The other day I noticed that Griffin had a small fluidy looking swelling hanging between his front legs. Then it was about the size of a baseball, a little smaller even. A few days later, and it was much larger! It jiggles when he walks. I also think that the chest area is puffy and inflamed, and the jiggly fluid filled sack is probably what is draining from his chest. It's hard to really see how it affects his locomotion, since he catwalks around me anyway, but I do think he is moving more stiffly.
Everything I know about diseases tells me that this sounds like pigeon fever, but in all my research online, it doesn't appear to occur in the midwest. It seems to be a western states thing. Pigeon fever, also called dryland strangles, is caused by the introduction of a bacteria called Corynebacterium pseudotuberculosis usually by flies and their bites. It can be very contagious and also spread to other horses by flies or poor sanitation practices. How Griffin might have come down with this, I'm not sure, although the bacteria does occur naturally in the soil in some regions.
But I could be way off. We also have Lyme disease here, but I don't think that would cause a reaction like I'm seeing.