by When the World Health Organization declared COVID-19 a pandemic on March 11, 2020, humans were the only species with reported cases of the disease. Although early genetic analyzes pointed to horseshoe bats as the evolutionary host of SARS-CoV-2, the virus that causes COVID-19, no reports had yet emerged suggesting that it could be transmitted from humans to animal species another.
Less than two weeks later, in a report from Belgium it was the first infection of a domestic cat – probably by its owner. In the summer of 2020 there were reports of outbreaks of COVID-19 and subsequent killings on mink farms across Europe and there were fears of similar killing calls in North America. People and other animals on and around mink farms have tested positive, raising questions about the potential for a secondary wildlife reservoir of COVID-19. That is, the virus could infect and establish a transmission cycle in a species other than the one from which it originated.
Researchers have documented this phenomenon of human-to-animal transmission, known as back-shedding or reverse zoonotic transmission, in both domestic and wild animals. Wildlife can be infected directly from humans or indirectly from domestic animals infected by humans. This cascading effect provides new opportunities for pathogens to evolve and can fundamentally change how they spread, as seen with influenza and tuberculosis.
For example, backwash has long been a threat to endangered great apes, even among populations with little human contact. The chimpanzees of Gombe National Park, made famous by Jane Goodall’s work, have suffered outbreaks of measles and other respiratory diseases likely caused by the environmental persistence of pathogens spread by people living nearby or at ecotourists.
We are researchers who study the mechanisms that drive cross-species disease transmission and how disease affects both wildlife conservation and humans. Emerging outbreaks have highlighted the importance of understanding how threats to wildlife health shape the emergence and spread of zoonotic pathogens. Our research suggests that looking at historical outbreaks can help predict and prevent the next pandemic.
Spills have happened before
Our research group wanted to assess how often backspills were reported in the years before the COVID-19 pandemic. Retrospective analysis not only allows us to identify trends or specific barriers to reporting spill events but also helps us understand where new threats are most likely to emerge.
We examined historical spill events involving different groups of pathogens across the animal kingdom, accounting for variations in geography, methods and sample sizes. We synthesized scientific reports going back nearly a century before the COVID-19 pandemic – from the 1920s to 2019 – covering diseases ranging from salmonella and intestinal parasites to human tuberculosis, influenza and polio.
We were also interested in determining whether detection and reporting bias could influence what is known about human-to-animal pathogen transmission. Charismatic megafauna – often defined as larger mammals such as pandas, gorillas, elephants and whales that evoke emotions in humans – tend to be overrepresented in wildlife epidemiology and conservation efforts. They receive more public attention and funding than smaller and less visible species.
Complicating this are difficulties in monitoring wild populations of small animals, as they rapidly decompose and are often scavenged by larger animals. This greatly reduces the amount of time researchers can investigate outbreaks and collect samples.
The results of our historical analysis support our suspicion that most reports described outbreaks in large charismatic megafauna. Many of them were captive, for example in zoos or rehabilitation centers, or semi-captive, like the great apes studied.
Despite the litany of papers published on new pathogens discovered in bats and rodents, few studies have examined pathogens transmitted from humans to these animals. However, small mammals with different ecological niches, including animals that live near human dwellings – such as mice, rats and gerbils – are more likely to share their pathogens with humans but become infected by human pathogens too.
COVID-19 and pandemic influenza
In our historical analysis of shedding prior to the COVID-19 pandemic, the only evidence we found to support the establishment of a human pathogen in a wildlife population were two reports from 2019 that described H1N1 infection in striped skunks. Like coronaviruses, influenza A viruses such as H1N1 are adept at transmigrating hosts and can infect a wide range of species.
Unlike coronaviruses, however, migratory waterfowl such as ducks and geese facilitate their widespread transmission. Exactly how these skunks became infected with H1N1 and for how long is unclear.
Shortly after we completed the analysis for our study, reports describing widespread COVID-19 infection in red-tailed deer throughout North America began to emerge in November 2021. In some areas, the prevalence of the infected as high as 80% despite little evidence of illness. in the deer.
This ubiquitous mammal has become a secondary reservoir of COVID-19 in North America. In addition, genetic evidence suggests that SARS-CoV-2 mutates three times faster in red-tailed deer than in humans, potentially increasing the risk of new strains being introduced into humans and other animals. . There is already evidence of deer-to-human transmission of an unprecedented variant of COVID-19.
There are over 30 million white-tailed deer in North America, many in agricultural and suburban areas. Surveillance efforts to monitor viral evolution in red-tailed deer can help identify emerging variants and further transmission from deer populations to humans or domestic animals.
Investigations of related species have shown that spill risk varies. For example, white-tailed deer and mule deer are susceptible to COVID-19 in the laboratory, while elk are not.
H5N1 and the US dairy herd
As of 2022, the spread of H5N1 has affected a wide range of bird and mammal species across the globe – foxes, skunks, raccoons, opossums, polar bears, coyotes and seals, to name but a few. Some of these populations are endangered or threatened, and aggressive surveillance efforts are underway to monitor viral spread.
Earlier this year, the US Department of Agriculture reported the presence of H5N1 in the milk of dairy cows. Genetic analyzes show that the virus was introduced into cows as early as December 2023, likely in the Texas Panhandle. Since then, it has affected 178 livestock herds in 13 states as of August 2024.
How the virus entered dairy cow populations is still uncertain, but it is likely that migrating waterfowl were infected with the virus. Efforts are underway to explain exactly how the virus moves between and among herds, although it appears to be caused by contaminated milking equipment rather than aerosol transmission.
Given the ability of influenza A viruses such as avian influenza to infect a wide range of species, it is critical that surveillance efforts focus not only on dairy cows but on animals living on or around farms. Monitoring high-risk areas for cross-species transmission, where livestock, wildlife and humans interact, provides information not only about how widespread a disease is in a particular population – in this case, dairy cows – but also allows identify susceptible researchers. species that come into contact with them.
So far, H5N1 has been detected in several animals found dead on affected dairy farms, including cats, birds and raccoons. Since August 2024, four people in close contact with infected dairy cows have tested positive, one of whom developed respiratory symptoms. Other species of wildlife and domestic animals remain at risk. Similar surveillance efforts are underway to monitor H5N1 transmission from poultry to humans.
People are only one part of the network
The language often used to describe cross-species transmission does not include its complexities and nuances. Given the number of species infected with COVID-19 during the pandemic, many scientists have called for limiting the use of the terms spillover and spillover because they describe the transmission of pathogens to and from humans. This suggests that disease and its implications begin and end with humans.
Considering humans as one node in a large network of transmission possibilities can help researchers more effectively monitor COVID-19, H5N1 and other emerging zoonoses. This includes systems thinking approaches such as One Health or Planetary Health that capture human interdependence with the health of the entire environment.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by Anna Fagre. Colorado State University and Sadie Jane Ryan, University of Florida
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Anna Fagre receives funding from the National Science Foundation (NSF BII 2213854) and the National Institutes of Health (K01OD037645).
Sadie Jane Ryan receives funding from the National Science Foundation (NSF BII 2213854; DBI 2016265; CCE 2200140).
