The "emerging threat" of zoonoses: what lies behind the spread of diseases like monkeypox

It is not a new phenomenon, but it is accelerating. The spread of diseases that originate in an animal and are transmitted to humans, the so-called zoonoses, are a "major" public health problem throughout the world, according to the World Health Organization (WHO), which they are booming. One of them is monkeypox., an endemic viral zoonosis in Africa that in the last week has been detected in 12 countries outside the continent. Of them, Spain is the one that has reported the most cases at the moment.

According to the data handled by the WHO, zoonoses are significantly driving emerging diseases in humans and constitute 75% of those that have emerged in the last 40 years. Two thirds of all pathogens transmissible between people are zoonotic, that is, they have jumped from an animal to a human being. "We have always lived with zoonoses, but in the last decade they are having a much more relevant impact. They can be pathogens that were unknown until now or that were already circulating in certain areas or regions and that appear in others", describes Ignacio García, professor and head of the Animal Health and Zoonosis Research Group of the University of Córdoba.

Zoonotic pathogens can be bacteria, viruses, parasites, or unconventional agents and spread to humans by direct contact with infected animals or through food, water, or the environment. The Nile virus, Zika, Ebola or influenza A are examples of zoonotic infections. The path that COVID-19 followed to end up becoming a pandemic is not known for sure, but the focus has always been on the wild animal market in Wuhan, China, and there is evidence linking mink as vectors of contagion of the virus. virus. The 2012 MERS or 2002 SARS epidemics also came this way.

In monkeypox, there are still more unknowns than certainties on the table, but it is known that vaccines against the eradicated smallpox produce immunity. "It was spread massively until 1980, so if there had been introductions of this type they would have had very little visibility because there was smallpox circulating and because almost all of us were protected. In the case of this family of viruses there is a growing threat because it is estimated that more than half of the population is not vaccinated," says Rafael Blasco, a researcher at the CSIC's National Institute for Agricultural and Food Research and Technology.

But in general, why this type of pathogen has emerged with greater intensity in recent decades responds "to a host of factors" of various kinds, says Ignacio García. Some, product of human action on nature, others linked to population changes. Both globalization and international mobility, as well as climate change, destruction of ecosystems and loss of biodiversitythe trade of species or the exponential increase in population have been identified as elements that favor the expansion of zoonoses.

"Human and animal mobility is a key conditioning factor and makes a virus that is in the center of Africa reach the other part of the world in 24 hours because someone takes a plane," illustrates the researcher from the University of Córdoba. To this is added the growth of the world population, which has gone from 4,500 million people in 1985 to 7,700 in 2020, and the human concentration in cities. "In general, infectious diseases are density-dependent, that is, the more susceptible individuals there are and the more concentrated they are, the easier it will be for them to be transmitted," explains García.

The researcher from the University of Córdoba also links the phenomenon with "the evolution of pathogens", which mutate to adapt and continue infecting: "A virus that is intelligent tries not to kill its host, but to infect it and continue transmitting itself Many viruses, during replication, mutate quite easily and variants are produced."

Some other factors of the greatest opportunities for expansion are linked to the destruction of natural habitats and the loss of biodiversity that has been caused by human activity. It occurs fundamentally in two ways: on the one hand, by destroying ecosystems and ending varieties of animals and plants, they cause "the alteration of a series of balances that usually keep these pathogens under control," says Luis Suárez, biologist and Conservation coordinator at WWF Spain. This analysisa joint effort of 500 scientists, calculated that 75% of the earth's surface has already been altered by human activities.

The greater the destruction of ecosystems, the less chance there will be of dissipating the disease. This occurs in a habitat where there are a variety of animals susceptible to infection, the chances of it occurring in a specific species will decrease and it may even end up in a non-vulnerable animal that will stop the infection. "In addition, there is less chance of it circulating because there is a balance between predators and prey that acts as a shield. A typical one is that of rodents, which are also vectors of some diseases. If predators decrease, there will be more, they will be more transmissible between them and more likely to spread it," says Suárez.

But, in addition, deforestation causes human communities to enter "to the heart" of the habitats in which animals that host pathogens can live. This was confirmed with some outbreaks of the so-called macaque malaria, which jumped to humans in deforested areas of Indonesia, one of the countries most affected by the destruction of forests to make room for palm cultivation.

To this is added that "the population trades, traffics and feeds on wild animals more and more, both legally and illegally," explains the researcher. An example is what happened in 2003 in the United States with monkeypoxthe first time it was exported outside of Africa due to the sale as pets of several prairie dogs that had been in contact with infected rodents imported from Ghana.

Ignacio García also points to climate change as another of the drivers of zoonoses because the vectors that can transmit diseases "have increased their distribution range" due to the general increase in temperature, which means "they are present in areas where they weren't before." Suárez gives the example of the aedes mosquito, which can be a carrier of dengue, yellow fever or chikungunya and which, according to the data, was present "in nine countries in 1970" and is currently present "in 128".

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