[This article has been written by Adelaida Sarukhan, scientific writer at ISGlobal, with the collaboration of Eduard Martín-Borregón, and it has been published in Spanish in The Huffington Post]
There is no doubt: in an increasingly interdependent world, health has become a global issue
On December 26, 2013, a two-year old toddler died of a little-known disease in Meliandou, a village in the forest zone of Guinea bordering Liberia and Sierra Leone. Eight months later, the
had spread across West Africa and Ebola epidemic the whole world was in panic over its possible spread outside Africa.
There is no doubt: in an increasingly interdependent world, health has become a global issue. This is mainly due to four closely related reasons:
1. Increased mobility of persons and goods
Until WWII, more people died from microbes introduced by the enemy than from war injuries
During much of human history, populations lived relatively isolated. In the last five centuries, the encounter of the Old and New worlds not only led to an exchange in flora and fauna, but also of pathogens such as smallpox and influenza. In fact, until WWII, more people died from microbes introduced by the enemy than from war injuries.
Ships have greatly contributed to the travel of pathogens and their vectors
Ships have greatly contributed to the travel of pathogens and their vectors. For example, the
, vector of many viruses including those that cause Aedes aegypti mosquito yellow fever, dengue or zika, travelled from West Africa to America in the 15th century on boats carrying slaves. More recently, rats in merchant ships have caused plague outbreaks in port cities such as Sidney, Bombay, San Francisco or Rio de Janeiro, while the Asian tiger mosquito, Aedes albopictus, vector of more than 20 viruses, has spread in the last decades to many countries in Europe and America due to ship transport of tires that contained larvae and eggs.
In 2015, 89,900 ships transported more than 1.7 billion tons of goods, which represent 80% of the total world trade and a great opportunity for pathogen spread.
A good example of the role of air transport in spreading disease is
HIV/AIDS: in the USA, its rapid spread between 1984 and 1990 follows a pattern that reflects the flow of air traffic between cities and the early cases could be traced back to cities visited by a Canadian flight attendant (called “Patient Zero”).
In the USA, the rapid spread of HIV/AIDS between 1984 and 1990 follows a pattern that reflects the flow of air traffic between cities
In 2015, 3.4 billion people travelled by air, 10 times more than in 1971. The current complex network of air routes explains the seemingly erratic spread of global diseases, as was observed with
. An infected individual travelling from Guangdong transmitted the infection to 16 guests in a Hong Kong hotel. These guests in turn travelled to Toronto, Singapour and Vietnam, where they seeded other outbreaks. A few weeks later, there were more than 8,000 SARS cases in 26 countries across 5 continents. SARS
Based on the world airport network, a model has simulated the spread of a hypothetical disease similar to influenza, in the first 53 days:
The pattern seems erratic on an ordinary map, but not so if represented on a tree map based on the connectivity and number of passengers in each airport:
Several studies conclude that the different outbreaks of Ebola over the last decades have one thing in common: deforestation
Returning to Ebola, several studies conclude that the different outbreaks over the last decades have one thing in common: deforestation. The
forest in Guinea, where the last outbreak originated, has been reduced to one fifth of its original surface. The invasion of jungles and forests by man favors the contact with (such as Ebola) that survive in animal reservoirs but can “jump” to human hosts. Deforestation has also been linked to a surge in malaria, most likely due to the replacement of indigenous species of zoonotic viruses Anopheles species by other species with increased transmission efficacy. For example, in Trinidad, deforestation followed by the plantation of bromeliads, whose flowers accumulate water, led to the proliferation of Anopheles bellator and an increase in malaria prevalence. In West Africa, deforestation and irrigation have led to a rise in malaria transmitted by Anopheles gambiae in villages close to the forest.
gives an idea of the terrifying rate of forest loss over the last 15 years: This map 3. Urbanization
A century ago, only 20% of the world population lived in cities (5% in less developed countries). As the following map shows,
urban growth has dramatically increased in the last 40 years.
4 billion people live in urban centres, with one out of three people living in slums. These megacities are incubators for new epidemics. To start with, they are much more efficient in propagating deadly diseases such as Ebola, since infected people can in turn infect many others in close proximity before dying. In addition, poor housing conditions favor the proliferation of diseases vectors such as rats and insects. An example is and that is on the rise in periurban areas of Latin America, or the the kissing bug that transmits Chagas disease Aedes aegypti mosquito that has adapted perfectly to live with and feed on humans and breed in cities (more than 75% of the population in the Americas lives in urban zones infested with the mosquito). 4. Climate Change
Before the identification of infectious agents, humans had already understood that climate conditions affect infectious diseases: the Romans, for example, moved to their villas in the hills during the summer to avoid getting malaria.
There is evidence that the risk of malaria epidemics increases five-fold in the year following an El Niño event
The rise in global average temperatures over the last decades (see the map) will forcefully have an impact on the prevalence and distribution of diseases and their vectors. The impact of climate is particularly true for disease transmitted by mosquitoes, since they cannot survive in low temperatures. There is evidence that the risk of malaria epidemics increases five-fold in the year following an El Niño event, and some models predict that an increase in 2-3º C will lead to several hundred million additional persons living in risk of malaria (climate-wise). However, predicting the long-term impact of climate change on the incidence of insect-borne diseases remains challenging.
What is more foreseeable is the short-term effect of climate change on water-borne diseases. Torrential rains result in the contamination of water supplies, while higher temperatures promote the proliferation of pathogens such as Vibrio cholerae, the bacterium that causes cholera.
5. The Zika virus example
in the Americas is a perfect example of a virus that spread thanks to the Zika virus epidemic widespread distribution of its vector, Aedes aegypti, and that provoked a large-scale epidemic among a population with no previous immunity against it. The result of this “journey”, which is shown in the map below, is the following: a relatively irrelevant and harmless virus has caused an epidemic of huge socioeconomic impact that falls disproportionately on the most vulnerable populations (pregnant women) and the poorest countries of the region.
The solutions to the health problems (...) must be addressed through an interdisciplinary and international approach
To say that health is global not only means that pathogens initially confined to remote areas can now spread with unprecedented speed and pose a global threat. It also implies that the
solutions to the health problems that currently affect us no longer depend on actions by individual countries, but must be addressed through an interdisciplinary and international approach.