Even as pandemic response is focused on understanding, controlling and preventing COVID-19 among humans, a ghost haunts epidemiological concerns about the disease: reverse-zoonotic sylvatic Covid. Or, the ability of SARS-CoV-2 to spill-back from humans to domestic or farmed animals, and from there to wild-life, where the virus can establish ineradicable disease reservoirs. This scenario is not far-fetched: the case of sylvatic plague in early-twentieth century California offers an example of how such processes can remain undetected until disease reservoirs are so firmly established that the disease itself can no longer be eradicated. The mirror of sylvatic plague reflects a potentially disturbing future should SARS-CoV-2 become firmly established in wildlife populations.
As early as April 2020, coverage of the fact that humans have been infecting farmed mink (Neovison vison) with SARS-CoV-2 emerged, but it was only after it was discovered that the virus had mutated in mink farms in Denmark and spilled-over back into 214 humans, 12 of which were infected a new variant of the virus, that epidemiologists began to be seriously concerned with this new zoonotic cycle of COVID-19.
According to the European Centre for Disease Prevention and Control (ECDC), “as of January 2021, the virus has been detected at 400 mink farms in eight countries in the EU/EEA.” Given that SARS-CoV-2 is assumed to be a zoonotic disease, with bats being a potential natural reservoir and contact with a still-unknown intermediary host leading to its emergence in humans, it is no surprise that this virus is able to infect non-human animals. We have known mink in particular to be susceptible to coronaviruses since the early 1990s. Yet no medical or government body drew on this knowledge to recommend or mandate the closing of mink farms in the early stages of the pandemic. In fact, mass culling of farmed mink only began in Denmark as recently as November 2020, and the Netherlands is the first country to announce the closing of all mink farms, by March 2021.
While the infection of humans from mink has drawn the most media attention, there is another, potentially more disconcerting spill-over process, with long-term, disease-ecological impact of the pandemic: the transmission of the virus from non-human animals to wild fauna. On December 11, 2020, the United States Department of Agriculture (USDA) National Veterinary Services Laboratories (NVSL) announced publicly they had confirmed, for the first time, evidence of SARS-CoV-2 in wild mink sampled in Utah.
Thus far, we have been relatively lucky: the joint investigation of the USDA, Centers for Disease Control (CDC), Utah Division of Wildlife Resources, and the USGS National Wildlife Health Center (NWHC) on “whether the virus has spilled over into other domestic, feral, and wild animals found both on and in proximity to infected [mink] farms” has found no “evidence for SARS-CoV-2 establishment in wildlife”. However, the report warns,
“the discovery of escaped mink with the opportunity to disperse and interact with susceptible wildlife, such as wild mink or deer mice, is concerning. In Utah, mink farms often overlap with designated critical mink habitats. Interactions or shared resources between escaped mink and wild mink or other wildlife species represent potential transmission pathways for spillover of SARS-CoV-2 into wildlife and could lead to health consequences or establishment of new reservoirs in susceptible wildlife.”
As summed-up in a recent Nature article, “years from now, when community spread has been suppressed, a reservoir of SARS-CoV-2 in free-roaming animals could become a recalcitrant source of new flare-ups.”
Early-twentieth century Californian history warns us of what can happen if such risks of disease spreading through non-human animals are neglected. In 1903, California newspapers began reporting on the appearance of a mysterious disease that was decimating ground squirrels – small burrowing rodents considered abhorrent crop pests – across Contra Costa County. The squirrel scourge was swift and deadly, and transformed entire burrow systems that honeycombed the hills into mass graves. Farmers felt their salvation had arrived: finally, they would be rid of these grain and fruit-devourers. Encouraged by county supervisors and advice in the press, some farmers appear to have actively attempted to spread the disease by collecting squirrel corpses and depositing them into burrows.
While squirrels were dying in droves in the Californian countryside, bubonic plague was killing humans and rats alike just 40 miles away in San Francisco. In 1900, plague was first detected in the city, and Chinatown swiftly became the target of a controversial and racially charged sanitary campaign (Risse 2012). However, by 1903, partially thanks to the efforts of Dr Rupert Blue of the US Marine Hospital Service, focus shifted towards “the movement of rats rather than people” (Risse 2012: 255). Medical authorities knew that plague could infect squirrels, and Blue suspected that the squirrel disease might have been plague. Yet comparatively little attention was devoted to it: and in fact, without even knowing its identity, many agriculturalists welcomed the disease as a liberator from the ravenous rodents.
When Public Health Officials finally began to devote serious attention to the squirrel disease, it was already too late. By 1908, efforts to eradicate plague in San Francisco through measures targeting rats had been successful, and no further cases were reported. Despite this urban victory, a new chapter in the history of plague was simultaneously opening in the countryside. Rupert Blue and George McCoy of the US Public Health and Marine Hospital Service procured evidence that Yersinia pestis had infected ground squirrels in Contra Costa County. Subsequent investigations revealed that it had become endemic in squirrel populations across much of the state. This was deeply troubling – the disease had established itself within a complex ecology of plague-susceptible animals in California and would soon spread eastward to do the same in Colorado, Arizona, New Mexico and other states. Ground squirrel burrows were known to sustain large communities of mobile animals: from black and brown rats who allegedly migrated between cities and farms, to burrowing owls who could fly between burrows, picking up the fleas they hosted. Plague, once localised in port cities, now threatened to spread across the entire US.
A collaborative endeavour between government officials and agricultural communities was swiftly inaugurated to drive Californian ground squirrels to near extinction. By December 1911, the San Jose Daily Mercury revealed that a sum of $1M (worth roughly $27.5M in 2021) had been expended on squirrel control, and 10,000 plague-infected animals had been found. Yet despite such efforts, infection had spread too far, and the rodents proved too resilient. With their prolific breeding, complicated burrows, and vast range, extermination of the sylvatic reservoir of plague was completely impossible.
How had this happened? The squirrel epizootic likely drew little initial attention from public health authorities due to a prevailing conception of plague as a disease of crowded cities and social underclasses (Engelmann, Henderson & Lynteris 2019), and to the delight of farmers thrilled to have found such a powerful tool of pest control. These histories seem important in our present times. By subordinating the health of the countryside to the needs of the city and placing the demands of large-scale agriculture ahead of public health, California earned the unenviable reputation of becoming a permanent habitat of Yersinia pestis which occasionally menaces rural communities and visitors to the region to this day. Similar processes of the disease establishing reservoirs in wild rodents since its introduction in the early 1900s have taken place across South America and in Africa, with Madagascar manifesting devastating annual epidemics of plague among humans as a result.
Of course, SARS-CoV-2 is not Yersinia pestis, and COVID-19 is not plague, a disease with hundreds of thousands of years of evolution and well adapted to infecting over 200 animals, most importantly rodents – these great in-betweens of the mammalian kingdom and closest involuntary associates of humans. And yet, raising the alarm among zoonosis watchers such as David Quammen, recent papers pending peer-review seem to suggest rodent susceptibility to SARS-CoV-2, and that deer mice in particular “have the potential to serve as secondary reservoir hosts that could lead to periodic outbreaks of COVID-19 in North America.”
The long-term epidemiological, environmental and socio-economic consequences of SARS-CoV-2 establishing itself in wild and peridomestic animals, including rodents, are presently unknowable but they are not unimaginable. In a recent letter to the BMJ, Stephen Green and Lorenzo Cladi argued that, in the context of the global COVID-19 pandemic and the numerous mutations in the virus, “scientific imaginations need to be exercised like never before to anticipate the next steps in this unfolding human tragedy,” especially as these relate to processes of reverse zoonosis. It is high time that we mobilise not only our scientific but also our historical imagination so as to anticipate and prepare for sylvatic reservoirs of SARS-CoV-2 resulting from mink farming in Europe and North America.
Jules Skotnes-Brown is a historian of science based at the University of St Andrews, and currently working on the Wellcome Trust funded project ‘The Global War Against the Rat and the Epistemic Emergence of Zoonosis’. His research focuses on animals, disease, knowledge production, and colonialism in the nineteenth and twentieth centuries. He has previously published articles on African elephants and national parks, British imperial expositions, and present-day videogames. Twitter: @J_SkotnesBrown
Christos Lynteris is a Senior Lecturer in Social Anthropology at the University of St Andrews. His work examines epistemological and biopolitical aspects of epidemics with a particular focus on zoonotic diseases. His recent publications include the books Ethnographic Plague (Palgrave 2016), Human Extinction and the Pandemic Imaginary (Routlege 2019), and, co-authored with Lukas Engelmann, Sulphuric Utopias: A History of Maritime Sanitation (MIT Press, 2020). He was the PI of the ERC-funded project Visual Representations of the Third Plague Pandemic and is currently the PI of the Wellcome Investigator Award funded project The Global War Against the Rat and the Epistemic Emergence of Zoonosis. Twitter: @visualplague
Acknowledgement: Research leading to this article was funded by the Wellcome Investigator Award project ‘The Global War Against the Rat and the Epistemic Emergence of Zoonosis’ [grant ID 217988/Z/19/Z]
Engelmann, Lukas, John Henderson & Christos Lynteris 2018. Plague and the City. Routledge.
Risse, Guenter B. 2012. Plague, Fear, and Politics in San Francisco’s Chinatown. Baltimore: Johns Hopkins University Press.