That's flubendazole, and it's a wormer - not a preventative though a good worming program does act as a sort of preventative. There's no real prevention for worms as they're in the environment.
As this is related to fenbendazole, it likely is a good wormer to use for rounds. Can you get that there in Guam? Do you have the website from where you get it? Or did you buy locally? After you talked to us the other day, I'd like to keep a nice Guam website on hand just in case.