Results from 2011 New England harbor seal die-off

9 08 2012

Katie Pugliares and Michael O’Neil with the New England Aquarium prepare to necropsy a seal pup. (Photo: NEAQ)

A full report in the microbiology journal mBio offers fascinating details on the nature of the flu virus that killed over a hundred harbor seal pups in northern New England last year. The virus, known as H3N8, was known to circulate in waterfowl, but had never been responsible for a seal die-off before.
Now, the full analysis of the virus’ molecular strategy is out, and it offers some troubling findings. First off, a bit of a primer on flu viruses. Viruses are tiny packets of genetic material wrapped in a protein covering. While it is possible to sequence that genetic material, it’s much easier to identify viruses by the protein flags they bear on their surfaces. In flu viruses, the two proteins used are hemagglutinin and neuraminidase (The “H” and “N” designations of the various flu variants.) There are many fine divisions within a particular H/N combination, as many, many mutations are possible in many, many enzymes or other proteins. This particular H3N8 virus appears to have been passing between waterfowl since 2002, and in that time, it’s now evident that it developed some new tricks.

Genes within the virus code for the surface proteins (including H and N). (Photo: wikimediacommons)

Flu viruses don’t make the jump between species very readily. For instance, the molecular markers the viruses use to stick to the inside of the trachea and successfully infect are different between species groups. So the marker on a duck’s trachea looks very different to a flu virus than that on a seal’s trachea. One of the mutations seen in the H3N8 of last year’s seal die-off gave the virus the ability to stick to those seal tracheal markers as well as the bird ones. Since those markers are common to many different mammal species, the virus could, in theory, then be able to infect additional non-bird victims.

It’s unclear whether all of the affected seals acquired this mutated, mammal-adapted version of H3N8 independently from waterfowl, or if any of the seals might have acquired it directly from other seals. That kind of transmission makes epidemiologists very nervous.

Highly pathogenic avian flu (H5N1) can infect and kill humans, but so far all its victims have had close contact with infected poultry. The possibility that H5N1 avian flu could acquire the ability to jump from human to human would vastly widen the circle of potential victims. No longer requiring a direct link to sick chickens, the virus could hopscotch across the globe in a flu pandemic. So far, it hasn’t happened. But what we know about flu is its limitless capacity for mutation and shape-shifting, and the unpredictable results when different flu strains mix in a single organism. These harbor seals, we now see, are capable of serving as just such a mixing bowl, infected with both avian and mammalian flu strains.

As with highly pathogenic avian flu, even if H3N8 were demonstrated to be able to infect humans, it would likely require several additional steps before the virus acquired the ability to jump from human to human. And the limited contact between seals and humans would offer some degree of a viral buffer zone. Still yet another reason to keep your (and your dog’s) distance from seals. As if the risk of shark bite weren’t sufficient.

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