COVID-19 could have been transmitted to humans by stray dogs --specifically dog intestines - which had eaten bat meat, according to a study, published in the journal Molecular Biology and Evolution.
The disease may have rapidly evolved in the intestines of dogs that consumed bats carrying an ancestor coronavirus.
"Our observations have allowed the formation of a new hypothesis for the origin and initial transmission of SARS-CoV-2," said study researcher Xuhua Xia from the University of Ottawa in Canada.
"The ancestor of SARS-CoV-2 and its nearest relative, a bat coronavirus, infected the intestine of canids, most likely resulting in the rapid evolution of the virus in canids and its jump into humans. This suggests the importance of monitoring SARS-like coronaviruses in feral dogs in the fight against SARS-CoV-2," Xia elaborated.
Xia has long-studied the molecular signatures of viruses in different hosts.
When viruses invade a host, their genomes often bear the battle scars from fighting off and evading the host's immune system through changes and adaptations found within their genomes.
Humans and mammals have a key antiviral sentinel protein, called ZAP, which can stop a virus in its tracks by preventing its multiplication in the host and degrading its genome.
The viral target is a pair of chemical letters, called CpG dinucleotides, within its RNA genome.
CpG dinucleotides act as a signpost that a person's immune system uses to seek and destroy a virus. ZAP patrols human lungs and is made in large amounts in the bone marrow and lymph nodes, where the immune system first primes its attack.
For the findings, the researchers examined all 1252 full-length betacoronavirus genomes deposited into GenBank to date.
Xia found that SARS-CoV-2 and its most closely related known relative, a bat coronavirus (BatCoV RaTG13), have the lowest amount of CpG among its close coronavirus relatives.
Xia applied his CpG tool to reexamining the camel origin of MERS and found those viruses infecting camel digestive system also had lower genomic CpG than those infecting camel respiratory system.
When he examined the data in dogs, he found that only genomes from canine coronaviruses (CCoVs), which had caused a highly contagious intestinal disease worldwide in dogs, have genomic CpG values similar to those observed in SARS-CoV-2 and eBatCoV RaTG13'.
This suggests that mammalian digestive systems are likely to be a key target infected by coronaviruses.
"This is consistent with the interpretation that the low CpG in SARS-CoV-2 was acquired by the ancestor of SARS-CoV-2 evolving in mammalian digestive systems and interpretation is further corroborated by a recent report that a high proportion of COVID-19 patients also suffer from digestive discomfort," said Xia.