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Stray dogs, possible origin of the coronavirus

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Snakes were first charged, then pangolins. However, to date no animal has been definitively identified as the intermediate host of the new SARS-CoV-2 coronavirus, causing the pandemic that has already infected more than a million human beings worldwide. Now, new research from the University of Ottawa, Canada suggests that stray dogs that fed on bats It could have been the transmitters. The study, for which more than a thousand virus genomes have been reviewed, is published in the journal “Molecular Biology and Evolution”.

Last February, a Chinese investigation indicated that the virus, which originated in bats, jumped to pangolins and later, possibly due to the sale of these animals in Wuhan wildlife markets, to people. Later studies found in these mammals coronaviruses closely related to that of Covid-19, but the degree of similarity was not sufficient to affirm that they had found the transmitter. Xuhua Xia, a professor of biology in Ottawa and an expert coronavirus tracker, is of the same opinion. In his opinion, the viruses isolated in these animals are “too divergent from SARS-CoV-2”, which seems to indicate “a common ancestor too far back in time, who probably lived in the 1960s.”

According to the new hypothesis, it was a bat coronavirus that infected the intestines of the canids, which probably caused the virus to evolve rapidly to remain hidden from the immune system. Thus reinforced, he gave a jump to the humans. The idea is supported by similar “battle” signals found in the new coronavirus, in a closely related bat virus (BatCoV RaTG13), and in dogs that affect their digestive system (CCoV).

Humans and all other mammals have a key antiviral sentinel protein, called ZAP, a soldier who patrols the lungs and can stop a virus on its way by preventing its multiplication and degrading its genome. The signal that the sentinel looks for to attack, the one that indicates the presence of the virus, are some chemical letters, called CpG dinucleotides, within its RNA genome. But coronaviruses like SARS-CoV can prevent ZAP by reducing these CpG signals, making ZAP powerless. It is the same cheating technique that HIV uses to bypass human antiviral defenses.

“Stealthy and dangerous”

Well, to carry out the study, Xia examined 1,252 beta-coronavirus genomes deposited in GenBank (a database of gene sequences available to the public) to date. In this way, he discovered that SARS-CoV-2 and its more closely related relative, a bat coronavirus (BatCoV RaTG13), have the least amount of CpG among their close relatives of coronavirus. That is, they hide very well in front of the immune system. “The survival of the virus indicates that it has successfully evaded ZAP-mediated antiviral defense. In other words, the virus has become stealthy and dangerous to humans, “he says.

When looking at the data in dogs, the researcher found that only the genomes of canine coronaviruses (CCoV), which had caused highly contagious intestinal disease in dogs worldwide, have CpG genomic values ​​similar to those seen in SARS-CoV -2 and BatCoV RaTG13. Second, as with camels, coronaviruses that infect the digestive system of dogs have a lower CpG than those that infect their respiratory system (canine respiratory coronavirus, or CRCoV).

Digestive symptoms

Furthermore, the known cellular receptor for SARS-CoV-2 entry into the cell is the enzyme ACE2, which is produced 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 a key target of the coronavirus. “This is consistent with a recent report that a high proportion of Covid-19 patients also experience digestive discomfort,” says Xia. “In fact, 48.5% had digestive symptoms as their main complaint.”

Humans are the only other host species observed by Xia with coronaviruses with low genomic CpG values. In a comprehensive study of the first twelve Covid-19 patients in the US, one patient reported diarrhea as the initial symptom before developing fever and cough, and stool samples from seven out of ten patients tested positive for SARS- CoV-2, including three patients with diarrhea.

Canids are often observed to lick their anal and genital regions, not only during mating but also in other circumstances. Such behavior would facilitate viral transmission from the digestive system to the respiratory system. “In this context, it is significant that the bat coronavirus (BatCoV RaTG13), as documented in its genomic sequence at GenBank (MN996532), was isolated from a faecal swab. These observations are consistent with the hypothesis that SARS-CoV-2 has evolved in the mammalian intestine or tissues associated with the intestine, “says the researcher.

Pets, not earned

The scientist believes that we should monitor dog coronaviruses more closely, but reassures pet owners about the possibility of being infected by them. “In order to transmit SARS-CoV-2 from animal to human, the animal must have a stable population of the virus in a tissue that is open to the environment. At the moment, there is no evidence that this occurs in dogs. This could say that they are immune to this virus or perhaps that we have not yet looked hard enough, “he explains.

In some Chinese cities, it has been decided to exterminate stray dogs, but Xia is not in favor of this measure. By acting in this way, “not only is the opportunity to detect various viruses lost in stray dogs, but also unjustified brutality is committed,” he says.

On the other hand, China’s recent decision to reclassify dogs as pets and not like cattle, “it has practical difficulties. If there is an ethnic minority that runs an autonomous region where dogs are traditionally raised and sold for meat, those people have a right to uphold their tradition. However, they could voluntarily give it up with government subsidies, “he argues. At the same time, “People in different cultures differ in their relationships with animals. For example, some people in China are much more resistant to eating horse meat than dog meat and are very surprised that there are people who do.

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