In the coastal waters surrounding Cape Cod, a fascinating and perplexing biological mystery has caught the attention of scientists studying marine mammals and infectious diseases. Harbor seals (Phoca vitulina) and gray seals (Halichoerus grypus), two closely related pinniped species sharing the same ecosystem, both are known to contract influenza viruses. Yet, the clinical outcomes of these infections differ dramatically: while harbor seals fall ill and sometimes succumb to the virus, gray seals appear remarkably resilient, showing no overt signs of sickness despite exposure. This counterintuitive difference in disease susceptibility has spurred a focused inquiry into the immunological mechanisms underlying these disparate responses.
At the heart of this investigation is the search for answers about the role of cytokines, a broad category of small proteins secreted by immune cells that orchestrate and regulate inflammation and immune defense. Milton Levin, Ph.D., an associate research professor of pathobiology and veterinary science at the University of Connecticut’s College of Agriculture, Health and Natural Resources (CAHNR), has spearheaded research aimed at elucidating whether variations in cytokine activity could explain why gray seals resist the pathological effects of influenza infections that devastate harbor seals. Cytokines are essential communicators in the immune response, responsible for activating defenses against pathogens and subsequently dampening inflammation to facilitate recovery.
Initial hypotheses posited that gray seals might possess unique or enhanced cytokine responses offering protective advantages. Yet, a surprising discovery emerged from Levin and his team’s study: when measuring cytokine levels in gray seal pups infected with influenza A virus, the anticipated elevation or alteration in cytokine profiles was conspicuously absent. The shields of the gray seals’ immune systems appeared not to be rallying against the viral invader in the measurable way scientists expected. This finding, published in the Journal of Wildlife Diseases, challenges longstanding paradigms that associate robust cytokine responses with effective antiviral immunity.
The implications of this finding are significant. The study suggests that the gray seals’ immune system either does not recognize or actively suppresses the inflammatory signaling cascade typically mobilized during viral infections. This subdued or non-existent cytokine response likely prevents the excessive inflammation that could damage tissues and organs, thereby explaining why gray seals do not display clinical illness despite being infected. In other words, their immune system may be opting for a “silent” defense strategy, tolerating the virus’s presence rather than aggressively attacking it.
This immune tolerance contrasts sharply with harbor seals, which not only become ill but can also die following influenza infection. Levin explains that this difference might hinge on the notion of a cytokine storm, an unregulated and excessive immune reaction observed in many species, including humans, which can cause more harm than the infection itself. This phenomenon occurs when cytokines flood the system uncontrollably, leading to systemic inflammation and tissue damage. Gray seals, by sidestepping this effect through muted cytokine signaling, avoid the detrimental consequences of a runaway immune system.
To delve into these immunological nuances, Levin’s team undertook an extensive field and laboratory project involving the collection of blood samples from over 100 wild gray seal pups inhabiting the Cape Cod region. Employing a commercially available cytokine detection kit originally designed for dogs—validated by previous research to work efficaciously in seals due to their shared evolutionary traits—the team assayed for thirteen different immune signaling proteins. Yet no significant differences emerged between infected and uninfected pups, reinforcing the notion of an atypical immune response.
This work not only sheds light on the specialized host-pathogen interactions occurring in marine ecosystems but also broadens scientific understanding of immune tolerance mechanisms across species. The gray seal’s apparent ability to tolerate the virus without mounting a damaging inflammatory assault may illuminate novel pathways of immune regulation that could inform human medicine. Understanding how these marine mammals modulate their immune systems could inspire new therapeutic strategies for inflammatory diseases characterized by cytokine storms.
Levin emphasizes that the next crucial step lies in characterizing the immune responses in harbor seals directly. Collecting analogous blood samples from harbor seal pups poses significant logistical challenges, mainly because harbor seals remain in close contact with their mothers for more extended periods and are more difficult to handle safely due to their size. Overcoming these obstacles will be essential to draw definitive comparisons that explain the immunological vulnerabilities of harbor seals compared to their gray seal counterparts.
Long-term, this line of research aims to untangle pathogen transmission dynamics within and between marine mammal populations and assess any potential zoonotic risks. Influenza viruses are known for their capacity to jump species barriers, and understanding whether humans contribute to viral transmission to seals—or vice versa—is imperative for public and wildlife health. By exploring the virology and immunology of seal influenza infections, scientists hope to better predict and manage emerging infectious diseases in these and other vulnerable populations.
The broader significance of this research lies in revealing how species occupying the same ecological niche can evolve contrasting responses to shared infectious threats. The gray seal’s muted cytokine response to influenza contrasts with the more reactive immune system of harbor seals, highlighting the diversity of evolutionary solutions to pathogen pressure. This discovery enriches the field of comparative immunology and underscores the complexity of immune strategies in nature.
Finally, Levin and colleagues hope that continued research will elucidate the molecular and genetic bases of these immune differences. Identifying key regulatory nodes that suppress detrimental inflammation in gray seals—or pinpointing specific viral factors that inhibit immune activation—could pave the way for breakthroughs in antiviral therapies. Such insights would have ramifications not only for understanding marine mammal health but also for advancing biomedical science more generally.
As our understanding of marine mammal immunology deepens, the enigmatic resilience of gray seals to influenza serves as both a scientific conundrum and an inspiring model. It challenges assumptions, raises new questions, and ultimately pushes the boundaries of how we comprehend host-pathogen interactions in the wild. With continued effort, this research may redefine paradigms of immunity and disease resistance, with ripple effects reaching beyond the ocean’s edge.
Subject of Research: Animals
Article Title: Gray Seal (Halichoerus grypus) Pups Fail to Mount an Inflammatory Cytokine Response to Influenza A Virus
News Publication Date: 11-Jul-2025
Web References: Journal of Wildlife Diseases Article
References:
Levin, M., et al. (2025). Gray Seal (Halichoerus grypus) Pups Fail to Mount an Inflammatory Cytokine Response to Influenza A Virus. Journal of Wildlife Diseases, 61(3), 628-XXX. DOI:10.7589/JWD-D-24-00166
Keywords: Influenza, Infectious diseases, Immune response, Cytokines, Marine mammals, Gray seals, Harbor seals, Pathobiology, Viral infections
