A monitoring program picked up a single case and no indications of wider spread.

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Did anyone have “mink farms” on their 2020 catastrophe bingo cards? It turns out that the SARS-CoV-2 virus readily spreads to mink, leading to outbreaks on mink farms in Europe and the United States. Denmark responded by culling its entire mink population, which naturally went wrong as mink bodies began resurfacing from their mass graves, forcing the country to rebury them. Because 2020 didn’t seem apocalyptic enough.
More seriously, health authorities are carefully monitoring things like mink farms because the spread of the virus to our domesticated animals raises two risks. One is that the virus will be under different evolutionary selection in these animals, producing mutant strains that then pose different risks if they transfer back to humans. So far, fortunately, that seems not to be happening. The second risk is that these animals will provide a reservoir from which the virus can spread back to humans, circumventing pandemic control focused on human interactions.
Heightening those worries, mid-December saw a report that the US Department of Agriculture had found a wild mink near a mink farm that had picked up the virus, presumably from its domesticated peers. Fortunately, so far at least, the transfer to wild populations seems very limited.
Down on the farm
The initial report surfaced on an email list that tracks disease outbreaks (the site played a key role in alerting the world to COVID-19). “The USDA National Veterinary Services Laboratories has confirmed SARS-CoV-2 by real-time RT-PCR and sequencing of a nasal swab collected from a free-ranging, wild mink sampled in Utah,” the report states. “To our knowledge, this is the first free-ranging, native wild animal confirmed with SARS-CoV-2.” (Spare a moment of sympathy for the researchers who are giving nasal swabs to wild predators.)
The report went on to say that the USDA was involved in a program that sampled wildlife around mink farms with SARS-CoV-2 outbreaks in three different states during the late summer and early autumn. It concludes that there’s no evidence that the virus is circulating in the broader wild mink population.
We asked the USDA’s Gail Keirn about its monitoring program, and she indicated that the program has since expanded to a fourth state: Utah, Michigan, Wisconsin, and Oregon are all included, and these account for all the sites where outbreaks at mink farms have occurred. The program is focusing on “mesocarnivores,” or animals with a diet that consists of at least 50 percent the meat of other vertebrates. Overall, 150 free-ranging animals near infected mink farms were tested for SARS-CoV-2. So far, only this lone mink has tested positive.
Risk of spread
According to Keirn, the four states where infected farms exist are all within the normal range of mink (which includes most of the country except the Southwest). But she also said that we don’t have good data on the population levels within that range. Depending on the quality of the habitat, researchers have found that individual mink may range within a territory from a quarter of a square kilometer to about 1.5 square kilometers. So, most mink probably won’t travel far enough to spread the virus themselves.
And there’s reason to think that they won’t spread it to other mink frequently, as mink are normally solitary, Keirn said. Thus, unless they have an extended window of infectivity compared to humans, they’re not likely to spend much time near other mink while infectious.
Given this information, there’s currently low concern about the virus being widely circulating in wild populations. But the finding reinforces the need for further monitoring. Keirn told Ars that monitoring was taking place under the umbrella of the CDC’s One Health program, and it involved the USDA, CDC, and US Geological Survey, along with state departments of agriculture, health, and natural resources. Mink monitoring has certainly not received the attention that human testing has, but it’s providing important information about how to protect humans.