Dog chewing leg obsessively

[amcal]

My parents have a female lab mix.  She maybe 6 years old.  About a year ago, she started chewing this small spot on her leg.  She chews it raw.  They took her to the vet, did tons of testing and didn't find anything.  So, they tried everything - they put a cast on it, they put this chain contraption on it, they tried allergy meds, creams, salves, deterrent sprays - nothing has worked.  The vet has no answer other than OCD.  He thinks -she started chewing and can't stop.  Well, now she has started on a second spot on her leg.  The first one is about the size of a quarter and, it's deep and now she's got another spot going.  They've done skin testing, various parasite, fungus etc.. Testing and everything is negative.  

​does anyone have any thoughts?  I'm almost wondering if something like Prozac might help?  

[Imakcerka]

My big dog does this.  Nothing wrong with her.  We think it's a nervous thing.  When she does it I ask her to stop and she holds her leg up and it kills her to just look at it.  But there is nothing wrong with her.