Novel H3N8 strain found in dead seals may pose human threat

Jul 31, 2012 (CIDRAP News) – A research team that analyzed the strain of H3N8 influenza linked to a baby seal die-off in New England last year found that it originated in birds and has adapted to mammals, signaling a possible threat to humans and animals alike.

The study, which appeared today in mBio, the online journal of the American Society for Microbiology (ASM), also revealed mutations that are known to make flu viruses more transmissible and able to cause severe disease.

In December 2011, scientists with the National Oceanic and Atmospheric Administration (NOAA) announced that an investigation of 162 seal deaths that fall revealed the H3N8 virus in samples from all five animals studied, the first time the strain had been linked to large-scale mortality in marine mammals. Study authors include researchers from NOAA, Columbia University, and several other institutions.

The infected seals had severe pneumonia and skin lesions, and most of them were less than 6 months old.

H3N8 is typically associated with wild birds, but in 2005 researchers found that a type of the strain that emerged in racing greyhounds had jumped from horses to dogs. The illness in dogs is generally mild and hasn't posed any known danger to humans.

Genetic and phylogenetic analysis of samples from the seals found that the virus was an avian subtype that was similar to one that has infected North American waterfowl since 2002, but has adapted to mammals with mutations that make it more transmissible and pathogenic. Scientists gave the new virus the provisional name influenza A/harbor seal/Massachusetts/1/2011.

The group found mutations previously detected in H5N1 viruses that infected humans and wrote that the H3N8 virus in seals had acquired the ability to bind sialic acid receptors that are commonly found in mammal respiratory tracts. Though the mutations are required for cell entry and replication in mammals, researchers wrote that more studies are needed to assess their functional significance.

Adaptations found in the study suggest the virus may be able to persist in seals and evolve into a new H3N8 clade, similar to what occurred with the canine and equine viruses, they reported.

Researchers concluded that natural emergence of a pathogenic virus that can transmit between mammals, and in a species that can be infected with multiple flu subtypes, is considered a significant threat to wildlife and human health.

They added that monitoring viruses such as the H3N8 strain found in seals is crucial, because it can help scientists better predict the emergence of new strains and prevent future pandemics.

Anne Moscona, MD, professor of microbiology and pediatrics at Weill Cornell Medical College who edited the report for mBio, said in an ASM press release that a new mammalian-transmissible virus that humans haven't been exposed to raises concerns. She said that the strain is a novel virus that infects mammals and may pass from animal to animal, traits that make it a potential threat to humans.

Also, she noted that the possibility that an avian flu virus could infect seals hadn't been considered before, which highlights the point that pandemic influenza can emerge in unexpected ways.

"Flu could emerge from anywhere, and our readiness has to be much better than we previously realized. We need to be very nimble in our ability to identify and understand the potential risks posed by new viruses emerging from unexpected sources," says Moscona.

In 2009, the emergence of a novel H1N1 virus—a reassortment of flu viruses found in birds, pigs, and humans—at the US-Mexico border surprised global health experts, who had predicted that the next flu pandemic would emerge from a country like Indonesia in which the H5N1 virus is endemic.

Ruben Donis, PhD, chief of the molecular virology and vaccines branch in the US Centers for Disease Control and Prevention's (CDC's) Influenza Division told CIDRAP News that the CDC received viral samples obtained from the seals several months ago and analyzed them with additional methods as part of its own risk assessment. He said overall, the virus appears to pose a low risk to humans with a low probability of causing zoonotic infections.

Similar concerns about H3N8 cropped up in 1963 when the virus made its jump from birds to horses, he added.

CDC officials have discussed issues surrounding the new virus with wildlife rescue teams and have emphasized precautions that personnel should take when working with the animals and requested that they obtain samples if they see seals that appear to have pneumonia. "We plan to keep in touch and follow up," Donis said.

Ian Lipkin, MD, director of Columbia University's Center for Infection and Immunity and coauthor of the mBio study, said in a university press release that the findings reinforce the importance of wildlife surveillance. "HIV/AIDS, SARS, West Nile, Nipah, and influenza are all examples of emerging infectious diseases that originated in animals. Any outbreak of disease in domestic animals or wildlife, while an immediate threat to wildlife conservation, must also be considered potentially hazardous to humans," he said.

Anthony SJ, St Leger JA, Pugliares K, et al. Emergence of fatal avian influenza in New England harbor seals. mBio 2012 Jul 31 [Full text]

See also:

Jul 31 ASM press release

Jul 31 Columbia University press release

Dec 20, 2011, CIDRAP News scan "New England seal deaths caused by H3N8"

Sep 7, 2005, CIDRAP News story "New canine flu virus came from horses"

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