WASHINGTON (Reuters) -- An extra enzyme helps make some strains of the flu virus especially deadly, researchers reported Monday.

They said knowing about the enzyme might help doctors find people infected with the most dangerous strains, and might provide a target for drugs.

Yoshihiro Kawaoka and colleagues at the University of Wisconsin said the most dangerous strain of influenza A makes use of an extra protein that helps it infect cells throughout the body.

The influenza A virus has two surface proteins -- hemagglutinin (HA) and neuraminidase (NA). To infect cells, the HA protein has to be cut into two pieces by enzymes known as proteases.

The proteases that can do this are normally common in the lungs and throat but not elsewhere -- which is why flu usually is just a respiratory illness.

Writing in the Proceedings of the National Academy of Sciences, Kawaoka's team said the worst strain of flu uses a different enzyme, plasmin, to help chop up HA. Plasmin is common throughout the body, meaning this strain can infect all sorts of tissues.

They looked at a virus descended from the strain that caused the 1918 pandemic of flu, blamed for killing more than 20 million people around the world.

"This is a mechanism that we never knew existed in human viruses," Kawaoka said in a statement.

His group tested 10 other strains of flu and did not find the same enzyme being used.

He said perhaps doctors could look for this enzyme in a flu outbreak to determine how likely it is to be dangerous. "Now we have additional markers that we can look for when a peculiar outbreak of human or animal flu occurs," he said.

"Their findings point us in a direction to better understand the pathology of these more virulent influenza viruses," Dominick Lacuzio, program officer for influenza at the National Institute for Allergies and Infectious Diseases (NIAID), which funded the study, said.