ClamAV definitions out of date
This could be because of a number of reasons. Check that the database is up to date by calling the freshclam command directly with verbose:
fab $environment -H asset-master-1.backend sdo:'freshclam -v'
If it reports the virus databases are up to date then you may need to check the ClamAV virusdb archive to investigate.This page is owned by #2ndline and needs to be reviewed