Danish Minks farmed for fur have proved to be carriers of COVID-19 that could harm public health. As a result, the animals are being culled.
Two decades ago, a seminal study from the University of Edinburgh compiled a list of all known human infectious diseases. It found a total of 1,415 different human pathogens and claimed that 61% were capable of spreading between humans and animals. Today, with the world put on hold by a deadly disease that seems likely to have spread first from bats to humans, we know the dangerous effects of such pathogens all too well.
The group of diseases that spread from animals to humans are collectively known as zoonoses. The term encompasses diseases such as measles, which first spread from cattle to humans thousands of years ago but now transmits exclusively between people, and Ebola, which periodically passes from bats to humans, where it then spreads from person to person. It can also refer to food-borne diseases caused by bacterias such as salmonella and campylobacter that we only get from the consumption of animal products and almost never pass from person to person.
Very occasionally, diseases pass the other way, from humans to animals. We call these “reverse zoonoses”. Covid-19 is both a zoonosis and a reverse zoonosis: it originally spread from animals to humans, before transmitting from humans back to domesticated animals such as cats. We know the virus multiplies in felines and can spread from cat to cat, although there is no evidence so far of it spreading from cats to people.
We also know that Covid-19 has passed between people and farmed mink. This was first reported in April, in the North Brabant region of the Netherlands, where mink were, until recently, still farmed for their fur. Analysis of the genetic code of the virus from mink suggested that it had passed from infected farmworkers to animals, where it then spread from mink to mink. Scientists also suspect that the virus passed from mink back to one worker.
A more alarming situation has recently emerged in Denmark, where the virus appears to have mutated in farmed mink and re-entered the human population. Thousands of mink are now being culled. The country has the largest mink-farming industry in the world, and so far over 200 people have reportedly been infected with a mink-associated strain of the virus. Twelve of those people were infected with a unique mink-related variant that hasn’t been seen before.
Mutations in viruses are expected during an outbreak. They occur more frequently in those with genomes made from RNA (a large group which includes coronaviruses), compared with those with genomes made from DNA, because RNA replication is more prone to error than DNA replication. In most cases, these mutations are “silent” – they do not change the amino acids that make up the virus’s proteins, so have no effect on any of its properties – but they have a signature that can be used to track the outbreak.
The mutations in Danish mink, however, have led to changes in the sequence of amino acids that make up the virus’s “spike protein”. This is the key that allows the virus to fit into a human’s (or other animal’s) lock. In the virus that causes Covid-19, the lock is a protein called ACE2 which straddles cell membranes. When the spike protein fits ACE2, the virus gains entry to the cell and can begin to replicate. Because the spike protein and the ACE2 receptor are close-fitting, changes to the amino acid sequence of either affect the virus’s ability to enter the cell.
Mink ACE2 is slightly different from human ACE2, so mink-to-mink transmission may have favoured a mutated virus with a change to its spike protein key that better fits the mink ACE2 lock. As the antibodies that humans generate to fight off Covid-19 target the spike protein on the external surface of the virus, this mink-adapted virus, which has a mutation in its spike protein, may be better able to circumvent our defences.
Danish researchers report that one of the variants of the virus found in mink isn’t as easily defeated by the antibodies that humans produce against Covid-19. One risk is that people who have recovered from Covid-19 may have antibodies that are less able to fight off the mink strain of the virus, leaving them open to reinfection. And most of the vaccines under development, on which we are pinning so much hope, are intended to induce antibodies that target the spike protein in the virus that causes Covid-19 in humans. While it’s too early to say for certain, one possibility is that these vaccines could be less effective against the mink virus, because it has a different spike protein.
A close relative of the mink, the ferret, has a similar respiratory tract physiology to humans, and experiences respiratory diseases such as Sars, Covid-19 and influenza much as we do. This is why they can be used in laboratories to study human respiratory diseases. But the fact that ferrets experience these diseases in a similar way to humans raises serious questions about keeping large numbers of mink confined in close quarters. Indeed, these conditions provide the perfect environment for respiratory viruses to spread.
In this age of zoonotic epidemics, with Sars, Mers and now Covid-19 all emerging in the past 20 years, we must think carefully about how we intensively farm mammals that are known hosts of human coronaviruses. Before the pandemic, the Netherlands was already in the process of stopping intensive mink farming. But many dangerous types of animal farming still continue. Consider the palm civet. This animal was implicated in the emergence of Sars in 2003, acting as a probable intermediary between the bats in which the Sars virus originated, and the people whom it later infected.
As an animal that might have helped to trigger a pandemic, are we now keeping the palm civet at a safe distance? Quite the opposite. In parts of Asia the palm civet is farmed intensively, fed coffee cherries, and the beans collected from its faeces are used to make the world’s most expensive coffee, kopi luwak.
Given the huge human, social and economic costs of pandemics, there’s a compelling argument that in order to prevent future catastrophes, we can no longer afford to take such risks.
Original source: https://www.theguardian.com