looks like Xeno 50 & 450 do roundworms only as far as worms go, xeno 200 parasite treatment does roundworms & hookworms.
but the xeno ultra spot on doesn't do any internal parasites or worms of any kind.
doesn't look like any of them cover lungworm.
Think i would go for the Panacur as it sounds like a tried & tested specific wormer, i used to use a form of Panacur on my previous dog& cat i think too.
i know my cousins dog was really ill when it was a puppy as it had a slug/snail eating habit, think they had to muzzle him for a while to stop him eating them it was so bad, he was a real mollusc junkie!
the vet would know if lungworm was covered in any of the above treatments, if it was deemed necessary seeing as it was only one slug.
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@Critter your a mine of information

xx