H5N1 avian flu outbreak marks the first cases in marine mammals in California and first detection in northern elephant seals
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Back in 2023 and 2024, a deadly strain of avian influenza – H5N1 – killed around 17,000 southern elephant seal pups on South American coastlines. Since then, researchers have kept a close eye on California’s northern elephant seals, which until now have remained free of the virus.
Lab tests, however, have revealed this week that the virus has breached this colony, marking the first time it’s been found in northern elephant seals. The research team collected samples from sick and dead elephant seals, which were analysed at the California Animal Health and Food Safety Laboratory System.
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As of 24 February, seven pups had tested positive for the virus. At the time of the lab results’ publication, 30 seals had died, with 29 of them weaned pups.
The elephant seal colony at Año Nuevo State Park is one of the most intensively studied in the world, with scientists monitoring the colony through flipper tagging and long-term tracking of individuals’ survival for more than six decades.
The highly infectious virus has been circulating the world as a panzoonotic – an animal pandemic – since 2020, infecting and killing approximately 700 species of birds and mammals on six continents, including cougars, polar bears, zoo tigers and rats. Animals that gather in large groups, like birds, are particularly vulnerable.
It has also infected humans – since 2024, 74 human cases have been diagnosed in the US, with two deaths. In these instances, most cases are attributed to hands-on contact with infected cows or poultry. Scientists stress that the current public health risk is considerably low, with no person-to-person transmission reported.
Since the colony was discovered to have been infected, California State Parks banned the public from the elephant seal viewing area of the Año Nuevo Coast Natural Preserve. Once confirmation had been made that H5N1 was responsible, more than 400 tours were cancelled for the rest of the season.
The virus is spread via contact with infected birds or animals, as well as their droppings. Animals can also catch H5N1 by eating an infected carcass.
Luckily, the virus outbreak among northern elephant seals was noticed very early on, thanks to monitoring by scientists at Año Nuevo and other locations in anticipation of a possible spillover into seals.
‘This is exceptionally rapid detection of an outbreak in free-ranging marine mammals’ said director of the Institute for Pandemic Insights at the University of California, Professor Christine Johnson. ‘We have most likely identified the very first cases here because of coordinated teams that have been on high alert with active surveillance for this disease for some time.’
What is H5N1?
The H5N1 virus was first discovered in 1996 in southeast China on a goose farm. It was transmitted within the poultry industry for several years, during which it spread over into wild birds and humans before spreading to Europe, the Middle East, Africa and later to North America, South America and, in early 2024, to Antarctica.
The current outbreak of H5N1 in North America was first detected in the winter of 2021 in Newfoundland and Labrador, Canada. It has now been confirmed in nearly all US states and Canadian provinces.
