Chthulhund had 'worms' in her eyes, but they could just as easily have been maggots.
Flies are immune to the Rash and other such diseases, and in fact often thrive on decay. Most beasts have lots of spots 'leaky' enough for a fly's taste, and let's face it, they ain't picky about what they liquify. Also, trolls make ideal candidates for blowflies looking for hosts for laying their eggs, moist, meaty(ish) and who probably are unlikely to take measures to repel them or kill the eggs.
Horseflies tend to take out small chunks of flesh (at least, some species do, I used to suffer in Michigan), therefore effecting a break in Reynir's skin suitable for pathogens to enter.