A new study published on the preprint server bioRxiv* in May 2020 reports the development of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in farmed mink in two locations in the Netherlands.
Even as the new COVID-19 pandemic continues to spread to new parts of the world, causing respiratory illness, which can be life-threatening in a significant percentage of cases, it is becoming apparent that it can also infect several animals in experimental conditions.
For instance, Rhesus and cynomolgus macaques, ferrets, and cats are known to allow viral replication in the cells of the respiratory tract, and some species develop mild to moderate disease. Domestic pets have also been known to develop infection transmitted from their human owners as well. Strikingly, several tigers in the Bronx were also found to develop respiratory symptoms after contracting the infection.
Study: SARS-CoV2 infection in farmed mink, Netherlands, April 2020. Image Credit: Gallinago_media / Shutterstock
The Mink Story Begins
In April 2020, two mink farms close to each other were reported to be harboring infected minks showing respiratory symptoms like runny noses. A few of the animals showed severe breathing trouble. The mortality between the date of reporting and April 30, 2020, was about two- to four-fold the expected mortality of 0.6%. The animals were found throughout the farms, which at this time consisted mainly of females carrying litters.
Lung tissue from three recent mink corpses was subjected to PCR analysis, with one sample per farm being sequenced as well. In the next week, 36 fresh corpses were necropsied, and PCR was done on nasopharyngeal and rectal swabs of each animal.
Pneumonia and Viral RNA Confirmed
The results at necropsy showed that 28/36 animals had interstitial pneumonia, while other organs appeared grossly normal. Mink without pneumonia showed changes that could be attributable to other diseases such as obstructed labor, septicemia, or chronic Aleutian disease.
Seven lungs were microscopically analyzed, confirming the diagnosis, but bacterial cultures were negative in all cases.
PCR of throat, lung, nasal, and rectal swabs from all seven animals for which microscopic tissue analysis had been done were positive for viral RNA. One liver and three intestine specimens were also positive, but no spleens.
Throat swabs from all necropsied animals were positive for viral RNA, and 34 of the 36 rectal swabs. Viral loads were higher in throat vs. rectal swabs.
The viral sequences from the index animals as well as another 9 animals from the second batch showed that the SARS-CoV-2 was probably introduced separately into the two farms. They vary by 9 and 15 nucleotide mutations, respectively, from the Wuhan reference genome. Sequences from each farm form one cluster, but the two index sequences, one from each farm, differ at 22 nucleotide sites. The deletion of 3 nucleotides at one specific location caused the amino acid, aspartic acid to be deleted at one site.
The Link to Human Infection
The public health services investigated for the source of the animal outbreak among the farmworkers. They found that one worker on one farm had symptoms of the disease since the start of April but had not been investigated further. On the second farm, one worker had been diagnosed and hospitalized with COVID-19 at the end of March. No viral RNA could be recovered from either worker.
Dust sampling in the air between the mink houses, at three different locations, showed that viral RNA was present in 4/9 samples.
The study reveals that human COVID-19 symptoms preceded mink symptoms, the viral sequences closely resemble those of the virus in the one confirmed human case and other human sequences, and the virus was probably introduced separately to each farm.
The Routes of COVID-19 Transmission in Mink
The mink probably spread the infection to each other once it was introduced by the human farmworkers. Mink cages are constructed separately, with non-porous cage partitions. Direct contact between mink is, therefore, not possible.
Indirect spread may have occurred through particles of feed or bedding which were provided by the workers, by infectious aerosols from other animals, or by fecal contamination of the bedding, which spread as dust. The presence of viral RNA in the dust, which was airborne and, therefore, subject to inhalation, shows that the infectious particles could spread via dust or droplets as well, as well as offering an occupational risk for other workers on the farm.
This is the first report on the occurrence of human COVID-19 infection in farmed mink. The conclusions are: “Mink are susceptible to infection with SARS-CoV-2 virus, may develop the respiratory disease with typical pathological findings of viral pneumonia, and can transmit the virus amongst each other.” The animals also present a potential source of infection to farmworkers, indicating the need for biosecurity measures to keep the infection isolated within the farms.
bioRxiv 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.