The fluke brings along clostridium, so once the eggs are laid in out of the "norm" places a dying fluke will trigger the clostridium and that bacteria produces solvents allowing the eggs to hatch and solvents allow the extraction of Retro Viruses.
Just like any defense structure, once penetrated it is much more difficult to mop up.
Now Buski isn't a very fast growing critter so you have 4 months to get it before any new eggs are produced but it is the larva stage etc that cause most of the hormone damage in relation to cancers.