A new and virulent subtype of flu bug has emerged among harbor seals in New England, researchers report.

Over a four-month period beginning last September, 162 harbor seals were found dead or dying along the coast of New England. An investigation by renowned virus hunter W. Ian Lipkin, MD, of Columbia University, identified the killer: a mutant flu bug transmitted to the seals by sea birds.

Virus isolated from the seals had undergone a series of important mutations:

It became able to spread among mammals, or at least from seal to seal.
It became able to infect the seals’ airways, destroying lung tissue or opening the door to fatal secondary infections.
It became more virulent.

“An additional concern is the potential [animal-to-human] threat that this virus poses, as it has already acquired mutations … that are often, though not exclusively, regarded as prerequisites for pandemic spread,” Lipkin and colleagues note in an article published in mBio, a journal of the American Society for Microbiology.