Tuesday, January 17, 2017

Sci Rpts: H5N8 - Rapid Acquisition of Virulence Markers After Serial Passage In Mice


The `good news' regarding avian H5N8 is that, so far, it has demonstrated little ability to infect non-avian species, and  - unlike H5N1 and H5N6 - no human cases have been reported.
We did see several reports during 2014-15 of dogs in South Korea ether testing positive, or developing antibodies, to the virus (see H5N8 Antibodies Detected In South Korean Dogs (Again)), proving mammalian infection is possible, even if the infection is mild or asymptomatic.
More recently a study (see EID Journal: HPAI A(H5Nx) Viruses With Altered H5 Receptor-Binding Specificity) suggested recent evolutionary changes to clade H5 viruses may have increased its host range, and thereby its ability to spread.

The reality is, the only constant with influenza is change.

Influenza A viruses are prolific, but sloppy replicators. They make millions of copies of themselves while they infect a host, but in the process, often make small transcription errors. Errors that can affect the way the virus acts; its ability to replicate, its transmissibility, and even its virulence.
Often, these changes are of little or no effect, or may even be detrimental to the survival of the virus. Those that favor replication in the host, however, tend to carry on to produce more progeny, and if `fit enough', can drown out the earlier `wild type’ virus in the host.

This process is called host adaptation, and while it can (and does) happen in the wild, it can be easily simulated in the laboratory as well via a classic serial passage study (see chart above).

You simply inoculate a host with a `wild type’ strain of a virus, let it replicate awhile, then take the virus from the first host and inoculate a second, and then repeat the process five, ten, fifteen times or more.

Over time, the virus tends to adapt to the new host (assuming there are no species barriers to prevent it).

Which is, in a nutshell, what scientists did with the H5N8 virus in a mouse model, and over only 5 passages the virus gained significant virulence markers. The sobering results of that study are published today in Nature's Scientific Reports.

I've only posted some excerpts from this open access report, so you'll want to follow the link and read it in its entirety.
It is a lengthy, highly detailed, and fairly technical read, so some readers may find parts of it tough slogging. The introduction and the last part of the discussion are pretty straightforward, however.

When you return, I'll have a bit more.

Rapid acquisition of polymorphic virulence markers during adaptation of highly pathogenic avian influenza H5N8 virus in the mouse

    Won-Suk Choi, Yun Hee Baek, Jin Jung Kwon, Ju Hwan Jeong, Su-Jin Park, Young-il Kim, Sun-Woo Yoon, Jungwon Hwang, Myung Hee Kim, Chul-Joong Kim, Richard J. Webby, Young Ki Choi & Min-Suk Song

    Scientific Reports 7, Article number: 40667 (2017)

Received:   30 August 2016
Accepted:   08 December 2016
Published online:  17 January


Emergence of a highly pathogenic avian influenza (HPAI) H5N8 virus in Asia and its spread to Europe and North America has caused great concern for human health. Although the H5N8 virus has been only moderately pathogenic to mammalian hosts, virulence can still increase.

We evaluated the pathogenic potential of several H5N8 strains via the mouse-adaptation method. Two H5N8 viruses were sequentially passaged in BALB/c mice and plaque-purified from lung samples. The viruses rapidly obtained high virulence (MLD50, up to 0.5 log10 PFU/mL) within 5 passages. Sequence analysis revealed the acquisition of several virulence markers, including the novel marker P708S in PB1 gene. Combinations of markers synergistically enhanced viral replication and polymerase activity in human cell lines and virulence and multiorgan dissemination in mice.

These results suggest that H5N8 viruses can rapidly acquire virulence markers in mammalian hosts; thus, rapid spread as well as repeated viral introduction into the hosts may significantly increase the risk of human infection and elevate pandemic potential.
         (BIG SNIP)

In this study, we used plaque-purified viruses; thus, determining the precise roles of all individual changes was not possible. Therefore, the roles of single mutations or multiple combinations of the markers need to be determined using reverse genetics to minimize potential confounders in future studies. However, adequate and sequential distribution of the individual mutations in the plaque-purified viruses can facilitate elucidation of their effects.
We observed dramatically enhanced virulence of H5N8 in a mammalian host within a short period of time, and its genetic plasticity made it possible to obtain polymorphic virulence markers. However, it is not guaranteed that a virus that has spread globally will stably maintain the genetic background that favours the avian host. Therefore, intensive monitoring of the genetic evolution of H5N8 viruses is essential, and minimizing human contact with avian species infected with H5N8 influenza will help control viral evolution at the frontlines of zoonotic transmission.
          (Continue . . . )

If history is any gauge, the odds that any single avian adapted influenza virus will `jump species' and adapt into a humanized flu strain are pretty long.

But today we are dealing with multiple co-circulating subtypes (H5N1, H5N2, H5N6, H5N8, H7N9 . . .and a list of also rans like H7N1, H7N2, H7N3, H7N7, even H9N2 . . . not to mention potential interaction with seasonal H1N1 or H3N2), each with multiple clades and sometimes dozens of genotypes, and all evolving and ressorting as they go down different evolutionary paths.
Those odds - whatever they might be - would seem significantly enhanced.

Of course, just because its possible, that doesn't mean it will happen.  Theres a certain amount of luck and timing involved (all bad for us), even if a virus happens to hit the genetic lottery.  It has to happen in a host, and at a location, where it can be spread effectively into the human population.

Luckily that doesn't happen very often.   But the more outbreaks of avian flu we see around the globe, the more spins of the wheel of misfortune these viruses get to take.

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