Friday, March 26, 2021

CDC: Investigating Possible Mink-To-Human Transmission Of SARS-CoV-2 In The United States


Although humans are the primary carrier of SARS-CoV-2, we are pretty certain it originated in bats, and we've seen it jump from humans to companion animals (dogs & cats) and to farmed mink, and laboratory experiments have demonstrated other species are susceptible to infection as well. 

Four months ago - weeks before news of the B.1.1.7 `UK' variant emerged - our attentions were focused on a mink variant that had successfully jumped back into Denmark's human population (see Denmark SSI: Increased Mink Variant COVID In Human Population - COVID Risk Assessment).

Between having a high susceptibility to the virus, and being housed in high density mink farms, mink-to-mink transmission of SARS-COV-2 has repeatedly resulted in the creation of mink variant viruses (see Preprint: Recurrent Mutations in SARS-CoV-2 Genomes Isolated from Mink Point to Rapid Host-Adaptation) as illustrated in the serial passage graphic below.


While most of the mink farm outbreaks have been reported from Europe, the United States has reported a handful, including last August (see USDA APHIS Confirms SARS-CoV-2 in Farmed Mink in Utah). The USDA maintains an interactive map (below) showing where SARS-CoV-2 has been detected in animals in the United States.

As of their last report, 16 mink farms in the United States have reported COVID-19. 

Last month, in EID Journal: SARS-CoV-2 Exposure in Escaped Mink, Utah, USA, we looked at a seroprevalence study done on wildlife captured in and around two infected mink farms, looking for signs of spillover. 

While 11 mink escapees tested positive for SARS-CoV-2 antibodies, somewhat reassuringly none of the other tested animals had a detectable antibody response.

Although Denmark's mink-variant problem has receded, due primarily to the introduction a more aggressive B.1.1.7 variant in December, concerns over the creation and emergence of additional variants from non-human mammals remain.  

Yesterday the CDC updated their COVID-19 and Animals webpage for the first time since February 10th. In their previous update, they stated:

Currently in the United States, there is no evidence of SARS-CoV-2 spreading from mink to people, but investigations are ongoing. More information will be shared when it becomes available. 

While most of the information remains unchanged, they substantially increased their section on Mink and SARS-CoV-2, including new data suggesting mink-to-human transmission might have occurred in the United States.

I've reproduced the revised section (including links to new guidance for mink farms) below.  

Mink and SARS-CoV-2

SARS-CoV-2 has been reported in mink on farms in multiple countries, including the United States.
  • In the United States, respiratory disease and increases in mink deaths have been seen on most affected mink farms. However, some infected mink might also appear healthy.
  • Infected workers likely introduced SARS-CoV-2 to mink on the farms, and the virus then began to spread among the mink. Once the virus is introduced on a farm, spread can occur between mink, as well as from mink to other animals on the farm (dogs, cats).
    • One wild mink found near an affected Utah farm was found to be infected with SARS-CoV-2. However, there is no evidence that the virus is currently circulating in free-living wildlife in the United States. 
  • Currently, there is no evidence that mink are playing a significant role in the spread of SARS-CoV-2 to people. However, there is a possibility of mink spreading SARS-CoV-2 to people on mink farms. Mink to human spread of SARS-CoV-2 has been reported in the Netherlands, Denmark, and Poland, and new data suggest it might have occurred in the United States. 
    • Investigations found that mink from a Michigan farm and a small number of people were infected with SARS-CoV-2 that contained unique mink-related mutations (changes in the virus’s genetic material). This suggests mink to human spread might have occurred.
    • The animals on the farm have since tested negative for SARS-CoV-2 twice, and the infected people have since recovered.
    • Finding these mutations in mink on the Michigan farm is not unexpected because they have been seen before in mink from farms in the Netherlands and Denmark and also in people linked to mink farms worldwide.
    • Currently there is limited information available about the genetics of the SARS-CoV-2 virus that has infected people living in the communities near the mink farm. Thus, it is difficult to know with certainty whether the mink-related virus mutations originated in people or in mink on the farm.
    • To confirm the spread of SARS-CoV-2 from mink to people, public health officials would need more information on the epidemiology and genetics of the virus in mink, mink farm workers, and the community around mink farms.
    • These results highlight the importance of routinely studying the genetic material of SARS-CoV-2 in susceptible animal populations like mink, as well as in people.
  • For most people in the United States, the risk of getting COVID-19 from animals is low, but there is a higher risk for people working on mink farms. 
    • Worker safety is critical to protect people and animals on mink farms. Mink farm workers with suspected or confirmed COVID-19 should avoid contact with animals, including mink, and should follow available guidance for farmed mink and other mustelids (animals such as weasels, ferrets, badgers, wolverines, and otters) to avoid introducing SARS-CoV-2 to mink on farms. 
  • CDC is aware of reports of a strain of SARS-CoV-2 virus in mink in Denmark that was also present in the local human population. 
    • This strain, called “Cluster 5,” had not been seen before and was made up of five mutations. The World Health Organization (WHO) reported that as of November 2020, the Cluster 5 variant was no longer circulating in Denmark.
    • Of mink and human samples tested so far in the United States, none have contained all the mutations that make up the Cluster 5 strain.

Mink Guidance

Guidance is available to protect worker and animal health, developed collaboratively by the US Department of Agriculture (USDA), CDC, and state animal and public health partners using a One Health approach:

With several highly competitive COVID variants already spreading across the United States (B.1.1.7, B.1.351 & P.1),the risks from any COVID spillover from mink to humans are likely low.  But it is important that we monitor, and analyse new variants, since we can never know from where the `next contender' will emerge. 

For now, the advice from the CDC on SARS-CoV-2 and animals remains pretty much unchanged. 

What you need to know
  • We do not know the exact source of the current outbreak of coronavirus disease 2019 (COVID-19), but we know that it originally came from an animal, likely a bat.
  • At this time, there is no evidence that animals play a significant role in spreading SARS-CoV-2, the virus that causes COVID-19, to people.
  • Based on the available information to date, the risk of animals spreading COVID-19 to people is considered to be low.
  • More studies are needed to understand if and how different animals could be affected by COVID-19.
  • We are still learning about this virus, but it appears that it can spread from people to animals in some situations, especially during close contact.
  • People with suspected or confirmed COVID-19 should avoid contact with animals, including pets, livestock, and wildlife.