We've an unusual notification today from the OIE of a laboratory confirmed outbreak of Highly Pathogenic (HPAI) H9N2 in a poultry flock in the southwest of Ghana, near their border with Cote d'Ivoire.
While H9N2 has been a frequent focus of our attention and has famously lent its internal genes to HPAI H5 & H7 viruses - until now - I've only seen it described as a Low Pathogenic (LPAI) virus in poultry.Now, this could be a lab error, or a typo, and could be nothing. But if the OIE report (below) is correct, then we've a bit of an outlier. First the report, then I'll be back with more.
While LPAI H9N2 can cause significant morbidity and mortality in poultry, what determines whether an an avian virus is Highly Pathogenic are specific mutations at the HA cleavage site.
Normally we see these occur when a Low Path H5 or H7 virus is introduced into a poultry flock, and it mutates from LPAI to HPAI. It is for this reason that all LPAI H5 and H7 virus outbreaks are reportable to the OIE.
An excerpt from Ecology and Evolution of Avian Influenza Viruses explains:
Avian influenza viruses are normally asymptomatic in wild birds, or cause mild disease in poultry, and therefore are termed low pathogenic avian influenza (LPAI) viruses. However, the acquisition of a multibasic cleavage site in the hemagglutinin protein genes of H5 and H7 subtype influenza A viruses can cause a highly pathogenic avian influenza (HPAI) virus resulting in severe disease in poultry and wild birds.Over the past few years we've seen a number of spontaneous H5 and H7 LPAI-to-HPAI mutations, both in Europe (see here, here, and here) and two years ago in the H7N8 outbreaks in Indiana (see Genome Announcements: LPAI-to-HPAI Mutation Cited in January's H7N8 Outbreak).
While LPAI H9N2 viruses have spread out of Asia over the past 25 years, into the Middle East, and most recently into Africa - and have caused considerable damage to the poultry industry (not to mention enabling HPAI H5 & H7 viruses) - it has not been considered a high path threat.
There are admittedly routes to increased pathogenicity other than the acquisition of a multibasic cleavage site in the HA, but technically - without that - they would still be categorized as a low path virus.Today's report provides us with an unusual classification for an H9N2 virus. And it comes on a day with an equally unexpected avian flu announcement from China ( Jiangsu China Reports 1st Novel H7N4 Human Infection).
Whether this report stands, or is revised, remains to be seen. But it is a reminder that influenza has the power to surprise us, and that what was true yesterday may not hold true tomorrow.