Good evening y'all!

So I came home from shift today and discovered that my mare has developed a ventral edema. Never dealt with this before so it's fairly new to me... what can y'all tell me about it as far as treating it and anything else I might need to know?

Is it warm when you touch that area, is she running a fever?

I'm guessing she is heavy bred?

Where are you located? We've had a TON of pigeon fever up here in the PNW, and the abscesses can form anywhere on the body, not just the chest/breast area. Is it hot to the touch? If it at all seems like it could be that, I would keep an eye on it and call your vet before the abscesses rupture- typically the area will get very hot and will lose hair just before. If your vet can come out, lance and drain the abscesses and catch everything that drains, that would be ideal. It's highly contagious and easily spread by biting flies. 
That might be worst case scenario- sorry OP, I don't mean to scare you if it isn't that! I might just be gun shy since we've seen so many cases up here! It could be something as simple as blunt force trauma if she smacked it on something, or if she's late in gestation it could be from that.

No, it's not warm. No fever. 

No she is not. 

My gelding had that! Huge flattish lumps they were firm not soft. I freaked out lol. I'm 2 hrs from a vet. So I put him on immune herbs, and made a clay poultice and put him on a high vitamin a /vitamin supplement. I didn't know what I was up against. It went away. My friend who is a vet said maybe it was a over reaction to gnats! I still don't know. 

Allergies could be the cause of this, I was worried about Pigeon fever but if its not warm to the touch and no fever then pretty sure its not that, unless shes got a lump coming up on her chest area or lower. When my gelding did this he was warm in the area and had a fever and a abscess came up on his chest, he had pigeon fever

Can be from circulatory/ heart problems. If she's had a respiratory infection, lately I'd have the vet check her for pneumonia. 

Did you ride the horse that day or day before?

I 2nd the it looks like it could be pigeon fever.   We had a few horses get it in the chest and one in the belly like that.  It tooks months for it to fully resolve in the belly and we had to flush the channels of puss out.   The pigeon fever was warm and painful to touch.   

Wondering how your mare is doing?

Thank yall for all your input! I ended up starting her on an antibiotic and monitored her for a temp and the edema is almost completely gone now.  

Was your vet able to diagnose what case the edema? 

It was a dietary issue!