Why not just Wis damage, which kills the wrong kind of creatures?
'Cause I didn't think of it, that's why! I picked Con damage because that's the standard fatal-effect damage, it's as simple as that.
The original game makes no mention of the symptoms of wererat lycanthropy. The supplement
Night Howlers (1992) has this to say:
Signs of lycanthropy appear in half that time. Aching joints, elongated teeth, and sudden hair growth are common symptoms. Restlessness, strange cravings, and stranger dreams inevitably plague the victim’s nights.
The victim may be cured if he can find help before lycanthropy runs its course and turns him into a full-fledged lycanthrope. A cleric of 11th level or greater can cure lycanthropy with a cure disease spell. Once a lycanthrope has undergone the first transformation to beast form, not even a wish is a guaranteed cure.
Demihumans learn from an early age that lycanthropy means death to them. Lycanthropy acts as a poison to elves, dwarves, halflings, and most humanoids (save vs. poison or be infected). A demihuman infected with lycanthropy immediately feels feverish and weak, suffering a –2 on all rolls to hit and on saving throws. He can survive for 1-6 days, during which the condition will worsen. A stricken character unable to find a cleric of 11th level or greater to cure the disease within that span will die.
I guess "feverish and weak" could mean anything.
How about we have Wis damage for all types of victim, plus they're Sickened while the cursed disease is running its course? That'd match the -2 penalty of the PC4 description.
You might notice that
Night Howlers says Lycanthropy kills demihumans in 1d6 days instead of the 2d12 days it takes to transform humans. All other BECMI sources say it takes the same time to affect demihumans as humans, so I would have the Wis damage be the same for both.
Oh, and in closing, the original 1977 Basic D&D version of lycanthropy did not put a caster level restriction on healing it - any cleric who could cast
cure disease could remove it.