You can use a screen as long as you are diligent about removing the lint to prevent possible fire hazard. Or buy some traps and place them near the vent.

I ordered a Heartland Dryer Vent(I am a sucker for advertisements). It is an enclosed vent. It has a baffle that floats when the dryer is in use. When the air flow stops the baffle lowers and closes off the bottom. It is supposed to help keep drafts out also. Here's to hoping it keeps the mice out!