Between September and January this year, mink on three Polish farms tested positive for the pandemic coronavirus, SARS-CoV-2, presenting a disturbing mystery as to how the animals became infected.
SARS-CoV-2 infections in mink are not particularly noteworthy or concerning on their own; it is well established that mink are susceptible to the virus. Awareness at the start of the pandemic led to large culls in Denmark and the Netherlands in 2020 and led to intensive monitoring and regulation of the remaining mink herds in many places, including Poland.
But the recent cases in Polish mink, reported this week in the journal Eurosurveillance, are unusual. While previous mink outbreaks have been linked to infected farmworkers and local circulation of the virus – indicating human-to-mink spread – none of the farmworkers or families on recently affected farms have tested positive for the virus. In fact, health investigators found that the infected mink carried a strain of SARS-CoV-2 that has not been seen in humans in the area for over two years (B.1.1. 307).
The finding suggests that humans were not responsible for infecting the mink, at least not directly. Rather, it suggests that another unknown species may have harbored and propagated the otherwise defunct strain stealthily for some time and managed to transport it to mink farms.
The suggestion raises more concerns about viral ‘rollback’. The term relates to the most recognized “spillover”, when a virus jumps from a host population – a reservoir – to a new population, such as humans. SARS-CoV-2 is believed to have originated in a horseshoe bat reservoir before reaching humans. Since then, it has become clear that it can also infect a wide range of animals, including rodents, cats, dogs, white-tailed deer, non-human primates, as well as ferrets and mink. Researchers fear the virus could spread to an animal population that could become a new reservoir from which the virus could periodically return to humans.
This fear drew attention when the omicron variant suddenly appeared with its surprisingly broad suite of genetic changes. Some researchers have speculated that the omicron may have cryptically spread and evolved in mice before reinfecting humans. Other researchers, however, hypothesize that the variant evolved in an immunocompromised person.
Cryptic cases
Farmed mink in Poland again highlights the risk of flashbacks by suggesting an unknown reservoir of SARS-CoV-2 in wild animals. In one investigation, researchers from the National Veterinary Research Institute and the Erasmus University Medical Center looked at cases at three farms within 8 km (about 5 miles) of each other. The first farm reported two infected mink (out of 15 tested and approximately 8,650 animals in total) on September 19, but they later tested negative and were skinned as expected. On November 16, a second farm of 4,000 mink reported six infected animals out of 15 tested, and they were carefully skinned. The third farm, with 1,100 mink, found 15 infected animals out of 15 tested on January 18, but they later tested negative in two rounds of testing in 50 days. All infected animals from the three farms were asymptomatic.
The researchers obtained eight complete genome sequences, four each from the second and third farms; there was not enough genetic material in the samples from the first farm. Genome sequences showed that they were nearly identical and most closely matched the B.1.1.307 lineage, which had not been seen in humans in Poland for more than two years. The viruses also had 40 small genetic mutations, some of which have previously been associated with circulation in mink, and could have been acquired quickly. None of the farm families or workers tested positive for SARS-CoV-2 at any of the three farms.
Researchers noted that all three farms had concrete fences 1.8 meters (6 feet) high and about 30 to 40 cm (about a foot) deep. There was no evidence of animals burrowing under the fences, but researchers noted overhanging tree branches that could have created a route for wild animals. Interviews with owners and staff revealed that the farms were occasionally visited by wild martens, weasel-like carnivores. And there were also wild cats around. The researchers tested feral cat droppings around the farms, but found that they were negative for SARS-CoV-2.
The researchers concluded that a wild animal – possibly martens, feral cats or even escaped mink – could have cryptically spread the SARS-CoV-2 lineage and introduced it to the three nearby farms in separate occasions. They called for more monitoring, not only in mink farms, but also in wild animal populations, such as martens, skunks and foxes.
“Animals from SARS-CoV-2 positive mink farms showed no signs of disease, which creates the possibility of independent viral evolution and may be a source of future outbreaks with new strains.” they wrote.