The wildlife trade carries a huge risk of the next disease outbreak, based on studies of more than 800 species.
The virus that causes COVID-19 was not the first pathogen to be transmitted from animals to humans, and certainly not the last. And while we will never be able to determine the exact origin of SARS-CoV-2, we can hone our search strategies to be fully prepared for the next outbreaks.
The new study quantified the risk of spread of naturally circulating viruses from animal hosts, specifically mammals, to humans — with a focus on the global wildlife trade, both legal and illegal.
The study found that a quarter of the mammals traded carry 75 percent of all known zoonotic viruses.
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“Each year, the international wildlife trade results in more than one billion direct and indirect contacts between wildlife, people and domestic animals,” says lead author and biologist Shivaprakash Nagaraju of The Nature Conservancy, India.
Simply put, this close contact increases the chance that animal-borne pathogens enter humans, becoming zoonotic diseases that can lead to disease outbreaks. Based on the results of this study, several key groups of animals carry the bulk of zoonotic viruses.
“By identifying the species that pose the greatest risk of human transmission of zoonoses, the study will help international health professionals prioritize where to focus their efforts to prevent the next global pandemic,” says Nagaraju.
But this is not a new risk — it just caught our attention. And the wildlife trade is not the only driving factor. Six out of every 10 infectious diseases in humans are zoonoses, with plague and rabies listed by the U.S. Centers for Disease Control and Prevention as zoonoses of concern along with emerging coronaviruses.
In the last decade alone, Ebola, HIV, Middle East Respiratory Syndrome (MERS), and Severe Acute Respiratory Syndrome (SARS) evolved into zoonotic viruses before the COVID-19 pandemic hit. So it would be foolish not to acknowledge your own hand in this.
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A growing body of evidence shows that human exploitative activities, such as deforestation and urbanization, which impact natural habitats and force wildlife into contact with humans, directly contribute to the spread of zoonotic diseases.
The two main sources of zoonoses are bats and rodents, which live in close-knit colonies and easily adapt to human-dominated environments. But this new study, a meta-analysis of data on 226 known zoonotic viruses from more than 800 mammalian species — wild, traded and domesticated — points to other potential sources that may have been overlooked.
In the current wildlife trade, primates and ungulates such as goats, cattle and pigs may pose an even greater risk to human health than bats and rodents — they carry a significant pathogen load, or about 30 percent of all known zoonotic viruses.
However, apart from the wildlife trade, rodents and bats are still the main reservoirs of zoonotic viruses in nature, the study found. Despite their valiant efforts, scientists have only touched the many millions of viruses circulating among wild animals. There are about 1.7 million undiscovered viruses in mammals and birds alone that we know nothing about.
“The viral load phylogenetic signal we found could be a function of both incomplete mammalian sampling for viruses and undiscovered viral diversity in mammals,” Nagaraju and colleagues write in their paper.
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However, other recent studies have similarly shown that we should look beyond bats and rodents in surveillance efforts, because these animals may not be the «special» reservoirs of viruses that past studies have made them. These groups of animals contain more species, which therefore may contain more viruses that can enter humans.
A 2009 analysis showed that between 2000 and 2006, the United States imported almost 1.5 billion live animals, and almost 80 percent of those shipments contained animals from wild populations. Many of these were from known zoonotic foci, and species identification was generally poor.
“If we want to stop the next pandemic before it starts,” says Joe Kieseker, also of The Nature Conservancy, “our results show that we should, among other things, focus our efforts on keeping rodents, bats, primates, ungulates and animals.»
“However, managing the wildlife trade is only part of the solution to preventing future zoonotic pandemics,” the researchers also note.
“An equally important threat to wildlife-associated zoonotic diseases is the expansion of industrial agriculture, infrastructure development and urbanization.”
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