California: Background on the Outbreak of H5N1 in Elephant Seals at Año Nuevo Natural Reserve

 
California Parks

#19,066

Yesterday, California Parks and UC Santa Cruz announced the first U.S. detection of H5N1 in Elephant Seals at the Año Nuevo Natural Reserve (see FluTracker's thread) after dozens of seals  were observed either sick or dying  and the virus was confirmed (on Tuesday night) by the USDA’s NVSL lab. 

(Excerpt)

On Feb. 19 and 20, Beltran’s team noticed seals at Año Nuevo Reserve with abnormal respiratory and neurological signs, including weakness and tremors. They collected samples from sick and dead elephant seals and took them to UC Davis for testing at the California Animal Health and Food Safety Laboratory System. Initial screening revealed the samples were positive for avian influenza, which the USDA’s NVSL lab now confirms is HPAI H5N1.

While the details of this outbreak have already been well covered by the media, this is just the latest in a series of concerning HPAI spillovers into marine mammals around the globe.

We've repeatedly looked at the devastation of colonies of pinnipeds (seals, sea lions, walruses) in South America, and elsewhere, due to this virus. 

Although exact numbers are impossible to ascertain, a little over a year ago in Nature Reviews: The Threat of Avian Influenza H5N1 Looms Over Global Biodiversity, we saw the following proffered butcher's bill.

An estimated half a billion domestic fowl have died of H5N1 (ref. 4), and this number is rising; mortality in wildlife is more difficult to quantify than in poultry, but the effects are substantial. By mid-2024, H5N1 infections were documented in at least 406 bird and 51 mammal wild species (according to reports on the World Animal Health Information System (WAHIS)), and available reports suggest that multiple millions of wild animal individuals may have died (Fig. 1a).

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.

While North America has seen occasional spillovers of HPAI H5 into marine mammals (primarily seals & dolphins see here, here, and here), they pale in comparison to what has been reported from South America, and have never included elephant seals until now.

(Note: Those will long memories will recall that in 2013 - in The 2009 H1N1 Virus Expands Its Host Range (Again) - we looked at a US Davis study which found that the 2009 H1N1 pandemic influenza virus had managed to jump to elephant seals as early as 2010.)

All of this is a concern - because while H5N1 hasn't managed to figure out how to transmit efficiently from human-to-human - there is growing evidence to suggest it may be close to doing so in marine mammals.  

• 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

• A year ago, we looked at Preprint: Pathology of Influenza A (H5N1) Infection in Pinnipeds Reveals Novel Tissue Tropism and Vertical Transmission.

• And last March, 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 H5N1 remains predominantly an avian adapted virus, over the past 5 years is has become increasingly `mammalian friendly'.  Dairy cows, domestic cats, and peridomestic animals like mice, skunk, and foxes have all been greatly affected, and each spillover is another opportunity for the virus to test out new combinations of mammalian adaptations. 

Last September, in ISIRV: Update on H5N1 Panzootic: Infected Mammal Species Increase by Almost 50% in Just Over a Year we looked yet at another concerning milestone for the H5 virus. 

And just last month, in Adv. Genetics (Review Article) : Evolution of H5N1 Cross‐Species Transmission - Adaptive Mutations Driving Avian‐to‐Human Infection, we looked at another cautionary report that warned of the dangers of mammalian spread.

 

These types of reports have become so common, that events that would have inspired screaming headlines 3 or 4 years ago are scarcely noticed today (see Avian Flu's New Normal: When the Extraordinary Becomes Ordinary).

While hopefully this spillover to elephant seals in California won't lead to the kind of carnage we've seen in South America, this should be a sobering reminder that while we continue to dither in our response to H5N1, it continues to explore new ways to expand its geographic and host ranges. 

And, for now at least, H5Nx seems to be on a roll.