Uronema shows up in eDNA tests all the time, because there are two “forms”; the falcultative intercellular parasite and the free living form that feeds on bacteria. The latter causes no disease issues. In fact, up to 75% of long term established tanks will have Uronema in them.
The reason that topical treatments don’t work on Uronema, is the drug never reaches the protozoan. The red lesions that you see is just the tip of the iceberg, you only see that after the infection has been raging in the fish’s muscle tissue for some time.
In theory, oral anti-protozoal drugs would work against intra cellular Uronema, but since one early symptom of this disease is that the fish stops eating, it is tough to do that.
Seahorses and seadragons develop another type of Uronema infection, that starts on the outside of the fish and works inwards. I’ve been able to treat that with formalin baths.