This is a problem experienced occasionally and could well be related to
Gut Extrusion as the early symptoms.
There is little you can do unfortunately. Try and make the snail comfortable. Personally, if it happened to me, I'd try and bathe the swollen area with tepid water. I'd also provide drinking water.
Snails exhibiting these problems are likely to die, so I'd try Aloe vera or something similar in application to try and soothe the condition.
So far it seems to be neither something that spreads to other snails, or something external. However, it is worth noting that snails seem to require soil flora and fauna to digest things. Changes in this micro-culture could cause digestion problems. For more reasoning on this and other possible causes see
Gut Extrusion.
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