If you could actually manage to remove the entire Yeerk . . . you'd remove decent-sized chunks of brain-matter with it. A Yeerk infesting a human has to tie itself to their neurons, cell by cell. At which point the Yeerk and the brain are basically a continuous unit. How else do you explain why the Yeerk dies when the host does? It'd take no less than a surgeon's scalpel to separate them, and even then, that would have to be one
heckuva surgeon. It should be noted that Essam was dying, and therefore already in the process of detaching himself from Hildy's brain, when Edriss pulled him out. If you pulled out a
healthy Yeerk, the effects would probably have been even
more severe.
And, do you honestly think a Vanarx would have evolved some kind of mystical ability to somehow separate the two, anyway? Evolution has no reason to give a crap about the host's well-being after the Vanarx is done with it. If the Vanarx doesn't get the whole Yeerk (or if it gets a few chunks of brain matter along with it), it just moves on to the next. Gedds are not exactly hard to hunt.
Hmm. Yanno, now that we're on the subject, why did the Vanarx ever evolve this ability at all? What nutritional benefit do Yeerks provide, that the Gedds they would be inside of, would not? Can't be Kandrona, because this is the Yeerk homeworld we're talking about, there'd be Kandrona everywhere. And, heck, why in the world wouldn't the Vanarx have just found itself a nice juicy Yeerk pool and gorged itself, without the need to evolve the oddly-specialized ability to pull Yeerks out of their hosts at all?
. . . I think we just found ourselves a KASU, and it is an entire alien creature's existence.