Reverse zoonosis: What is the actual risk?

The SARS-CoV-2 pandemic has highlighted the zoonotic risk associated with the transmission of pathogens from animals to humans. But the reverse—the transmission of pathogens from humans to animals—is also possible. What impact does this “spillback” actually have on wildlife conservation and public health? Researchers studied the consequences of 97 reverse zoonotic events—or zooanthroponoses—to try to determine the extent of the threat.   Global industrialization is characterized by a rise in multihost pathogens. Driven by deforestation, land conversion, and climate change, these pathogens are spreading from animals to human populations at an increasing rate, posing a significant threat to public health. More recently, concerns have arisen regarding the reverse transmission of pathogens from humans to wild animals, which could pose problems for wildlife conservation and human health. These potential risks have notably led to far-reaching policy decisions, such as widespread moratoriums on bat research in the context of the SARS-CoV-2 pandemic. However, the overall frequency of reverse zoonosis events and their underlying factors remain poorly understood. There is documented evidence of the impact of human pathogens on wild animal populations, but few critical analyses regarding the scale of the phenomenon. This lack of knowledge limits our understanding of the actual and potential impacts on wildlife conservation and public health, as well as on zoonotic transmissions from animals to humans. These gaps leave three questions unanswered. Fundamentally, it is unclear to what extent spillback (human-to-animal) and spillover (animal-to-human) are symmetrical and comparable processes. There are also uncertainties regarding the pathogens involved in spillover and the host animals, making it difficult to assess the impact of the rise in human infections. Finally, the actual threat these pathogenic organisms pose to biodiversity remains to be assessed, particularly in comparison to other anthropogenic pressures, such as habitat destruction and urbanization. These unknowns make it particularly difficult to estimate the consequences of the preventive measures that have been implemented. Researchers therefore sought to conduct a concrete assessment of the outcomes of 97 past reverse zoonoses involving a wide range of pathogens. Overall, relatively few spillbacks resulted in significant morbidity or mortality. Furthermore, few of these events led to the establishment of a human pathogen in a new animal reservoir or to secondary spillover with subsequent transmission back to humans. The study’s findings therefore suggest that the impacts of reverse zoonosis transmission from humans to animals represent an apparently minor threat to wildlife conservation and public health, particularly when compared to other anthropogenic stressors such as climate change or habitat destruction. It should be noted, however, that most of the animals included in the study were nonhuman primates or large animals living in captivity. In conclusion, the authors propose a list of experimental and observational evidence that will help better assess the risk of future reverse zoonoses—a significant tool given current outbreaks.