From lions and tigers to big hairy armadillos, more and more animals are becoming infected with coronaviruses.

We consider COVID-19 to be a human pandemic, but it is much more than that. The virus that causes the disease, SARS-CoV-2, can infect a wide and growing range of animals, both captive and wild.

So far, the virus has been found in more than 100 domestic cats and dogs, as well as captive tigers, lions, gorillas, snow leopards, otters and spotted hyenas, according to the U.S. Department of Agriculture. U.S. zoo staff have documented one positive case in a bear beaver, long-nosed raccoon, cougar, domestic ferret, fishing cat, bobcat, mandrill and squirrel monkey.

According to the U.S. Department of Agriculture, only three wild species - mink, mule deer and white-tailed deer - have tested positive in the United States. Cases have been found in wild black-tailed marmots, big hairy armadillos and leopards in other parts of the world.

But testing of wild animals is infrequent, and emerging research is beginning to suggest that COVID-19 may be affecting more species. Joseph Hoyt, a disease ecologist at Virginia Tech, said, "I think the spread of this disease to wildlife is much greater than previously thought."

How did SARS-CoV-2 infect such a wide range of species, and what were the effects?

Receptor linkage

One major reason is that all mammals have a complex receptor called ACE-2. This receptor plays an important role in regulating blood pressure and other physiological functions.

Once the SARS-CoV-2 stinger protein enters the body, it begins to infect host cells by binding to the ACE-2 receptor, which is widely found in the upper airways and sinuses of humans and many other mammals.

Compared with other similar proteins, the ACE-2 receptor has relatively little variation in physical structure in vertebrate species, says Yale virologist Craig Wilen. Even so, there are enough small variants that scientists initially thought some mammals were unlikely to be infected.