A coronavirus variant that was first discovered in Britain may have come from dogs, according to a study from Chinese scientists.
The Shanghai-based researchers tracing the early evolution of variant B117, which has caused a new wave of cases in several countries, failed to find its footprint in viral samples collected from humans worldwide.
But when they expanded the search to include animals, they discovered some early forms of B117 on dogs, including one sample taken in the United States last July.
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“Such progenitor variants comprised most or all of the mutations of the early variant B117 within the Canidae family populations, and they may have spilled back to humans after a rapid mutation period,” wrote professor Chen Luonan and colleagues at the State Key Laboratory of Cell Biology in a non-peer-reviewed paper posted on biorxiv.org last Friday.
The emergence of the B117 variant has puzzled researchers. After being isolated from two patients in Kent in southeast England and London last September, it quickly became the dominant strain in the United Kingdom and many other countries, spreading more quickly than previous strains.
Some experts believe the variant might have emerged from local communities under the selective pressure of antiviral drugs used during the pandemic. According to one prevailing theory, it appeared suddenly in the UK and subsequently spread to other parts of the world.
But the variant has nine distinct mutations that were rarely, if ever, found in earlier human strains, according to Chen and colleagues.
These mutations did not happen in adjacent genes, but spread out sparely across the entire viral genome. The chances of all these mutations appearing at the same time is extremely low.
The Shanghai team believes that these nine mutations built up one after another. Their model suggested the variant might have originated outside the UK and acquired the mutations on a non-human host. Dogs were the most likely suspects, followed by minks or cats.
Qu Liandong, a professor of virology at the Harbin Veterinary Research Institute, who was not involved in the study, said more solid evidence would be needed to substantiate the theory.
The strains found on dogs were not entirely the same as the first identified in the British patient. Though the number of genome sequences obtained by researchers globally has reached hundreds of thousands, it is small compared to the total number of patients out there.
Some early forms of the B117 variant could be missed by sampling programmes, according to Qu.
But if pets such as dogs turn out to be the host, “we will have a big problem”, said Qu, who studies infectious diseases shared by humans and animals.
“Nearly all of our measures to fight the pandemic so far consider only humans. If animals are involved, it will change the game completely,” he said.
When avian flu breaks out in a chicken farm, all of the chickens there must be killed, according to worldwide standard practices. If the disease can infect humans, all susceptible animals - including healthy ones - have to be eliminated in the affected area.
Dogs are important human companions, but if they were proven capable of carrying or producing mutated variants of the Sars-CoV-2 virus, they could be culled as well, Qu said.
An alternative is to give the animals vaccines. “But we can’t give the dogs human vaccines. We may need to develop some completely new versions. We are already struggling to vaccinate humans. How can the programme be expanded to cover dogs or other animals?” Qu added.
There are increasing concerns that B117 could make dogs very ill. Veterinarians near London noticed a sudden increase in pets - including dogs and cats - suffering myocarditis, a serious heart disease early this year, and many of these animals tested positive for the variant, according to a report by Reuters in March.
Humans and animals have different immune systems and it is usually difficult for a virus to leap from one species to another.
The Sars-CoV-2 virus is believed to have originated in bats, but it could have taken decades to adapt to humans. When and where it made the jump from animal to humans remains unclear.
Chen’s team said that the B117 variant had a unique evolutionary strategy of increasing its infectiousness, so it can spread more easily from one host to another, but at the same time reducing the number of copies it makes in a host.
Whether this strategy helped the variant cross the species gap requires further study, according to the researchers.
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