This does seem to be true. I suspect that it's because the nematodes can't get in to the snail body very easily whereas they can crawl through the slug's breathing hole which is on their right hand side under the mantle. This probably explains why it is the mantle that seems to be the nucleus of the infection when a slug begins to die.

Have a look at the pictures in this article:

http://www.just-green.co.uk/8b-Photo...pr05_Slugs.pdf