Research published in the journal Emerging Infectious Disease has shown that Highly Pathogenic Avian Influenza (HPAI), widely known as bird flu, is responsible for the deaths of hundreds of seals on the New England coast in June and July 2022.
The study documents the first large-scale mortality of wild mammals in relation to bird flu. The H5N1 subtype of the virus has resulted in huge death tolls among wild bird populations, as well as tens of millions of poultry deaths in Europe and North America.
Spillover into mammals from birds has been documented during the global outbreak that started in 2020, but until now this was only known in small and localised events. The team behind the research demonstrated that more than 330 Harbour Seals and Grey Seals died in relation to bird flu on the New England coast in June and July 2022.
Grey Seals (pictured) and Common Seals were involved in the mass mortality event, the first described in mammals that has been linked to bird flu (Chris Teague).
The lead authors of the study, Wendy Puryear and Kaitlin Sawatzki, work at Cummings School of Veterinary Medicine at Tufts University. They both have an extensive history of researching viruses in seals and say the bird flu findings are down to a strong dataset formed by information from wildlife clinics and rehabilitation centres in New England.
Puryear said: "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. And we have pairings of samples, sometimes literally from a bird and a seal on the same beach."
Tufts Wildlife Clinic has monitored the virus in birds and some mammals since January 2022, recording a minimum of three strains that had crossed over from Europe. They were also able to test almost all seals that came through the Greater Atlantic Region Marine Mammal Stranding Network, regardless of whether the seal was visibly sick, and gather genetic data on any viruses detected.
Sawatzki said: "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."
The team found that the mass mortality event in seals coincided with the virus hitting local gull populations severely. It is thought seals could easily pick up the virus from gulls as they share the same habitat and can easily come into contact with gull excrement.
H5N1 is nearly 100% fatal in domestic and wild birds, other than wildfowl, and this appears to be the case in mammals too. All seals testing positive for bird flu were either dead or dying, with no survivors, though it is possible that potential cases of asymptomatic or recovered animals would not have reached the stranding network. It is also uncertain whether the virus has been passed between seals, or the animals have only picked the disease up from gulls.
In Peru, where 60,000 pelicans, penguins and gulls died of bird flu, it has been announced that 3,500 sealions succumbed to the virus. Seal mortality possibly related to bird flu has also been reported from Canada and Russia. No cases of bird flu have been detected on the Atlantic coast since the end of summer 2022, but the 'stranding season' is due to come around again soon and may reveal that the virus persists.
Authorities say that the risk to public health from bird flu remains low, with less than 10 cases of H5N1 reported in humans since December 2021 and all of these related to people with direct exposure to infected poultry. A single-dose vaccination is available for poultry but is not currently used on a large scale, and there are currently no solutions for infections in wildlife populations.
The researchers said that tight biosecurity is vital for limiting the spread of the virus within and between species, advising thorough monitoring and strict separation of domestic and wild animals.
Puryear, W, Sawatzki, K, et al. 2023. Highly pathogenic avian influenza A(H5N1) virus outbreak in New England seals, United States. Emerging Infectious Disease. DOI: 10.3201/eid2904.221538