Mink farming and SARS-CoV-2: staying vigilant at the human-animal interface
[Last Updated: February 2, 2021]
SARS-CoV-2 and mink farming
SARS-CoV-2, the virus that causes COVID-19, emerged in the human population from an animal reservoir (believed to be bats) in early 2019 and rapidly spread across the globe. Since then, the virus has infected nearly 100 million people and has shown the capacity to transmit from humans to other species with bilateral transmission possible between people and mink.
Mink are farmed around the world for the production of pelts that are used for garments, and oil that is used in medical products, cosmetics, and to preserve leather. Mink carcasses are used for fishing bait and fertilizer. In 2018, Canada had 98 mink farms in 8 provinces producing approximately 44 million CAD worth of pelts. Comparatively, Denmark, as one of the largest producers in the world has 1,500 farms that produce 1.1 billion Euros worth of pelts.
Why are farmed mink and SARS-CoV-2 in the headlines?
In April 2020, SARS-CoV-2 was identified in farmed mink in the Netherlands and has since been found on farms in a growing number of countries including Canada. On these farms, the virus has shown the capacity to move from humans to mink, rapidly spread among the mink and, in a few instances, move back to humans.
Transmission of the virus to mink poses three interrelated public health concerns. First, farmed mink present a set of evolutionary pressures distinct from those posed by humans, such as different immune defence mechanisms and housing in very large, dense groups. These different evolutionary pressures are significant because they may select for novel genetic mutations in the virus that have public health significance. Mutations that occur in mink are important because they may affect the characteristics of SARS-CoV-2, which could have implications for the transmissibility of the virus among people, disease outcomes, and/or the effectiveness of vaccines, therapeutics, or diagnostics.
The second concern is that farmed mink could become a reservoir of the virus and enable an ongoing risk of mutation and subsequent transmission back to humans. Finally, farmed mink could transmit SARS-CoV-2 to wild mink and/or other animals, which could become reservoirs outside of direct human control.
What do we know about the public health risks of SARS-CoV-2 in farmed mink?
Novel genetic mutations of public health significance
There is currently a high level of uncertainty about the magnitude of the risks posed by SARS-CoV-2 in farmed mink populations. However, mutations that were previously rare or non-existent in people have arisen on mink farms and spilled back into the community in several instances, indicating that circulation among mink does increase the risk of developing novel genetic variants of the virus. The risk posed by mutated strains was highlighted when one mink variant, known as cluster 5, was identified as a variant of public health significance because preliminary work suggested it had decreased sensitivity to the antibodies that neutralize the viral variants common in humans. Yet, epidemiological evidence indicates that mink variants in people are not noticeably different than human variants with respect to transmissibility, characteristics, and severity of disease.
At this time, it is unknown whether existing mink variants have any direct impacts on vaccines, therapeutics, or diagnostic tests. Current surveillance has not detected the cluster 5 variant since September 2020, though Denmark has reported increased prevalence of another mink-associated variant. However, whole genome sequencing is required to detect many variants and different jurisdictions have different capacities for use of this specialized and expensive tool.
Establishment of a long-term reservoir in farmed mink
Regardless of any immediate hazards posed by existing mink variants, the long-term risk of establishing a new non-human reservoir of the virus at the human-animal interface is unclear but important to understand. When the virus is first introduced into farmed mink, likely through human interactions with the animals, it transmits rapidly among individual mink and infects a large proportion of the population. This is facilitated by high susceptibility of mink to the virus, and by farming practices that promote close contact between mink. For example, individual mink are housed in wire mesh cages spaced centimeters apart and sometimes separated by a plastic/wood divider. There are typically thousands to tens of thousands of individuals on a single farm.
The exact mode of transmission of the virus between mink is unknown, but is likely via infectious droplets and dust or contaminated substances such as bedding and feed. Some experts have indicated that this rapid spread may enable herd immunity on mink farms and recent evidence suggests that herds may clear the virus relatively quickly. However, the duration of immunity and whether the herd can be reinfected by the same or another strain of the virus remains unknown. Despite these considerations, the rapid transmission through thousands of animals gives the virus many opportunities to mutate.
Transmission from farmed mink to wild animals
One of the most important considerations in the longer-term risk of SARS-CoV-2 on mink farms is the potential for transmission to wild mink and other animals, leading to the establishment of reservoirs outside of direct human control. Transmission to wild animals may occur through a number of different routes. First, farmed mink are housed in open-sided sheds that may allow for the entry of wild animals, where direct or indirect contact with mink may occur. Second, the feces and carcasses of farmed mink are often collected and stored in compost heaps prior to being spread as manure, which may enable exposure to wild animals. Finally, mink may escape from the farms into the wild.
While farms may have protections such as fences, traps, and dogs to prevent contact between farmed and wild animals, these activities may have varying levels of effectiveness (see ‘Protection of wild animals’ below). Indeed, the virus has been identified in feral cats on mink farms in the Netherlands and it was recently detected in a wild mink during environmental monitoring on farms in Utah. Together, these cases suggest that transmission from farmed mink to wild animals is occurring. However, the ability of the virus to become established in wild populations depends on many factors, including the behavior of the species (solitary vs. social), survivability of the infected host, and the transmissibility of the virus between animals.
Magnitude of these risks
Overall, the magnitude of the risks posed by SARS-CoV-2 in farmed mink populations are not fully understood. Several ‘rapid risk assessments’ have attempted to quantify each of these risks as ‘low’, ‘medium’, or ‘high’ to inform immediate decision making and guidance for public health officials and farmers. However, each assessment has highlighted the large degree of uncertainty due to the very limited available evidence. For instance, the Canadian assessment indicated that the probability of wildlife exposure to SARS-CoV-2 via farmed mink was “most likely low”, but acknowledged that the risk estimate ranged from negligible to high due to the uncertainty. Such guidelines may be helpful for immediate decision making and allocation of limited resources but more information is needed to better assess future risk.
What can be done about SARS-CoV-2 in farmed mink?
Prevention and mitigation measures in farmed mink
A number of options are available to prevent, manage, and eliminate the risks associated with SARS-CoV-2 on mink farms. Given the rapid spread of the virus that can occur on mink farms and the associated risk of mutations, prevention is critical. Recommendations for prevention largely focus on protection of animals from infected persons, reinforcement of public health measures among staff, ongoing surveillance of workers, mink, and exported products, and regular cleaning and disinfection of surfaces and equipment. Protecting animals from infected persons requires equipment, training, restriction to essential workers only, no movement of workers between farms, active monitoring of workers for infection, and onsite protocols (Note: Danish website. Requires translation.), including PPE and hygiene guidelines, to decrease the risk of transmission. Farms may also consider re-examining farming practices which promote close contact between individual mink and between mink and people. Surveillance of mink and their products relies on syndromic monitoring and submission of dead animals/products for testing. Symptomatic surveillance presents a challenge because infection in mink may produce few to no symptoms, underscoring the importance of preventative measures and enhanced surveillance that includes routine testing of live/dead animals, products, and the environment. There are also several SARS-CoV-2 vaccines for mink currently in development.
If the virus enters the mink population, management options focus strongly on heightened measures to isolate farms and close potential routes of viral exit. Management also requires active monitoring of workers, mink, products, and the environment. These efforts largely focus on containment rather than elimination because the generally low mortality rate of SARS-CoV-2 among mink (1-10%) has relatively low economic consequences for farmers.
De-population of mink farms is the most extreme option. While it can potentially eliminate risks if done with proper precautions, such as protective equipment for workers and safe carcass disposal, culling introduces other risks. For example, culling results in a naïve population of mink when animals are reintroduced and it could result in reduced cooperation of farmers in surveillance programs. Massive culls have occurred in several countries and both Denmark and the Netherlands decided to kill all farmed mink. In the Netherlands, this decision advanced a law to ban all mink farming by 2024. Other countries, such as Canada and the USA, have focused on prevention and containment and have not recommended culling. In Canada, mink farms tend to be small, isolated, located in rural locations, and have few staff, potentially presenting a lower risk for mink variants to spread into the community.
Protection of wild animals
In terms of the transmission from farmed mink to wild animals, prevention is the only functional option. Wildlife cannot effectively or morally be culled and several studies have shown that culling wildlife to control zoonotic diseases may paradoxically increase those very risks. For example, Lee et al. 2018 found that rat pest control activities were linked with a significantly increased prevalence of Leptospira interrogans among surviving rats, potentially due to a disruption of social interactions associated with transmission of the bacteria. Similar impacts of culling have been observed for Sin Nombre virus in mice, bovine tuberculosis in badgers, and Marburg virus in bats. Given these challenges, educating the public about avoiding contact with wildfire may be one of the only feasible management measures available. Once there is evidence of SARS-CoV-2 circulation in wild populations, it is too late to take any decisive mitigation action. Therefore, efforts to prevent the entrance and exit of wild animals on mink farms are essential.
While many farms have wildlife exclusion fences and trapping, farming regulations vary and such measures may be ineffective against some species, such as rodents. Exclusion measures must be reassessed in consultation with wildlife specialists and pest control professionals.
Despite recognition of the potential risks and prevention measures taken to date, SARS-CoV-2 has continued to enter mink farms around the world and has been identified in wild animals around infected mink farms. It is not yet clear whether farms have been infected because prevention measures were ineffective or because the measures were not adequately implemented. However, in light of the potential risks associated with viral transmission to farmed mink, it is clear that strong efforts to administer, evaluate, and enforce measures to prevent the movement of SARS-CoV-2 onto and off of mink farms are paramount. Given the global impacts of the current pandemic and the costs of ongoing control efforts, an ounce of prevention is clearly worth a thousand pounds of mitigation.
Michael Lee is an environmental epidemiologist at the British Columbia Centre for Disease Control and is a PhD candidate in the School of Population and Public Health at the University of British Columbia.