Sheath swelling

Question
My horses sheath is swollen on, one side. When he starts to work and gets warmed up it gets hot on that side. When he continues through a full workout it has shrunk significantly, almost back to normal. Then over the course of 2 to 3 days of inactivity it swells back up.
He does not seem to be in any pain or discomfort when it is examined or I clean his sheath.
What could this be? What tests could be done to determine what it is? What are some possible treatments?

Answer
Norma:

Not sure I can tell you for certain what this might be, but I would investigate it with blood work and an ultrasound exam at the very least. It could be a lymph node or a small area of inflammation or infection. Swelling in the sheath also occcurs readily with disruptions in the blood protein levels. An ultrasound exam would facilitate the ability to directly visualize this and if its a mass of some sort, a needle can be used to aspirate it to determine what the tissue is and if its inflammatory, infectious or other tissue. I hope this helps you some. Let me know if I can answer any other questions for you.