A new collaborative project led by scientists in the US and Canada has identified at least 887 viruses present in wildlife that carry a risk of spreading to humans. Of those listed in the ‘SpillOver’ watchlist, only 38 are known zoonotic diseases such as Covid-19, Ebola and rabies, and the list doesn’t even include influenza or any viruses associated with insect carriers and domesticated animals.

Scientists have long known that viruses from wild animals pose a threat to humans, especially with our ever-advancing encroachment into non-human habitats and the hunting and selling of wild animals. The most topical example is of course Covid-19, thought to have passed to humans via a yet unidentified species of animal sold at wet markets in China, while Ebola and HIV are associated with the African ‘bushmeat’ trade.

Surprised by the lack of research regarding the threat that wild animal viruses pose to humans, a team working out of the US and Canada has pulled together more than a decade of work by thousands of other scientists working in more than 30 countries to identify animal viruses, discovering many new ones in the process.

The study published in April in the journal Proceedings of the National Academy of Sciences USA also led to the development of a new ‘SpillOver’ tool, essentially an online watchlist that allows anyone to see the list of 887 animal viruses and their associated risk of spilling over to humans, and any scientists to send in data on existing or newly discovered species.

The list includes both known and potentially zoonotic diseases - that is viruses of animal origin that can infect humans - with Covid-19 taking second place after Lassa virus. Many of the top 20 viruses listed are linked with significant human outbreaks, such as SARS and Nipah, but among the highest risk viruses are several not yet classified as known zoonoses according to the data, including Coronavirus 229E (bat strain), rousettus bat coronavirus HKU9 and SARS-related betacoronavirus Rp3.

The risk score - a number out of 155 - does not reflect the severity of the resulting infection or mortality rates, only how likely a disease is to spread from animals to humans based on 30 factors including the virus’s genetics and global distribution, the ecology of its hosts and other environmental factors. For example, Covid-19 ranks in second place with 87 out of 155 due to its high detection rate in humans, whereas macaque foamy virus isn’t classed as a known zoonotic disease yet ranks 30th due in part to the frequent contact between humans and macaques, specifically in agriculture and deforestation.

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Jonna Mazet, an epidemiologist and disease ecologist at the University of California, Davis, led the work for more than 10 years before she and her team decided to create a new framework for interpreting their findings. “We wanted to move beyond scientific stamp collecting [simply finding viruses] to actual risk evaluation and reduction,” Mazet told Scientific American.

Conspicuous by their absence from the list is all the many subtypes of influenza, such as avian flu and its many strains. These, Mazet said, would have ‘topped the list’ for risk of spreading to humans, and would have been included if they weren’t already monitored closely elsewhere. 

Also omitted were any viruses linked to domesticated animals - which would include pigs and other farmed animals - and also insect-borne diseases such as Zika which made headlines in 2015 and 2016 when the media highlighted its spread in South America and the Caribbean and the threat it is thought to pose to pregnant women and their unborn children.

So much of the focus that we as animal justice campaigners place on zoonotic diseases centres on influenza and the fetid breeding ground for novel strains that is industrial farming. Yet even without the inclusion of avian flu - such as H5N8, H5N1 and H7N9, regarded by many as each being just a mutation or two away from causing the next devastating outbreak in humans - the list of 887 wild animal viruses logged so far reveals many risk factors that relate directly to the way we humans treat animals and our environment.

In the case of macaque simian foamy virus, as mentioned above, ‘agricultural system change’ and deforestation were both listed as medium risk contributors, while human population density in macaque ecosystems was a high-risk factor. Without going through the entire list, humans going into rainforests to cut them down and make way for land to graze cows or plant soy for animal feed, or otherwise venture into once wild environments to plunder the natural world for resources, are all associated with outbreaks of zoonotic diseases.

Although Mazet and her team hope their work will prompt governments and health authorities to take action to prevent future spillovers, the study on which the list and online tool is based makes no mention of how exactly they are to stop another devastating pandemic. For us, the answer is clear, we need only look at one of the biggest risk factors within our control linked to both wild and domesticated animals. We must end animal agriculture.

We discuss all the major outbreaks of modern times and their zoonotic origins, including viruses from wild and domesticated animals, in our Covid-19 and animal exploitation: preventing the next global pandemic white paper.

Andrew Gough is Media and Investigations Manager for Surge.

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