A mink mother and two babies scout their path from underneath a rock. Some minks have been infected ... [+]
If all the animal species that could host the coronavirus were put together, you'd have a respectable zoo. A recent study suggests that the virus SARS-CoV-2 can infect a broad range of mammals, which means that if given the chance, the virus could spill over into these species.
Scientists are concerned that if this coronavirus, SARS-CoV-2, gains a foothold in other species, new variants could form that would be more transmissible or harder to defend against. "To prevent a future outbreak we need to monitor these species to prevent animal to human transmission," said Qiang Ding, senior author of the new study and assistant professor at the Centre for Infectious Diseases at Tsinghua University School of Medicine.
This recent study, published in the journal Proceedings of the National Academy of Sciences, adds to the evidence that dozens of mammals might be susceptible to Covid-19. This study only looked at the susceptibility of different animals' cell receptors to the virus rather than studying animals themselves, but real-world evidence is backing up the data.
It’s not unusual for a virus to infect multiple species. SARS-CoV-2, infects a wide range of species, but that's typical of coronaviruses according to University College London microbiology professor Joanne Santini. "Coronaviruses generally have a broad host range," she said.
SARS-CoV-2 infects humans, as well as other animals, through cells' ACE2 (angiotensin converting enzyme 2) receptor proteins. ACE2 is important for blood pressure regulation. In the new PNAS study, a team of scientists looked at genetic variations of the ACE2 receptors among 295 animal species (mostly mammals, birds and fish), to see just how broad this coronavirus's host range may be. Similar studies have looked at how well SARS-CoV-2 binds to ACE2, but this study went further by assessing how effectively SARS-CoV-2 could infect cells, too.
Ding's group analyzed ACE2 protein sequences from 75 bird species, 4 alligator species, 4 turtle species, 9 lizard species, 130 mammal species and 68 fish species. The researchers saw that 80 of these animals, all mammals, had sequences that would allow ACE2 to serve as a receptor for the coronavirus.
They next looked at whether SARS-CoV-2 could enter cells through ACE2 receptors of 48 out of these 80 species. The scientists chose to zero-in on species that are often in close contact with humans, used in medical research or are endangered. Engineering human lung cells to express each species' ACE2 receptor, they tested the virus' ability to bind to the receptors and enter and infect cells.
They found that 44 of the 48 species could be susceptible to Covid-19, including great apes, baboons, macaques, rabbits, ferrets, minks, pigs, sheep, cattle, horses, dogs, cats and even some marine mammals.
But these results aren't definitive. The researchers only looked at one aspect of infection: the interaction between ACE2 and SARS-CoV-2's spike protein. The sequence of ACE2, "it's the most important genetic determinant of the host range," said Ding. But, he adds, "the most important doesn't mean it's everything," and other components of the virus or host cells could play a role in infection.
Though not definitive, the results seem to reflect reality: ferrets, minks, cats, dogs, gorillas and more have already caught Covid-19. Ding and his colleagues write that it's urgent to figure out which animals are susceptible to Covid-19, as these animals could serve as reservoirs for the virus and lead to future outbreaks.
"Animals that can act as a reservoir, they live in in high enough density to spread that virus between themselves," said Santini, who was not involved in Ding's study.
If the virus establishes itself in an animal population, it could change as it replicates. This would introduce the possibility that animals, if they have direct or indirect contact with people, could transmit new variants to humans. Such variants could have harmless mutations, but the concern among scientists is the possibility that new variants could emerge that undermine the vaccines.
Fortunately, this concern hasn’t been realized yet, although a variant from animals, mink, has emerged. In the spring of 2020, farmed minks were exposed to infected humans in Denmark, and beginning in June 2020, people associated with the mink farms began testing positive for a new mink-associated variant. The good news is that the variant didn’t have any harmful mutations. And, given that farmed minks live in a controlled environment, a farm, those outbreaks could be contained.
In wild populations, however, the virus could spread unchecked. Luckily, the first wild animal to test positive was also a mink, which isn’t a very social creature. "Mink in the wild, they're solitary animals," Santini said, which makes them unlikely to serve as a reservoir. But animals susceptible to the virus that live in large groups could pass it along to each other.
"Targeted surveillance of those animals that you know are in contact with humans…would be a wise thing," said Santini. It would allow scientists to track changes in the virus and prevent animal groups from becoming a reservoir. It’s one thing to know which animals are susceptible, but "we're only really going to see what animals are getting infected by doing wide surveillance looking for transmission," said Santini.
