Is there any risk that they may take my work and use it anyway?
But it rarely makes sense. Think about it, interviewing is an expensive process that takes a lot of resources, and it's very rare that something like this would actually be a good use of time or money. It would be much more effective to post for contract-gigs on any number of sites.
Clarifying sub-questions because, of course, this is a literal possibility: What actions could I take should my design for them be stolen? (I'm in the USA)
You could sue them, but that's probably a bad idea. (Expensive, time-consuming, difficult to prove, and would ruin your reputation.) It's one of those things in life where you simply have very little recourse.
What should I do in the future to prevent similar work from being stolen?
Nothing. You could copyright it, watermark it etc., but back to the previous question, it's going to cost more than it's worth to try to enforce, and people who sue prospective employers have a hard time getting job offers. Often in life, there's just nothing you can do about getting screwed over. This is one of those things. Better to just approach it on the assumption of good-faith and not waste you time and money on the things you can't do anything about.