Nowhere in the DEF/ST scoring rules is there anything about Fumbles lost, so they don't count as anything
Similair situation is if on an interception return the defender fumbles, no scoring there either
Stewie: [hitting on some co-eds] I must say, the most recent campus sporting event was quite spectacular.
Co-ed: Aw. Are you in a fraternity, little boy?
Stewie: Not yet, but I'm thinking of joining I Felta Thigh.