Some research teams focus on the ACE2 receptor, a protein present on the cell surface of many species. The spiny protrusions of the coronavirus allow it to bind to these receptors, like a key in a lock, and enter cells.
In 2020, a group of scientists compared the ACE2 receptors of hundreds of vertebrates, mostly mammals, with those of humans to determine which species the virus might infect. (The ACE2 receptors of birds, reptiles, fish, and amphibians are not similar enough to ours to cause concern.)
“Predictions have been very good so far,” said Harris A. Lewin, a biologist at the University of California, Davis and author of the study, in an email. Scientists have predicted, for example, that white-tailed deer are at high risk of infection.
But some predictions turned out to be totally wrong: the paper identified farmed mink as a species of “very low concern” – then, in April 2020, the virus raged through mink farms.
Indeed, ACE2 only offers a snapshot of susceptibility. “Viral infection and immunity are much more complex than just a cell-binding virus,” Kaitlin Sawatzki, a virologist at Tufts University, said in an email.
And of nearly 6,000 species of mammals worldwide, scientists have sequenced the ACE2 receptors of just a few hundred of them, creating a skewed dataset. These sequenced species include model organisms used in experiments, species that carry other diseases, and charismatic zoo dwellers, not necessarily the animals people are most likely to encounter.
“If a pandemic were to have come out of a squirrel, we’d be like, ‘God, what’s wrong with us? We haven’t even measured the basic biology of a squirrel,” Dr. Han said.