I will start her on the sulfatrim tomorrow and post updates here. Please keep Duck in your thoughts!
For example, my chickens were treated for possible gape worms - a parasite that lives in the throat - with fenbendazole (Safe-Guard). Gape worms interfere with breathing and a chicken can repeatedly open and close its mouth, trying to breath better. Fenbendazole happens to be one of the antiparasitics listed in my old copy of Carpenter's Exotic Animal Formulary for giardiasis (all species) and mice (cestodes, pinworms). There seem to be a lot of rodent treatments for pin worms (guessing that cyclically, they would pass out of the digestive system and perhaps even be visible to the naked eye).
All this said, it is unusual and I don't recall any guinea pig on the forum having parasites in the throat/respiratory system so this is just a wild guess.
Gape worms seem to be mainly a problem for birds. More about gapeworms in chickens:
Some cool slides identifying various parasite eggs, including gape worm, which could look like a grain of rice (perhaps smaller than what you found, though). A tutorial on identification of internal parasites in mammals and birds:
https://www.slideshare.net/VanessaVanRh ... nd-draft-2