Study Explains How Gray Seals Resist Influenza While Harbor Seals Fall Ill

In the waters around Cape Cod, two species of seals display very different responses to influenza infection. Harbor seals often become sick and may die from the virus, whereas gray seals can carry the infection without developing symptoms. This puzzling contrast prompted Milton Levin from the University of Connecticut and his collaborators to investigate the role of cytokines in shaping these outcomes.

Cytokines are small proteins produced by immune cells to regulate immune and inflammatory responses. As Levin explains, “They help initiate an immune response and then they help tamp down the immune response once a threat is gone.” Given their central role in immunity, the researchers expected to see distinct cytokine profiles in gray seal pups infected with influenza compared to those not infected. However, their results, published in Journal of Wildlife Diseases, showed no differences.

This finding suggests that gray seals either fail to mount an immune response to influenza or that the virus suppresses the response to such an extent it is not detectable. Levin notes, “Right now, it seems that the seals are not responding at all to influenza, and that’s probably why we’re not seeing clinical signs and why they don’t die.” This pattern stands in contrast to observations in many other species, where viral infection typically alters cytokine activity.

To carry out the study, Levin and his team collected blood samples from over 100 gray seal pups. In the laboratory, they measured the concentration of 13 different cytokines using a kit originally developed for canines. Levin’s earlier work confirmed that this tool is effective for studying seals due to their evolutionary relationship with dogs. The absence of a cytokine response may act as protection for gray seals. In humans, certain infections can trigger a “cytokine storm,” a dangerous overreaction of the immune system that causes greater harm than the pathogen itself.

The researchers now aim to perform similar tests on harbor seals, which suffer more severe outcomes from influenza. However, collecting samples from harbor seal pups presents difficulties. Unlike gray seals, which wean quickly and remain on beaches accessible to researchers, harbor seal pups stay with their mothers for several weeks, and adults are too large to handle safely.

Ultimately, the decade-long research effort seeks to clarify how viruses spread among marine mammals, why some species become ill while others remain unaffected, and whether transmission occurs between seals and humans. As Levin explains, “We’re trying to understand how pathogens, viruses, and influenza in particular, are being passed between species and if it is being transmitted to humans or are humans transmitting it to seals.”