Winter is approaching, and with it, flu season. Cases of both influenza (flu) and the respiratory syncytial virus (RSV) tend to experience their peak seasons between December and February. This year, the outbreak of both flu and RSV cases has arrived early and is spreading rapidly. And in the wake of a wave of early flu and RSV hospitalizations, researchers have made a new discovery about the way that the flu and RSV co-infections may occur in some people, observing for the first time a flu-and-RSV hybrid virus capable of evading the immune system and infecting the lungs.
Flu and RSV Co-Infections
The lead author of the new study, MRC-University of Glasgow Centre for Virus Research’s Dr. Joanne Haney, knew that it was possible for a person to have simultaneous flu and RSV infections. Indeed, these co-infections are believed to be relatively common. However, it has not been clear why these RSV and flu co-infections can often lead to much more severe complications, symptoms, and outcomes for patients.
To understand why this is, Dr. Haney and her team needed to observe how RSV and flu viruses would behave if they were both injected into the same cell.
What they found was that, instead of competing against each other as expected, the RSV virus and the influenza virus fused to form a kind of new hybrid virus that has never been seen before.
Exploring the New Hybrid Virus
The research team described this new hybrid virus as “palm tree-shaped,” with the RSV virus forming the “trunk,” and the influenza virus forming the “leaves.”
This kind of interaction between two competing viruses has never been seen before. As the study’s supervisor, Professor Pablo Murcia, explains, “We are talking about viruses from two completely different families combining together with the genomes and the external proteins of both viruses. It is a new type of virus pathogen.”
The research team also observed the novel way in which this hybrid virus manages to infect other cells, even when it encounters influenza antibodies. While the influenza antibodies stick to the influenza proteins on the hybrid virus’ “leaves,” the virus is still able to spread to lung cells by utilizing the RSV proteins on the “trunk” of the hybrid virus. In other words, as Murcia puts it, “Influenza is using hybrid viral particles as a Trojan horse.”
This immune-system evading mechanism also means that this hybrid virus is more easily able to spread and infect an even wider range of lung cells than each individual virus is typically able to on its own.
Could this hybrid virus trigger potentially fatal complications, such as viral pneumonia, that can result from flu and RSV co-infections? It’s possible, but researchers caution against jumping to conclusions just yet. It’s still unclear exactly if and how this hybrid virus may be implicated in human diseases. In cases where an RSV and flu co-infection leads to viral pneumonia, for example, the immune system-evading hybrid virus may be to blame. Or it may simply be a complication of RSV itself, which, as it travels lower in the lungs, tends to cause the more serious infections associated with lower lung infections.
Research into this hybrid virus is ongoing. And while this discovery may help illuminate future understanding of how RSV and flu infections spread, scientists are equally as interested in understanding what this means about the potential of other hybrid viruses. “We need to know if this happens only with influenza and RSV, or does it extend to other virus combinations as well,” Professor Murcia concludes. “My guess is that it does. And, I would hypothesize that it extends to animal [viruses] as well. This is just the start of what I think will be a long journey, of hopefully very interesting discoveries.”
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