What are the SARS-CoV-2 exposure risks for workers on mink farms?

Within just a year, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) – the causative pathogen of the coronavirus disease 2019 (COVID-19) pandemic – has caused more than 91.23 million recorded infections and over 1.99 million deaths.

Key to this is the rapid transmissibility of SARS-CoV-2, which is thought to have jumped the species barrier – from a Chinese horseshoe bat species, through an unknown intermediate, and then on humans hosts. Many smaller mammals were then reported to be susceptible to infection. Subsequently, large-scale infection was then detected in farmed mink, first in the Netherlands, but then also in the US, Denmark and in other countries across Europe.

A new preprint on the medRxiv* server reports on the risk of COVID-19 among workers on these farms, and in the communities living around such farms. The study was aimed at measuring the levels of the virus in the air on mink farms by portable samplers. The researchers also looked at the outdoor air, in order to understand the dispersion of the virus in the environment. Finally, they examined surfaces and materials in the mink cages, for SARS-CoV-2 contamination.

The researchers found that viral RNA was detected in every farm in at least a third of samples collected over six hours each. These samples represented dust in the air, that could be inhaled. In the second farm, one of two eight-hour samples was positive. The mean concentration in these four samples was 4 x 103 RNA copies/m3 (Ct values 35 to 36).

Settled dust showed the presence of viral RNA in over 80% of samples. All the Electrostatic Dustfall Collectors (EDCs) used showed positive results at all farms, at the first visit, and for the second and third visits, all EDCs at one farm were positive, whereas 73% to 80% were positive at the second, and 64% and 27% at the third, visit, for the other two farms, respectively.

Thus, viral RNA reduced significantly over time at all farms, by a four to fivefold reduction each week. EDCs close to the mink cages recorded viral RNA titers that were on average threefold higher than those placed at a greater distance.

None of the residential air samples were positive over seven days.

At the fourth farm, which was tested earlier in the outbreak, three of six samples collected over six hours each, and both the samples collected over eight hours, were positive, with higher concentrations in personal air samples relative to the stationary air samples. Both personal particulate matter ( PM10) samples, and two of six stationary PM10 samples, were positive, but the concentrations were lower than with inhalable dust samples.

Outdoor samples were contaminated at locations within 1.5 meters from the farm entrance, as well as much farther away. Inhalable dust samples were also contaminated.

Mink cages were almost universally contaminated, as was the bedding, and about half the fecal samples. Drinker cups and some residual food samples were contaminated, at about a third and a tenth, respectively. The fourth farm, NB4, mentioned above, had sevenfold higher viral loads in cage swipe samples, and 50 times higher levels in beddng, relative to the farms tested later. Similarly, mink cages where the animals had recently died were heavily contaminated, as were drinker cup swabs in such cages, vs those housing live animals.

Post-culling samples had much lower viral RNA titers compared to those sampled pre-culling at the fourth farm, but almost 15% and over 20% of cage swipe samples and fecal samples were positive. Bedding was positive in over 55% (top bedding layer) and 85% (bottom bedding layer), but the viral load was 100-fold less in the top layer. The bottom bedding layer showed only a tenfold reduction, however.

The study confirms the presence of heavy contamination with viral RNA, whether in the air, as inhalable dust, or on surfaces, or settling dust. Concentrations of viral RNAs in outdoor air were very low. This finding suggests that workers on these farms are at high risk of infection, while neighboring communities are at negligible risk.

In this scenario, environmental contamination was so heavy and so prevalent as to suggest a prominent role for this factor in the transmission of this virus between minks, as well as between minks and workers. Animal handling, which peaks in the period April to June, is one probable route of exposure for farmworkers, but in other months, when animals are not typically handled, zoonotic transmission is also likely.

Air also contains smaller particles less than 10 micrometers in size as well as larger particles, both of which may be inhaled and may transport the virus to the respiratory tract. Inhalable dust contained high concentrations of viral RNA.

The question is whether airborne SARS-CoV-2 RNA is due to sneezing or coughing, by the infected animal, or through shedding into the environment, which then becomes contaminated. Both probably operate together. The highest occupational exposure was when the outbreak among minks was at its peak, in the acute phase of the outbreak, so that settling dust from this period was found to be contaminated by viral RNA in empty rows even several meters away from the animals.

The last cleaning of the mink cages occurred several months before the outbreak began, making its results independent. Sporadic cleaning was inadequate to contain or prevent the transmission of infection. RNA breakdown is influenced by the local temperature, humidity level, chemicals in the environment, including alkylating agents, and radiation such as ultraviolet radiation. Even two weeks after culling, viral RNA contaminates the environment. Thus, proper cleaning is required, along with precautions against touching or inhaling contaminated objects, surfaces or air.

Our occupational and environmental risk assessment supports earlier reported whole genome sequencing research showing mink-to-human transmission in farm workers but no direct zoonotic transmission events to nearby communities.”

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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