SARS-CoV-2 was not the first coronavirus that raised fears of a pandemic; There were concerns about SARS and MERS before it arrived. But flu viruses have also been a regular source of concern, as they can often be spread from farm animals to us. Earlier this week, a report was released describing a flu virus with what researchers who identified it called “pandemic potential.” The virus is currently leaping from farm animals to us, but currently cannot spread from human to human.
Under surveillance
Institutions to which some of these researchers are affiliated, the Key Laboratory for Animal Epidemiology and Zoonosis, the Chinese National Influenza Center and the Center for Influenza Research and Early Warning, provide some indication of how seriously China has been taking the risk of newly evolved influenza strain.
For seven years, these centers supported researchers by doing something that makes everything you did for your thesis research seem enjoyable: taking nasal samples from pigs. Almost 30,000 of these swabs come from random pigs that showed up in slaughterhouses, plus another 1,000 from pigs brought into veterinary practices with respiratory problems. Why the pigs? Well, on the one hand, some historical pandemics, named for their species of origin, are called swine flu. And there is a reason for this: pigs are known to be infected with influenza viruses native to other pigs, birds, and humans, to whom they are often very close.
All of these were checked for the presence of influenza viruses, and several of the viruses underwent complete genome sequencing. The good news is that influenza viruses are not very prevalent in China’s agricultural system; only 0.45 percent of the randomly selected pigs carried a flu virus. Among those who entered the vet while suffering from respiratory problems, the percentage wasn’t even particularly high: 4.23 percent. So influenza is a relatively small but constant feature in China’s agricultural system.
There are many strains of influenza virus that can be distinguished by specific characteristics in the sequences of their genomes. And most viruses consist of a complicated mix of these due to a distinctive feature of the influenza genome. Like the human genome, it is not a single molecule. Instead, the influenza virus genes are spread through eight different RNA molecules. If a single cell is infected by more than one virus, these eight segments will be randomly mixed into each virus produced within that cell. So the typical virus the researchers found was made up of parts of more than one well-characterized virus.
Take over
Since the surveillance took place over eight years, the researchers were able to track changes in the viruses that circulated among pigs in China. And, over time, there was a clear trend: A virus we’ll call G4 (short for G4 EA H1N1) came to dominate the swine virus population in the later years of the study. It is made up of parts of three different strains of influenza that have been described above, but is primarily based on an avian virus isolated in Europe.
As this was a surveillance project, the researchers were naturally curious about whether the virus could pose a threat to humans. And the disturbing answer is yes. The original lineage of the virus was already known to interact with human cells, and the researchers confirmed that it could infect human respiratory tract cells that grow in a culture dish. Ferrets are often used as a model for the spread of the flu virus, and this virus was able to infect ferrets either by direct contact or by respiratory droplets. That means that there is the potential to infect humans in the real world.
The researchers then turned to real humans. They looked at the blood plasma of hundreds of farm workers, with members of non-farm households as controls. The bad news: More than 10 percent of farm workers had antibodies that could recognize this virus, and the numbers increased over time. The virus had also spread to some members of the general population. The good news is that all infected people there had close contact with the pigs, either living near a farm or working in food processing. Nothing indicates that the virus is spreading among humans at this time.
Now, it is still possible for these people to come into contact with a different but closely related virus and generate antibodies that cross-recognize the pig virus. Of course, that would be troubling in its own way.
So is it time to panic about this? The answer is clearly no – there is no indication that the virus is spreading among humans at this time. It has the potential to generate a pandemic, but that potential is definitely not being realized at this time. But the research is exactly why we have surveillance programs like this in the first place. Current flu vaccines would not protect against this virus, and it is definitely something we want to keep looking at. Fortunately, China and its international health partners can continue to stay current.
PNAS, 2020. DOI: 10.1073 / pnas.1921186117 (About DOI).
