I lost a few guppies to the viscitudes of the camallanus worm, but have managed to cure the survivor.

Apparantly there are no effective fish targetted treatments for camallanus worm. There are two treatments available for either birds or reptiles (Levamisole HCl or Fenbendazole, usually with Piperazine as a secondary ingredient). Fenbendazole is more difficult to use, there is a danger of overdose.

I tried the easy approach first. I put a diluted solution of Levamisole onto the fish food, let it soak in for 10 minutes then fed the fish (I didn't bother to dry it as some have recommended). I used the dosage recommended on the packet. This had no effect. I then used the solution directly on the food (no dilution). This did work. In three days (three feedings) the worms were gone. By the fourth day the guppy looked completely normal (no swelling, no stretching of the anus).