COLUMBUS, Ohio – The influenza D virus that researchers say has been flying under the radar since its detection in animals in 2011 can vigorously make copies of itself in human cells and lung tissue samples, a new study shows.
The findings suggest influenza D has strong potential to spill over to humans, the scientists say. Some people who work with cattle, considered the primary host for this virus, have been found to have influenza D antibodies in their blood, but no active human infection has been discovered to date.
Results showed that several genetically different strains of influenza D that were isolated from cattle and pigs, another animal host, were as effective at replicating in cells from the human respiratory tract as influenza A viruses – the types of flu linked to both modern seasonal illness and historic pandemics.
"All of the viruses, despite their genetic distance from each other, had similar replicative capacity in these human cells and human tissues," said lead author Cody Warren , assistant professor of veterinary biosciences at The Ohio State University.
"It seems like there are many animal species that are susceptible to influenza D viruses, suggesting that maybe they can evolve differently in different hosts. What's obvious is that zoonotic infections are happening.
"This is an opportunity, if we are indeed in front of it, to invest in surveillance and basic understanding of its biology such that we could be prepared if it were to emerge in the future."
The study was published this week in Proceedings of the National Academy of Sciences .
The influenza D viruses from pigs used in the study were collected as part of long-term surveillance for swine flu – a type A influenza virus – at county and state fairs led by Andrew Bowman , professor of veterinary preventive medicine at Ohio State.
"Flu D popped up in our samples and that led us to question what the risk is at the human-animal interface," said Bowman, a co-author of the study.
"The currently held belief is that bovines are the natural host for flu D. So pigs may be a secondary host, or not. The fact that it was in pigs is something we were trying to understand: Is that a potential pathway for the virus to adapt into a more transmissible virus in humans? So the pig component is trying to understand what the role of the pig is with influenza D."
Warren's lab began by testing influenza D's replication in cells that mimic the human airway by differentiating patient-derived lung epithelial cells at the air-liquid interface – cellular conditions the virus would encounter naturally.
"When the virus started growing in those just as well as influenza A, we decided to leverage another physiologically relevant system to evaluate the breadth of cell types targeted by this virus: tissues," Warren said.
The team compared influenza types D and A growth in both human and swine lung tissues, finding that both virus types replicated efficiently in lung tissues from both species.
The experiments in cells and tissue also showed a key difference between the two flu types: Influenza D, despite growing well, did not stimulate a robust antiviral immune response in infected cells, while the influenza A virus did.
Separate tests in cell cultures showed influenza D growth was restricted if the cells were first primed by the protein interferon – suggesting that human cells have potent mechanisms to restrict viral infection.
In infected animals, influenza D generates respiratory symptoms that cattle and pigs recover from, suggesting their immune systems are at work fending off the infection. In humans, most viral infections trigger the release of interferon, which produces inflammation and raises body temperature – leading to the symptoms that make us feel sick.
These initial findings prompt important questions: Could humans have been infected but not felt sick, meaning influenza D is not a serious health risk? Or is it so stealthy that it can hide from the immune system, leaving us unable to put up a fight?
"Those are gaps that we don't quite understand," Warren said. "The virus replicates to really high levels but doesn't elicit a robust interferon response. Would it behave differently in the body of a person versus in these cell or tissue-based systems? That's up for debate.
"It seems the only smoking gun we're missing is a virus isolated from a person. There's been evidence of a history of exposure, but not necessarily any demonstration of active infection."
Bowman has collected many more influenza D viruses from pigs through ongoing swine flu surveillance, and plans to analyze the samples to assess whether the viruses are changing, and how.
"What's giving influenza D attention now is that it's recognized as a cause of respiratory disease in cattle, and we've certainly seen it cause disease outbreaks in pigs," he said. "Causing disease in some host species makes us somewhat concerned about what it might do in humans. It may not cause disease in this particular form, but in an evolved form, there may be that potential."
This work was supported by the U.S. Department of Agriculture, the National Institute of Allergy and Infectious Diseases, the National Institutes of Health and the Ohio State Enterprise for Research, Innovation and Knowledge.
Additional co-authors include Christina Sanders, Mine Liu, Jovanna Fusco, Elizabeth Ohl, Natalie Tarbuck, Emily King, Devra Huey and Amanda Panfil of Ohio State; Thomas Fabrizio and Richard Webby of St. Jude Children's Research Hospital; and Phylip Chen and Mark Peeples of Nationwide Children's Hospital.
#
