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Bird flu killed hundreds of seals in New England last year

A new study from the Cummings School of Veterinary Medicine has revealed that the deaths of hundreds of seals in New England were caused by bird flu. The research is the first to link a large scale mortality event in wild mammals to avian influenza. 

In the summer of 2022, more than 330 New England harbor and gray seals died along the North Atlantic coast. The researchers have connected this shocking event to an outbreak of avian influenza (HPAI) in the region.

The Tufts Wildlife Clinic has been conducting avian influenza surveillance since January 2022, which is shortly after the H5N1 strain arrived from Europe. The testing has revealed that at least three strains of avian influenza have crossed the Atlantic, and there have been consistent waves of bird infections.

In collaboration with NOAA’s Greater Atlantic Region Marine Mammal Stranding Network, the team also tested seals for bird flu. 

“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 study co-first author Wendy Puryear. “And we have pairings of samples, sometimes literally from a bird and a seal on the same beach.”

“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 co-first author Kaitlin Sawatzki.

Domestic poultry and seabirds are highly susceptible to the H5N1 strain of bird flu. This particular strain is responsible for about 60 million poultry deaths in the U.S. since October 2020. 

The experts determined that at the time of the seal mortality event in New England, seagulls were getting hit hard by the H5N1 virus. There are lots of ways gulls and other birds may transmit the virus to seals, and not just through direct contact, noted the researchers.

The highly pathogenic H5N1 is associated with a death rate of nearly 100 percent among domestic and wild birds, and the Tufts research indicates that there is a similar fatality rate for wild mammals. According to the study authors, none of the stranded seals that tested positive for bird flu recovered.

“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 team found evidence that the virus had mutated during the mortality event, but there has not been a case of bird flu in seals along the Atlantic coast since the end of summer 2022.

The CDC reports that the risk to the public remains low, with less than 10 human cases of H5N1. In these cases, the individuals had direct contact with infected poultry.

The ultimate public health threat associated with bird flu, however, is not yet known. According to the World Health Organization, 868 cases of human infection with H5N1 have been reported worldwide since 2003, and the fatality rate was over 50 percent.

The study is published in the journal Emerging Infectious Diseases.

By Chrissy Sexton, Editor

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