Frog slime contains a molecule with the power to kill flu viruses, scientists have found out. Known as 'urumin', it also appeared to protect mice from lethal flu infections.

Influenza A viruses, which cause the disease commonly known as the 'flu', severely affects between three and five million people worldwide every year – resulting in 250,000 to 500,000 deaths.

Effective vaccines now exist to protect humans against the flu, but during pandemics, antiviral treatment becomes the first line of defence against the disease, because there is often insufficient time to produce vaccines.

The problem is that resistance to antivirals is growing, so scientists are trying hard to find new antiviral drugs that could be given to patients.

In the study, now published in the journal Immunity, scientists investigated molecules known as 'host defence peptides' produced in the mucus on the skin of the South Indian frog, showing that they could kill influenza A.

Slime vs virus

Most animals are known to make anti-microbial hosts defence peptides as part of their immune system but it's only in recent years that scientists have started to document them.

Frogs have received a lot of attention, because it is easy to isolate peptides from the slimy layer of mucus on their skin. Additionally, other studies with amphibians have highlighted the potential of their host-defence peptides against a range of different viruses, in-vitro.

Researchers from the Rajiv Gandhi Center for Biotechnology in Kerala, India, had been isolating peptides from the slime of local frogs. A team, led by Joshy Jacob from Emory University, decided to screen 32 of these peptides against an influenza A strain. They discovered that four of them were able to act against the virus.

The scientists then tested these four peptides on human red blood cells in a dish. They found that three proved toxic. Only one, urumin appeared to be harmless to human blood cells – but it managed to kill different influenza A strains.

In another part to the study, the scientists tested the action of urumin in vivo by testing the extent to which peptide treatment could protect mice infected with the flu. Urumin appeared to offer a certain degree of protection against the virus as fewer mice who had received the peptide died from the virus.

More research is needed to understand the way urumin works, but so far, the researchers have established that it targets a viral surface protein called hemagluttinin. "The virus needs this hemagglutinin to get inside our cells," Jacob said. "What this peptide does is it binds to the hemagglutinin and destabilizes the virus. And then it kills the virus."

Although antivirals based on urumin are still a long way away, these findings represent a first promising step.