Spillback: How often do humans transmit diseases to animals?


Zoonosis: Much of humanity has become intimately familiar with this term over the past couple of years. This describe an “infectious disease caused by a pathogen that has passed from an animal to a human”. After taking birth in a creature, probably a bat, then potentially infecting an intermediate species, possibly a pangolin, raccoon dog, mink, or fox, SARS-CoV-2 jumped to humans and spread across the world, causing the disease COVID -19. We are now the main hosts of the virus. Thank you, wild animals!

But infection is a two-way street. Yes, pathogens like coronaviruses can “spread” from animals to humans, but they can also “spread” from humans to animals.


This concept has been the subject of a recent review published in the journal Ecology Letters. With all the attention zoonoses have received in recent times, an international team of experts in ecology, microbiology and infectious diseases sought to shed light on when and how humans transmit diseases to animals, a process called reverse zoonosis or spill.

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By reviewing the scientific literature, they found 97 documented cases. About half of them have occurred in captive environments like zoos, where humans have regular contact with wild animals and are much more likely to notice their illnesses. Sixty percent of the total cases were in non-human primates like chimpanzees and gorillas, which are closely related to us and therefore more susceptible. Elephants, long-lived birds, hoofed animals and rodents were some of the other creatures affected.

Obviously, the 97 cases were limited to those that had been scientifically recorded, so they are definitely an undercount of all human-to-animal disease transmission. For example, five snow leopards died of COVID-19 in US zoos, but their spillover cases were reported in the popular press, not the scientific literature, and therefore did not count researchers. All of these big cats probably contracted the disease from their keepers.

Additionally, a significant amount of fallout could occur in nature when humans interact with animals, but we may simply not notice it, perhaps due to lack of oversight or because human-to-human infections cause rarely noticeable effects. Squirrels, for example, are known to transmit many viruses to humans, but there is no mention in the scientific literature of humans transmitting pathogens to them. Given how often we interact with these ubiquitous rodents, we’ve surely infected them with something.

The spreading disease could sicken or kill animals or possibly return to reinfect humans. (Source: Fagre et al., Letters of Ecology, 2022)

Overflow to overflow to overflow again

When we transmit our diseases to animals, it is often through one of three routes: wildlife spends time near our copious, concentrated sewage and runoff; the fauna picking up all the parasites living on us or in us; or wildlife breathing one of our respiratory pathogens.

What happens after that varies. A rollback could trigger mass mortality, putting the animal population at risk. Alternatively, the animal could become a reservoir for the pathogen, where the pathogen could potentially mutate and possibly even spread to humans. We have already transmitted SARS-CoV-2 to populations of mink in Spain and to white-tailed deer in North America. Scientists fear that at some point these animals will infect us with a new form of virus that evades our hard-earned immunity.

As the COVID-19 pandemic has made clear, zoonotic disease surveillance and research is extremely important, especially as the Earth becomes increasingly populated and its inhabitants more connected. “Continued increases in human population density, epidemic and pandemic risk, human-wildlife contact, and anthropogenic stressors on animal health are expected to increase the rate of transmission of pathogens from humans to animals. wildlife over the next few decades,” the researchers warn.


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