Characterization of multidrug-resistant influenza A/H3N2 viruses shed during 1 year by an Immunocompromised child

Background. Development of influenza drug resistance is an important problem in immunocompromised children that could result in treatment failure and viral transmission to others. Methods. A total of 17 influenza A/H3N2 isolates were recovered over a period of 1 year from an immunocompromised child who was initially treated with oseltamivir and then with amantadine and zanamivir for viral pneumonitis. Drug susceptibility phenotypes to oseltamivir, zanamivir, and peramivir were evaluated by neuraminidase (NA) inhibition assays, and sequence analysis of key viral genes (i.e., M2, NA, and hemagglutinin [HA]) was performed. The impact of NA mutations identified in oseltamivir-resistant isolates was analyzed using recombinant NA proteins. Results. An influenza A variant with NA mutations E59G, E119V, and I222V was first detected after 38 days of oseltamivir treatment. In an NA inhibition assay, this variant was 274 times more resistant to oseltamivir than the original isolate but was susceptible to zanamivir. The I222V substitution enhanced the level of oseltamivir resistance that was primarily conferred by the E119V mutation in recombinant NA proteins. Remarkably, the E119V mutation persisted for 8 months after cessation of oseltamivir. Amantadine therapy led to rapid emergence of the M2 mutation S31N, which is known to confer amantadine resistance. The patient shed the virus intermittently while receiving nebulized zanamivir therapy despite the absence of a resistance phenotype, which could be the result of nonoptimal drug delivery and impaired host immunity. Conclusions. This study highlights the potential for emergence and persistence of multidrug-resistant influenza isolates in immunocompromised subjects even after cessation of treatment, reinforcing the need for development of new anti-influenza compounds.