Bird flu associated with hundreds of seal deaths in New England in 2022
Tufts University
The study was published on March 15 in the
journal Emerging Infectious Disease.
HPAI is more commonly known as bird flu,
and the H5N1 strain has been responsible for about 60 million poultry deaths in
the U.S. since October 2020, with similar numbers in Europe. The virus was
known to have spilled over from birds into mammals, such as mink, foxes, skunk,
and bears, but those were mostly small, localized events. This study is among
the first to directly connect HPAI to a larger scale mortality event in wild
mammals.
The co-first authors on the paper -- virologist and senior scientist Wendy Puryear and post-doctoral researcher Kaitlin Sawatzki, who both work in the Runstadler Lab at Cummings School -- have been researching viruses in seals for years.
They credit their findings in
the new study to a unique and robust data set made possible by a collaboration
with wildlife clinics and rehabilitation and response organizations in the
region, in particular with Tufts Wildlife Clinic and director Maureen Murray,
V03, associate clinical professor at Cummings School, and an author on the
paper.
"We have a better resolution and
greater depth of detail on this virus than before because we were able to
sequence it and detect changes almost in real time," said Puryear.
"And we have pairings of samples, sometimes literally from a bird and a seal
on the same beach."
The clinic has been conducting avian
influenza surveillance on birds and some mammals since January 2022, shortly
after this strain of avian influenza took a trans-Atlantic journey from Europe
into the U.S. Through this testing, the team found a wide range of flu viruses,
including at least three strains that crossed the Atlantic, and they witnessed
consistent waves of infection in birds.
At the same time, in collaboration with
NOAA's Greater Atlantic Region Marine Mammal Stranding Network, they were able
to screen nearly all seals that came through the network, whether or not the
animal appeared sick. The stranding network is composed of experts from state
and federal wildlife and fisheries agencies, non-profit rehabilitation and
response facilities, aquariums, and academic institutions who respond to
strandings.
"Because of the genetic data that we
gathered, we were the first to see a strain of the virus that's unique to New
England. The data set will allow us to more meaningfully address questions of
which animals are passing the virus to which animals and how the virus is
changing," said Sawatzki.
How HPAI Is Transmitted
In addition to poultry, H5N1 also has had a
huge impact on wild birds, especially sea birds. Multiple locations around the
globe have experienced large die-offs, such as recently in Peru, where the
virus killed 60,000 pelicans, penguins, and gulls.
At the time of the seal mortality event in New England, the virus was hitting gulls particularly hard, the researchers found. There are lots of ways gulls and other birds may transmit the virus to seals, they said.
Seals and sea birds are coastal animals living in the same
areas that have environmental contact, if not direct contact, since they share
the same water and shoreline. A seal may contract the virus if it comes in
contact with a sick bird's excrement or water contaminated by that excrement,
or if it preys upon an infected bird.
The accepted knowledge is that H5N1 is nearly 100% fatal for domestic and wild birds other than waterfowl, and the same is proving true when it comes to spillover in wild mammals. All the seals that tested positive for HPAI were deceased at the time of sampling or succumbed shortly after.
None of the animals that tested positive recovered.
However, it's possible some asymptomatic or recovered cases never came into the
stranding networks.
In addition to the seal mortality event in
New England, which was the first time H5N1 was detected in marine mammals in
the wild, other locations have lost marine mammals to the virus. Peru announced
about 3,500 sea lions died from the virus, Canada reported a seal mortality
event along the St. Lawrence Estuary, and there was a similar event with seals
in the Caspian Sea, according to news reports from Russia.
A hotly debated topic among scientists is
whether there has been mammal-to-mammal transmission of HPAI between seals.
"It's not surprising that you might
have transmission between the seals, because it has happened with low
pathogenic avian influenza," said Puryear. "However, we can't say
definitively whether or not there has been mammal-to-mammal transmission of
HPAI."
"To get strong evidence of mammal-to-mammal transmission, you need two things: lots of infected animals and time," explained Sawatzki.
"Time for the virus to mutate, and
time for the mutated virus to be transmitted to another seal. As the virus
acquires mutations, we can see shared mutations in the sequences that are
specific only to mammals and that haven't been seen in a bird before. We had
the numbers, but this outbreak didn't last long enough to provide evidence for
seal-to-seal transmission."
The research team found evidence that the
virus mutated in a small number of seals. But fortunately, they have not seen a
case of bird flu in seals along the Atlantic coast since the end of last
summer. However, stranding season is about to start for harbor seals and gray
seals, so they are bracing themselves for what might happen.
Prevention and Risk to Humans
The risk to the public remains low,
according to the Centers for Disease Control and Prevention. Since December
2021, less than 10 human cases of H5N1 have been reported globally, and those
cases occurred in people with direct exposure to infected poultry. There are no
documented cases of human transmission for this variant.
However, there is the possibility it could
become a larger issue for human health. Avian influenza emerged in 1996, and
since 2003, 868 cases of human infection with H5N1 have been reported
worldwide, according to the World Health Organization. Of those, 457 were
fatal, roughly a 50% fatality rate.
"And that's why people get nervous
about it," Puryear said.
There is a single-dose vaccine available
for poultry, but it's not currently administered on a large scale -- in part
because of cost and logistics, and in part because there's some concern it may
make future surveillance of the virus more difficult. There's not much that can
be done in terms of responding to the virus for wildlife, particularly given
the scale at which infection is occurring.
Biosecurity is important in limiting the
ways in which the virus can spread between and within species, the researchers
said. For example, wild birds should be kept separate from domestic birds, such
as backyard chickens. In addition, thorough and timely surveillance of domestic
animals and wildlife is key to understanding how the virus is evolving to
prepare the best possible vaccines and treatments.
Citation: Research reported in this article was supported by the National Institutes of Health's National Institute of Allergy and Infectious Disease under award 75N93021C00014. Complete information on authors, funders, and conflicts of interest is available in the published paper.