The Omicron variant of the virus that causes COVID-19 might have evolved its massive number of mutations in a mouse, scientists say.
Omicron differs from previous variants of the SARS-CoV-2 virus, being more related to early strains, rather than later variants like Alpha and Delta. The currently favoured theory is that an immunocompromised person infected long ago has struggled to wipe the virus from their system, giving it the chance to evolve in unique ways before escaping and infecting others.
It was first detected in southern Africa, which has struggled for decades with an AIDS epidemic and has a higher-than-usual proportion of immunocompromised people, as well as low vaccination rates.
But a new paper published online suggests an alternative theory - that it came from mice.
"The molecular spectrum… of mutations acquired by the progenitor of Omicron was significantly different from the spectrum for viruses that evolved in human patients, but was highly consistent with spectra associated with evolution in a mouse cellular environment," the study, published in the Journal of Genetics and Genomics, reads.
"Furthermore, mutations in the Omicron spike protein significantly overlapped with SARS-CoV-2 mutations known to promote adaptation to mouse hosts… Our results suggest that the progenitor of Omicron jumped from humans to mice, rapidly accumulated mutations conducive to infecting that host, then jumped back into humans, indicating an inter-species evolutionary trajectory for the Omicron outbreak."
It's believed the original SARS-CoV-2 virus that infected humans back in 2019 originated in bats, like the original SARS virus of the early 2000s, perhaps passing through another animal on the way.
The US Centers for Disease Control says there's little evidence at present that animals - such as mink and cats - infected with COVID-19 "play a significant role" in spreading it amongst humans.
The main problem with the mouse theory is the virus reportedly struggles to infect them - scientists have to use "transgenic mice" in their experiments else it's too difficult to infect them, experts say.
It's possible part of Omicron's enhanced ability to infect hosts is because it doesn't rely on the same tricks though, Medical Xpress reported.
The study says if their hypothesis is correct, the number of mutations suggests the virus was circulating and evolving in mice for about a year before making the leap back into humans.
"Given the ability of SARS-CoV-2 to jump across various species, it appears likely that global populations will face additional animal-derived variants until the pandemic is well under control," the scientists said.
"Viral surveillance and sequencing in wild animals will likely help to prevent future outbreaks of dangerous SARS-CoV-2 variants."
