Chatham House and a professor at the London School of Hygiene and Tropical Medicine, said that “even megacities in the most impoverished countries are just an airline trip away from the rest of the world.” He notes that “once a disease starts in a developing country, it’s likely to find its way into more prosperous ones as well”

Nowhere was this more apparent than the concerns regarding the spread of the Zika virus during the 2016 Rio de Janeiro Olympics.“Dense urbanization may not have created Zika, which causes newborns to have unusually small heads,” said Dr. Seth Berkley, CEO of GAVI, the Global Vaccine Alliance. “It has accelerated its spread from a mere handful to a current tally of 1.5 million cases this year”. Since this statement was made in 2016 the number of Zika cases has sharply fallen. However, the World Health Organization (WHO) remarks that the threat remains, with resurgence a very real possibility.

Some reported that the fears of the Zika virus were overblown. This was due to the winter setting of the Olympics, a time when mosquito populations are at their lowest. However, Zika is not the only infectious disease to find a home in Rio, and media hype surrounding the Zika virus — predominantly focused on its effects on newborns — may have been a distraction from other infectious diseases that were given a means to spread across the globe.

Travellers to the Olympics were far more likely to run the risk of contracting foodborne illnesses and diseases such as influenza, according to the European Centre for Disease Prevention and Control.


Sanitation in developing megacities

One of the key issues in developing megacities is a lack of adequate sanitation infrastructure. This directly ties in with the relative economic state of the city. Chicago – the only potential new megacity in a more economically developed nation – has an economy twelve times larger than that of many of those in developing nations. In poorer cities there may be lapsed sanitation, poor healthcare infrastructure, and potentially high levels of pollution.

Sanitation can play a role in the spread of illnesses such as cholera or rotavirus. One long-term study, based on 22 years of rotavirus cases in Dhaka, Bangladesh, found infection rates for rotavirus during the monsoon season were around ten times higher in the densely populated centre than the quieter periphery.

Dhaka has a population in excess of fifteen million, and so presents a case study for the potential future of all developing megacities.

Rates of mosquito-borne diseases may also surge in larger cities. Most developing countries are in zones with breeding populations of mosquitoes. Factors within these cities, such as the construction of more buildings, often means more stagnant water sources. This allows for ample breeding grounds for existing mosquito populations. Without adequate control measures, this presents a great risk to the inhabitants of the cities. Along with Zika, there is the potential for the spread of other vector-borne conditions such as malaria, dengue fever and chikungunya.

Dengue fever has been singled out as being particularly prone to thrive in areas of increased population. The consistent rise in population density, increased construction and ongoing travel of humans and mosquitoes around the world have led to a thirty fold rise in urban dengue outbreaks between 1960 and 2010, according to the WHO.

Current knowledge of epidemiology in existing large cities could play a role in developing strategies to combat disease in expanding cities. Effective strategies could also be replicated from wealthier cities that have had success in fighting disease cases, though many nations may not have the budget necessary to implement the same measures.


Infectious disease is not all there is to worry about

Noncommunicable diseases (NCDs) may also be a major health problem for inhabitants flocking to urban centres including megacities.

Studies have shown that the transition from rural to urban living comes with considerable lifestyle changes. Many of these will outweigh the potential for greater access to healthcare and income that typically come with urban living.

Rural lifestyles, particularly in developing nations, often involve physical work activities such as farm labour. While often low-income, these jobs tend to keep rural workers physically active, which is known to be highly protective against various NCDs such as diabetes and heart disease.

Diets also tend to be radically different between rural and urban settings. Rural areas have far greater access to freshly grown fruit and vegetables, granting residents a considerably healthier diet. Megacities in particular have very little remaining arable land with which to grow fresh crops due to their denser population.

Those living in megacities are therefore far more likely to have a diet filled with processed foods. There are obvious implications to this, higher levels of obesity — a prominent risk factor in developing many NCDs — as well as increased sugar consumption, another factor leading to obesity as well as directly contributing to the development of diabetes.

There are also less obvious concerns. Xenoestrogens are one group of chemicals that have been implicated as a great concern. These chemicals are known to be at high levels in commercially raised meats and dairy products. Studies in Egypt have shown that consumption of these xenoestrogens are far higher in larger cities, and there is a distinct correlation with higher breast cancer rates in women compared to those in rural areas.


Air pollution: Yet another deadly development

NCD rates are again worsened in urban settings due to the higher incidence of pollution. Megacities can present extreme situations where pollution hits such high levels that the city is covered in a dense layer of smog. This smog is tremendously unhealthy for all those breathing it. It is often projected far past the city limits, affecting the local area for miles around.

Nowhere is this better illustrated than in India. The country houses seven of the world’s ten most polluted cities. Pollution in India has hit such extreme levels that an estimated 1.2 million individuals were thought to have died due to air pollution in 2017.

The WHO reports that pollution is responsible for 29 percent of all deaths due to lung cancer. In India, pollution is driving elevated rates of lung disease even among people who don’t use tobacco and other NCDs such as dementia, and diabetes. Globally, pollution also accounts for seventeen percent of all deaths and illnesses related to acute lower respiratory infection as well as 43 percent of all deaths and disease from chronic obstructive pulmonary disease. It also contributes to rising rates of asthma.

Lung conditions are not the only condition caused by pollution. Pollution causes 24 percent of deaths from stroke and 25 percent of ischaemic heart disease cases. This is due to the ability of PM2.5 (a form of small particulate matter) to enter the blood through the lungs, causing damage to other organs.

A recent study, published in The Lancet shows a notable increase in levels of diabetes, even in areas where average pollution rates are high but lower than what the WHO defines as “safe” limits.


Learning from existing megacities

As more and more individuals flock to the cities — an estimated 68 percent of the world’s population by 2050 — an ever increasing number of people are exposed to the environmental and epidemiological risks of urban living. Health risks are present in rural areas too, but it is important to recognise the epidemiological shift in nations’ disease burdens which will inevitably accompany migration to urban settings.

These risks, particularly with regard to infectious disease, are not linearly increased with the rise in population, but amplified significantly due to the relative density of the population. The inevitable formation of urban slums create areas where impoverished individuals are at particular risk of disease, often with far lower access to healthcare.

These situations can be observed in larger cities across the globe. By learning from the success stories of some, and the problems of others, the newly growing megacities may put into place policies and construction plans to avoid some of the worst situations. Sanitation and access to healthcare must be prioritised if large-scale epidemics of communicable illnesses and rapid rises in NCDs are to be avoided.