Credit https://doi.org/10.1098/rstb.2024.0320
#19,090
Just over 3 weeks ago, in California: Background on the Outbreak of H5N1 in Elephant Seals at Año Nuevo Natural Reserve, we looked at early reports from California Parks and UC Santa Cru on the first detection of HPAI H5N1 in North American Elephant Seals.Initial reports suggested that at least 7 seal pups had tested positive after dozens of seals at California's Año Nuevo Reserve had developed respiratory and neurological symptoms.
Since then, according to local news outlets (see CIDRAP report), the number of HPAI positive seals has increased to 16, along with an otter and a sea lion. The actual count is likely much higher, as only a limited number of mammals have been tested.
Compared to what we've seen in South America, this remains a small outbreak, but over the past 5 years we've seen evidence of this virus becoming more `mammalian friendly'.
Dairy cows, domestic cats, and peridomestic animals like mice, skunk, and foxes have all been affected, but marine mammals appears to be close to achieving sustained mammal-to-mammal transmission.
- Two years ago, in a research letter (see EID Journal: HPAI A(H5N1) Viruses from Multispecies Outbreak, Argentina, August 2023), researchers wrote: ". . . it seems likely that pinniped-to-pinniped transmission played a role in the spread of the mammal-adapted HPAI H5N1 viruses in the region . . ."
- In the summer of 2024, we looked at a Preprint: Massive outbreak of Influenza A H5N1 in elephant seals at Peninsula Valdes, Argentina: increased evidence for mammal-to-mammal transmission
- And a year ago, in Nature Comms: Cross-species and mammal-to-mammal transmission of clade 2.3.4.4b HPAI A/H5N1 with PB2 adaptations, the authors wrote:
- Several mutations were detected months later in sea lions in the Atlantic coast, indicating that the pinniped outbreaks on the west and east coasts of South America are genetically linked. These data support sustained mammal-to-mammal transmission of HPAIV in marine mammals over thousands of kilometers of Chile’s Pacific coastline, which subsequently continued through the Atlantic coastline.
Although exact numbers are impossible to come by, a little over a year ago in Nature Reviews: The Threat of Avian Influenza H5N1 Looms Over Global Biodiversity, we saw a partial tally.
The most noteworthy mass-mortality events include more than 200,000 wild birds in coastal areas of Peru6; 24,000 sea lions in South America7; 20,500 wild birds in Scotland8; 6,500 Cape cormorants in Namibia9; and 17,400 elephant seals, including >95% of the pups in Argentina10.
These figures, however, largely underestimate actual mortalities, owing to a pervasive lack of monitoring, testing and reporting — particularly in inaccessible areas and in disadvantaged countries4,7.
Aside from the devastating impact on these mammals and the marine ecology, the possibility exists that HPAI could make strides towards human adaptation as it spreads through other mammalian species.
All of which brings us to a study published earlier this month in Philosophical Transactions B which details the impact of HPAI on pinnipeds, and offers some recommendations on how its impact might be managed.
Since this article was submitted months before the current outbreak in California, I've included some excerpts from a UC Davis press release which incorporates these latest developments.
The full 17-page PDF can be downloaded at the link below.
Elizabeth Ashley; Ralph Eric Thijl Vanstreels; Michelle Barbieri; Wendy Puryear; Frances Gulland; Cara Field; Christine Kreuder Johnson; Marcela Uhart Published online: 05 Mar 2026 https://doi.org/10.1098/rstb.2024.0320
Abstract
Since 2020, H5Nx high pathogenicity avian influenza viruses (HPAIVs) have caused widespread disruptions not only to global agriculture and trade but also to the health of free-ranging wildlife. Pinnipeds have experienced greater mortality from H5Nx HPAIV than any other mammalian taxa. Emergent virus strains, persisting over long time periods and vast geographic distances, have repeatedly triggered large-scale mortality events in pinniped populations.
Of particular concern is the spread of H5Nx HPAIV to the Southern Hemisphere—including the emergence of a marine mammal-adapted clade in South America and detections in the sub-Antarctic and Antarctic—and to other remote locations such as the Hawaiian Islands. These developments elevate concern for the world’s endangered, isolated and endemic pinnipeds.
While managing HPAIV in any animal population is a formidable task, working with free-ranging marine mammals poses unique challenges. In this review and perspective piece, we attempt to synthesize complexities at this intersection. We describe lessons learned from HPAIV investigations in marine wildlife, highlight gaps in knowledge and capacity, and discuss the incorporation of outbreak risk assessment and countermeasures into pinniped conservation. Finally, we propose ways in which pinnipeds—and marine wildlife broadly—could be better integrated into existing systems for HPAIV intelligence, control and prevention.
From the press release:
Global strategies to protect seals and sea lions from avian influenza
A birds-eye view of the impacts of H5N1 on pinniped conservation
University of California - Davis
News Release 19-Mar-2026
(EXCERPT)
A study from the University of California, Davis, steps back to look at the overall impact of the virus on pinnipeds worldwide and offers recommendations for moving forward to monitor, characterize risk and build resilience in the affected species. It also suggests ways to help prevent the virus from reaching currently unaffected but vulnerable pinniped species, such as the endangered Hawaiian monk seal or Galapagos sea lion.
(SNIP)
“There is a huge, unprecedented conservation risk,” said corresponding author Christine Johnson, director of the Institute for Pandemic Insights at the UC Davis Weill School of Veterinary Medicine. “Influenza is constantly changing, and that is a big problem now that it’s widely circulating in birds and marine mammals.”
(SNIP)
In late February, northern elephant seals in California marked the first cases of HPAI H5N1 in a marine mammal in the state. The speedy detection was due to routine surveillance for H5N1 that was set up over a year prior by UC Davis and Año Nuevo Natural Reserve in collaboration with UC Santa Cruz’s long-term monitoring of the northern elephant seal colony at Año Nuevo State Park.
At the end of 2025, in response to a growing number of H5N1 cases in Bay Area seabirds, the team increased surveying efforts, walking the length of the reserve to document and sample any sick or dead bird or mammal throughout the elephant seal breeding season.
These efforts in advance of the outbreak allowed teams to quickly respond to changes in the seals’ health and collect samples for testing at UC Davis. Johnson called it an “exceptionally rapid detection of an outbreak in free-ranging marine mammals,” and an example of the kinds of preemptive efforts to detect and respond to outbreaks effectively.
The paper’s key recommendations include:
- Fund and support long-term wildlife monitoring, and conduct surveillance both between and during outbreaks to detect trends early and respond swiftly before outbreaks spread.
- Build stronger communication and coordination networks among local, national and global researchers, agencies and academic partnerships to prepare for outbreaks. This includes working with public health practitioners and social scientists to engage and protect people at risk of disease exposure.
- Make wildlife health surveillance a routine part of conservation research and management activities.
- Improve technologies for non-invasive monitoring. For example, the UC Davis Institute for Pandemic Insights brings together engineers and wildlife health experts to deploy auditory and thermal imagery with satellite imagery to better understand key events or tipping points that may indicate an outbreak is likely.
- Pursue high-level policy changes and international agreements that address the root causes of avian influenza outbreaks.
- Address concurrent conservation threats. The authors emphasize that avian influenza is just one of many stressors affecting marine wildlife. Many species face challenges including habitat loss, declining food supply and climate change. Small populations are especially vulnerable.
“H5 avian influenza viruses are an emergent threat to seal and sea lion populations already facing numerous conservation pressures,” said first author Elizabeth Ashley, a graduate student researcher pursuing a dual degree in veterinary medicine and epidemiology at UC Davis. “Understanding how this virus spreads in coastal ecosystems is critical for protecting vulnerable marine wildlife.”
(Continue . . . )
Even if HPAI H5Nx somehow proves incapable of sparking a human pandemic, its impact on our fragile and interconnected biosphere could be devastating.While I can't tell you exactly what negative impacts the loss of a Billion+ wild birds, a quarter of a million marine mammals - or unfathomable numbers of peridomestic mammals - might have on our society, one thing is certain.
We are well on our way to finding out.
