An alarming increase in harbor seal deaths along the New England coast can now be attributed to a new variant of influenza, U.S. researchers said in the journal of the American Society of Microbiology, mBio.
Between September and December 2011, 162 harbor seal pups washed up along the coastline, from southern Maine to northern Massachusetts. This is four times higher than the expected mortality of seals during this time period. Most of the seals were under six months of age, and more common causes of death, such as malnutrition, were promptly dismissed.
An investigation of five of the deceased animals revealed that a new variant of the H3N8 influenza A subtype was passed from waterfowl to seals and is responsible for the clinical and pathological symptoms observed in these pups.
This finding has ignited a storm of headline writing, most of which alarm the public to bird flu in seals. However, some clarification is needed. Let’s start with the basics.
There are three types of influenza virus: influenza A, influenza B, and influenza C. Influenzas A and B are more common and are responsible for seasonal flu epidemics. Influenza C is very mild, and there is no vaccine to protect against it.
Influenza virus A has different subtypes, and this is where it starts to get complicated.
The Disease Daily spoke with HealthMap’s resident MD, David Scales to learn more about the influenza virus.
“Influenza viruses have wide genetic variation, and this variation determines what animal(s) it can infect (including humans) and [the] virulence in each species. Since genetic sequencing tests on the virus require relatively specialized laboratories, scientists have devised a way to differentiate between influenza viruses based on molecules that appear on the surface of the virus that are relatively easy to detect. Two molecules in particular, hemagglutanin, which allows influenza to bind to other cells, and neuraminidase, an enzyme, give the "H" and "N" descriptions found in the famous "H5N1" virus often called bird flu.”
The different subtypes of influenza A virus (H3N2, H1N1, H3N8 etc.) are caused by “antigenic shift,” which means that there is a new hemagglutinin or hemagglutinin and neuraminidase combination. Because the combination of H’s and N’s are different, humans may not have immunity to the virus.
Different subtypes of the virus can change, too. So, there are variants of H1N1, for example. This happens through “antigenic drift,” which describes the small changes that happen within a virus over time. Eventually, the virus may be different enough to escape recognition by the body’s immune system. This is why flu shots are needed every year.
Back to our seals.
H3N8 is not a new subtype of influenza, and it is not unusual that it was present in birds, though it mostly occurs in dogs and horses. Calling it “bird flu,” is slightly misleading because it is not the highly pathogenic H5N1 virus prevalent in Asia that people normally think of when they read “bird flu.” Instead, this variant of influenza virus travels in between species. What is significant is that the virus mutated while in the waterfowl creating a new variant that is capable of jumping from waterfowl to seals.
The mbio article deliberately highlights the importance of evolving variants of influenza viruses. What is unique about this variant is that the seals act as an intermediary; one that can host both bird and mammalian flu viruses. This is similar to pigs, which acted as intermediaries during the swine flu (H1N1) outbreak, also in 2009. Additionally, seal H3N8 presents a number of distinctive mutations that indicate enhanced virulence and transmission in mammals.
There have been a number of incidences where bird flu viruses infected single mammals but did not spread. However, it is uncommon that a flu virus adapts to its mammalian host and spreads to others of the same species. Seal H3N8 has gained the mutations to do just that. Therefore, the ability of this, and other avian influenza viruses, to mutate and adapt is an important factor in the emergence of global pandemics.
Interestingly, this is not the first time the influenza A virus was discovered in New England harbor seals. An article published in 1984, also by the American Society of Microbiology, reveals the presence of an H4N5 virus variant in a number of dead harbor seals found between 1982 and 1983. It was discovered that this new variant was most closely related to one previously only detected in ducks and turkeys.
Thanks to Katharina Schwan and David Scales