Why is this syndrome not documented, or is it a bug ?
Others have had issues with this since 2010 ( Google search ), there are no syndromes listed in the wiki that do this, there are no syndromes in the game's files that have this effect ... and no, it's not acid or boiling rain.
If the cause is indeed the evil sludge (which seems likely), then there won't be any documentation because syndromes for evil weather, as for FBs, Titans, and demons, are procedurally generated for individual worlds.
As for why dwarves don't get it, as others have said the syndrome could be caused by ingestion: only cats licking themselves would have cause to ingest the sludge, so naturally only they'd be affected.
However, it may still be contact-based: dwarves tend to wear shoes/socks, which would prevent their bare feet from picking up the sludge, but cats are barefoot.
As for why it doesn't affect adult cats, I admit I don't have a sure answer for that. My guess is that it has to do with size, for one of two reasons: 1) some syndromes are diluted with size, so perhaps the syndrome is weak enough to be shrugged of by adult animals, while kittens are still small enough to be affected; 2) larger creatures naturally have more blood to lose, so even if adults
are affected by the syndrome they might not die from the bleeding.
Another possibility: is your fort in a "Scorching" region? Long exposure to such heat can melt fat and cause bleeding (which, again, smaller creatures might not be able to survive).
Either way, my advice is to quarantine your cats indoors. If the problem is sludge-related, a few kittens might still succumb (from picking up the syndrome before they were quarantined), but eventually the deaths will stop. At least this way you can test the hypothesis: if the deaths continue long after the quarantine... I don't know what your problem is.