Researchers have seen two viruses – influenza A and respiratory syncytial virus – merge to form a single hybrid virus.
Although competition between viruses has been studied in detail, this new finding provides researchers with an unusual example of one virus co-opting another for its own benefits.
“This type of hybrid virus has never been described before,” said virologist and lead author Pablo Murcia. The Guardian. “We are talking about viruses from two completely different families that combine with the genomes and external proteins of both viruses. This is a new type of pathogenic virus.”
The hybrid virus looks like a gecko’s foot under a microscope, with respiratory syncytial virus (RSV) forming the legs and influenza A virus forming the toes.
It was discovered during a laboratory experiment designed to analyze the interactions between viruses during infection to better understand clinical outcomes, pathogen behavior and transmission.
Human lung cells were exposed to both viruses, as well as each virus individually as a control group. An assortment of microscopy techniques then revealed filamentous structures consistent with a hybrid of the two viral particles.
When these two viruses join forces, influenza A appears to infect a larger number and wider range of human cells. Influenza A particles have been shown to evade the immune system by displaying RSV surface proteins, giving the virus a survival advantage.
The hybrid also spread in cells that lack influenza receptors, which could allow influenza A to travel further through the airways into the lungs and cause more serious infections.
Unfortunately for RSV, this fusion is not so important, the presence of influenza A considerably reducing its ability to replicate.
The experiment was limited to a laboratory environment, which “cannot fully capture the spatial and physiological complexity of the entire airway,” the researchers explain.
However, the enhanced fitness of influenza when fused into a hybrid virus suggests that such blatant theft of another virus’s toolkit may play a role in viral pneumonia.
“RSV tends to travel lower in the lungs than the seasonal flu virus, and you’re more likely to get more severe disease the further down the infection goes,” says virologist Dr. Stephen Griffin. at the University of Leeds who did not participate in the study.
“This is another reason to avoid getting infected with multiple viruses, because it [hybridisation] is likely to happen all the more if we don’t take precautions to protect our health,” he says.
Influenza A alone causes more than 5 million hospitalizations each year, while RSV is the most common cause of acute lower respiratory tract infections in infants, with reinfection common later in life.
The study “raises questions about the fundamental rules that govern viral assembly”, and there may be other hybrid viruses yet to be discovered, the researchers write.
“Respiratory viruses are part of a community of many viruses that all target the same region of the body, like an ecological niche,” says virologist and lead author Joanne Haney.
“We need to understand how these infections occur within each other to get a more complete picture of the biology of each individual virus.”
This article was published in Natural microbiology.