We’re currently all too familiar with zoonotic disease — that is, a disease that jumps from non-human animals to humans. In this study, researchers at UC Davis’ One Health Institute studied the sources of zoonotic diseases: where they come from, how they originate, and how they spill over to humans. The researchers studied 5,335 different species of land mammals and 142 zoonotic viruses. They also used data from the International Union for Conservation of Nature’s Red List of Threatened Species, which categorizes different species based on their conservation status. This list provides labels — like “least concern” in the case of the gray squirrel, or “near threatened” in the case of the jaguar — to easily reference a species’ conservation status. Using these labels and species population data, the researchers were able to determine which species carry the most zoonotic viruses. Their results also support current theories about the conditions that allow zoonotic viruses to develop. 

When controlling for other factors, the researchers found that the more endangered a species is, the less likely it is to share viruses with humans. This is somewhat intuitive — if there are fewer animals, they have fewer opportunities to interact with humans and transmit a virus. Some important factors that cause zoonotic viruses to develop are large, densely-packed animal populations that cover a wide territory. Fewer animals in a smaller habitat means that they both develop fewer viruses and spread viruses less frequently when they do develop. However, we don’t see zoonotic virus rates spread evenly across threatened species. Threatened species with naturally smaller habitat areas hosted 1/5th as many viruses as species threatened for other reasons, such as habitat destruction or exploitation by humans. If we look specifically at species with reduced populations due to exploitation (like hunting and the wild animal trade) we find that they host twice as many zoonotic viruses as species threatened for other reasons. This supports the idea that repeated close contact with animals creates more opportunities for virus spillover.

It’s important to note, however, that while only 12 species of animals in this study were classified as domesticated, those species host 50% of known zoonotic viruses. Indeed, domesticated animals occupy 8 of the top 10 virus-sharing slots. Those species are pigs, cows, horses, sheep, dogs, goats, cats, and camels, with virus numbers ranging from 31 in pigs, cows, and horses, to 15 in camels. However, the researchers noted that our relationships with domestic animals mean that we’re monitoring them more closely. We’re more likely to detect zoonotic viruses in domestic animals than we are in wild animals, which might bias the data.  The researchers recommend better health monitoring for people who work closely with domestic and wild animals so that we have more complete data. 

The largest number of zoonotic viruses appear to come from non-human primates, bats, and rodents. Together, these groups act as hosts for 75.8% of all zoonotic viruses. In non-human primates, this likely happens because we share a close genetic relationship, which allows viruses to jump between species more easily. In the case of bats, it seems like this is caused by specific bat behaviors. Bats roost in large numbers, which creates ideal conditions for viruses to spread from host to host. Some bat species also migrate, which exposes them to other environments and creates more opportunities to spread viruses. Rodents like the house mouse and black rat live in many of the same habitats as humans. They are also commonly kept as companion animals and used in research. These close relationships create plenty of opportunities for zoonotic viruses to develop.

All of this evidence supports the theory that the main driver for virus spillover is animal-human interactions. The researchers recommend that those involved in high-risk activities, such as working with farmed or wild animals, should be regularly monitored for a fever to catch new viruses. It’s likely that virus transmission happens much more frequently than we currently record, but those transmissions don’t result in noticeable outbreaks. Extra surveillance for people at risk will give us more information about virus transmission rates, and can help us detect outbreaks in their earliest stages. 

The researchers also note that we are living in a period of increasing urbanization and food production. As our habitats collide with animals’ habitats and we raise more animals for food, we may see more zoonotic viruses. The researchers recommend limiting contact between wild and domestic animals. We should also be aware of the impacts of urbanization and the risks involved with moving into animals’ habitats. We need to be especially conscious of our relationships with animals to prevent future pandemics. This is an area where animal advocates will play a crucial role in informing the public.