Study claims coronavirus may have jumped from 'intermediary' stray dogs to humans but experts say no proof yet
Scientists from across the world are scrambling to identify the source of the novel coronavirus. Some have said that it is likely that the SARS-COV-2, the virus which causes the COVID-19 disease, was passed from bats to a mystery animal species that then passed it on to humans.
Now a new study claims that stray dogs may have been the intermediary host. An earlier study had claimed that pangolins could have transferred the new coronavirus from bats to humans.
Canada-based University of Ottawa's biology professor Xuhua Xia, who traced coronavirus signatures across different species, has proposed that stray dogs — specifically dog intestines — may have been the origin of the COVID-19 pandemic.
"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 dogs, most likely resulting in the rapid evolution of the virus in dogs and its jump into humans. This suggests the importance of monitoring SARS-like coronaviruses in feral dogs in the fight against SARS-CoV-2," says Xia in the paper published in Molecular Biology and Evolution.
However, experts say that there is no evidence or data to support this. "I think the data do not support these conclusions," Pleuni Pennings, an assistant professor of ecology and evolution at San Francisco State University, told Live Science.
"I find it difficult to understand how the author has been able to conclude anything from this study or to hypothesize much, let alone that the virus causing COVID-19 may have evolved through dogs," University of Cambridge veterinary medicine expert James Wood told MailOnline.
"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," said Wood.
What study says?
Humans and mammals have a key antiviral 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," says the study.
"But it's been shown that viruses can punch back. Single-stranded coronaviruses, like SARS-CoV, can avoid ZAP by reducing these CpG signposts, thus rendering ZAP powerless," it adds.
For the study, Xia examined 1,252 full-length beta coronavirus 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 systems also had lower genomic CpG than those infecting camel respiratory systems.
When he examined the data in dogs, he found that only genomes from canine coronaviruses, which had caused a highly contagious intestinal disease worldwide in dogs, "have genomic CpG values similar to those observed in SARS-CoV-2 and BatCoV RaTG13."
Dogs, like camels, also have coronaviruses infecting their digestive system with CpG lower than those infecting their respiratory system, says the researcher. The study further says that dog coronaviruses can affect their host's digestive system, entering via a protein called ACE2, which is also made in humans' intestines.
"The known cellular receptor for SARS-CoV-2 entry into the cell is ACE2. ACE2 is made in the human digestive system, at the highest levels in the small intestine and duodenum, with relatively low expression in the lung. This suggests that mammalian digestive systems are likely to be a key target infected by coronaviruses," the findings state.
It further explains, "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."
"In fact, 48.5% presented with digestive symptoms as their chief complaint. Humans are the only other host species Xia observed to produce coronavirus genomes with low genomic CpG values," states the research.
The researcher says that dogs are often observed to lick their anal and genital regions, and such behavior may facilitate viral transmission from the digestive system to the respiratory system.
Based on his results, Xia has presented a scenario in which the coronavirus first spread from bats to stray dogs eating bat meat and subsequently jumped onto humans to become a "severe human pathogen".
"While the specific origins of SARS-CoV-2 are of vital interest in the current world health environment, this study more broadly suggests that important evidence of viral evolution can be revealed by consideration of the interaction of host defense with viral genomes, including selective pressure exerted by host tissues on viral genome composition," says the study.
Pennings told Live Science, "There are many viruses with lower (CpG) values than SARS-CoV-2. When you look at all viruses, the [CpG] value is not strange at all."