The One Health concept was introduced in the early 2000s, encompassing a notion that has been known for more than a century: that human and animal health are interdependent and linked to the ecosystems in which they coexist. Complex and global problems, such as a pandemic, must also be approached from a global perspective.

According to data of the World Organisation for Animal Health, 60 percent of known human infectious diseases and 75 percent of emerging human diseases are of animal origin. Most are zoonoses, diseases that are naturally transmitted from animals to humans or vice versa. Many others are transmitted to humans by animal vectors, from rodents, fleas and ticks to flies and mosquitoes. More than 100 different arthropod-borne infections are known, and the mosquito is probably the most dangerous animal on the planet: it causes more than 725,000 deaths each year, transmitting hundreds of different pathogens. Diseases such as rabies, influenza, Ebola, salmonellosis, HIV, brucellosis, trichinosis, hantavirus infections, anthrax, Q fever, bubonic plague, tularemia, dengue, leishmaniasis, zika, malaria, toxoplasmosis and, of course, the SARS, MERS and SARS-CoV-2 coronaviruses, as well as a long etcetera, are of animal origin or transmitted by vectors. It is not only human density that influences disease transmission; overcrowding on farms is also a factor to watch out for.

Controlling many of these diseases in humans requires first controlling them in animals, so it is hard to understand why animal infectious disease specialists, veterinarians, do not have a more prominent role in public health. It has been a big mistake not to count on the experience of veterinarians in this pandemic, they are professionals accustomed to control large epidemic outbreaks among animals. The health of people depends on the health of animals.

Different factors, including the pathogens themselves and the external environment, condition their evolution from their origin in animals to their adaptation to humans. Climate, for example, is one of the factors conditioning global health. On the one hand, small changes in humidity and temperature can modify the geographical distribution of many vectors, such as ticks and mosquitoes, which transmit infectious diseases. This is the reason why some so-called tropical diseases cease to be tropical and emerge in other latitudes. This is the case of dengue fever, malaria, Zika, West Nile fever and a long etcetera, which can already be found in Europe or North America. In addition to climate, human-induced ecological transformations can alter the migrations of animals such as birds, modify the population density of rodents, favor the contact of humans with wild or wild animals... and all this can contribute to the emergence or spread of new pathogens. Thus, changes in bird migration patterns and in the population of waterfowl can modify the transmission cycle of the influenza virus and its survival and persistence outside the host. This, in turn, may influence the risk of exhibition to influenza virus in domestic birds and humans.

Climate change involves not only extreme events, such as major floods and droughts, but also an increase in the flow of infectious agents between other living beings and humans..

In the specific case of COVID-19, we have no evidence that environmental degradation has had any direct influence on its emergence, but that does not mean that it has not had an influence in other past pandemics or that it may not have an influence in the future. The destruction or alteration of ecosystems, environmental pollution, climate change, global movement - not only of people, but also of plants and animals, each with accompanying infectious agents, intensive livestock practices, the abuse of pesticides, fertilizers and antibiotics that select for resistant mosquitoes (vectors) and bacteria, and our way of life are all factors with a catalytic effect on the appearance or geographic distribution of infectious agents that increase the risk of new infections. The health of the planet directly affects our health. In this sense, the pandemic has once again raised the question of whether it is possible to maintain continuous economic growth on the planet.

The consequences of this constant growth affect the health of the planet and consequently our own. We are part of the ecosystem; we are tenants, not owners of the planet. In reality, we are newcomers to a planetary ecosystem that has been evolving for millions of years before we appeared on it. In the last century, the agricultural internship has increased by 70 to 80 percent, especially intensive monoculture, due to the enormous demand for animal protein; more than half of the world's population lives in large cities and the issue of megacities with more than ten million inhabitants continues to grow; less than 20 percent of the world's population has access to 80 percent of the planet's resources; the risk of soil erosion, ecosystem degradation, salinization and greenhouse gas emissions continue to increase; the global temperature of the planet and the frequency and intensity of extreme natural phenomena are on the rise; the effect on animal and plant biodiversity has led some to speak of a new mass extinction.

There is therefore an interdependence between human health, animal (and plant) health and ecosystem health. The One Health strategy aims to bring together human, animal and environmental health professionals, because everything is connected. To achieve this, it is necessary to promote and promote spaces for communication and exchange between the different professionals over and above trade union interests. It is also necessary to implement this global vision, multidisciplinary, in university programs of study plans: it would not be so complicated to introduce at least one subject on the One Health strategy in the training of physicians, veterinarians, biologists and environmentalists. Medicine, for example, is still very isolated from the other disciplines and at the same time very compartmentalized within it. We should promote this joint vision of research with specific calls, congresses and publications, and, at the institutional level, with Departments or mixed or interdisciplinary commissions. Experience is showing us that, in order to control new infectious diseases and future pandemics, we must consider the human population and the planet as a unit. Health problems and challenges are a global issue.

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Ignacio López-Goñi
Full Professor of Microbiology, University of Navarra(ilgoni@unav.es / @microbioblog)
This article was originally published in The Conversation.