Assessing the susceptibility of highly pathogenic avian influenza H5N1 viruses to oseltamivir using embryonated chicken eggs

BACKGROUND & OBJECTIVES: The susceptibility of influenza viruses to neuraminidase inhibitors (NAIs) is studied using enzyme-based assays, sequence analysis and in vitro and in vivo studies. Oseltamivir carboxylate (OC) is the active prodrug of the NAI oseltamivir. There is lack of information on the use of embryonated chicken eggs for studying susceptibility of highly pathogenic avian influenza (HPAI) H5N1 viruses to antiviral drugs. The aim of the present study was to assess the use of 10 day old embryonated chicken eggs for studying antiviral susceptibility of HPAI H5N1 viruses. METHODS: Two HPAI H5N1 viruses isolated from India were used in the study. Fluorescence-based NAI assay was performed to determine antiviral susceptibility of these viruses. In ovo antiviral assays were carried out using 10 day old embryonated chicken eggs. The virus dilutions were incubated with 14 μg/ml of OC and inoculated in the allantoic cavity. In the eggs, 50 per cent egg infectious dose (EID(50)) titres as well as mortality were quantitated. RESULTS: The two viruses used were susceptible to OC in the NAI assay. It was found that there was a significant drop in EID(50) titres; however, no significant protection from mortality after OC treatment was observed. INTERPRETATION & CONCLUSIONS: By measuring viral titres, the egg model was suitable to study the susceptibility of HPAI viruses to antiviral drugs along with NAI assay. The present study highlights the use of eggs as a model to study susceptibility of HPAI viruses to OC.