Researchers have seen two viruses – influenza A and respiratory syncytial virus – fuse to form a single, hybrid virus.
While competition between viruses has been studied in some detail, this new finding provides researchers with an unusual example of a virus cooperating with another for its own benefit.
“This kind of hybrid virus has never been described before,” said virologist and senior author Pablo Murcia The guardian. “We’re talking about viruses from two completely different families fused together with the genomes and external proteins of both viruses. It’s 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 feet and influenza A forming the toes.
It was discovered during a laboratory experiment designed to analyze 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 separately as a control group. Subsequently, a variety of microscopy techniques revealed filamentous structures consistent with a hybrid of both virus particles.
When these two viruses join forces, influenza A appears to infect a greater number and a wider range of human cells. Influenza A particles were found to evade the immune system by displaying RSV surface proteins, giving the virus a survival advantage.
The hybrid also spread to cells that lacked influenza receptors, which could allow influenza A to travel further down the respiratory tract into the lungs and lead to more severe infections.
Unfortunately for RSV, this fusion is not so great, as the presence of influenza A greatly reduces its ability to reproduce.
The experiment was limited to a laboratory setting, which “cannot fully capture the spatial and physiological complexity of the entire respiratory tract,” the researchers say.
However, the enhanced ability of influenza when fused into a hybrid virus suggests that such blatant theft of another virus’s toolbox may play a role in viral pneumonia.
“RSV tends to go lower in the lung than the seasonal flu virus, and you’re more likely to get more severe disease the further down the infection goes,” says Dr. Stephen Griffin, a virologist. in the the University of Leeds which was not involved in the study.
“It’s one more reason to avoid getting infected with many viruses, because that [hybridisation] it’s likely to happen even more if we don’t take precautions to protect our health,” he says.
Influenza A alone causes over 5 million hospitalizations each year, while RSV is the most common cause of acute lower respiratory tract infections in infants, with reinfection common in later life.
The study “raises questions about the fundamental rules governing virus assembly,” and there could be other hybrid viruses out there that have yet to be discovered, the researchers write.
“Respiratory viruses exist as part of a community of many viruses that all target the same area of the body, like an ecological niche,” says virologist and lead author Joanne Haney.
“We need to understand how these infections occur in the context of each other to gain a more complete picture of the biology of each individual virus.”
This work was published in Nature Microbiology.