How can I trigger an SNMP trap if a particular string shows up in a
log file on a unix host?

For example, let's say I have an application that writes an error
"critical alert: blah" to /var/log/blah.  I'd like to have some way to
trigger an snmptrap that will notify another host.

I can find plenty of info about how to trigger traps for conditions
that are pre-defined in a MIB, but I'm confused how I'd trigger a
trap for something of this sort that is arbitrary and undefined.