Like humans, horses, pigs, and canines . . . marine mammals are susceptible to influenza infection, although due to limited surveillance we don't really know how often it happens.
It has been documented, however, that:
- During the winter of 1979-1980 seals were found suffering from pneumonia on the Cape Cod. In that instance, the culprit turned out to be an H7N7 influenza. (see Isolation of an influenza A virus from seals G. Lang, A. Gagnon and J. R. Geraci)
- In 1984 influenza subtype H4N5 – a strain previously only seen in birds – was determined to be behind the deaths of a number of New England seals in 1982 and 1983 (cite Are seals frequently infected with avian influenza viruses? R G Webster et al.)
- And in 1995, in the Journal of General Virology, authors R. J. Callan, G. Early, H. Kida and V. S. Hinshaw wrote of the The appearance of H3 influenza viruses in seals during the early 1990s.
- Seals have also been shown susceptible to influenza B (cite Influenza B virus in seals. Osterhaus AD, Fouchier , et al.).
- In 2013 we saw a report (and a study) from UC Davis showing the human 2009 pandemic H1N1 virus had jumped to wild California Elephant Seals just one year after that virus emerged (see The 2009 H1N1 Virus Expands Its Host Range (Again)).
- And in November of 2011, we saw a die off of seals in New England that was eventually determined to be due to a Mammalian Adapted avian H3N8 virus.
The biggest incident we've seen, however, came in northern Europe (mostly Denmark and Germany & Sweden) in 2014, when as many as 3,000 harbor seals reportedly died from avian H10N7 (see Avian H10N7 Linked To Dead European Seals), prompting warnings to the public not to touch seals.
Human infections with avian H10N7 have only rarely been reported, but known cases include two children in Egypt in 2004 (see Avian Influenza Virus A (H10N7) Circulating among Humans in Egypt) and a handful of abattoir workers in Australia in 2012 (see EID Journal: Human Infection With H10N7 Avian Influenza).
While these were mild illnesses, other avian H10 infections (notably H10N8 in China) have proved quite pathogenic (and deadly) in humans.
Yesterday the journal PLoS One published a research article that first looked at the pathogenesis of H10N7 infection in seals and then sought to evaluate the virus's zoonotic potential through experimentally inoculated ferrets.
In addition to finding that secondary bacterial pneumonia contributed heavily to seal mortality in this outbreak, the authors found that ferrets `. . . were susceptible to infection and developed respiratory disease, which suggests that Seal/H10N7 is potentially virulent for human beings.'The full study is available online, but I've posted the abstract below. When you return, I'll have a bit more.
Influenza A (H10N7) Virus Causes Respiratory Tract Disease in Harbor Seals and Ferrets
Judith M. A. van den Brand, Peter Wohlsein, Sander Herfst, Rogier Bodewes, Vanessa M. Pfankuche, Marco W. G. van de Bildt, Frauke Seehusen, Christina Puff, Mathilde Richard, Ursula Siebert, Kristina Lehnert, Theo Bestebroer, Pascal Lexmond, [ ... ], Wolfgang Baumgärtner
Published: July 22, 2016
Avian influenza viruses sporadically cross the species barrier to mammals, including humans, in which they may cause epidemic disease. Recently such an epidemic occurred due to the emergence of avian influenza virus of the subtype H10N7 (Seal/H10N7) in harbor seals (Phoca vitulina).
This epidemic caused high mortality in seals along the north-west coast of Europe and represented a potential risk for human health.
To characterize the spectrum of lesions and to identify the target cells and viral distribution, findings in 16 harbor seals spontaneously infected with Seal/H10N7 are described.
The seals had respiratory tract inflammation extending from the nasal cavity to bronchi associated with intralesional virus antigen in respiratory epithelial cells. Virus infection was restricted to the respiratory tract.
The fatal outcome of the viral infection in seals was most likely caused by secondary bacterial infections.
To investigate the pathogenic potential of H10N7 infection for humans, we inoculated the seal virus intratracheally into six ferrets and performed pathological and virological analyses at 3 and 7 days post inoculation. These experimentally inoculated ferrets displayed mild clinical signs, virus excretion from the pharynx and respiratory tract inflammation extending from bronchi to alveoli that was associated with virus antigen expression exclusively in the respiratory epithelium. Virus was isolated only from the respiratory tract.
In conclusion, Seal/H10N7 infection in naturally infected harbor seals and experimentally infected ferrets shows that respiratory epithelial cells are the permissive cells for viral replication. Fatal outcome in seals was caused by secondary bacterial pneumonia similar to that in fatal human cases during influenza pandemics. Productive infection of ferrets indicates that seal/H10N7 may possess a zoonotic potential. This outbreak of LPAI from wild birds to seals demonstrates the risk of such occasions for mammals and thus humans.
While primarily piscivorous, harbor seals (along with other marine mammals) are known to occasionally dine on aquatic birds, which continually exposes them to a diverse and growing constellation of avian influenza viruses.
We've already seen avian H7N7, H4N6, H3N3, H3N8, and now H10N7 emerge in seals, and it seems likely we'll see even more subtypes in the future.
Admittedly, birds and pigs are viewed as the most likely source of the next influenza pandemic virus. But marine mammals are a legitimate part of influenza's ecology, making their continued study and surveillance more than just of academic interest.