Epidemics and pandemics, unfortunately, are nothing new. A glance at the history of humanity is enough to show that our species’ struggle against infectious diseases has been constant. Not to mention the recent Covid, Black Death, cholera, tuberculosis, flu, typhoid or smallpox are just a few examples of those that left indelible traces…
Each disease requires specific action and the implementation of different prevention, response and treatment mechanisms. It is therefore essential to identify the origins and modes of appearance of pathogens.
In this sense, approximately 60% of emerging infectious diseases reported worldwide are zoonoses (transmitted between animals and humans). It is estimated that approximately one billion people worldwide become ill and millions die each year as a result of zoonotic events. And of the more than 30 new human pathogens detected in recent decades, 75% originated in animals.
The recent emergence of various zoonoses – H5N1 avian influenza, H7N9 avian influenza, HIV, Zika, West Nile virus, severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), Ebola or Covid-19 (SARS-CoV) -2) among others – posed serious threats to human health and global economic development.
They are often unpredictable, as many originate in animals and are caused by new viruses that are only detected after the fact. However, there are at least ten factors that we already know for sure are linked to the emergence of a future epidemic or pandemic. They are gathered and explained below.
1. Wars and famines
The damage caused by war is obviously numerous and complex: the deaths, injuries and mass displacements of people to flee fighting are the most obvious. But the emergence of infectious epidemics is also closely linked to conflict.
In 2006, cholera outbreaks were reported in 33 African countries, 88% of them in conflict-affected countries. In recent years, several countries in the Middle East and Africa have experienced infectious epidemics as a direct consequence of war, exacerbated by food and water shortages, displacement and damage to infrastructure and health services.
2. Land use change
Land use change is a major ecosystem modification induced directly by human populations. The consequences are very far-reaching.
These changes can, in fact, affect the diversity, abundance and distribution of wild animals and make them more susceptible to infection by pathogens. In addition, by creating new opportunities for contact, they facilitate the circulation and spread of pathogens between species, which can lead to human infection.
Through deforestation and forest fragmentation, we promote the extinction of specialist species in these habitats and the development and establishment of more generalist species. Some wildlife species that host pathogens, particularly bats and other mammalian species such as rodents, are relatively more abundant in landscapes thus transformed, such as agricultural ecosystems and urban areas, than in adjacent undisturbed locations.
Establishing pastures, plantations or intensive livestock operations near forest edges can also increase the flow of pathogens from wildlife to humans.
4. Uncontrolled urbanization and population growth
Changes in population size and density due to urbanization again affect the dynamics of infectious diseases. For example, influenza tends to have more persistent epidemics in more populous and dense urban areas.
5. Climate change
Climate change increases the risk of viral transmission between species. Many virus species are still unknown, but they probably have the ability to infect our species. Fortunately, the vast majority of them currently circulate silently among wild mammals.
However, the expected increase in temperatures with climate change will lead to massive migrations of animals in search of milder environmental conditions, which will facilitate the emergence of “biodiversity hotspots” (a threatened biogeographic area with at least 1,500 species of plants and animals endemic). If they reach areas with high human population density, particularly in Asia and Africa, new opportunities for zoonotic spread to humans will arise.
According to recent predictions based on climate change scenarios, by 2070, transmission of viruses between species will increase approximately 4,000-fold.
Globalization has facilitated the spread of many infectious agents to all corners of the world.
The transmission of infectious diseases is the best example of the increasing porosity of borders. Globalization and increased connectivity accelerate the potential emergence of a pandemic and its rapid spread, due to the constant movement of microorganisms through international trade and transport.
7. Hunting, trading and consumption of bushmeat
The transmission of zoonoses can occur at any point in the bushmeat production chain, from hunting in the forest to the point of consumption. The pathogens that have been transmitted to humans by the meat of wild animals are numerous and include but are not limited to HIV, Ebola virus, simian foamy virus and monkeypox virus…
8. Illegal Money Trafficking and Wildlife Markets
An ecosystem with high species richness reduces the rate of encounter between susceptible and infectious individuals, which decreases the likelihood of pathogen transmission. On the other hand, live animal markets and other hidden illegal trade venues are places where the most diverse species are crammed into overcrowded cages.
Under these conditions, they not only share the same unhealthy and unnatural space, but also disease-carrying ectoparasites and endoparasites. Animals bleed, drool, defecate and urinate on each other: this leads to the exchange of pathogenic microorganisms and parasites, forcing interactions between species that should never have happened.
9. Microbial Evolution
Microorganisms are constantly evolving, naturally and in response to direct and indirect selection pressures from their environment. A well-established example is that of influenza A viruses, whose ancestral reservoir is waterfowl, from which they managed to infect other types of animals.
The worldwide development of many types of antimicrobial resistance in common human pathogens is a clear demonstration of the enormous capacity of microorganisms to adapt quickly.
10. Collapse of public health systems
In recent decades, in many countries, there has been a gradual withdrawal of financial support to public health systems.
This decimated the critical infrastructure needed to deal with sudden outbreaks. The recent and rapid emergence of new infectious disease threats such as Covid-19, along with the resurgence of older diseases such as measles and tuberculosis, has important implications for global public health systems.
We must be aware that preparing for possible future epidemics and pandemics requires a careful and careful study of possible factors that facilitate the emergence of infectious diseases. Careful and critical analysis will help design future prediction and prevention strategies.