For the first time, two common respiratory viruses proved to have the ability to fuse and make one hybrid virus capable of evading the human immune system and infecting lung cells, a study has found. 

Researchers of the study observed that the discovery could verify why co-infections (when an individual is infected with two viruses simultaneously) sometimes lead to significantly worse diseases, such as hard-to-treat viral pneumonia, for some patients. 

Although co-infections are believed to be a common occurrence, it remains unclear as to the response the viruses would give if they found themselves located in the same cell.

Globally, almost 5 million people are admitted to hospitals with influenza A, but the respiratory syncytial virus (RSV) is considered the main cause of acute lower respiratory tract infections in children aged under five while it can cause severe illness in some children and adults.

Dr. Joanne Haney from the MRC-University of Glasgow center for virus research, who led the research, said, “Respiratory viruses exist as part of a community of many viruses that all target the same region of the body, like an ecological niche,” adding, “We need to understand how these infections occur within the context of one another to gain a fuller picture of the biology of each individual virus.”

For the process of analysis, Haney and her colleagues infected human lung cells with both influenza A and RSV, and instead of fighting off one another as some viruses are known to do, they fused to form a palm tree-shaped hybrid virus – with RSV as the trunk, and influenza as the leaves.

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Supervising the research, Professor Pablo Murcia, published in Nature Microbiology, “This kind of hybrid virus has never been described before,” he said, adding, “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.”

Upon formation, the hybrid virus exhibited the ability to infect neighboring cells despite the presence of antibodies against influenza intended to block infection. Even though the antibodies clung to the influenza particles on the new virus' surface in an attempt to block it, the virus resorted instead to neighboring RSV proteins to infect lung cells to which Murcia said, “Influenza is using hybrid viral particles as a Trojan horse.”

Alongside the capability to evade the human immune system, it also indicated its facilitated track to accessing wider selections of lung cells, and whereas influenza infects nose, throat, and windpipe cells, RSV aims to target windpipe and lung cells. 

According to Dr. Stephen Griffin, a virologist at the University of Leeds, this could potentially trigger a severe and, possibly, fatal lung infection called viral pneumonia, but he advised for more research in the field to prove that hybrid viruses are connected to human disease. “RSV tends to go lower down into the lung than the seasonal flu virus, and you’re more likely to get the more severe disease the further down the infection goes.”

“It is another reason to avoid getting infected with multiple viruses because this [hybridization] is likely to happen all the more if we don’t take precautions to protect our health," he added. 

As part of this breakthrough discovery, the research team showed that the hybrid viruses could reach cultured layers of cells and individual respiratory cells. “This is important because the cells are stuck to one another in an authentic way, and the virus particles will have to go in and out in the right way,” commented Griffin.

In regards to what comes next, what follows is to verify and confirm if hybrid viruses can form in co-infection patients, and if that proves possible, then which ones. “We need to know if this happens only with influenza and RSV, or does it extend to other virus combinations as well,” said Murcia. “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.”