Frog slime may provide a new way to prevent flu epidemics, research suggests.
Scientists have discovered a potent anti-flu molecule in mucus secreted from the skin of an amphibian from southern India.
In laboratory experiments the peptide from the frog Hydrophylax bahuvistara neutralised dozens of influenza strains, dating from 1934 to the present day.
Researchers named the protein building block "urumin" after the urumi - a traditional whip-like sword with a flexible blade from the frog's home province of Kerala, southern India.
Dr Joshy Jacob, from Emory University in the US, whose team identified urumin and three other flu-fighting peptides in the frog slime, said: "I was almost knocked off my chair. In the beginning, I thought that when you do drug discovery, you have to go through thousands of drug candidates, even a million, before you get one or two hits. And here we did 32 peptides, and we had four hits."
Frogs are known to secrete a wide range of defensive peptides from their skin that protect against bacterial infection.
But the new discovery shows that some of them also combat viruses, including strains that cause flu.
Flu-fighting frog peptides could be useful when vaccines are unavailable or circulating flu strains become resistant to existing anti-viral drugs, say the scientists whose research appears in the journal Immunity.
Seen through an electron microscope, urumin appears to "dismantle" the virus after attaching itself to an area less likely to mutate into resistant new forms. The same "stalk" region is also the target of proposed universal vaccines effective against a wide range of flu strains.
In the tests, urumin delivered through the nose protected unvaccinated mice from lethal doses of flu viruses.
It specifically targeted the H1 strain family which includes the swine flu sub-type that led to a major pandemic in 2009 and is now just another form of seasonal flu. Urumin was not effective against other current strains, such as H3N2
The other anti-flu frog peptides proved toxic to human red blood cells. Only urumin seemed to be harmless to human cells but lethal to multiple flu virus strains.
Developing infection-fighting peptides into effective drugs is a challenge because enzymes in the body break them down.
Dr Jacob's team is now looking at ways to stabilise anti-viral peptides such as urumin. The scientists are also searching for more frog-derived peptides that are active against other viruses, such as dengue and Zika.
Urumin is thought to target a viral surface protein called haemagluttinin - the H in H1.
"The virus needs the haemagglutinin to get inside our cells," said Dr Jacob. "What this peptide does is it binds to the haemagglutinin and destabilises the virus. And then it kills the virus."