Unfortunately, epidemics and pandemics are nothing new. A simple look at the history of mankind is enough to show that The fight of our species against infectious diseases has been constant. Not to mention the recent Covid, the Black Death, cholera, tuberculosis, influenza, typhus, and smallpox are just a few examples of those that have left indelible marks…
Each disease requires a specific action and the implementation of different prevention, response and treatment mechanisms. For this reason, it is essential to identify the origins and modes of appearance of pathogenic agents.
In this sense, approximately 60% of the emerging infectious diseases reported worldwide are zoonoses (transmitted between animals and humans). It is estimated that approximately one billion people in the world get sick and that millions die each year as a result of zoonotic events.
And of more than 30 new human pathogens detected in recent decades, 75% originated in animals.
The recent appearance of several zoonoses: avian influenza H5N1, avian influenza H7N9, HIV, Zika, West Nile virus, severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), Ebola or Covid-19 (SARS-CoV -2), among others, has posed serious threats to human health and global economic development.
They are generally 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 numerous and complex: the deaths, the injuries, and the mass displacement of people to flee the fighting are the most evident. But the emergence of infectious epidemics is also closely related to the 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 an important modification of the ecosystem induced directly by human populations. The consequences are very wide.
These alterations can affect the diversity, abundance and distribution of wild animals and make them more susceptible to infection by pathogens. Furthermore, by creating new opportunities for contact, they facilitate the circulation and spread of pathogens between species, which can ultimately 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 general species.
Some wildlife species that harbor pathogens, particularly bats and other mammalian species such as rodents, are relatively more abundant in transformed landscapes, such as agricultural ecosystems and urban areas, than in adjacent undisturbed sites.
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 populated and dense urban areas.
5. Climate change
Climate change increases the risk of viral transmission between species. Many virus species are still unknown but are likely to have the ability to infect our species. Fortunately, the vast majority of them currently circulate quietly among wild mammals.
However, the expected increase in temperatures with climate change will cause massive migrations of animals in search of milder environmental conditions, which will facilitate the appearance of “biodiversity hot spots” (threatened biogeographical areas with at least 1,500 species of plants and endemic animals). If they reach areas with high human population density, mainly in Asia and Africa, new opportunities for zoonotic spread to humans will arise.
According to recent predictions based on climate change scenarios, by 2070 the transmission of viruses between species will increase approximately 4,000 times.
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 transportation.
7. Hunting, trade, and consumption of bushmeat
Transmission of zoonoses can occur at any point in the bushmeat supply chain, from hunting in the forest to the point of consumption. Pathogens that have been transmitted to humans through bushmeat are numerous and include, but are not limited to, HIV, Ebola virus, monkey foam virus, and monkeypox virus. ..
8. Illegal Cash Trafficking and Wildlife Markets
An ecosystem with a high species richness reduces the encounter rate between susceptible and infectious individuals, which decreases the probability of pathogen transmission. On the contrary, live animal markets and other hidden places of illegal trade are places where the most diverse species are crowded into overcrowded cages.
Under these conditions, not only do they 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, thus forcing interactions between species that should never have occurred.
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 rapidly adapt.
10. Collapse of public health systems
In recent decades, in many countries, there has been a gradual withdrawal of financial support for 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, have 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 the potential factors that facilitate the appearance of infectious diseases. A careful and critical analysis will help design future forecasting and prevention strategies.
Raúl Rivas González, Professor of Microbiology, University of Salamanca
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