If blood and urine tests were normal, this would rule out most liver and kidney problem, which are the most common causes of edema.
Vasculitis can be a cause, this is the blood vessels becoming inflamed and then leaking fluid. Autoimmune diseases can cause this (where the immune system attacks itself). So testing for this could be done. We look for what are called anti-nuclear antibodies in the blood and also a biopsy of the edema area to look at the blood vessels can be useful.
Another cause that can show up as edema is called Potomac Horse Fever. This is caused by a bacteria named Ehrlichia risticii. Typically animals are not feeling too well, but it can show up as just edema. A blood test can be run to check for this. Here is website that has a paper on this Click here
Also, an ultrasound of the abdomen should be done to look to see if there are any masses in the abdomen that can be blocking the flow of blood back to the heart. This would cause increased pressure in the vessels and edema.
Also a heart evaluation can be done to make sure it is working well. Although not typical in horses, if the heart is not pumping properly, then fluid can be backed up and cause edema.
All of these suggestions are based on the fact that a blood test was normal. It may not actually hurt to repeat the blood test again, paying close attention to the total protein and albumin. If these are low, then it could explain the edema and then would need to look for a cause as to why these are low.
Hope this answers your concerns. Let me know if you have any questions left.
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