Classic serial passage study, albeit with ducks instead of ferrets
# 8428
When novel flu viruses jump species, they rarely do so fully adapted to the new host. There is generally period of transition – which may be measured in days, weeks, years . . . or even decades - before the virus can adapt to the new host species.
Sometimes the gulf between the original host and the new species is too great, and this adaptation never takes place.
Even though we don’t fully understand how it works, the concept is fairly simple.
When a flu virus infects a cell, it immediately sets upon making thousands of copies of itself. But single-stranded RNA flu viruses are notoriously sloppy replicators, and some of these copies will invariably carry small transcription errors. Most of these `variants’ will prove either neutral or detrimental to the survival and propagation of the virus, but occasionally a change will occur increases its biological fitness in the new host.
Those, as you might expect, are the ones that thrive and perpetuate themselves.
Which is why we are always concerned whenever a novel flu virus jumps to humans, as each instance is another opportunity for the virus to `figure us out’.
One of the classic lab experiments used to `hurry’ this evolutionary process along is called a serial passage experiment (see graphic at top of this post), where an test subject (usually a mouse or ferret) is infected with a virus, and that virus is then collected and used to inoculate another test subject. This process is repeated a number of times.
After 10 or so iterations, the virus is then examined for `adaptive changes’ and/or changes in behavior (ie. virulence, transmissibility). Sometimes, after multiple passes through a series of hosts, the virus picks up mutations that favor its survival in the new species.
This is essentially how Ron Fouchier created a `mammalian-adapted’ H5N1 virus in the laboratory in 2011, and it mimics what viruses do in the wild, albeit at an artificially enhanced speed.
For a flu virus to spark a pandemic, it basically needs to meet three criteria:
- It needs to be able to infect humans
- It needs to be pathogenic in humans (causes disease)
- It needs to be efficiently transmitted from human-to-human
The avian influenza viruses we’ve been watching (H5N1, H7N9, H9N2, H7N7, etc) all appear to meet the first two criteria (although severity of disease varies greatly between subtypes), but so far item #3 remains absent.
The primarily barrier to a pandemic appears to be a lack of `airborne transmission’ between humans.
Since ferrets are highly susceptible to influenza, and exhibit a similar respiratory response to infection to humans (coughing & sneezing), they are often used for transmissibility studies.
Infected ferrets are placed in cages adjacent to healthy ferrets, but any direct contact is prevented. If the healthy ferrets catch the virus, its a pretty good indication of airborne transmission.
Today, we’ve a study appearing in the Journal of Virology that takes the avian H7N1 virus – passes it serially through ferrets 10 times – and then tests the virus for both transmissibility and virulence. The end result was an H7N1 virus that was transmissible via the airborne route (in ferrets) with no apparent loss of virulence.
Airborne Transmission of Highly Pathogenic H7N1 Influenza in Ferrets
Troy C. Sutton1, Courtney Finch, Hongxia Shao, Matthew Angel, Hongjun Chen, Ilaria Capua, Giovanni Cattoli, Isabella Monne and Daniel R. Perez
Avian H7 influenza viruses are recognized as potential pandemic viruses as personnel often become infected during poultry outbreaks. H7 infections in humans typically cause mild conjunctivitis; however, the H7N9 outbreak in the spring of 2013 has resulted in severe respiratory disease. To date, no H7 viruses have acquired the ability for sustained transmission in humans.
Airborne transmission is considered a requirement for the emergence of pandemic influenza, and advanced knowledge of the molecular changes or signature required for transmission would allow early identification of pandemic vaccine seed stocks, screening and stockpiling of antiviral compounds, and focused eradication efforts on flocks harboring threatening viruses.
Thus, we sought to determine if a highly pathogenic influenza A H7N1 (A/H7N1) vrus, with no previous history of human infection, could become airborne transmissible in ferrets.
We show that after 10 serial passages, A/H7N1 developed the ability to transmit to co-housed and airborne contact ferrets. Four amino acid mutations (PB2 T81I, NP V284M, M1 R95K, and Q211K) in the internal genes and a minimal amino acid mutation (K/R313R) in the stalk region of the HA protein were associated with airborne transmission. Furthermore, transmission was not associated with a loss of virulence.
These findings highlight the importance of the internal genes in host adaptation and suggest that natural isolates carrying these mutations be further evaluated. Our results demonstrate that a highly pathogenic avian H7 virus can become airborne transmissible in a mammalian host, and support on-going surveillance and pandemic H7 vaccine development.
Importance: The major findings of this report are that a highly pathogenic strain of H7N1 avian influenza can be adapted to become airborne transmissible in mammals without mutations altering the receptor specificity. Changes in receptor specificity have been shown to play a role in the ability of avian influenza viruses to cross the species barrier and these changes are assumed to be essential. The work herein challenges this paradigm, at least for the influenza viruses of the H7 subtype, which have recently become the focus of major attention as they have crossed to humans.
A bit surprisingly, four of the five amino acid changes that were associated with airborne transmission were found to occur in the internal genes (PB2, NP, M1) of the virus. Given the history of H9N2 donating its internal genes to novel reassortants (see Study: Sequence & Phylogenetic Analysis Of Emerging H9N2 influenza Viruses In China), these findings may help identify potential surveillance targets in that subtype, and others.
This isn’t the first time we’ve seen evidence of airborne transmission of an H7 virus in ferrets.
In 2013, in Nature: Limited Airborne Transmission Of H7N9 Between Ferrets & Science: H7N9 Transmissibility Study In Ferrets), we saw lab experiments that showed this H7 avian flu virus could be transmitted between ferrets (albeit at low levels) via respiratory droplets.
So far, while airborne transmission has been demonstrated (in ferrets) in the lab, we’ve yet to see evidence of sustained and efficient airborne transmission of these novel avian viruses in humans.
But nature’s lab is open 24/7, new reassortant flu viruses are appearing all the time, and just because it hasn’t happened yet doesn’t mean it can’t happen sometime in the future.
