A strain of highly pathogenic avian influenza (H5N1), or bird flu, has been detected in sea lion populations, posing a concern for marine mammal health. The virus is causing significant mortality in these animals following a spillover from bird populations. This emergence in a new type of host signals a shift in its behavior and raises questions about its potential trajectory.
Transmission from Birds to Mammals
The primary route of transmission for the H5N1 virus from birds to sea lions is through direct contact with infected avian species. Sea lions often prey on or scavenge sick or dead seabirds, which brings them into close contact with the virus. This predator-prey interaction is the most probable pathway for the virus to jump from its avian hosts into a mammalian population.
Another transmission route is environmental. Wild birds shed the influenza virus through their saliva, mucus, and feces, which can contaminate coastal areas where sea lions rest and breed. As sea lions move through these environments, they can inhale or ingest the virus, leading to infection.
The specific virus strain at the center of these outbreaks is the H5N1 clade 2.3.4.4b. This lineage of the virus spread from Eurasia to North America in late 2021 and has since been detected in a wide range of wild birds. The frequent interaction between migratory birds and resident sea lion populations has created a persistent opportunity for these spillover events to occur along coastlines in both North and South America.
Clinical Signs and Population Impact
Once infected with H5N1, sea lions exhibit severe and often fatal symptoms, primarily affecting their nervous system. Observers have documented infected animals showing a lack of coordination, tremors, and intense seizures. The neurological damage can progress to paralysis, rendering the animals unable to move, hunt, or escape predators. These signs indicate the virus is attacking the brain and central nervous system.
In addition to neurological symptoms, respiratory distress is also a common clinical sign. Infected sea lions may have nasal discharge and show abnormal or labored breathing. The combination of neurological and respiratory illness leads to rapid deterioration of the animal’s health.
The impact on sea lion populations has been substantial, with mass mortality events reported in several regions. In South America, outbreaks have led to the deaths of tens of thousands of sea lions, affecting local colonies. For example, Peru and Chile have reported over 30,000 sea lion deaths combined. These die-offs are particularly devastating during pupping seasons, as the loss of adult females leads to the starvation of their dependent pups.
Potential for Mammal-to-Mammal Spread
A primary concern for scientists is whether the H5N1 virus has adapted to spread directly between mammals, a development that would significantly increase its threat. Research from outbreaks in South America provides strong evidence that a new strain of the virus has evolved to a point where it can be transmitted from one marine mammal to another.
Scientists are analyzing the genetic makeup of virus samples from deceased sea lions. They are searching for specific mutations that would allow the virus to more easily bind to receptors in mammalian cells, which facilitates mammal-to-mammal transmission. While the risk to the human public is currently low, the possibility of the virus becoming more adept at spreading between mammals serves as a warning.
These outbreaks in marine mammals are viewed as a natural experiment, showing how the influenza virus can adapt to new hosts. The ability to spread between sea lions suggests the virus has overcome a significant biological barrier. This evolution highlights the need for continued monitoring, as a virus that can efficiently spread among mammals could potentially jump to other species, including humans.
Scientific and Public Health Response
In response, government agencies and wildlife health organizations have intensified surveillance. This involves monitoring wild bird populations, the primary reservoir for the virus, and marine mammal colonies for signs of illness. Teams collect samples from dead animals to test for the virus and perform genetic sequencing, which helps track its spread and evolution.
Public health officials have issued guidance to protect human and animal health. The public is advised to avoid contact with sick or dead wildlife, including birds and marine mammals, and to keep pets away. Any sightings of animals exhibiting strange behavior or found deceased should be reported to local wildlife or public health authorities.