“Ping-pong zoonosis”: COVID spreads from humans to animals and back

There are some signs that the COVID pandemic is abating in the United States, with January 2023 being less severe than January 2022. Cases, hospitalizations and deaths are all down slightly, according to data from the Centers for Disease Control and Prevention. But our baselines have changed, altering our definition of normal. These metrics, especially deaths, are still much higher than what we have historically tolerated. And while things aren’t as bad as they were a year ago, as we’ve seen time and time again throughout the pandemic, it does have a way of backtracking when we lower our keep.

Experts are particularly concerned that the virus jumps from humans to animals and then comes back, each time presenting new opportunities for new mutations in different creatures. And many experts fear we’re not doing enough to monitor the situation, which means a nasty and surprising new twist could emerge from this viral game of human-to-animal-to-human ping pong.

“This back-and-forth transmission is also called ‘ping-pong zoonosis’ or ‘zooanthroponosis’, as in the case of a circulating variant of SARS-CoV-2 like BQ.1.1 or BF.7 which is transmitted from humans to mammals.”

Granted, things have changed a lot since the initial 2020 outbreak. We have excellent vaccines for the virus and we know that high-quality masks are effective in mitigating the spread. We also have many drugs and therapies to fight SARS-CoV-2, the virus that causes COVID.

But, as viruses do, SARS-CoV-2 continues to mutate. Viruses are pathogenic microorganisms, living or not (scientists are still actively debating this question). When they infect us, they hijack the genetic code of our cells to make copies of themselves. These viral Xeroxes can be a bit sloppy and errors can make the virus more or less destructive. This is normal and even expected.

Viruses of lesser concern tend to disappear. But the most concerning variants – think alpha, delta, or the many offshoots of omicron – can spread disease, disability and death around the world. As immunity wanes – also a normal and expected problem with coronaviruses – slightly different variants of the virus can evade our body defenses and all the therapies we offer it, although vaccines remain effective against serious diseases and the death.

Experts have been closely monitoring problematic mutations since SARS-CoV-2 was first sequenced in January 2020. A new article in the journal Nature Reviews Microbiology attempts to summarize some of these recent changes and help public health experts attempt to manage them.

It won’t be easy. The authors, including Professor David Robertson, a virologist at the University of Glasgow, detail the many ways the virus has mutated to evade our immunity, both from vaccines and in recovery. (Getting vaccinated in this case is less likely to prevent you from getting sick, but vaccines still largely protect against death and hospitalization.) There are some “unpredictable implications,” they observe, that are particularly concerning.

The first is that the virus could spread from humans to animals and vice versa. The second is that we’re not doing enough to monitor these mutations by doing less sequencing, which is a method of studying the contrasting genetic makeup of organisms – playing a kind of very complex tiny version of spotting the difference.

“There are many countries with low sequencing capacity, or places with previously good oversight that are decreasing or eliminating sequencing altogether,” Robertson and colleagues wrote. “This is troublesome because a lack of genomic surveillance will mean that future variants will be detected much later or could be circulating at low levels before eventual detection. There is therefore a need for widespread and equitable surveillance coverage to detect quickly potential news [variants of concern] among these individuals and communities before they spread more widely.”


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When a virus jumps from animals to humans, it’s called a zoonosis, from the Greek zōon “animal” and nosos “disease”. So far, there’s not much evidence that animals transmit COVID to us, even though we’ve done it to them many times. (Of course, the prevailing theory about the origin of SARS-CoV-2 is that it originated in bats.) Once COVID went global, humans shared it with deer, dogs, hamsters, zoo animals and many other species of mammals. But no creature has absorbed more virus from us than mink, a species in the weasel family that is bred for its fur.

Dr. Rajendram Rajnarayanan, assistant dean of research and associate professor at the New York Institute of Technology campus in Jonesboro, Arkansas, has been tracking these reverse zoonotic transfers for some time. It counted 133 cases in cats, 323 in deer and 1,320 in mink. In general, mink farms are a bad idea, with some having recently spread bird flu.

Animal reservoirs are the reason smallpox was eradicated in the 1980s through a global vaccination campaign – because, quite simply, there are no animal reservoirs for smallpox.

“This back-and-forth transmission is also called ‘ping-pong zoonosis’ or ‘zooanthroponosis’, as in the case of a circulating variant of SARS-CoV-2 like BQ.1.1 or BF.7 which is transmitted from humans to mammals,” Rajnarayanan told Salon in an email. When this happens, it creates a “reservoir” for the virus, a place where it can always hide and potentially come back. We see this with the occasional outbreak of bubonic plague, which is caused by the bacteria Yersinia pestis and lives in wild rodents like groundhogs. Fortunately, while the plague killed millions of people in the Middle Ages, today it can be treated with antibiotics.

We are not always so lucky. To take one example, the H1N1 influenza virus that caused the 2009 pandemic is sometimes referred to as a “quadruple reassortant” virus, because segments of the virus originated from humans, birds, and two species of pigs, North American and Eurasian. This is an example of a reverse zoonosis that the CDC estimates has killed more than half a million people worldwide.

Animal reservoirs are the reason smallpox was eradicated in the 1980s through a global vaccination campaign – because, quite simply, there are no animal reservoirs for smallpox. Animals cannot spread smallpox, so they cannot hold it in and pass it on to us. This is not the case with COVID or other viruses, like Ebola or mpox. As with bubonic plague, we are unlikely to completely eradicate a virus, bacteria or pathogen that occurs naturally in animals.

“Animal reservoirs also keep a lineage circulating and potentially reintroduce it into circulation at an opportune time,” Rajnarayanan said.

“Just as many pathogens can jump from nonhumans to humans, some can also jump from humans to nonhumans,” said Dr. T. Ryan Gregory, evolutionary and genome biologist at the University of Guelph in Canada. E-mail. “The result could be that even if we manage to eliminate the pathogen from human populations, it could reappear later and cause an epidemic by returning to humans of another species. [Additionally], if a pathogen circulates in another species, then it will evolve under the immunity conditions of that species, which are likely to be different from ours. This could result in a variant that is quite different and against which we would not have strong immunity if it came back to humans.”

As we approach the third anniversary of COVID stay-at-home orders and the initial (justified) panic of the pandemic, so many people are so exhausted that they have declared the pandemic over. On Jan. 17, Rep. Brett Guthrie (R-KY) introduced the “Pandemic is Over Act,” a bill sponsored by more than a dozen other House Republicans, in an effort to overturn the public health emergency that Americans introduced in the spring of 2020. But President Joseph Biden said essentially the same thing a few months ago, though his quote that the pandemic is “over” was mostly taken out of context.

Yet the pandemic is not over. Not only are thousands still dying every week, but China is currently experiencing one of its worst outbreaks to date, with around 80% of the population (about 1.12 billion people) infected in the latest wave. Each infection is a new opportunity for the virus to mutate into something that our defenses will fight against. While there’s no guarantee China’s outbreak will create huge problems for the rest of the world, we’ve seen omicron and delta emerge from similar waves in South Africa and India respectively.

Meanwhile, as the Nature Reviews Microbiology article noted, many countries are calling back surveillance precisely when we should be watching for the emergence of new variants, animal-based or otherwise. Currently, we have about 700 omicron sublines, according to Rajnarayanan, which is sometimes called a “soup of variants”. It should be noted that climate change is a major reason for all recent pandemics and viral illnesses are expected to worsen as the planet heats up.

“The evolution of variants is becoming increasingly complex, and it is more important than ever to detect, characterize and track variants as they evolve and spread,” Gregory said. “Wastewater is particularly useful in identifying the presence of ‘cryptic’ variants, that is, those that are not widely distributed but are still potentially important.”

Unfortunately, some conspiracy theorists want to blame this whole mess on vaccines, even though that’s not how vaccination works. Although immunity to vaccines or past infections creates significant selective pressure, according to Gregory, it is mostly infections that drive the evolution of variants.

“At the end of the day, it’s the fact that a lot of viruses circulate, mutate, and are subject to natural selection that’s the biggest problem,” Gregory said. “That’s why mitigation is also important. Vaccines do a good job of preventing serious acute illnesses, but they don’t stop transmission and so we need other measures. Fortunately, the ones we have, such as wearing high-quality (N95) masks, ventilation, air filtration, and reducing indoor crowd sizes are all variant-proof.”

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