Ideas on cause of swollen sheath in stud colt? I have a yearling stud colt that developed a swollen sheath...?

I rescued a yearling stud colt, well I BOUGHT him and his same-aged pasture-mate, two days ago. They are both very underfed, but very good natured. I let them settle for a day, then wormed them with Equimectrin, and they both have passed massive amounts of round worms. Today when I went out, I noticed the smaller of the two's sheath is very swollen. Not sure if all of these factors, or the stress of having a NORMAL life plays a part in this... just want some insight or ideas. I'm calling the vet regardless.