Folks,
There is no known worm that causes lesions like this in aquarium fish. The parasite that has been associated with HLLE is a protozoan of the Hexamita group, it is an internal parasite - in the fish's gut. Early studies of these "hole-in-the-head" lesions in fish consistantly turn up nothing more than normal bacteria, no pathogens at all. Like I said, the "worms" are just mucus plugs.
The relationship with HLLE and Hexamita is more concrete in freshwater fish - treating discus or oscars with oral metronidazole will usually fix them right up (unless they are too severely scarred). I have never had any luck treating HLLE in marine fishes with metro. How can a gut parasite cause head lesions? Good question! It is presumed that the higher-then-normal numbers of Hexamita in the gut "steal" nutrients from the fish, their health degrades, and the head lesions develop.
IMO: for marine HLLE, the only PROVEN cure is to move the fish to a new (presumably better) aquarium. Look at the other thread going on right now where a person moved a hepatus tang to a new tank and it almost immediately started to get better. They are attributing it to a better diet, but that is awfully quick for a dietary change to happen - besides, that also presumes that the fish was being fed poorly before it was moved, and there is no indication of that. All of the "vitamin C" studies I have heard of that definatively worked ALSO included moving the fish to a new tank. HLLE is NOT contagous, there are no confirmed cases of a fish with HLLE transferring it to a new, unaffected tank. Aquarium systems that have never had HLLE tend to stay that way, unless something major is changed in the lifesupport system.
Grounding probes have long been ruled out as a cause of marine HLLE - but this old saw is taking a long time to die. There is one proven cause, and I've said this many times here - particulates, especially from certain types of carbon, free-floating in the water, have been proven to cause HLLE. What is the mechanism? I'm applying for funding to try to determine that next year.....
Jay