People have been panicking about Covid-19 in animals since the very start of the pandemic. There is now ample evidence that SARS-CoV-2 – the coronavirus that causes Covid-19 – can pass from humans to other animals. This is known as go back. The virus is capable of infecting a range of species, hamsters To gorillas.
It is reassuring that the vast majority of animals do not get as seriously ill from an infection as humans do. In addition, at present there are very few documented cases of animals subsequently transmitting the infection to humans. But a new concern is now under discussion: what if SARS-CoV-2 could replicate unnoticed in animals and mutate? Could new variants emerge that can re-infect humans and create more havoc?
SARS-CoV-2 has evolved in humans throughout the pandemic, leading to numerous new variants arising, and there is two factors which seem to have contributed to the emergence of variants. The first is the large number of infections in people around the world, as the virus has the ability to mutate each time it reproduces. The second is the much lower number of chronic infections that occur in people whose immune systems are not fully functioning. Faced with a weakened immune system, the virus is not quickly eliminated and therefore has time to develop ways of evading immunity.
Is it possible that these evolutionary scenarios are also happening in animals, but we didn’t know they are happening?
To know if this is a risk, we first need to know how many infections occur in animals. This will help identify any hidden reservoirs of the virus. To this end, SARS-CoV-2 infections in animals are undergoing in-depth study in many places around the world. Scientists are studying exactly which species are susceptible to infection, as well as the frequency of the virus in different animal populations.
To find out which species are susceptible, many different animals – both domesticated and wild – have now been exposed to the virus under experimental conditions. This made it possible to understand precisely which animals can be infected – they understand cats, ferrets, deer mouse and Virginia deer. And to find out how common animal infections are, the SARS-CoV-2 antibody test is also used to find animals that have previously been naturally exposed to the virus.
Most studies of natural infections in animals have focused on cats and dogs, as these are the species that live most closely with humans. A recent UK preprint (research that has yet to be reviewed by other scientists) found that only six of 377 pet dogs and cats tested between November 2020 and February had specific antibodies to SARS-CoV-2.
This shows that the infection is not widespread and goes unnoticed in most of our pets. The first results of another study in the Netherlands (who is also still awaiting review) found higher antibody levels in the animals tested (54 out of 308 dogs and cats were positive), but likely due to different sampling strategies. British research looked at blood samples from a random set of animals, while the Dutch study specifically sampled pets in the homes of people known to be infected with Covid-19.
So it’s reasonably safe to say that our pets are unlikely to act as a significant reservoir for ongoing infections that could allow new variants to emerge. But what about other species?
The animal that has raised the most concern is the mink. The only documented cases of animals transmitting SARS-CoV-2 to humans involve these animals.
These were first identified in the Netherlands in May 2020 and mink-related variants have been identified in Denmark in November 2020. Fortunately, very effective containment measures quickly brought mink infections in these areas under control, but these animals will need to continue to be monitored closely.
What about the other source of viral variants: chronic cases of Covid-19? Could these occur in animals, allowing greater evolution of the virus within a single host?
Typically, chronic SARS-CoV-2 infections occur in people whose immune systems are not fully functioning – often because of other the medical conditions they have or the treatments they receive. These immunocompromised patients therefore receive extensive medical care throughout their infection.
Animals can also be immunosuppressed for a variety of reasons, but even the most beloved pets rarely experience the kind of extended hospitalization that could allow viral progression. As for immunosuppression in other animals, such as wildlife, this would be a significant drawback for survival.
Animals with weakened immune systems are unlikely to survive long enough for much of the development of an acute virus like SARS-CoV-2 to occur. However, minor mutations have been reported in experimental infections (again in early research pending review), suggesting that some evolution is theoretically possible even over a short period.
Going forward, it is essential that we continue to monitor SARS-CoV-2 in all kinds of animal populations. Particular attention should be paid to animals that live more closely with humans – it is pets and farm animals that are most likely to be accidentally exposed to high doses of the virus from an infected person. . Special attention should also be given to wildlife known to be susceptible to infection.
If there is evidence of natural animal-to-animal spread or chronic infections in animals, strict control measures should be put in place quickly. At this time, there is no need to consider general control strategies for animals similar to those used in humans, but we need to keep an open mind for the future.
Sarah L Shopping Cart is a clinical researcher in viral immunology and veterinary surgeon at the University of Cambridge.
This article first appeared on The conversation.