Researchers at the Cummings School of Veterinary Medicine at Tufts University found that an outbreak of highly pathogenic avian influenza (HPAI) was linked to the deaths of more than 330 New England harbor and gray seals along the North Atlantic coast in June and July 2022. and The outbreak was linked to a wave of bird flu in birds in the area.
The study was published March 15 in the journal Emerging Infectious Disease.
HPAI is better known as bird flu, and the H5N1 strain is responsible for about 60 million poultry deaths in the US as of October 2020, with similar numbers in Europe. The virus was known to have spread from birds to mammals such as mink, foxes, skunks and bears, but these were mostly small, localized events. This study is among the first to directly link HPAI to a larger-scale mortality event in wild mammals.
The paper’s first authors — virologist and senior scientist Wendy Puryear and postdoctoral researcher Kaitlin Sawatzki, both of whom work in the Runstadler Lab at the 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 collaboration with wildlife clinics and rehabilitation and response organizations in the region, particularly the Tufts Wildlife Clinic and director Maureen Murray, V03, associate clinical professor at Cummings. School, and a writer on paper.
“We have better resolution and greater depth of detail in this virus than before because we were able to follow it and detect changes in near real time,” Puryear said. “And we have pairs of specimens, sometimes literally from a bird and a seal on the same beach.”
The clinic has been conducting bird flu surveillance in birds and some mammals since January 2022, shortly after this strain of bird flu made a transatlantic journey from Europe to the U.S. Through this testing, the team found a wide range of flu viruses , such as at least three strains that crossed the Atlantic and witnessed continuous waves of infection in birds.
At the same time, working with NOAA’s Greater Atlantic Region Marine Mammal Stranding Network, they were able to check almost all the seals that came through the network, whether the animal looked sick or not. The Stranding Network consists of experts from state and federal wildlife and fisheries agencies, non-profit rehabilitation and response facilities, aquariums, and academic institutions that respond to strandings.
“Because of the genetic data we collected, we were the first to see a strain of the virus that is unique to New England. The data set will allow us to more meaningfully answer questions about which animals transmit the virus to which animals and how the virus changes,” Sawatzki said.
How HPAI is spread
In addition to poultry, H5N1 also had a huge impact on wild birds, particularly seabirds. Many locations around the world have experienced large die-offs, such as recently in Peru, where the virus killed 60,000 pelicans, penguins and seagulls.
At the time of the New England seal mortality event, the virus was hitting seagulls particularly hard, the researchers found. There are many ways that seagulls and other birds can transmit the virus to seals, they said. Seals and seabirds are coastal animals that live in the same areas that have environmental contact, if not direct, since they share the same water and coastline. A seal can contract the virus if it comes into contact with the droppings of a sick bird or water contaminated by those droppings, or if it preys on an infected bird.
The accepted knowledge is that H5N1 is almost 100% lethal to domestic and wild birds, except for waterfowl, and the same appears to be true when it comes to spillover into wild mammals. All seals tested positive for HPAI died at the time of sampling or succumbed shortly thereafter. None of the animals that tested positive recovered. However, it is possible that some asymptomatic or recovered cases never entered the stranding nets.
In addition to the New England seal mortality event, which was the first time H5N1 was detected in marine mammals in the wild, other locations have lost marine mammals to the virus. Peru reported about 3,500 sea lions died from the virus, Canada reported a seal mortality event along the St. Lawrence Estuary and a similar seal event in the Caspian Sea, according to news reports from Russia.
A hotly debated issue among scientists is whether there has been mammal-to-mammal transmission of HPAI between seals.
“It’s not surprising that you can have transmission between seals because it happened with low-pathogenic bird flu,” Puryear said. “However, we cannot say for sure 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,” Sawatzki explained. “Time for the virus to mutate, and time for the mutated virus to spread to another seal. As the virus mutates, we can see common mutations in the sequences that are specific to mammals that have not been seen in We had the numbers, but this outbreak did not last long enough to provide evidence of seal-to-seal transmission.”
The research team found evidence that the virus mutated in a small number of seals. But thankfully, they haven’t seen an outbreak of bird flu in seals along the Atlantic coast since late last summer. However, stranding season is about to begin for fur seals and gray seals, so they are preparing 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. As of December 2021, fewer than 10 human cases of H5N1 have been reported worldwide, and these cases occurred in people with direct exposure to infected poultry. There are no documented cases of human transmission for this variant.
However, it has the potential to become a bigger issue for human health. Bird flu emerged in 1996, and since 2003, 868 cases of human infection with H5N1 have been reported worldwide, according to the World Health Organization. Of these, 457 were fatal, a fatality rate of about 50%.
“And that’s why people are nervous about it,” Puryear said.
There is a single-dose vaccine available for poultry, but it is not currently administered on a large scale — partly because of cost and logistics, and partly because there is some concern that it might make future surveillance of the virus more difficult. There is not much that can be done in terms of response to the virus for wildlife, particularly given the scale at which the infection is occurring.
Biosecurity is important to limit 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, close and timely surveillance of pets and wildlife is key to understanding how the virus is evolving to prepare the best possible vaccines and treatments.
Citation: The research reported in this article was supported by the National Institute of Allergy and Infectious Diseases of the National Institutes of Health under award 75N93021C00014. Full information about authors, funders and conflicts of interest is available in the published work.