Nipah and Hendra viruses are recently emerged bat-borne paramyxoviruses (genus 
Henipavirus) causing severe encephalitis and respiratory disease in humans with 
fatality rates ranging from 40-75%. Despite the severe pathogenicity of these 
viruses and their pandemic potential, no therapeutics or vaccines are currently 
approved for use in humans. Favipiravir (T-705) is a purine analogue antiviral 
approved for use in Japan against emerging influenza strains; and several phase 
2 and 3 clinical trials are ongoing in the United States and Europe. Favipiravir 
has demonstrated efficacy against a broad spectrum of RNA viruses, including 
members of the Paramyxoviridae, Filoviridae, Arenaviridae families, and the 
Bunyavirales order. We now demonstrate that favipiravir has potent antiviral 
activity against henipaviruses. In vitro, favipiravir inhibited Nipah and Hendra 
virus replication and transcription at micromolar concentrations. In the Syrian 
hamster model, either twice daily oral or once daily subcutaneous administration 
of favipiravir for 14 days fully protected animals challenged with a lethal dose 
of Nipah virus. This first successful treatment of henipavirus infection in vivo 
with a small molecule drug suggests that favipiravir should be further evaluated 
as an antiviral treatment option for henipavirus infections.