The term “mink” is usually applied to at least two different animals from the family Mustelidae; the European mink (Mustela lutreola) and the American mink (Neovison vison). The two minks, as in other mustelids, are small carnivores with characteristically elongated bodies. Mustelids contain around 60 known species, including, in addition to mink, otter, ferret, polecat, marten, ermine, badger, sable, and wolverine (Canuti et al., 2020). Several mustelids, including mink, sable, and ermine, have been hunted since prehistoric times for their fur. One of the major economic stimuli for Russian expansion into Siberia and Far East, as well as French and English expansion in North America, was the abundance of furbearers in these territories.

Some mustelids have been domesticated. The ferret (Mustela putorius furo) and the tayra (Eira barbara) are kept as pets. The ferret has been kept in Europe for a long time; the famous Leonardo da Vinci painting of the “Lady with an Ermine” holds in her hands, the albino form of domesticated ferret. As for mink, its aggressive behavior prevents their use as pets, but their fur has led to their breeding on mink farms. In recent centuries, however, fur farming, notably of American mink, has also become widespread, and provides the majority of the fur brought to market.

Mink are semi-aquatic, living along waterways. They are solitary, and rather sedentary.

The European mink was widely distributed over most of continental Europe a century ago (Karath, 2017; Skorupski, 2020). Currently, only a few thousand (an estimated 5,000 individuals) persist in the wild in Spain, France, and the Danube delta. In Russia, sightings have become so rare that the species is considered to be on the brink of extinction (Skorupski, 2020). The European mink now occupies less than 3% of its former habitat. It is currently considered one of the most endangered mammal species in the world (Skorupski, 2020). Reintroductions of populations have been carried out in Estonia, Germany, and Spain (Karath, 2017; Skorupski, 2020). Even if the disappearing habitat and hunting may play a role in this situation, the main cause is the introduction of the invasive American mink (Karath, 2017; Skorupski, 2020).

The American mink is native to North America, where it is found throughout Canada and most of the United States, except in Arizona and the arid parts of California, Nevada, Utah, New Mexico, and western Texas. First imported in Europe by fur farmers for their superior pelt in the 1920s, some animals escaped and thrived in the wild. Thus, escapees have established populations in England, France, Germany, Iceland, Ireland, Norway, Poland, Scotland, and Sweden (Bevanger and Henriksen, 1995; Lariviere, 2021). Bigger, more adaptable, more aggressive, and more fertile, they have simply replaced the native species (Karath, 2017).

The ability of the species to colonize new habitats is excellent, and it is estimated that all of Sweden was invaded in approximately 35 years (Gerell, 1967). Neovison vison was also brought to South America for fur farming in the 1930s, and numerous populations have been recorded in the wild since 1960–1961 (Lariviere, 2021).

The American mink is currently the most important species in fur-farming operations (Tomson, 1987). The main fur producing countries are Denmark, the Netherlands, Poland, and China, where the first mink farms were established in the 1950s (Figure 1). China is the leading market for fur. According to the Fur Commission USA, in 2012, the United States exports of mink pelts to China reached a record high of $215.5 million (https://furcommission.com/u-s-mink-manufacturers-eye-growing-chinese-demand-for-fur/p, Accessed February 03, 2021). At the same time, China doubled its domestic mink production, contributing to a record 80 million pelts produced worldwide.

Currently, the annual production of Chinese mink is over 20 million (20.7 in 2018); the economic benefits are considered significant (Gong et al., 2020). Overall, 95% of fur farms are concentrated in the Northern provinces: Shandong (greatest concentration), Liaoning, Heilongjiang, Jilin, and Henan (Gong et al., 2020). According to the European Centre for Disease Prevention and Control, Europe has ≈2,750 mink farms. There are ≈ 1,200 mink farms in Denmark (Hammer et al., 2021); ≈ 125 in the Netherlands, with an average of 5,000 female breeding animals (Oreshkova et al., 2020); ≈ 900 in Finland; and ≈ 300 in Poland, where data must be adjusted, as numerous mink farms have been closed in the past 4 years. European production was 34.7 million mink pelts in 2018. In the United States, there are ≈ 245 fur farms that produced 3.1 million pelts in 2018. In Canada, 1.76 million mink pelts were produced in 2018 on ≈ 60 farms. In Russia, according to the Russian National Association of Fur Breeders, published in the official journal of the Russian Federal Service for Veterinary and Phytosanitary Supervision, “Veterinaria i zhizn,” 22 enterprises in 14 regions of the country are engaged in breeding mink, with a broodstock of about 300 thousand individuals.

Female mink are bred in March and whelp in May (Tomson, 1987). The young are weaned in July and are then placed in individual cages to prevent fighting and damage to the fur. They have a fur molt in early fall and are killed (pelted) when their pelt is “prime”; that is, when the pigment has migrated from skin follicles into hair shafts. The young that are desired for breeding are held over for the next breeding season. Farm mink are usually fed with a commercially produced wet feed, consisting of a mix of animal by-products and slaughter offal, e.g., by-products from the fishing and meat industries and plant origin, such as corn gluten meal, soybean oil, and extruded cereals (Lyhs et al., 2019).

Hundreds to thousands of cages in close proximity are frequently housed in a small area, in a single shelter or building. Disease problems are those caused by intensive farming practices, marginal nutrition, and poor sanitation. Contagion is facilitated by the proximity of animals and their low genetic diversity (reproduction using a few males selected for their fur).

Several microorganisms can affect mustelids (Canuti et al., 2020). The most studied are those commonly found in mink on fur farms. Aleutian disease, considered as the “most serious” infectious disease affecting farmed mink, is caused by a highly contagious and environmentally resistant amdoparvovirus. The disease is characterized by a chronic wasting syndrome involving disruption of the immune system, with an impact on mortality and reduced mink reproduction (Gong et al., 2020). Distemper is caused by a morbillivirus. It is fatal to unvaccinated mink. Dogs are the common source. However, the role of wildlife reservoirs, such as the fox, has been observed during a major epidemic in mink farms in Denmark. Most mink farms have outer perimeter fences to exclude feral canines from their mink (Trebbien et al., 2014). Mink enteritis is caused by a highly contagious virus closely related to feline panleukopenia virus and canine parvovirus type 2 (Wilson et al., 2015). In outbreaks, mortality is very high, reaching 75% in weaned mink. The most common bacterial diseases include type C botulism and hemorrhagic pneumonia caused by specific strains of Pseudomonas aeruginosa (Wilson et al., 2015). Mink are also often affected by coccidiosis (Tomson, 1987).

Since 2010, new epidemics have been reported in mink farms. In 2011, a new orthoreovirus was reported in mink on a farm in Hebei Province, China. Almost all mink were infected, with an estimated mortality of <5% (Lian et al., 2013). In 2014, an invasive outbreak of swine pseudorabies occurred in mink on a farm in Shandong Province, China, with a mortality rate of 87% (3,522/4,028; Wang et al., 2018). In 2014, Newcastle disease, due to avian paramyxovirus serotype-1, was described in mink on a farm in Heilongjiang province, China, responsible for hemorrhagic encephalitis and pneumonia, with a death rate of 95% in the 9% of affected mink (Zhao et al., 2017). In 2015, epidemics due to an H5N1 avian influenza virus were reported in two mink farms, 200 km apart, in Northeast China. The death rates were 56% (128/230) and 64% (242/376; Jiang et al., 2017).

Interestingly, many emerging infectious diseases reported in mink have high zoonotic potential. It seems that farmed mink are susceptible for the infections of different vertebrate, including birds, pork, and humans.

The ferret, M. putorius furo, is another member of the genus Mustela in the family Mustelidae, and can be considered as the “cousin” of the mink. It is also the domesticated form of the European polecat. Ferrets are usually considered a good animal model for viral respiratory diseases (Belser et al., 2016; Hewitt et al., 2020). Based on several experimental models, ferrets were shown to be highly susceptible to SARS-CoV-2 infection (Blanco-Melo et al., 2020; Hewitt et al., 2020; Kim et al., 2020; Shi et al., 2020). In most models, ferrets are infected with SARS-CoV-2 via the intranasal route to better mimic the natural route of infection in COVID-19 patients (Blanco-Melo et al., 2020; Hewitt et al., 2020; Kim et al., 2020; Shi et al., 2020). The inoculum dose varies from 5 × 10⁵ to 5.5 × 10⁶ plaque-forming units of the virus. Various SARS-CoV-2 strains have been used, including human strains [e.g., NMC-nCoV02 (Kim et al., 2020), CTan-H (Shi et al., 2020) strains, and Victoria/1/2020 SARS-CoV-2 (Ryan et al., 2021)] and environmental strains [e.g., the F13-E strain collected from an environmental sample in the Huanan Seafood Market in Wuhan (Shi et al., 2020)].

Infected ferrets usually develop mild clinical symptoms, including fever 2–8 days post-infection, reduced activity, and occasional cough (Kim et al., 2020). Clinical signs usually disappear spontaneously within 2 weeks of infection; no fatalities have been reported (Kim et al., 2020; Ryan et al., 2021). Viral RNA can be detected in nasal washes from 2 up to 20 days post-infection (Kim et al., 2020; Ryan et al., 2021), with the highest viral load occurring at 4 days [e.g., 3.8 log₁₀ RNA copies/ml (Kim et al., 2020)], but also in blood, saliva, urine, and feces. Less frequently, viral RNA was detected in lungs, kidney, intestine, and fecal samples between 4 and 8 days post-infection (Kim et al., 2020). In contrast, the virus was not detected in the heart, liver, spleen, pancreas, and brain samples from these animals (Shi et al., 2020). About 2–3 weeks following infection, viral RNA was no longer detectable in nasal washes or any organs (Shi et al., 2020; Ryan et al., 2021).

Infectious viruses could be isolated in cell cultures from nasal washes, saliva, trachea, and lungs collected from 2 to 4–6 days post-infection (Kim et al., 2020). Specific immunohistopathological findings have also been reported and are summarized in Table 1 (Kim et al., 2020; Shi et al., 2020; Ryan et al., 2021). Antibodies against SARS-CoV-2 were detected in infected ferrets by ELISA and neutralization assays at 2–3 weeks post-infection (Kim et al., 2020). Serological titers ranged from 32 to 128 at that time (Kim et al., 2020). Finally, infected ferrets can also transmit SARS-CoV-2 to other ferrets, with significant animal-to-animal transmission through direct contact and the aerosol route (Table 2; Kim et al., 2020; Richard et al., 2020; Schlottau et al., 2020).

As early as April 23 and 25, 2020, two closely situated (17 km apart) mink farms in the North Brabant province of the Netherlands, housing 21,200 animals, reported increased mortality in mid-April 2020 (Oreshkova et al., 2020; Oude Munnink et al., 2021). Animal necropsy enabled the detection of SARS-CoV-2 (Molenaar et al., 2020). Post-mortem findings also showed acute interstitial pneumonia in almost all mink examined (Molenaar et al., 2020). Overall, the impact of SARS-CoV-2 infection in mink ranged from asymptomatic to death, with a spectrum similar to humans (Molenaar et al., 2020). Despite strict quarantine measures, subsequent official investigations identified additional infected farms in the same region. In farms, animal-to-animal transmission was facilitated by high animal density. From June 6, mink from infected farms were culled. In early November, Dutch health authorities decided to cull all mink in the country and ban mink farming. In the Netherlands, the minks at non-infected farms were not culled. All animals, including those of the breeding stock, at non-infected farms were pelted, as per normal procedure in November and December. Thus, only animals at the infected farms were culled as of June, as soon as infection was detected. Mink farming in the Netherlands is banned as of January 2021.

After the first Dutch cases in April, Denmark, the largest European mink pelt producer, reported mink farm infections in May 2020 in the Jutland region (Hammer et al., 2021). In mid-June, the Danish government imposed the culling of infected animals in infected farms. On November 4, 2020, due to the emergence of 12 cases of human infections caused by a mink SARS-CoV-2 variant (referred to as “Cluster 5”; Larsen and Paludan, 2020), a culling of all mink was decreed. In the Netherlands and Denmark, COVID-19 cases were diagnosed among farm workers before infections in mink were detected, suggesting that the animals were infected by humans. Human and mink viral strain genome sequences, although slightly different by a few mutations, clustered together (Oude Munnink et al., 2021).

In the United States of America, the outbreaks seem to be well documented and reported to the World Organization for Animal Health. The first cases of infection in farmed mink were reported in Utah on August 17, 2020, in Wisconsin and Michigan on October 8 and 9, 2020 and in Oregon on November 27, 2020, with mortality rates of mink infected by SARS-CoV-2 varying according to the farms (Table 3). To date, in addition to the Netherlands (69 mink farms), Denmark (290 mink farms), and United States (17 mink farms), SARS-CoV-2-infected farms have been reported in France (1 mink farm), Greece (22), Italy (1), Spain (1), Sweden (13), Poland (1), Lithuania (2), and Canada (2; https://www.oie.int/en/, Accessed February 03, 2021). Overall, cases of infections in mink farms have been reported in Europe and North America. No cases of COVID-19 infection in breeding mink have been diagnosed in Russia (https://www.vetandlife.ru/vizh/sobytiya/kak-v-rossii-zashchishchayut-norkovye-fermy-ot-covid-19/?sphrase_id=5987, Accessed February 03, 2021). To the best of our knowledge, no cases have been reported in China.

In Spain and Italy, infected mink farms have been suspected to have played a role in the regional spread of SARS-CoV-2. At the end of June, an outbreak of COVID-19 caused by a variant named 20A-EU1 began in the Aragon region of Spain and then rapidly spread to other European countries because of tourist travel (Hammer et al., 2021). The region where the outbreak started is known to host several mink farms where animal infections were detected. In Italy, the role of mink is also suspected, as the emergence of the D614G mutation occurred in the Lombardy region, where Italian mink farms are located.

Since August 2020, Utah has been battling outbreaks of COVID-19 in mink farms (https://www.kuer.org/health-science-environment/2020-12-15/novel-coronavirus-detected-in-a-wild-mink-near-infected-utah-fur-farm, Accessed February 03, 2021). A state veterinarian said in November that nearly 11,000 mink have died from the disease. In mid-December, a wild mink, found while federal officials were surveying the area around these farms for the virus, tested positive for the SARS-CoV-2. It is believed to be the first confirmed case in a free-ranging native animal. A report on December 13, 2020 from the United States Department of Agriculture suggests the animal acquired its infection from farmed mink. The wild animal harbored a virus that appears identical to what was seen in nearby farmed mink (https://promedmail.org/promed-post/?id=8015608, Accessed February 03, 2021).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.