Something seems strange when you stroll along the rocky coastlines of southern Peru or the beaches of central California. The number of seals being hauled out on the rocks has decreased. Locals no longer find the carcasses particularly noteworthy. However, because H5N1 avian influenza is killing these animals and is spreading in ways that were unlikely even a few years ago, scientists are keeping a close eye on the situation.
Marine spillover is a term that researchers are increasingly using to describe this pattern. It describes the point at which a virus that started in wild birds spreads to coastal marine mammals, such as dolphins, sea lions, elephant seals, and even porpoises, usually through direct contact with infected seabirds that share feeding or resting grounds. What started out as a concerning side note in the larger H5N1 narrative has evolved into something more akin to a crisis.
The figures in Peru were astounding. Nearly simultaneous mass deaths of sea lions and pelicans were caused by H5N1’s arrival along the coast in late 2022. Dolphins, sea lions, cormorants, and sanderlings are five species from two taxonomic classes that were infected during the same outbreak window, according to samples collected by researchers. Tens of thousands of seabirds and at least 585 sea lions perished. The animals that lived long enough to be seen were displaying neurological symptoms, such as seizing on the beach, circling in the water, and being unable to dive. In all honesty, it was disastrous.
California came next. The first known case of H5N1 in marine mammals in the state was found in northern elephant seals at Año Nuevo State Park earlier this year. Elephant seals accounted for the majority of the at least eighteen marine mammal deaths that were reported in that cluster, along with one sea otter and one sea lion. Since not every carcass on a remote stretch of coastline is recovered and tested, it’s possible that these figures understate what actually occurred.
It’s not just the mortality that makes the situation worth closely observing. It’s the transmission’s geographic location. Contact with infected seabirds appears to be the main way that H5N1 is spreading to marine mammals; the spillover is primarily one-way, from sky to shore.
For example, sustained transmission from whale to whale or dolphin to dolphin is not supported by phylogenetic analysis of cetacean isolates. Instead of establishing itself as a self-sustaining chain within marine mammal populations, the virus appears to arrive, cause harm, and then retreat. Although it doesn’t completely reassure, that distinction is important.
This worry has been greatly heightened by the southern elephant seal situation, especially in the South Atlantic near South Georgia. According to a recent survey, the virus killed almost half of the breeding females in the largest population of southern elephant seals in the world during a single outbreak period. Half. That is a population-level blow to a species that takes years to recover demographically, not an epidemic on the periphery. As you watch that develop, it’s difficult to avoid the impression that the problem’s speed has outpaced the public discourse surrounding it.
Additionally, a peculiar new thread involving vampire bats has emerged from Peru; when it first surfaced, researchers were genuinely taken aback. It turned out that a tiny proportion of coastal vampire bats had H5N1 antibodies after consuming infected sea lions, pelicans, and cormorants. The virus was not effectively spread by the bats themselves; in any given colony, seroprevalence remained below 8%.
However, the structural issue is that these same bats also consume livestock at mixed-diet inland locations, potentially forming a corridor between terrestrial farm animals and marine wildlife that was unknown two years ago. Whether that corridor leads anywhere hazardous is still unknown. As of right now, it’s more of a surveillance gap than a real threat.
The more fundamental problem is that H5N1 continues to find new hosts in its current form. Minks in Spain. American dairy cattle. Peruvian sea lions. Elephant seals in South Georgia and California. Porpoises and dolphins in the Pacific and Atlantic. Although each spillover event is technically isolated, the overall picture points to a virus that is subtly and covertly broadening its range of viable hosts. Virologists are particularly concerned about cetaceans because they are difficult to monitor and because researchers have discovered mutations in important viral proteins in H5N1 isolates from dolphins and porpoises that may indicate some degree of adaptation to mammalian biology. Not enough to make an emergency declaration. Enough to continue observing.
It has long been said that marine mammals are sentinel species, meaning that the health of the ocean systems they inhabit is reflected in their well-being. The coasts are currently sending a signal that is hard to misinterpret.
