Tuesday, February 12, 2019

J. Virology:Genetic Compatibility of Reassortants Between Avian H5N1 & H9N2 Influenza Viruses


In the fall of 2014 Egypt began reporting on what would turn out to be the largest human H5N1 outbreak on record, one that over the ensuing six months would result in 160+ infections, and 51 deaths (see EID Dispatch: Increased Number Of Human H5N1 Infection – Egypt, 2014-15).
Since then, human infection with H5N1 in Egypt has dropped off a cliff, with 10 cases reported in 2016, 3 in 2017, and none in 2018. 
While the exact reasons behind this change in direction aren't known, the H5N1 virus is highly mutable, and it is likely the virus has attenuated slightly over time, much as it has in Vietnam, Indonesia and China.

Influenza viruses evolve via two well established routes; Antigenic drift & Antigenic Shift (reassortment).
  • Antigenic drift causes small, incremental changes in the virus over time. Drift is the standard evolutionary process of influenza viruses, and often come about due to replication errors that are common with single-strand RNA viruses (see NIAID Video: Antigenic Drift).
  • Shift occurs when one virus swap out chunks of their genetic code with gene segments from another virus.  This is known as reassortment. While far less common than drift, shift can produce abrupt, dramatic, and sometimes pandemic inducing changes to the virus (see NIAID Video: How Influenza Pandemics Occur).
In either case, most mutations/reassortments are evolutionary failures.

They die out quickly because they are not as biologically `fit’ as the parental virus they must compete with. Only rarely does a mutation convey enough of an evolutionary advantage to allow it to become `fixed' in a host, and potentially transmitted onward.
While the recent downward momentum of H5N1 human infections is a very welcome trend, there is no reason to suspect that another reassortment, or the right combination of amino acid changes due to antigenic drift - or both - couldn't reverse this trend. 
As discussed often before (see The Lancet: H9N2’s Role In Evolution Of Novel Avian Influenzas), the avian H9N2 virus is a highly promiscuous virus, reassorting with many other subtypes easily.  In addition, we've seen the H9N2 continue to evolve towards a more `humanized' virus (see Virology: Receptor Binding Specificity Of H9N2 Avian Influenza Viruses).
While H9N2 has circulated in the Middle East for more than two decades, it was only first detected in Egyptian poultry in 2010 (cite), prompting concerns over how it might interact with HPAI H5N1.
The H5N1 virus already carries some internal genes from the H9N2 virus, and so the concern is another exchange - of more recent, potentially more `mammalian adapted'  H9N2 genes - could happen again with uncertain results. 
As surveillance and reporting has deteriorated badly in Egypt over the past 6 years (see Revisiting Egypt’s Murky H5N1 Battle), we don't have a very good handle on what is happening with avian flu viruses in that part of the world.
All of which serves as prelude to a 2018 open-access study, published in the Journal of Virology, that looks at the results of experimental reassortments of Egyptian H5N1 and H9N2 viruses in the laboratory.

This is a lengthy, and at times technical, report. The abstract, however, is pretty straightforward. I've only posted a few excerpts, so follow the link to read it in its entirety.

Genetic Compatibility of Reassortants between Avian H5N1 and H9N2 Influenza Viruses with Higher Pathogenicity in Mammals
Yasuha Arai, Madiha S. Ibrahim, Emad M. Elgendy, Tomo Daidoji, Takao Ono, Yasuo Suzuki, Takaaki Nakaya, Kazuhiko Matsumoto, Yohei Watanabe
Stacey Schultz-Cherry, Editor

DOI: 10.1128/JVI.01969-18


The cocirculation of H5N1 and H9N2 avian influenza viruses in birds in Egypt provides reassortment opportunities between these two viruses. However, little is known about the emergence potential of reassortants derived from Egyptian H5N1 and H9N2 viruses and about the biological properties of such reassortants.
To evaluate the potential public health risk of reassortants of these viruses, we used reverse genetics to generate the 63 possible reassortants derived from contemporary Egyptian H5N1 and H9N2 viruses, containing the H5N1 surface gene segments and combinations of the H5N1 and H9N2 internal gene segments, and analyzed their genetic compatibility, replication ability, and virulence in mice. Genes in the reassortants showed remarkably high compatibility.
The replication of most reassortants was higher than the parental H5N1 virus in human cells. Six reassortants were thought to emerge in birds under neutral or positive selective pressure, and four of them had higher pathogenicity in vivo than the parental H5N1 and H9N2 viruses.
Our results indicated that H5N1-H9N2 reassortants could be transmitted efficiently to mammals with significant public health risk if they emerge in Egypt, although the viruses might not emerge frequently in birds.

IMPORTANCE Close interaction between avian influenza (AI) viruses and humans in Egypt appears to have resulted in many of the worldwide cases of human infections by both H5N1 and H9N2 AI viruses. Egypt is regarded as a hot spot of AI virus evolution. Although no natural reassortant of H5N1 and H9N2 AI viruses has been reported so far, their cocirculation in Egypt may allow emergence of reassortants that may present a significant public health risk.
Using reverse genetics, we report here the first comprehensive data showing that H5N1-N9N2 reassortants have fairly high genetic compatibility and possibly higher pathogenicity in mammals, including humans, than the parental viruses. Our results provide insight into the emergence potential of avian H5N1-H9N2 reassortants that may pose a high public health risk.

The cocirculation of H5N1 and H9N2 viruses in Egypt provides opportunities for coinfections to produce reassortant viruses. Using reverse genetics, we found high genetic compatibility of H5N1-H9N2 reassortants, resulting in higher virulence of reassortants in human cells and mice than the parental H5N1 and H9N2 viruses. To our knowledge, this study presents the first broad-spectrum data on the emergence potential of H5N1-H9N2 reassortants with distinct phenotypes.
Previous studies using reverse genetics in the genetic background of an AI virus and of a seasonal influenza virus showed various degrees of genetic compatibility between the two viruses, with occasional high pathogenicity in humans (2326). In contrast, our study showed that reassortment between the AI viruses, even two subtypes of a bird species, could generate hybrid viruses with significantly high public health risk for humans.
In conclusion, our analyses indicated a substantial emergence potential of influenza virus reassortants derived from the H5N1 and H9N2 viruses currently cocirculating in Egypt, as well as the possibility of their high public health risk for humans relative to the parental H5N1 and H9N2 viruses. Cocirculation of the two influenza virus subtypes in birds may accelerate the emergence of novel viruses that may be a public health risk.
Our results underscore the necessity for enhanced influenza virus surveillance strategies to monitor reassortment events in nature and reduce the public health threat posed by the H5N1 and H9N2 viruses cocirculating in Egypt.

(Continue . . . )

For more on the ubiquitous (in Asia & the Middle East), and constantly evolving H9N2 virus, you may wish to revisit:
EID Journal: Two H9N2 Studies Of Note
Avian Bio. Research: Sequence & Phylogenetic Analysis Of Avian H9N2 HA Genes - Iran

Macao Health: Guangdong Province Reports Human H9N2 Infection
Sci Rpts:Reassorted H9N2:pH1N1 Virus Transmission After Serial Passage In Swine