The global coronavirus pandemic may have started after a stray dog ate bat meat, a study has indicated.
Feral canines are the most likely animal intermediate host for the transmission of Sars-CoV-2 into humans, suggested Professor Xuhua Xia after studying coronavirus signatures across different species.
He hypothesises that the ancestor of the new coronavirus and its nearest relative, a bat coronavirus, infected the intestines of dogs, where it evolved before jumping to humans.
The findings of Prof Xia, from the University of Ottawa's biology department, are published online in the advanced access edition of the journal Molecular Biology and Evolution.
Humans and mammals can fight viruses through a key antiviral protein called Zap which prevents the infection from multiplying, while regions of DNA called CpG dinucleotides direct the immune system to attack the virus.
But single-strand coronaviruses can avoid the body's natural defences by reducing CpG, in a similar fashion to HIV.
In his study, Prof Xia analysed all 1,252 full-length betacoronavirus genomes deposited into the open access GenBank database.
He said that Sars-CoV-2 and its closest known relative, a bat coronavirus (BatCoV RaTG13), which shares 96% sequence similarity, have the lowest amount of CpG among its close coronavirus relatives.
Only genomes from canine coronaviruses, which have already caused a highly contagious intestinal disease worldwide in dogs, have similar genomic values, according to the study.
Dogs also have coronaviruses which affect their respiratory systems as well as their digestive systems, but the digestive kind has much lower CpG values, the paper notes.
The cellular receptor for Sars-CoV-2 entry into the cell is called ACE2, which is "pervasively expressed in the human digestive system ... with relatively low expression in the lung", the study says.
It adds: "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.
"The interpretation is further corroborated by a recent report that a high proportion of Covid-19 patients also suffer from digestive discomfort.
"In fact, 48.5% presented with digestive symptoms as their chief complaint.
"In particular, live Sars-CoV-2 virus was isolated from the stool of a Covid-19 patient. In this context, it is significant that BatCoV RaTG13 ... was isolated from a fecal swab.
"These observations are consistent with the hypothesis that Sars-CoV-2 has evolved in mammalian intestine or tissues associated with the intestine."
However, the paper was criticised by professor James Wood, head of the department of Veterinary Medicine and researcher in infection dynamics at the University of Cambridge.
He said: "I find it difficult to understand how the author has been able to conclude anything from this study, or to hypothesise much, let alone that the virus causing Covid-19 may have evolved through dogs.
"There is far too much inference and far too little direct data. I do not see anything in this paper to support this supposition and am concerned that this paper has been published in this journal.
"I do not believe that any dog owners should be concerned as a result of this work."
Prof Xia said: "Our observations have allowed the formation of a new hypothesis for the origin and initial transmission of Sars-CoV-2.
"The ancestor of Sars-CoV-2 and its nearest relative, a bat coronavirus, infected the intestine of canids, most likely resulting in a 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."
Nearly two million people have contracted Sars-CoV-2 since it first appeared at a seafood market in Wuhan, China, in late 2019, according to a global tracker by Johns Hopkins University.
More than 120,000 people have died after developing Covid-19, the disease caused by the virus.