Selection and viral load kinetics of an oseltamivir-resistant pandemic influenza A (H1N1) virus in an immunocompromised patient during treatment with neuraminidase inhibitors

Prolonged viral excretion in immunocompromised hosts leads to long oseltamivir treatment and to the subsequent development of oseltamivir-resistant pandemic influenza virus selection. We report the selection and nasopharyngeal shedding kinetics of an oseltamivir-resistant strain in a hospitalized immunocompromised patient with prolonged influenza illness. Viral load quantification and genotyping methods were performed from 7 serial nasopharyngeal samples. Before initial oseltamivir treatment, the viral load was 5.78 log10 copies/mL of sample and only wild-type virus population was detected. The nasopharyngeal viral load remained above the detection limit although there was a second course of oseltamivir treatment. Twelve days after the onset of symptoms, an oseltamivir-resistant strain was selected. After 12 days of inhaled zanamivir treatment, the patient was discharged asymptomatic. The study emphasizes the importance of viral load quantification and surveillance of emergence of resistant strains prospectively because the information provided has important implications in the clinical management of the patient.