While previous studies suggested that the virus was passed to humans by pangolins, a Canadian team now claims that dogs were actually responsible for passing the virus to humans
It’s one of the worst pandemics in history, and now scientists believe they may know how the coronavirus was spread to humans.
The virus is believed to have originated in bats, before being passed to humans via an intermediate animal.
While previous studies suggested that this animal was a pangolin, a Canadian team now claims that a different animal is responsible for passing the virus to humans.
Researchers from the University of Ottawa claim that coronavirus was actually spread to humans by dogs.
Lead author Professor Xuhua Xia suggests a scenario in which “the coronavirus first spread from bats to stray dogs eating bat meat.”
The specific origins of Covid-19 are “of vital interest in the current world health crisis,” he said.
Professor Xia, a biologist at the University of Ottawa, went on: “Our observations have allowed the formation of a new hypothesis for the origin and initial transmission.
The ancestor of Covid-19 and its nearest relative, a bat coronavirus, infected the intestine of dogs, most likely resulting in a rapid evolution of the virus and its jump into humans.
“This suggests the importance of monitoring SARS-like coronaviruses in feral dogs in the fight against Covid-19.”
Pangolins were previously believed to have provided the key ‘staging post’.
But a complex analysis of Covid 19 mutations has ruled both them out as the ‘human reservoir’ – as well as bats.
It found the common ancestor of Covid-19 would have to be dated to around 1882 or 1966, respectively.
This is simply “too far back in time”, said Prof Xia.
Scientists agree that covid-19 almost certainly began in bats. They have been around for millions of years and have picked up a lot of viruses that have killed people – including Ebola, rabies and Sars.
But humans and bats don’t interact – so another animal is usually involved. Finding out what it is is crucial to eradicating Covid-19.
In the study, the researchers looked at a key antiviral protein, called ZAP, which can stop a virus in its tracks by preventing its multiplication and degrading its genome.
The viral target is a pair of chemical letters, called CpG dinucleotides, within bits of DBNA called RNA.
These 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.
But it’s been shown that viruses can punch back. Coronaviruses can avoid ZAP by reducing these CpG signposts – rendering it powerless.
Professor Xia said: “Think of a decreased amount of CpG in a viral pathogen as an increased threat to public health, while an increased amount of CpG decreases the threat of such viral pathogens.
A virus with an increased amount of CpG would be better targeted by the host immune system, and result in reduced virulence, which would be akin to natural vaccines.”
He found only genomes from dog coronaviruses have CpG values similar to those observed in Covid-19.
They also affect their digestive system and enter via a protein called ACE2 which is also made in humans’ intestines.
Professor Xia said: “This is consistent with the interpretation the low CpG in Covid-19 was acquired by the ancestor evolving in mammalian digestive systems.
“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 the researchers observed to produce coronavirus genomes with low genomic CpG values.
A study of the first 12 patients in the US found one reported diarrhoea as the initial symptom before developing fever and cough.
Stool samples from 7 out of 10 tested positive for COVID-19, including three with diarrhoea.
Dogs are often observed to lick their anal and genital regions, not only during mating but also in other circumstances.
Such behaviour would facilitate viral transmission from the digestive to the respiratory system.
It also fits the picture of a pathogen that causes gastrointestinal, respiratory and lung disease.
Professor Xia added: “In this context, it is significant the bat coronavirus (BatCoV RaTG13) was isolated from a foecal swab.
“These observations are consistent with the hypothesis that Covid-19 has evolved in mammalian intestine or tissues associated with intestine.”
However, not all scientists are convinced by the theory.
Prof Mick Watson, Personal Chair of Bioinformatics and Computational Biology at the Roslin Institute, said: “Whole genome comparisons show that the most similar sequenced relative to SARS-CoV2 came from a bat. That does not mean that SARS-CoV2 came from a bat, but it is the best evidence we have right now.
“The theory that SARS-CoV2 originated in dogs seems to stem from speculation about CpG and high ZAP expression. This is speculative at best and certainly isn’t strong evidence for the link.”
Meanwhile, Caroline Allen, Chief Veterinary Officer at the RSPCA, said: “It is really important to note that this is just a theory. There is currently no evidence that dogs were the origin of this virus and no evidence that they are playing a role in the spread of Covid19, which is a disease transmitted between humans.”