I'm pretty sure that most Marine Toads are wild caught, although I hope that someone is starting a captive breed program out there somewhere. Anyhow, I have two of them. One was given to me at the end of a reptile show just because the vendor didn't want to have to bother bringing him back with him. My local petshop also tired of one and also gave him to me after having no luck selling him. I had them seperated for months, then finally decided to put them in together when I didn't see any signs of ill health. They are the best of buddies it seems.
If you do get a Marine, you could collect a fecal sample and take it to your local vet to have it analized. You shouldn't even have to take the toad in with you when you do this, and this would save you an office visit. Hopefully, all will be well, but your vet will know the best route to take if anything looks suspicious. I'm not sure what the treatments would consist of either.