by Jared Reed  

Australian researchers have reported what they say is the first evidence of resistance to zanamivir (Relenza) in seasonal influenza A (H1N1) strains.  

Writing in the Journal of Virology, researchers from the Melbourne-based WHO Collaborating Centre for Influenza say their analysis of 391 samples of H1N1 influenza virus from the region found that nine (2.3%) had a markedly reduced zanamivir susceptibility.  

The H1N1 samples were collected from patients who were not being treated with the drug, and the resistance appeared to be conferred by a previously undescribed neuraminidase mutation known as Q136K. The mutation was unique to zanamivir and would not confer resistance to oseltamvir, they write.   

They note that there have been many reports of oseltamivir-resistant seasonal A (H1N1) viruses but until now there have been none relating to zanamivir.  

“The recent spread of oseltamivir-resistant A(H1N1) viruses ... has shown that neuraminidase resistant viruses can spread rapidly and circulate widely in the global community,” they warn.  

And given that the neuraminidase resistance mutations  have no apparent impact on viral fitness, “there is concern that one or both of the mutations may occur in highly pathogenic avian influenza A(H5N1) viruses thereby reducing the effectiveness of neuraminidase inhibitor treatment of infected patients,” they say.    

“Both oseltamivir (Tamiflu) and zanamivir (Relenza) have been stockpiled by many countries worldwide for use in the event of an influenza pandemic. It is therefore essential that the neuraminidase inhibitor sensitivity of A(H5N1) avian influenza viruses is constantly monitored so that the potential effectiveness of available treatments is known,” they conclude.