My theory for the reason why they specifically cut Ethan’s wrist - other than the fact that Alcina is way too elegant to

My theory for the reason why they specifically cut Ethan’s wrist - other than the fact that Alcina is way too elegant to be taking a bite off some random mortal her daughters dragged in - is that Alcina’s bite is contagious of whatever virus is behind their mutations. 

In vampire lore it is not unusual for the bite of a powerful vampire to cause strong side effects, if not to induce transformation altogether. Considering their cult is female-only and being turned into a “full vampire” is probably one of the greatest honors among them, it would not do to accidentally infect this one guy, much less when Mother Miranda appears to need him alive. 

In the picture she seems to be drinking with her mouth nearly closed - almost as if to avoid her saliva from touching the wound. We have also never seen her bite anyone, while that appears to be her daughters’ preferred mode of attack.